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EarthERSC 3001 Mineralogy I

Pre-requisites: ERSC 2003 or ERSC 2102

Sultan Qaboos University,   College of Science

Department of Earth Science

Instructor:  Prof. Dr. Sobhi Nasir

 

Course Description

Earth, the planets, meteorites, and comets consist mainly of minerals. Minerals and assemblages of minerals are, therefore, a major source of information about processes in our solar system. Closer to home, minerals are many of the resources we think of as commodities and raw materials - thus all national economies, to a large extent, depend on mineral resources. Modern agriculture is dependent on various fertilizers and soil conditioners, and thus the ability to grow many of the foods we eat is dependent upon mineral resources. Many environmental processes, such as those controlling the chemistry of waters, are strongly influenced by mineral reactions. For these reasons, all scientists, industrialists, economists, environmentalists - and basically every informed citizen - needs to know something about mineralogy and mineral occurrences on Earth.

Mineral crystals are beautiful and obvious examples of the universe's inherent order and symmetry. The study of crystals has increased our understanding of the size of atoms, and how they are arranged in compounds. For these reasons, mineralogy can be an important enhancement to studies of mathematics, geometry, and chemistry.

Mineralogy is a foundation course for geology. Rocks are made of minerals, and the occurrences and arrangements of minerals in rocks gives us much of the information we have on Earth's present and past processes. Interpreting Earth's various activities through the study of rocks requires a basic knowledge of mineralogy.

 

The goals of this course are to teach students to:

  1. have an appreciation of the relationships among the structure of a mineral, its chemical composition, and its physical properties;

  2. be able to identify samples of the common rock-forming minerals in samples of the common rocks;

  3. be familiar with the equipment and other resources available at Oman and elsewhere to characterize and identify minerals;

  4. begin to understand factors that affect the stability and occurrence of minerals and assemblages of minerals ("rocks").

Geology is a very "hands on" science, and we will spend much time on the examination of hand samples of minerals and rocks.

Because we will cover a rather large body of material in a mere 40 hours of lecture and 20 hours of lab, a tight schedule must be followed. The general class outline will be supplemented on a weekly basis with more specific outlines, reading assignments, laboratory assignments, and problem sets. You should expect to spend at least 5-10 hours per week on this course in addition to the lectures, laboratories, and field trips.

There are approximately 3500 known minerals, with more being discovered daily. Of the known minerals, we will study about 80 to 100 in detail, in terms of their structure, occurrence, chemical formula, economic and crustal significance. This can seem a formidable task - but no whining will be allowed. For an analogy, consider that chemists must learn everything about the 100 or so known elements; as the periodic chart is a device to group elements by common properties, we will systematically study minerals in groups based on their chemistry and structure. Beginning in about the third week of class, you will be asked to examine minerals in groups of 10-15 samples. There will be weekly quizzes in lab to help you along.

Scientists rarely work in a vacuum and seldom work alone. In encourage you to use all of the resources available to help you conquer the challenges of mineralogy. Mineralogy texts can be very valuable reference tools - you don't need to memorize mineralogical data if you know where to look up the required information. The bookstore has copies of Manual of Mineralogy, by Klien and Hurlbut, available for purchase. This text will be used extensively in this course and can be a valuable reference text for future use.

I also urge you to help one another in the lab, on take home problem sets and other assignments. Joint efforts are only a problem if the members of the group are not equal partners in the endeavor. Don't let other people do your thinking for you. (Note that there should be no consultation on "take home" tests.) Also, once you've careful considered a problem, concept, or idea, don't hesitate to ask for help from me. Everyone in the class will benefit from your voicing questions, both in and outside of the classroom.

Computers will be of considerable use in mineralogy, petrology, and many other geology courses. I will encourage you to use resources of the Internet, general programs such as Word and Excel, and mineralogy-specific programs in this course.

Mineralogy will be, for many of you, your first "upper level" underagrduate course. Mineralogy is not an introductory level course, and although it does not assume too much requisite material, the style of the course is dramatically different than introductory courses. For example, the text is a detailed and comprehensive resource that you can refer to over and over agian, not a topical survey. The down side of this, at first, is that the text will seem difficult to read. A second difference is that you are responsible for synthesizing information and understanding relationships among concepts. Mineralogy, as most geology courses, also requires visualization of 3-D relationships, which can be tricky. In contrast, most introductory courses emphasize rope memorization - you will be miserable in Mineralogy if you try to memorize your way through - so don't.

The study of minerals and geology is, after all, supposed to be interesting, useful, and fun.

 

Earth Science 3001: Mineralogy

Objectives and Evaluation

 

Text: Manual of Mineralogy by Klein and Hurlbut, 21st edition.

Lectures: Saturday through Monday, 10:00 AM, Room 0042N 

Labs: Tuesday , Room 0042N

Course Objectives: This course is designed as an introduction to the study of crystals, minerals, and assemblages of minerals (rocks). Specific goals of the course include familiarizing you with the common, rock-forming minerals, teaching you how to identify samples of these minerals, and providing you with an understanding of important relationships among their physical properties, chemical composition, crystal structure, and geologic occurrence. The syllabus listed on the accompanying pages is a tentative outline of lecture topics and associated readings. Additional assignments, such as problem sets, will be announced in class during the quarter; in-class quizzes may be unannounced.

Evaluation:

Test #1 10%
Test #2 20%
Final Exam (cumulative) 40%
Problem sets and quizzes  10%
Labs and Lab Final: 20%

Note: It is very important that you complete the assigned readings and all work on time. Warning: Getting behind in Mineralogy can be detrimental to your physical and mental well-being! In order to help you keep up, I will not accept late, unexcused assignments. Absences from class or lab may be excused in keeping with the University guidelines as expressed in the SQ University Bulletin. However, it is your responsibility to make arrangements for an absence ahead of time if at all possible, or to notify me of the absence promptly. I can always be reached through the Earth Science department office during normal business hours.

Course Calendar

Day 1

Class Introduction; Definition of a mineral

Begin Reading Chapter 1 in text.

Day 2

Symmetry Operations and Point Groups

p. 21-47

Day 3

The 32 Classes and Six Systems of Crystals

p. 43-53; 63-107 (main points)

Day 4

From finite to the infinite: 2D Lattices

p. 108-115

Day 5

Lattices and Miller Indices of the Crystal Systems

p. 123-128; 38-43

Day 6

Symmetry Operations with Translation

p. 129-133

Day 7

Other Considerations of Crystalline Materials

p. 150-169

Day 8

Atoms, Ions, the Periodic Chart, Bonding

p. 170-190

Test #1

Is all the preceeding stuff on the first test?

 

Day 9

X-ray Analytical Methods

p. 276-288, 228-231

Day 10

Packing, Coordination, Charge Balance

p. 190-201

Day 11

Pauling's Rules, Solid Solution

p, 201-210; p. 233-237

Day 12

Calculation of Mineral Formulas, I

p. 240-249; skim 315-330

Day 13

Mineral Physical Properties; Native Elements

Ch. 6; p. 334-350

Day 14

Sulfides, Oxides and Hydroxides

p. 350-398

Day 15

Halides, Sulfates, Phosphates, and Carbonates

p. 398-439

Day 16

Silicate Mineral Structures; SiO2 Polymorphs

p. 440-444; 524-532

Day 17

Tectosilicates: Feldspars

p. 532-543

Day 18

Tectosilicates: Feldspathoids, Scapolite, and Zeolites

p. 543-548, 548-557

Test #2

This is not much fun right now....

 

Day 19

Phyllosilicate Mineral Structures

p. 498-506

Day 20

Phyllosilicates: Serpentines and Clays

p. 506-515

Day 21

Phyllosilicates: True Micas

p. 515-522

Day 22

Inosilicates: Pyroxenes and Pyroxenoids

p. 474-485

Day 23

Inosilicates: Amphiboles

p. 485-498

Day 24

Asbestiform Minerals: Uses and Health Risks

class handouts, Internet

Day 25

Mineral Formulas, Part II

class handouts; p. 240-249

Day 26

Cyclosilicates and Sorosilicates

p. 462-474

Day 27

Nesosilicates: Olivines

p. 444-451

Day 28

Nesosilicates: Garnets

p. 451-454

Day 29

Nesosilicates: Aluminum Sliciates, etc.

p. 454-462

Day 30

Review of Crystallography and Crystal Chemistry

Ch. 9: p. 309-315

Day 31

Igneous Mineralogy and Rock Classifications

Ch. 14: p. 558-569

Day 32

Sedimentary Mineralogy and Rock Classifications

Ch. 14: p. 569-581

Day 33

Metamorphic Mineralogy and Rock Classifications

Ch. 14: p. 581-590

 

Test #3 - Final Exam

 

 

 

 

 

 

 


 

Earth Science 3001 Laboratory

 

Materials needed for lab:

In general, always bring your copy of Klein and Hurlburt's text, pencils, rulers, a protractor, and a pocket calculator;

Purchase a hand lens and a small magnet by the fourth week of the quarter.

Special note:

Please be very careful with materials provided for your study in lab. You can do this in two ways: 1) put samples and other materials back in their original location when you are through studying them; and, 2) make sure the lab door is closed and locked when you leave.

The weekly schedule for labs, for the ten weeks of the quarter, are as follows:

Lab #1: Introduction to the lattice, unit cell, and Miller indices. Crystal systems, classes, and forms.

Lab #2: Crystal systems and crystal classes, continued.

Lab #3: X-ray Diffraction Analysis.

Lab #4: Measurement of Physical Properties of Minerals; Native Elements, Oxides, Hydroxides, and Sulfides.

Lab #5: Halides, Phosphates, Sulfates, and Carbonates.

Lab #6: Silicates I: Tectosilicates. Quartz, Feldspars, Feldspathoids, Scapolites, Zeolites.

Lab #7: Silicates II: Phyllosilicates

Lab #8: Silicates III: Inosilicates.

Lab #9: Silicates IV: Cyclosilicates, Sorosilicates, and Nesosilicates.

Beginning with Lab #5, there will be quizzes in lab over the precceding week's material.

A lab final will be given in your last lab meeting, in the week following the Thanksgiving break.

 

 

 

 

 

 

Day 1


 A mineral is a naturally occurring, inorganic material with a fixed composition and a repeating internal structure.

In addition to the many obvious minerals that you are already familiar with, such as quartz, garnet, and diamond, consider the following - are they minerals?



Day 2


Symmetry (n): 1. A relationship of characteristic correspondence, equivalence, or identity among constituents of a system or between different systems. 2. Exact correspondence of form and constituent configuration on opposite sides of a dividing line, or plane, or about a center or axis. 3. Beauty as a result of balance or harmonious arrangement.

All two-dimensional patterns and three-dimensional objects, including minerals, may be grouped according to the symmetry they possess. Symmetry can be defined as "invariance to an operation". A symmetry element is the geometrical feature, such as a point, line or plane, that we can use to visualize order in an arrangement. A symmetry operation is the act of changing an object or arrangement of objects, by inversion, rotation, or reflection. Symmetry can be defined as "invariance to an operation, which is to say that one or more points in a design, etc. will appear unchanged, or unmoved, by the symmetry operation. Objects that are invariant to rotation are said to contain a rotation axis. Objects invariant to a reflection are said to contain a mirror plane.

If we ignore the possibility of translation (that's covered in lecture #6), symmetry operations also have the property that repeating the operation will eventually return a part of an object or pattern to its original position.

For example, rotation of 180 followed by another rotation of 180 is equivalent to a rotation of 360 (or 0). Thus the axis for rotation of 180 is called a "two-fold axis," because repeating the operation twice returns the object to its original position. Crystals can have only 1-fold, 2-fold, 3-fold, 4-fold, and 6-fold rotation axes. 5-fold, 7-fold, and higher order rotation axes cannot occur in crystals (Why, you should ask...), although they may occur in an object or motif.

The collection of symmetry operations (rotation, reflection, inversion and their combinations) that characterize an object all intersect at its center, i.e., in a point. Therefore, the collection of symmetry operations that characterize a crystal are termed the point symmetry, point group, or crystal class of the object.

Objects invariant to the combined operation of rotation about an axis followed by reflection across a plane perpendicular to that axis contain a rotoreflection axis. If an object possesses 2-fold rotoreflection about one axis, it possesses 2-fold rotoreflection symmetry about all axes. For this reason, 2-fold rotoreflection symmetry is given a special name: inversion. Because the orientation of the axis does not matter, the symmetry element is termed an inversion center (the one point unmoved by this combined operation). Every rotoreflection operation may be represented instead by the combined operations of rotation followed by inversion (rotoinversion). For various reasons, our text (and most other texts) use rotoinversion axes rather than rotoreflection axes.

A crystal may possess only certain combinations of symmetry elements. Only 32 possibilities exist and these are the 32 crystal classes or crystallographic point groups (see pages 31-32). Every mineral belongs to one of these crystal classes. Details of the 32 point groups are given in K&H (p. 63-100).

Only certain combinations of symmetry elements are possible and, for the same reasons, some symmetry elements require the presence of others. It is useful to remember that the line of intersection of any two mirror planes must be a rotation axis. If the mirror planes intersect at 90, the line of intersection will be a 2-fold axis; at 60 = 3-fold, at 45 = 4-fold, at 30 = 6-fold. Think about it....


Day 3

 


 

The 32 crystal classes may be grouped into six crystal systems according to the following criteria and constraints (compare with table 2.2 of K&H):

Crystal System

Rotational Symmetry Constraints

Axis Length Constraints

Axis Angle Constraints

Triclinic

One 1-fold or axis

No Constraints

No Constraints

Monoclinic

One 2-fold or (=m) axis

No Constraints

a=g=90

Orthorhombic

Three perpendicular 2-fold or axes

No Constraints

a=b=g=90

Tetragonal

One 4-fold or axis

a=b

a=b=g=90

Hexagonal

One 6-fold, 3-fold

a=b

a=b=90, g=120

Isometric

Four 3-fold or axes

a=b=c,

a=b=g=90

The 'lattice constraints' of the chart above are developed in chapter 3 of K&H, and in lecture #4.

Of the six crystal systems, which has the highest number of minerals? In other words, are minerals distributed evenly among the six crystal systems, or is their a tendency toward higher or lower symmetry among crystalline materials?

The crystal classes are identified in the textbook using Hermann-Mauguin notation (see K&H, Table 2.9). The Hermann-Maugin notation identifies rotation or rotoinversion axes (if present) along particular directions and mirror planes (if present) perpendicular to the same directions. The directions for writing out the HM notation for each of the six crystal systems as follows:

Triclinic

no particular setting

-

-

Monoclinic

the b-axis only

-

-

Orthorhombic

the a-axis

the b-axis

the c-axis

Tetragonal

the c-axis

the a-axis

the [110] direction

Hexagonal

the c-axis

the a-axis

the [210] direction

Isometric

the c-axis

the [111] direction

the [110] direction

We will define what is meant by various axes and directions in a subsequent lecture. The main point is that symmetry elements are listed in a crystal from the highest level of symmetry present (e.g., a 4-fold axis of rotation), to the lowest level present. Moreover, the symmetry of the crystal systems tends to increase from the top to the bottom of the table above.

We can also recap previous discussions by restating Neumann's Principle:

The symmetry elements of any physical property of a crystal must include the symmetry elements of the point group of the crystal.

Thus any physical property we may measure for a mineral - e.g., hardness, thermal conductivity, refractive index, x-ray diffraction - must vary with direction in symmetrical ways such that all of the symmetry elements of the point group (and perhaps more) are present for that physical property.

 

Day 4


The periodic internal structure of minerals causes minerals to occur with regular external forms - as crystals. The angles between two faces of a crystal is identical for all forms of the same mineral that exhibit the corresponding faces. This is true regardless of the size or place of origin of the crystals! The consistency of interfacial angles was first reported by Nicolas Steno in 1669.

Interfacial angles for well-developed specimens of mineral crystals are measured perpendicular to the line of intersection of two crystal faces. The angle reported is always the acute angle. Interfacial angles can be measured with a contact goniometer, or more precisely with a reflection goniometer.

The observation of the consistency of interfacial angles led to the suggestion that crystals are built according to a pattern from building blocks that are small relative to the size of a macroscopic crystal. The concept of these building blocks is essentially the same as that of the unit cell in modern crystallography.

This pattern, or ordered atomic arrangement, implies that a certain atom is present in exactly the same structural (atomic) site throughout an infinite atomic array. To say that an atom is in the "same atomic site" means that it is surrounded by an identical arrangement of neighboring atoms.

To create a pattern requires a repeating unit or motif and instructions for how to perform the repeat. The recipe for a specific pattern may be discovered from the pattern by making some careful observations:

  1. Identify points in the pattern that have identical surroundings, but which are offset from one another. The set of all translationally equivalent points is called the lattice for the pattern (in two dimensions, the "plane lattice");

  2. Chose one lattice point to serve as an origin;

  3. Connect the origin with adjacent lattice points (two for a two dimensional pattern, and three for a three dimensional crystal) with lines (called translations or lattice vectors), making sure that the lines are not collinear or (for three-dimensions) coplanar. The parallelogram (or parallelepiped) defined by these two (or three) translations is called the unit cell for the pattern or crystal and contains within its boundaries the complete recipe for constructing the pattern.

Note that the lattice can be thought of as an imaginary pattern of points (or nodes) in which every point has an environment that is identical to that of any other point in the pattern. A lattice has no specific origin, as it can be shifted parallel to itself. The lines along which the lattice can be shifted , i.e., translated, are the lattice vectors.

The various symmetry elements discussed thus far, with the addition of glide planes (translation), leads to formation of 17 Plane Groups, as shown on page 121 in K&H.

For any given lattice, there are many possible choices of unit cell. Normally, the unit cell translations are selected so that there is one unit cell for each lattice point (i.e., for an infinite lattice, the number of unit cells equals the number of lattice points). Any such unit cell would be a primitive unit cell. All primitive unit cells have the same area (or volume). In some instances, however, it is convenient to choose a larger unit cell such that there is one unit cell for every two (or more) lattice points. These are called centered unit cells. Centered unit cells are usually chosen to have a rectangular geometry.

Unit cell translations may be used to define a coordinate system for a crystal. Points are located in crystallographic coordinate systems by measuring parallel to the unit cell translations.

Unit cell translations and the axes of crystal coordinate systems are labeled a, b, and c. Their positive directions are selected to define a right-handed coordinate system: to the right and front and upward are positive directions, to the left and back and down are negative. The angles between unit cell translations are identified by the Greek letter corresponding to the name of the opposite (i.e., perpendicular) translation. (Note, my html editor won't do no Greek!) Gamma is the angle opposite c (i.e. the angle between positive a and positive b). Similarly, the angles opposite a and b are alpha and beta, respectively.

 


 

Day 5


Three dimensional lattices can be constructed by adding one additional translation direction (vector) to the plane lattices discussed yesterday. The five types of nets can be stacked ten ways to create 14 possible space lattices or Bravais lattices. The 14 Bravais space lattices represent all possible ways that a motif can be repeated in 3D space.

The bravais lattices also serve as our introduction to the six crystal systems: cubic, hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic. Note carefully the angles and axis lengths for each of the bravais lattice types and the relationship to the six crystal systems.

Note - Greek and other symbols may not come out correctly on this page - compare with your text.

Crystal System

Constraints to lengths of crystallographic axes

Constraints to angles between crystallographic axes

Triclinic

no constraints

no constraints

Monoclinic

no constraints

alpha=gamma=90

Orthorhombic

no constraints

alpha=beta=gamma=90

Tetragonal

a=b

alpha=beta=gamma=90

Hexagonal

a1=a2=a3

alpha=beta=90; gamma=120

Isometric

a=b=c

alpha=beta=gamma=90

Vector space is referred to the three, familiar, non-coplanar axes x,y, and z; unit cell vectors are referred to as a, b, and c.

Rational directions in a mineral may be located by extending a vector from the lattice point that is the origin of the unit cell to any other lattice point. The direction is labeled with the coordinates of the lattice point placed in square brackets without commas. For example, the direction parallel to the b-axis of a crystal would be [010]. This is the same direction as [020], [030], etc. By convention, [010] is used instead of [020], [030], etc.

Rational planes are perpendicular to corresponding rational directions: Rational planes in a mineral are defined by Miller indices, which may be determined for any plane (or any line in two dimensions) from the intersections of the plane (or line) with the crystallographic axes.

The recipe for Miller indices is:

  1. Determine the intercepts of the plane of interest with the crystallographic axes (i.e., the axes of one unit cell containing the origin) ;

  2. Invert the intercepts (so that x becomes 1/x);

  3. Multiply all terms by the lowest common denominator to clear fractions. The indices determined by this recipe are placed in parentheses without commas.

For example, a plane that intersects the a-axis at 2, the c-axis at 1, and is parallel to the b-axis, would have a Miller index of (102): (2,,1) - (1/2,1/,1/1) - (2/2,2/,2/1) - (102). Note that planes parallel to a crystallographic axis will have a zero in the Miller index for that axis. If the intercepts are negative, a bar is placed over the appropriate index (my html editor won't do this). When saying out loud a Miller index that contains a negative number, one says "bar" before the number.

( = "infinity" in the text above)

Note in the previous paragraph that Miller indices can be positive and negative; remember that crystal axes are a Cartesian coordinate system, in which positive directions are taken as 'to the front, right, and top.' Therefore, if you have to move back, left, or down to intersect a line or plane, the Miller indices of that feature will contain negative numbers.

Miller indices also conveniently give the equation for the plane of interest. In two dimensions, if A and B are the intercepts of a line on the x and y axes, the equation of the line would be: x/A + y/B = 1 if A = 1/4 and B = 1/3, then the equation of the line becomes 4x + 3y = 1. If A=3 and B=4, the equation becomes x/3 + y/4 = 1 or 4x + 3y = 12 Thus the Miller indices are the rational (whole number) coefficients for the equations for any set of planes with hx +ky + lz = constant

By convention, when one refers to a set of planes for which one or more axis intercept is unspecified, the letters h, k, and l may be used for the unspecified a, b, and c indices, respectively. Thus a general Miller index would be (hkl), which would refer to any or all rational planes. All the planes parallel to a given line in a crystal are said to belong to a single zone. For example, all (h0l) planes are parallel to the b-axis.

As an example of the interaction of symmetry elements that limits the

Day 7


Recall from our first day of class the definition of a mineral. Now, ask yourself the following questions:

  1. Can two different minerals have the same chemical compositions?

  2. Can two different minerals have identical structural arrangements?

The answer to both of these questions is yes - the combinations of chemistry and structure are unique for each mineral, but the individual chemistries, or structures, of different minerals may be similar. This leads us to consider the following special considerations of the crystalline character of minerals:

Isostructuralism: minerals with differing chemistry may have analogous structures. For example, halite (NaCl) and Galena (PbS) have analogous structures - in each, cations and anions are arranged in cubic lattices. Crystals in which the centers of the constituent atoms occupy geometrically similar positions, regardless of the size of the atoms and the absolute dimensions of the structure, are said to belong to the same structure type (K&H, p. 151).

Polymorphism: the ability of a specific composition of elements to crystallize into more than one type of structure. A constant chemical composition may crystallize into more than one structure because different structures may represent different levels of internal (structural) energy, and this energy may be a function of pressure, temperature, or both. For example, there are six polymorphs of SiO2, the most common of which is quartz. Although the composition of each is one silicon to every two oxygen, the structural arrangements among the SiO2 polymorphs differ.

Because differing structures can form from one compound, in response to differing conditions of temperature and pressure, mineral polymorphs are ideal indicators of pressure and temperature conditions in geologic processes. For example, the SiO2 polymorph coesite has a very dense and compact structure, which on Earth forms only at pressures reached at the base of the crust - or within the mantle. Therefore, if coesite is in a rock, a geologist can be certain the rock once experienced very high pressures. In addition to the SiO2 polymorphs, several other common groups of polymorphs are important indicators of pressure and temperature conditions (such as the aluminum silicates, the potassium feldspar polymorphs, and calcite-aragonite).

If a chemical compound crystallizes with a particular structural arrangement under high pressure and temperature conditions, will the structure change as the pressure and temperature changes, or will the original structure persist? If we look at a mineral in a basalt, for example, it is obviously not at the same temperature and pressure at which it crystallized (unless you're holding a very fresh basalt straight from a Hawiian volcano - ouch!). So ... would the minerals in a basalt change or remain unchanged as the rock cooled?

The answer to both is ... sometimes yes, sometimes no. Energy is also required to change the structure from one type to another - and the energy needed may be so great as to prevent the change from taking place. Some polymorphs formed at high temeprature and pressure conditions remain at low (surface) temperatures and pressures for billions of years. However, some polymorphs only occur at high temperatures and pressures, and change instantly to others when T and P are lowered.

Why the difference? It is because polymorphism, and the structural transformations between polymorphs, occur through three, very different mechanisms:

Displacive polymorphism: change or reformation of a crystal structure by rotating components in the structure. Changes from one polymorph to another through a displacive transformation do not require the breaking of chemical bonds. Analogous to a 'bending' of the structure or components in the structure. This type of transformation can occur very rapidly. For example, there is a 'high-quartz' polymorph (see fig. 152) that is only stable above 600 deg. C; the displacive transformation from high to low quartz (the quartz we know and love) is so quick that even in a laboratory, you cannot grow high quartz, quench it, and preserve the high-temperature structure. Stated differently, the kinetics of displacive transformations can be very rapid.

Reconstructive polymorphism: changes to a polymorph's structure through extensive rearrangement of atomic bonds, and reassembly of atoms. This type of structural arrangement requires a lot of energy, and thus only tends to occur at high temperatures or pressures. Once formed, the high P-T polymorphs tend to hang around for a long time. Samples of lunar igneous rocks, billions of years old, contain the high-temperature SiO2 polymorph tridymite.

Order-Disorder Polymorphism: the degree to which elements are ordered ('systematically arranged') among the possible sites of a crystal structure affects the symmetry of that structure, and different structural polymorphs can be formed by different degrees of disorder ('randomizing'). This one is tricky to visualize and appreciate, but it is very significant for some important minerals. As a first step, consider that perfect order, in nature, only exists in crystalline materials at absolute zero (this is the third law of thermodynamics)*. For a hypothetical crystal at absolute zero (0 K), there is only one possible arrangement of the constituent atoms. As minerals are at higher temperatures, elements tend to occur in more random distributions among possible sites.

For example, there are three polymorphs of potassium feldspar (KAlSi3O8): Sanidine, one of the potassium feldspar polymorphs, has a high degree of structural symmetry and a relatively random distribution of Si and Al (both of these elements can fit into sites, or 'holes', surrounded by four oxygens); upon cooling, the structure constricts, and there is a tendency for Al to go into some of the smaller sites, and Si to go into some of the larger ones, i.e., the distribution of aluminum and silicon tends to become more ordered. The low-temperature polymorph formed from sanidine by disorder polymorphism is microcline. The ordering of elements in the sanidine-microcline transition actually lowers the structural symmetry, as sanidine has a 2-fold rotation axis and mirror plane which microcline lacks.

Question: would you consider sanidine or microcline to be more common in slowly-cooled plutonic rocks, such as a granite? The answer is ... you have to think about it.

*In fact, it is thermodynamically impossible for matter to reach absolute zero. Therefore, there are no 'perfect' crystals. "Wait just a minute...", you may say, "I thought you said on Days 1 and 2 that minerals are perfectly ordered materials with infinitely repeating structures - I have it right here in my notes!". Well, er, ah ... we (myself, the authors of your book, and the rest of the 'mineralogy establishment') lied just a bit at first. A certain degree of randomness and disorder exists in all materials, at least on the atomic scale. So there. Like it or not, you can't get away from statistics and probabilities, and given even just a little chance, atoms will have some random variation in their structural arrangements.

Which, by chance, bring up yet another type of polymorphism:

Polytypism: mineral polymorphs may form through differences in the stacking order of idential units. These particular polymorphs are called 'polytypes'. For example, there are numerous polytypes of the micas resulting from different stacking orders of Si:O sheets. The differences in stacking order may result from the statistical probability that structural components may link together in a given way, or from a dependence on temperature and pressure conditions.

Defects are very important in controlling variations in the physical properties of minerals, such as hardness, electrical conductivity, mechanical deformation properties, and color. Consider that one means of 'tempering' is to press a mass of hot steel into a sheet, fold it, press, fold, etc., and then finally quench it (picture the blacksmith beating on iron, then plunging it into a bucket of water). This process introduces defects at high temperature, and prevents them from annealling ('healing'). The process can be reversed - heating tempered steel for a time allows the metal structure to anneal. If you've ever put an axe or hammer head in a fire to remove a broken wood handle, then you probably lowered the hardness of that tool.

There are six basic types of defects pictured on page 163 of your text:

The types of defects discussed above may form during the growth of a mineral, or through later deformation. The processes of radioactive decay may also distrupt a mineral lattice and introduce defects, through metamictization. The energy and particle emissions accompanying radioactive decay, of uranium, for example, can break atomic bonds and disrupt structures sufficiently to alter the mineral's physical characteristics (color, hardness, cleavage, etc.). The degree of metamictization tends to increase with higher concentrations of radioactive isotopes, and with time.

Question: Which would tend to be more metamict - a uranium-bearing zircon crystal in an Archean (>2.8 billion years old) granite, or one in a Cenozoic (< 65 Ma) granite?

Pseudomorphism occurs when one mineral has the outward appearance of another. This does not imply that the internal structure is the same (which be isostructuralism) - only that the outer shape and form is similar. Pseudomorphs also occur when a mineral of one crystal system grows in forms that look similar to those of other crystal systems. For example, aragonite is CaCO3 with a orthorhombic structure. A common habit, or general shape (see pg. 52), of aragonite is as six-sided crystals. In this way aragonite can occur as hexagonal pseudomorphs, and only the careful mineralogy sleuth who measures the angles among six-sided aragonite crystals would note they are not 120, and thus cannot be truly hexagonal.

Pseudomorphs can also form through replacement of one mineral by another. If one mineral is replaced by another, through a chemical reaction, the new mineral may take the shape of the former one. A common example of this is the oxidation of pyrite, which may result in formation of limonite cubes that are pseudomorphs after the original pyrite.

Twinning refers to the formation of identical, or similar, objects.

From the biology of twinning: Monozygotic twins derive from the division of a single zygote(fertilized ovum) during the first developing stages of the embryo after fertilization, hence the term monozygotic. These separate cell masses become embryos which are genetically identical and will be the same sex. Monozygotic twins are contained within the same chorionic membrane and three-quarters of MZ twins share the same placenta. This also means they have the same blood type. Incomplete or late division of the zygote and subsequent cell masses can result in conjoined or Siamese twins. Monozygotic twins make up about a third of all twin births but their occurence has nothing to do with heredity, unlike dizygotic twinning. MZ twinning occurs randomly in all racial groups and follows no discernable hereditary pattern.

Mineralogical twins are analogous to monozygotic, Siamese twins. Mineral twinning occurs when two (or more) crystals of the same mineral are joined along symmetry elements that are not normally present in individual crystals of the mineral (see pg. 97-103 and 146-149 in text). The twinned crystals are, by definition, compositionally and structurally identical. Twinning occurs for many different minerals, and across many of the chemical and structural groups of minerals. Most mineral twins, at least those visable to the eye, form during crystal nucleation and growth. Mineral twinning during crystal growth may be random, but the particular symmetry operations that join mineral twins are constrained by the crystal structure.

The operations, or twin elements, which may relate a twinned crystal to it's counterpart are rotation (a twin axis), reflection (a twin plane), and inversion (a twin center). Twinning is defined by a twin law, which indicates whether there is a center, axis or plane of twinning, and also defines the crystallographic orientation of the twin element (usually with Miller Indices). Certain twin laws are more common in particular crystal systems, and they are also diagnostic features for many minerals.

There are two main types of twins:

Contact Twins are related by reflection;

Penetration Twins are related by rotation or inversion.

Repeated, or multiple, twins are three or more twins repeated by the same twin law. If the twin law defines a plane, then polysynthetic twinning results (such as the twinning on (010), common in plagioclase and called 'albite law twinning'). If the twin law defines a rotation axis, then cyclic twinning results. Cyclic twinning of aragonite, which has an orthorhombic structure, can result in formation of crystals with a nearly perfect 6-fold rotation axis. The six-sideness of aragonite twins results from twinning on {110} faces, which are nearly 60 degrees apart.

Growth twinning results from the emplacement of atoms on the outside of a growing crystal in such a way that the regular arrangement of the lattice is interrupted. Growth twinning therefore reflects a misstep during crystal nucleation and/or growth, analogous to the formation of siamese twins. Transformation twinning occurs in pre-existing crystals and thus is a secondary form of twinning. Transformation twinning commonly results from structural changes during cooling, as in the formation of intersecting albite- and pericline-law twinning in sanidine (monoclinic) as it changes to microcline (triclinic). Glide twinning (deformation twinning) results in some minerals as they change shape in response to stresses of deformation; calcite crystals readily accomodate changes in shape during deformation by twinning.

Twinned minerals have their own following in the mineral collecting hobby - see other discussion and a list of minerals that commonly form specimen-quality twins in The Mineral Galeries twin page:

 


 

Day 8


Bond-mechanisms and the periodic table

The major types of bonds are:
Ionic: a form of electron capture or "swapping" between atoms of greatly differing valence (e.g., NaCl)
Covalent: electron sharing between two or more atoms of similar, or identical, valence (Diamond)
Metallic: Electron sharing whereby atoms move freely through a structure (native metals)
Van der Waal's: Weak polarized attraction among atoms, no electron transfer (graphite)

Ions are atoms which have a net electrical charge: electronegativity is a measure of the ability of an atom to attract electrons to it's nucleus. Metals have low electronegativity and are electron donors. They tend to become cations. Nonmetals have high values of electronegativity and are electron receptors, and tend to be anions. The difference in electronegativity can be used to estimate the 'bond character', which can be thought of as the degree of ionic and covalent character to a particular atomic bond.

Ionic Radii: The size, and radius, of an atom is not a fixed quantity, but rather depends upon the size of the atom in a neutral state, the valence of the atom in a compound, and the coordination of the atom (number of other atoms it is joined to) in a compound.



Day 9


Xrays are about 1x10-8 cm, and of wavelength intermediate to Ultravoliet and Gamma radiation.

X-rays are produced when electrons "boiled" from a filament are caused to strike a target of atoms by the force of a high voltage field. Decelleration of electrons as they approach atoms in the target creates a "white" background of radiation called the Brehmstrallen radiation. Superimposed on this background are peaks of intense x-rays that have wavelengths that depend on the target-atom involved. The peaks of characteristic wavelength are produced when an atom looses an electron from an inner shell, due to a collision with an accellerated electron, and then compensates by having an electron from an outer shell fill the partly vacant inner shell. The peaks are labeled Ka, Kb, La, etc., depending on the energy involved. This is the principal behind an electron microprobe, which uses a beam of electrons to cause emission of characteristic x-rays from a material, and thus identify the elements in the material.

Two of the most fundamental aspects of a mineral, it's space group and unit cell dimension, can be determined from X-ray diffraction experiments. To understand these experiments, which we do in lab, we must explore the physics of diffraction. Diffraction, generally defined as a departure of a ray from the path expected from reflection and refraction, was first observed for light in the early 19th century. Sets of narrow slits and ruled gratings were observed to produce diffraction patterns when the spacing of the slits is similar to the wavelength of light used. Because all of the slits in a diffraction grating are illuminated by the same source of light, the set of slits may be considered to be a set of light sources all in phase with one another. Light rays travelling perpendicular to the diffraction grating will remain in phase. Light rays at an angle f to the perpendicular will not be in phase, except for special angles such that S sin f = n l, where S is the spacing of the slits, l is the wavelength of light, and n is an integer. We may use this expression to determine l for a laser or S for a diffraction grating from a measurement of the spacing of the diffraction pattern.

Diffraction of x-rays by crystals is possible because the spacing of planes of atoms in crystals is similar to the wavelengths of x-rays. The atoms in crystals behave like little x-ray sources as they scatter incident x-rays. Although an X-ray experiment can be designed to be very similar to an optical diffraction experiment (a Laue experiment), most experiments involve x-ray "reflections". For the diffracted X-rays to be in phase, the geometry of the experiment must satisfy Bragg's law:

n*lambda = 2 d(hkl) sin(theta)

d(hkl) is the spacing between the parallel planes of atoms with Miller index (hkl), theta is the complement of the incident angle, lambda is the wavelength of the x-rays and n is an integer. Note that each set of planes (hkl) may produce more than one diffracted ray, each with different values of n.

When the beam of monochromatic light strikes the powder mount, all possible diffractions take place simultaneously. If the orientation of particles in the mount is truely random, for each family of atomic planes with its characteristic interplanar spacing (d), there are many particles whose orientation is such that they make the proper theta angle with the incident beam to satisfy the Bragg Law. Different families of planes with different interplanar spacings will satisfy the Bragg Law at appropriate values of theta for different integral values of n, thus giving rise to separate sets of "reflected" rays.

To view diffraction from more than one set of planes (hkl), it is necessary to change both the angle theta and the orientation of the crystal. The buerger precession camera is designed to do both, so that diffracted x-rays from the set of all planes that belong to a single zone are recorded on the film. The precession camera also moves the film that records the diffraction in such a way that preserves the symmetry of the diffracting pattern.

The chart on which the record is drawn is divided into tenths of inches and moves at a constant speed, such as for example, 0.5 inch per minute. At this chart speed and a scanning speed of the detector of 1 per minute, 0.5 inch on the chart is equivalent to a 2-theta of 1 degree. The position of peaks on the chart can be read directly, and the interplanar spacings giving rise to the peaks can be determined by use of the Bragg equation.



Day 10


Atomic Packing Arrangements

Anions are generally larger than cations, and the outer orbitals of electrons about most anions are s-orbitals. For these reasons, it is convenient to describe and view many atomic structures as consisting of identical spheres packed in some arrangement.

There are many examples in nature and the world around us where similar objects pack and arrange to fill a particular volume. One trip through a grocery store illustrates some simple packing schemes of atoms.

 Simple Cubic Packing, or 'Body Centered Cubic Packing', results when each layer of atoms has a regular, square lattice arrangement. Alternate layers are translated along a body diagonal (see the body-centered isometric lattice, Fig. 3.17-13, p. 126).




 

 Closest packing structures result when layers of atoms are arranged in a hexagonal pattern. In closest packing arrangements, each atom ( or grapefruit, in the upper right box) is in closest-contact with six others in the layer. (Imagine that each layer extends indefinitely, and count how many grapefruit are arranged about each individual fruit). Closest packing arrangements lead to higher density structures, as the spaces between atoms are smaller.

  There are two closest packing arrangements - Cubic Closest Packing (CCP) and Hexagonal Closest Packing (HCP). In hexagonal closest packing, every other layer is identical, and thus the upward 'repeat' of layers in a structure is ABABABAB.... (As in the two layers at left).
 

In cubic closest packing, it is every third layer that is identical, and thus the repeat is ABCABCABCABC.... Considering the stacks of grapefruit above, the third layer would come into a position different than the first two.

In both HCP and CCP, each sphere is in contact with tweleve others in the three-dimensional structure.

  The arrangements of spaces, or voids, in HCP and CCP is fundamentally different. You can see the spaces by comparing the figure above with the one at the left. A space with six corners is empty above, and filled by a Key lime, at left. This is an octahedral void in the closest-packing arrangement of grapefruit, and the lime is octahedrally coordinated with respect to the surrounding grapefruit.

Tetrahedral voids, those with four corners and which are smaller, are also are present throughout the structure. In the photos above, each grapefruit sits on top of three in the layer below, thus surrounding a space with four corners. (Herein lies another way to 'see' differences between octahedral and tetrahedral sites, viewed from above - you can see the octahedral sites, but the tetrahedral ones are 'hidden' by the top layer.)

The Key lime above would not fit into one of the tetrahedral sites. However, a smaller round fruit (maybe a muscadine grape) would fit into a tetrahedral site. Thus the sizes of voids is dependent on the number of atoms arranged around the void. Similarly, the size of speres that can be placed into the void depends on the relative size of the void. Therefore, polyhedra of higher coordination number accomodate larger atoms, as indicated in the table below.

CN

RR

Type

Polyhedra/Form

2

<.155

Linear

Line

3

.155

Triangular

Triangle

4

.255

Tetrahedral

Tetrahedron

6

.414

Octahedral

Octahedron

8

.732

Cubic

Cube

12

1.0

Cuboctahedral

Cuboctahedron


Oxygen is, by far and by any measure, Earth's most abundant atom. Therefore, when considering values of radius ratio and the way cations fill atomic structures, it is most important to consider the radius ratio of cations to oxygen. Table 4.9 in the text tabulates the radius ratio and typical CN for various cations and oxygen.

Not all of the possible sites in an atomic structure will, necessarily, be filled. In order for a mineral to be stable, the total number of ions must be such that the whole structure is electrically neutral.

Local Charge Balance: CaF2, CN=8. Ca+2, 8 F at -1/4 each (cubic coordination). Work out examples for SiO4 (tetrahedral coordination), and CO3 (in triangular coordination).

Pauling's Rules:

Rule #1

A coordinated polyhedron of anions is formed about each cation, the cation-anion distance equaling the sum of their characteristic packing radii and the coordination polyhedron being determined by the radius ratio.

Rule #2

An ionic structure will be stable to the extent that the sum of the strengths of the electrostatic bonds that reach an ion equal the charge on that ion.

Rule #3

The sharing of edges, and particularly faces, by two anion polyhedra decreases the stability of the ionic structure.

Rule #4

In a crystal containing different cations, those with high valency and small coordination number tend not to share polyhedron elements with one another.

Rule #5

The number of essentially different kinds of constituents in a crystal tends to be small.



Day 11


Substitutional Solid solution.
Recalling Day 1 and our definition of a mineral, the chemical composition of minerals is fixed, but fixed within limits. In other words, most minerals can have some variability to their composition, and we can write mineral formulas in a way to express this variation. You can think of a solid solution as one mineral that has the chemistries of two or more ideal, end-member minerals 'dissolved' or mixed into it.

How does solid solution occur? As reviewed in Pauling's Rules, both the size and charge of ions determines how they will enter structures. Therefore, ions of similar size and charge might be expected to substitute for, or take the place of, one another in minerals. As long as radius-ratio considerations and the principle of local charge balance are met, these substitutions will not decrease the stability of the structure.

Solid solution may be complete to partial depending on radius ratio constraints. Complete binary solid solutions exist when a complete spectrum of chemiical compositions is possible between two 'pure' endmembers. Partial solid solutions exist where there is a mixing 'gap' between the end-members. Solid solutions may be simple, involving only one ion-pair, or coupled, and they can also involve substitutions of vaciencies to maintain charge balance.

Simple cationic
Example: Mg+2 = Fe+2
Since iron and magnesium have about the same ionic radius, 0.72 and 0.78 angstroms, respectively, and the same charge, they can substitute for one another extensively in mineral structures. The radius ratio constraints predict that both enter into octahedral coordination with oxygen (1.36 angstroms, thus the radius ratios are about 0.55). Important mineral groups in which Fe-Mg solid solution is near-complete include: the olivines, (Fe,Mg)2SiO4, and the orthopyroxenes, (Fe,Mg)SiO3.

Simple anionic
Br-1 = Cl-1

Coupled cationic
Na-1Si+4 = Ca+2Al+3
(plagioclase; sensitive function of pressure)

AlVIAlIV = Mg+2Si+4 (Tschermak exchange)

Omission
Vac-4 = Na-1Al+3 (alkali site in micas or amphiboles) Compositional variations in minerals are a function of: ionic size: Substitution is possible if a size difference is less than about 30% charge: electrical neutrality must be met (Paulings rule #1) temperature and pressure

Effects of Temperature and Pressure Increasing pressure and temperature favors substitution of smaller cations into sites, i.e., Temperature and pressure changes alter the the constraints of radius ratio for coordination polyhedra.

Exsolution
Imagine a nice bubbling pot of beef stew, with all the ingredients mixed; upon cooling, the stew will separate into different components - and a fat-rich layer will 'exsolve', or come out of solid solution, on the top. (Ready to scoop up and eat for breakfast - Mmmmm!) In our stew example, convection (and maybe a spoon) tend to mix the fat throughout the stew. This is a pretty good analogy for exsolution in minerals. At high temperatures and pressures, the constraints of size and valence may permit a given substitution to occur - however, as conditions change, that substitution may not remain stable, and the mineral will tend to 'unmix' and separate into different components. So, mineral exsolution refers to the process whereby an originally homogeneous solid solution separates into two or more minerals of distinct composition. Mineral components in solution can 'exsolve', or come out of solution, upon cooling or a drop in pressure.

Why would changing pressure and temperature affect the stability of a given atomic stubstitution? Well, solids tend to expand with increasing temperature, and the sites for cations will also tend to expand. Therefore, we would predict that high temperatures would favor larger cations in a given site. Similarly, solids contract under increasing pressure, and thus we would expect sites to favor smaller cations at higher pressures.



Day 13


NATIVE ELEMENTS

Copper

Cu

Isometric

Sulphur

S

Orthorhombic

Graphite

C

Hexagonal

Diamond

C

Isometric

Gold

Au

Isometric

Silver

Ag

Isometric

Metal Structures -
Metals generally have one of three packing arrays, and metallic bonding:

  1. cubic closest packing, in which each atom is surrounded by 12 neighbors, and a face-centered cubic lattice is defined

  2. hexagonal closest packing, in which 12 surround each, and defines a hexagonal lattice

  3. 'body centered cubic' packing, each is surrounded by 8 neighbors

Diamond and Graphite structures: covalently bonded structure; covalent and van der Waals's structure, contrast hardness, conductivity, etc. Diamond, covalent bonds in which each C is in tetrahedral coordination with 4 other C - high hardness, very low conductivity due to lack of free electrons Graphite, each C is covalently bonded to three other C, leaving one electron free - low hardness, Van Der Waals bonds, high electrical conductivity



Day 14


SULFIDES

Galena

PbS

Isometric

Sphalerite

ZnS

Isometric

Pyrrhotite

Fe(1-x)S

Monoclinic/

Hexagonal

Chalcopyrite

CuFeS2

Tetragonal

Molybdenite

MoS2

Hexagonal

Pyrite

FeS2

Isometric

Covellite

CuS

Hexagonal

Bornite

Cu5FeS4

Tetragonal/

Isometric

Marcasite

FeS2

Orthorhombic

Sphalerite - ZnS

isostructural with diamond, face-centered cubic lattice with 1/2 of carbon represented by Zn and the other half by S; radius ratio for Zn:S (0.32) predicts tetrahedral coordination of Zn within 4 S.

Sphalerite structure - analogous to the diamond structure, in which 1/2 of the C are replaced by Zn and the other half by S - mixture of ionic and covalent bonding results in ionic complex with very different properties than diamond

Sphalerite-Chalcopyrite Structure - formed by regular substitution of Fe and Cu for Zn, leading to a doubling of the unit cell. Chalcopyrite structure is twice the sphalerite structure, with 1/2 of the Zn replaced by Fe and the other half by Cu.

Pyrrhotite (Fe1-xS) has vacancy substitutions to compensate for the fact that some Fe is +2 and some is +3; in 1:1 ratio, can only be +2. Electron interactions associated with the vacancy cause the magnetism. - many hexagonal and monoclinic polymorphs of pyrrhotite exist.

Pyrite-Marcasite - NaCl type structure, cubic, S-pairs for Cl ions in Halite-type structure, octahedral coor. - Marcasite, orthorhombic polymorph, octahedral coordination, single sulfur in coord. - arsenopyrite is derivative of marcasite structure with substitution of arsenic

Pyrite is isostructural with NaCl, with Na represented by Fe, and Cl by diatomic sulfur; Marcasite also has diatomic sulfur, but it is arranged around Fe in a body centered 'cubic' lattice (orthorhombic).

OXIDES

Hematite

Fe2O3

Hexagonal

Corundum

Al2O3

Hexagonal

Ilmenite

FeTiO3

Hexagonal

Periclase

MgO

Isometric

Pyrolusite

MnO2

Tetragonal

Rutile

TiO2

Tetragonal

Spinel

MgAl2O4

Monoclinic

Magnetite

Fe3O4

Isometric

Chromite

FeCr2O4

Isometric

Simple oxides have a formula either of XO2, XO, or X2O3; multiple oxides have the formulas XY2O4 with two different metal sites (one octahedral and one tetrahedral). Oxides include minerals of great economic importance, as well as some diagnostic of geological environments The bond type in metals is strongly ionic, meaning that is fairly accurate to consider the radius ratio constraints and packing models for viewing structures and coordination.

Hematite Group (X2O3):
Hematite Fe2O3 Hexagonal
Corundum Al2O3 Hexagonal
Ilmenite FeTiO3 Hexagonal

HCP of oxygens with all cations in octahedral coordination, and only 2/3 of the octahedral sites filled with trivalent cations. Each oxygen is shared between four octahedra, a result of the local charge balance. Octahedra within layers share edges, and faces are shared across layers.

Simple Oxides (XO):
Periclase MgO Isometric

Simple cubic packing, identical to halite, in which each layer of octahedra is filled. The structure is analogous to the hematite group structures, yet with no vacancies as each oxygen is shared among six octahedra (cations are 2+ rather than 3+).

Rutile Group (XO2):
Pyrolusite MnO2 Tetragonal
Rutile TiO2 Tetragonal

Based on hexagonal closest packing of oxygen, with Ti filling half of the octahedral interstitial positions, ABABABAB stacking of octahedra in 'chains' is evident. The +4 cation are arranged in chains of octahedra, parallel to the c-axis; rutile has a prismatic habit due to the chains parallel to c.

Spinel Group (XY2O3):
Spinel MgAl2O3 Monoclinic
Chromite FeCr2O4 Isometric
Magnetite Fe3O4 (Fe
+2Fe+32O4) Isometric

Oxygen is in CCP along (111) planes in the spinels. The differing cations are either in octahedral or tetrahedral coordiantion, depending on radius ratio considerations. In 'normal spinels', the X cations occupy the tetrahedral sites (8 per unit cell) and the Y cations occupy the octahedral sites (16 per unit cell). In the 'inverse spinels' the X cations are in octahedral coordination, and 8 of the 16 Y cations are in tetrahedral coordination.

Solid solution among the oxides is variable. Among the spinels it is extensive.

HYDROXIDES

Goethite

aFeO(OH)

Orthorhombic

Gibbsite

Al(OH)3

Monoclinic

Brucite

Mg(OH)2

Hexagonal

All characterized by a hydroxyl (OH)- group or H2O molecules. The presence of the hydroxyl group causes bond strengths to be weaker than for oxides.



Day 15


HALIDES

Halite

NaCl

Isometric

Fluorite

CaF2

Isometric

Sylvite

KCl

Isometric

Halides are characterized by the presence of a halogen atom; both the large cations and ions behave as spherical bodies, and the packing leads to structures of highest symmetry. Bonding in the halides is strongly ionic.

SULFATES

Gypsum

CaSO4-2H2O

Monoclinic

Anhydrite

CaSO4

Orthorhombic

Celestite

SrSO4

Barite

Barite

BaSO4

Orthorhombic

(SO4)2- complexes are the basis of the minerals.

PHOSPHATES

Apatite

Ca5PO43(F,Cl,OH)

Hexagonal

Monazite

(Ce,La,Y,Th)PO4

Monoclinic

(PO4)3- complexes are the basis of the minerals

CARBONATES

Aragonite

CaCO3

Orthorhombic

Calcite

CaCO3

Hexagonal

Dolomite

CaMg(CO3)2

Hexagonal

Ankerite

CaFe(CO3)2

Hexagonal

Magnesite

MgCO3

Hexagonal

Siderite

FeCO3

Hexagonal

Rhodochrosite

MnCO3

Hexagonal

Carbonates have a (CO3)2- complex in the structure; radius ratio relations predict one carbon surrounded by three oxygen in a ring with a residual charge of -2/3 on each oxygen, and a bond of 1 1/3 between each oxygen and the carbon. In the presence of hydrogen, the (CO3)2- group breaks down to form H2O + CO2.

The radius ratio of Ca:O is sufficiently close for there to be two CN for Ca: 6-fold coordination occurs for calcite and 9-fold for aragonite. Calcite is hexagonal, and aragonite is orthorhombic. The oxygen coordinated to Ca are in the CO3 rings, which for calcite and aragonite are in layer perpendicular to c; in Calcite the rings all point the same way, and in aragonite they point in opposite directions from layer to layer.



Day 16


Silica Polymerization and the Silicates

Silicate Structures: Silicon and oxygen constitute more than 95% of the earths crust by volume.

92% of crustal minerals are silicates, by volume.

In all minerals of Earth's crustal and upper mantle environments, silica occurs in tetrahedral coordination with oxygen. With four oxygen at the apices of a tetrahedron, coordianted about the silicon, the charge between each silicon-oxygen is 1, and 1 remains with each oxygen for bonding. The (SiO4)-4 unit is a fundamental packing unit of the silicates.

The various ways that (SiO4)-4 tetrahedra link, or are polymerized, to other cations, or other (SiO4)-4 tetrahedra, defines the basic silicate types*.

Nesosilicates
('orthosilicates')

(SiO4)-4

Isolated (SiO4)-4 tetrahedra

Garnet, Aluminum Silicates,
Olivine, Staurolite, etc.

Sorosilicates

(Si2O7)-6

Pairs of (SiO4)-4 tetrahedra

Epidote Group Minerals

Cyclosilicates

(Si6O18)-12

Rings of (SiO4)-4 tetrahedra

Beryl, Tourmaline, Cordierite

Inosilicates

(Si2O6)-4/
(Si4O11)-6

Infinitely extending strips of (SiO4)-4 tetrahedra

Single Chain: Pyroxenes, Pyroxenoids;
Double Chain: Amphiboles.

Phyllosilicates

(Si2O5)-5

Infinitely extending sheets of (SiO4)-4 tetrahedra

The micas

Tectosilicates

(SiO2)0

Three-dimensional frameworks of (SiO4)-4 tetrahedra

Silica Polymorphs, Feldspars,
Feldspathoids, Scapolite, Zeolites

*Deep mantle, meteorite impact, and other ultrahigh-pressure settings may result in a more densely-packed SiO2-structure, with silicon in octahedral (6-fold) co-ordiantion with oxygen. This is the structure basis for stishovite, which is isostructural with rutile (TiO2, with Ti in octahedral coordination).


Tectosilicates are the silicate class built from three-dimensional networks of corner-sharing SiO4 tetrahedra. About 65% of Earth's crust is composed of tectosilicates. This group of minerals includes the SiO2 polymorphs, feldspars, feldspathoids, scapolites, and zeolites.

The SiO2 Polymorphs
Considering minerals composed of only SiO2 (along with water, in the case of amorphous SiO2), there are several possible symmetries and distinct structures possible. The SiO2 polymorphs comprise an extremely important group with distinctive pressures and temperatures of occurrence.

Quartz

SiO2

Hexagonal

Common in igneous, metamorphic, and sedimentary rocks

Tridymite

SiO2

Hexagonal

A high-temperature, low pressure SiO2 polymorph; certain volcanic rocks.

Crystobalite

SiO2

Isometric

A high-temperature, low pressure SiO2 polymorph; certain volcanic rocks.

Coesite

SiO2

Monoclinic

A high pressure polymorph; deep crustal and mantle rocks

Stishovite

SiO2

Tetragonal

A high pressure polymorph; meteorites and impact metamorphism

Opal

SiO2-nH2O

amorphous
spheres in hcp

Forms by replacement or direct precipitation in sedimentary and hydrothermal environments.



Day 17


(Further Development of this page is in progress)

Tectosilicates: Feldspars

Feldspars are the single most common group of minerals in Earth's crust. To put it into perspective, the total volume of plagioclase in the crust is greater than that of all the water in all the oceans! The Moon, also, has a crust composed mainly of feldspars - as the highlands (which appear white) are made of the plagioclase-rich rock 'anorthosite', and the Maria (darker, circular regions) are underlain by the plagioclase-rich rock 'basalt'. The very name 'feldspars' comes from Old German words meaning 'Field Stones', in reference to their abundance. Feldspars, along with quartz and the feldspathoids, are used as a basis for classifying and naming felsic igneous rocks (granites, rhyolites, etc.). The identity of feldspar polymorphs, composition, and textures can tell us a lot about the crustal setting in which a rock formed. Feldspars are important economic minerals, for the manufacture of porcelain, glass, and some ornamental stones.

Feldspar Series Common feldspars can be expressed in terms of the system orthoclase - albite - anorthite. The members of the series orthoclase-albite are known as the alkali feldspars, and those in the series albite-anorthite are the plagioclase feldspars.

Felsspar structures can be described as an infinite network of SiO4 and AlO4 tetrahedra, which is a 'stuffed' derivative of the SiO2 structures, with substitution of Al for some Si into tetrahedral sites, and accomodation of Na, K, and Ca in voids. Na, K, Ca, and Al balance the substitutions for Si+4 as follows:

Si4O8 - Si+4 + (NaAl)+4 = NaAlSi3O8
Si4O8 - Si
+4 + (NaAl)+4 = NaAlSi3O8
Si4O8 - 2Si
+4 + (CaAl2)+8 = CaAl2Si2O8.

It is particularly important to consider Al substitution into the tetrahedral sites of feldspars, because of the similar ionic radii of Al and Si, and the crustal abundance of Al; other trivalent cations such as chrome and ferric iron, can substitute into tetrahedral sites of feldspars. The larger voids in feldspar structures favor larger cations, such as Na, K, Ca, and sometimes Pb, Rb, Cs. Water can also be incorporated into feldspar structures. Although water, lead, and ferric iron are typically, at most, very minor constituents of feldspars, the color of feldspars is strongly influenced by these elements. For example, the alkali feldspars are commonly white or clear and colorless, but small amounts of ferric iron commonly give them a pink to red color, and small amounts of water and lead give them a blue to green color (as in the semiprecious stone amazonite).

Potassium Feldspars

Sanidine

KAlSi3O8

Monoclinic

Orthoclase

KAlSi3O8

Monoclinic

Miocrocline

KAlSi3O8

Triclinic

You can think of the feldspar structure as having four-membered rings of (Si,Al)O4 tetrahedra linked into chains that parallel the a-axis, shaped like double-crankshafts, and the larger sites fitting into large spaces that alternate with the tetrahedra.

In the high-temperature polymorph of KAlSi3O8, Sanidine, Al and Si are completely disordered. That is to say, the occurrence of Al and Si in the tetrahedral sites of sanidine is random. This results in a higher symmetry of sanidine, as there are fewer 'special positions' that are restricted to either Al or Si (as in the more ordered polymorphs, orthoclase and microcline).

The alkali feldspar series (Albite to Orthoclase/Microcline/Sanidine) show complete solid solution only at high temperatures. For example, sanidite-high albite solid solutions exist at high temperature, but given time during cooling, the mixture will exsolve to form an interlocking network of low-albite and microcline 'perthite'  at low temperature. In otherwords, there is ideal substitution of albite and potassium feldspar at high temepratures, but a miscibility gap and incomplete solid solution at lower temperatures.

Composition and structures of the feldspar minerals thus are often functions of cooling rate, and we can evaluate the thermal history of a rock by examining the feldspar textures.

Review Feldspar T-X diagrams.

Plagioclase Feldspars

Albite

NaAlSi3O8

Triclinic

Anorthite

CaAl2Si2O8

Monoclinic

Plagioclase series

In view of their importance and characteristic occurrences, compositions of the plagioclase solid solution series are subdivided, and named as follows: Albite (An0-10), Oligioclase(An10-30), Andesine(An30-50), Labradorite(An50-70), Bytownite(An70-90), and Anorthite(An90-100). (Find a way to remember this - such as, "All orangutans are laughable, baboonish animals".)


Day 18


Tectosilicates, Continued:
The Feldspathoids

Feldspathoids, principally the minerals leucite, nepheline, kalsilite and sodalite, are anhydrous framework silicates similar to feldspars, but with lower SiO2 content. The feldspathoids tend to form from melts rich in alkali's and poor in silica. Although they are far less common than the feldspars, the unique settings in which feldspathoids occur are often of great tectonic and economic importance. Where present in sufficient quantity and purity, nepheline is mined for use in the manufacture of glass and porcelain.

Feldspathoids

Leucite

KAlSi2O6

Tetragonal (high-T)/
Isometric

Nepheline/
Kalsilite

NaAlSiO4/
KAlSiO4

Hexagonal

Sodalite

Na8(AlSiO4)6Cl2

Isometric

Lazurite

(Na,Ca)8(AlSiO4)6(SO4,S,Cl)2

Isometric

The feldspathoids have lower specific gravity (<2.5) than the feldspars (>2.5), and permit substitution of large cations, because of larger cavities in 4- and 6-member SiO4 rings. Large anions and anionic complex are housed in these cavities, and coordinated to oxygen in the enclosing rings.

Formulas for the feldspathoids may be derived simply by considering Ab-An solid solution, and taking out one SiO2 (for leucite), or Si2O4 (for nepheline). The sodalite formula is as six sodium nepheline, with 2 more Na and Cl to balance. Similarly, Lazurite is as six nepheline, but with Ca and sulfate in addition to Na and Cl.

Scapolite Group

Na-Scapolite

3NaAlSi3O8-(NaCl)

Tetragonal

Ca-Scapolite

3CaAl2Si2O8-(CaSO4)
3CaAl2Si2O8-(CaCO3)

Tetragonal

Scapolites are tectosilicates with compositions suggestive of feldspars. The formulas are basically three time albite or anorthite, with a halite- or anhydrite-like complex.

Scapolite structures consist of corner-sharing Al and Si-tetrahedra that enclose large cavities. These cavities hold the anion complexes. There is complete solid solution in the scapolite series. Scapolite is a common and distinctive mineral in metamorphosed, siliceous limestones.

Zeolites

In a manner similar to the feldspars, feldspathoids, and scapolite, the zeolites are framework silicates with anions, anion complexes, and H2O housed structural voids. The hardness and specific gravity of the zeolites are among the lowest of the tectosilicates, reflecting their open structure. The voids in zeolite are actually interconnected spaces or channels, and thus zeolites have both porosity and permeability. In addition, zeolites can incorporate or loose water, and cations bound to (OH), as a function of temperature. In other words, heating and cooling zeolites causes them to loose or incorporate both volatile anions and certain cations. These properties, and fact that zeolites can now be commercially synthesized, results in uses of zeolites as 'molecular sieves' or absorbants in many household and industrial applications (everything from cat-litter to catalytic converters).

Day 19


These pages are under construction, and contian class notes....

Phyllosilicates (p. 498-524)

Serpentine Group
Antigorite Mg3Si2O5(OH)4 Monoclinic
Chrysotile Mg3Si2O5(OH)4 Monoclinic

Clay Group
Kaolinite Al2Si2O5(OH)4 Triclinic
Pyrophyllite Al2Si4O10(OH)2 Monoclinic
Talc Mg3Si4O10(OH)2 Monoclinic
Glauconite (K,Na,Ca)0.5-1(Fe,Al,Fe,Mg)2(Si,Al)O10nH2O Monoclinic

Chlorite Group
Chlorite (Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6 Monoclinic

Mica Group
Muscovite KAl2 (AlSi3O10)(OH) 2 Monoclinic
Paragonite NaAl2(AlSi3O10)(OH) 2 Monoclinic
Margerite CaAl2(Al2Si2O10)(OH) 2 Monoclinic
Lepidolite K(Li,Al)2-3(AlSi3O10)(OH)2 Monoclinic
Phlogopite KMg3(AlSi3O10)(OH)2 Monoclinic
Biotite K(Fe,Mg)3(AlSi3O10)(OH)2 Monoclinic

Structural and Chemical Development of the Micas

The name of the group is derived from Greek phylon, or leaf, as in phyllo pastry. Structure consists of an infinite sheet of SiO4 tetrahedra, in which three of the oxygen are shared with adjacent tetrahedra (down 1 from the four of tectosilicates) and the apical oxygen of the sheets have a net charge of -1; sharing of oxygen leads to a 2:5 ratio of Si:O in the tetrahedral sheets, and thus the formula for the sheets if (Si2O5)-2. Each sheet, if undistorted, has a 6-fold symmetry.

Most phyllosilicates are hydroxyl bearing, with the (OH)-1 group located in the center and the same height of the rings of unshared apical oxygens. When ions, external to the (Si2O5OH)3- sheets, are bonded to the sheets they coordinate with two oxygen and the single (OH)-; the size of the triangle formed is close to that of an ideal XO6 octahedron (with X commonly Fe or Mg - more discussion of the consequences of misfit will follow). This means that each tetrahedral (Si2O5OH)3- sheet will coordiante with a sheet of regular octahedra, in which each octahedron is tilted onto one of it's triangular sides. When such tetrahedral and octrahedral sheets are joined , the general geometry of the kaolinite and antigorite structures are formed. (An 'open-faced' t-o sandwhich).

The phyllosilicates are divided into two major groups: trioctahedral and dioctahedral. The trioctahedral micas have divalent cations (mainly Fe+2 or Mg+2) in the octrahedral sites and each octahedral site is occupied. Each oxygen of the trioctahedral micas is surrounded by and coordinated to three cations in adjacent, filled octahedral sites. For the dioctahderal micas, trivalent cations in the octahedral sheets (generally Al, Fe+3, or Cr+3) are also bound to three oxygen or (OH)- groups, but to maintain charge balance one third of the octahedral sites are left empty (as for the corundum-ilmenite-hematite structures), thus each oxygen or (OH)- group has cations in two adjoining octahedra.

Brucite, Mg(OH)2, consists of two (OH)- planes between which Mg octahedra are coordinated. The sheets of the brucite structure can be noted as Mg3(OH)6. If we replace two of the (OH)- on one side with the apical oxygen of an Si2O5 sheet, then we obtain Mg3Si2O5(OH)4, which is the formula and structure for the trioctahedral mica antigorite. In short, the antigorite and chrysotile structures are built from one tetrahedral sheet and one octahedral sheet, giving them t-o layers that are electrically neutral and held together by van der Waals bonds. The equivalent structure of dioctahedral micas is kaolinite, Al2Si2O5(OH)4 (built from Gibbsite, Al(OH)3, note the missing cation to maintain charge balance).

Trioctahedral Micas Oct. Brucite Layer Tet. Si-O layer Antigorite 3Mg(OH)2 + (Si2O5) -2 = Mg3Si2O5(OH)4 + 2(OH)-

Dioctahedral Micas Oct. Gibbsite Layer Tet. Si-O layer Kaolinite 2Al(OH)3 + (Si2O5) -2 = Al2Si2O5(OH)4 + 2(OH)-

If we bind Si2O5 sheets to both sides of the octahedral layers we can derive additional phyllosilicates which each have t-o-t layers: as for talc, Mg3Si4O10(OH)2, and phyrohyllite, Al2Si4O10(OH)2. In each case it can begin with either brucite or gibbsite, and replace two (OH)- with two apical oxygen of Si2O5 groups.

If some Al+3 substitutes for Si+4 in the tetrahedral sheets, then we can carry the evolution of phyllosilicates. This substitution causes a charge to be located along the surfaces of the t-o-t layers. An ideal substitution of Al for Si occurs when one of every four Si is replaced by Al: this results in a sufficient charge to bind a cation in 12-fold coordination between the bases of SiO5 sheets. This binding of t-o-t layers increases hardness, etc., relative to other micas.

Phlogopite KMg3(AlSi3O10)(OH)2 Monoclinic
Muscovite KAl2 (AlSi3O10)(OH) 2 Monoclinic

If half the Si are replaced by Al, then two charges per t-o-t layer become available. Then such ions as Ba and Ca may enter between and bind the t-o-t layers. The ionic bonds are stronger, hardness is increased, forming the "brittle micas"

Margerite CaAl2(Al2Si2O10)(OH)2 Monoclinic

Addional members of the micas can be developed by considering the layers of their structures as different "minerals": Chlorite can be considered as two layers of talc , Mg3Si4O10(OH)2, separated by a layer of brucite.

Mismatch between the octahedral and tetrahedral layers causes distortion of structures. This misfit is accommodated by bending the larger octahedral layers around the tetrahedral layers.

Note also that the octahedral sheets are staggered relative to the tetrahedral sheets. This gives rise to the monoclinic symmetry of most micas. Polytypism results from changing the stacking and staggering of micas: because of the three-fold symmetry of Si2O5 sheets, there are three alternate directions that octahedra can be staggered.

Class Discussisons:

Survey of Hydrous Phyllosilicates

Structure of Clay Minerals and resulting uses - expandable structure - perfect basal cleavage, low hardness - fine grain size

Chemistry of Clay Minerals and resulting uses: - aluminum-rich clay minerals - bonding with polar molecules - serpentine producing reactions - kaolinite producing reactions

Geologic Occurrences of select clay and serpentine group minerals. - Crysotile-Antigorite - Vermiculite - Kaolinite


Day 20


These pages are under construction, and contian class notes....

Phyllosilicates (p. 498-524)

Serpentine Group
Antigorite Mg3Si2O5(OH)4 Monoclinic
Chrysotile Mg3Si2O5(OH)4 Monoclinic

Clay Group
Kaolinite Al2Si2O5(OH)4 Triclinic
Pyrophyllite Al2Si4O10(OH)2 Monoclinic
Talc Mg3Si4O10(OH)2 Monoclinic
Glauconite (K,Na,Ca)0.5-1(Fe,Al,Fe,Mg)2(Si,Al)O10nH2O Monoclinic

Chlorite Group
Chlorite (Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6 Monoclinic

Mica Group
Muscovite KAl2 (AlSi3O10)(OH) 2 Monoclinic
Paragonite NaAl2(AlSi3O10)(OH) 2 Monoclinic
Margerite CaAl2(Al2Si2O10)(OH) 2 Monoclinic
Lepidolite K(Li,Al)2-3(AlSi3O10)(OH)2 Monoclinic
Phlogopite KMg3(AlSi3O10)(OH)2 Monoclinic
Biotite K(Fe,Mg)3(AlSi3O10)(OH)2 Monoclinic

Structural and Chemical Development of the Micas

The name of the group is derived from Greek phylon, or leaf, as in phyllo pastry. Structure consists of an infinite sheet of SiO4 tetrahedra, in which three of the oxygen are shared with adjacent tetrahedra (down 1 from the four of tectosilicates) and the apical oxygen of the sheets have a net charge of -1; sharing of oxygen leads to a 2:5 ratio of Si:O in the tetrahedral sheets, and thus the formula for the sheets if (Si2O5)-2. Each sheet, if undistorted, has a 6-fold symmetry.

Most phyllosilicates are hydroxyl bearing, with the (OH)-1 group located in the center and the same height of the rings of unshared apical oxygens. When ions, external to the (Si2O5OH)3- sheets, are bonded to the sheets they coordinate with two oxygen and the single (OH)-; the size of the triangle formed is close to that of an ideal XO6 octahedron (with X commonly Fe or Mg - more discussion of the consequences of misfit will follow). This means that each tetrahedral (Si2O5OH)3- sheet will coordiante with a sheet of regular octahedra, in which each octahedron is tilted onto one of it's triangular sides. When such tetrahedral and octrahedral sheets are joined , the general geometry of the kaolinite and antigorite structures are formed. (An 'open-faced' t-o sandwhich).

The phyllosilicates are divided into two major groups: trioctahedral and dioctahedral. The trioctahedral micas have divalent cations (mainly Fe+2 or Mg+2) in the octrahedral sites and each octahedral site is occupied. Each oxygen of the trioctahedral micas is surrounded by and coordinated to three cations in adjacent, filled octahedral sites. For the dioctahderal micas, trivalent cations in the octahedral sheets (generally Al, Fe+3, or Cr+3) are also bound to three oxygen or (OH)- groups, but to maintain charge balance one third of the octahedral sites are left empty (as for the corundum-ilmenite-hematite structures), thus each oxygen or (OH)- group has cations in two adjoining octahedra.

Brucite, Mg(OH)2, consists of two (OH)- planes between which Mg octahedra are coordinated. The sheets of the brucite structure can be noted as Mg3(OH)6. If we replace two of the (OH)- on one side with the apical oxygen of an Si2O5 sheet, then we obtain Mg3Si2O5(OH)4, which is the formula and structure for the trioctahedral mica antigorite. In short, the antigorite and chrysotile structures are built from one tetrahedral sheet and one octahedral sheet, giving them t-o layers that are electrically neutral and held together by van der Waals bonds. The equivalent structure of dioctahedral micas is kaolinite, Al2Si2O5(OH)4 (built from Gibbsite, Al(OH)3, note the missing cation to maintain charge balance).

Trioctahedral Micas Oct. Brucite Layer Tet. Si-O layer Antigorite 3Mg(OH)2 + (Si2O5) -2 = Mg3Si2O5(OH)4 + 2(OH)-

Dioctahedral Micas Oct. Gibbsite Layer Tet. Si-O layer Kaolinite 2Al(OH)3 + (Si2O5) -2 = Al2Si2O5(OH)4 + 2(OH)-

If we bind Si2O5 sheets to both sides of the octahedral layers we can derive additional phyllosilicates which each have t-o-t layers: as for talc, Mg3Si4O10(OH)2, and phyrohyllite, Al2Si4O10(OH)2. In each case it can begin with either brucite or gibbsite, and replace two (OH)- with two apical oxygen of Si2O5 groups.

If some Al+3 substitutes for Si+4 in the tetrahedral sheets, then we can carry the evolution of phyllosilicates. This substitution causes a charge to be located along the surfaces of the t-o-t layers. An ideal substitution of Al for Si occurs when one of every four Si is replaced by Al: this results in a sufficient charge to bind a cation in 12-fold coordination between the bases of SiO5 sheets. This binding of t-o-t layers increases hardness, etc., relative to other micas.

Phlogopite KMg3(AlSi3O10)(OH)2 Monoclinic
Muscovite KAl2 (AlSi3O10)(OH) 2 Monoclinic

If half the Si are replaced by Al, then two charges per t-o-t layer become available. Then such ions as Ba and Ca may enter between and bind the t-o-t layers. The ionic bonds are stronger, hardness is increased, forming the "brittle micas"

Margerite CaAl2(Al2Si2O10)(OH)2 Monoclinic

Addional members of the micas can be developed by considering the layers of their structures as different "minerals": Chlorite can be considered as two layers of talc , Mg3Si4O10(OH)2, separated by a layer of brucite.

Mismatch between the octahedral and tetrahedral layers causes distortion of structures. This misfit is accommodated by bending the larger octahedral layers around the tetrahedral layers.

Note also that the octahedral sheets are staggered relative to the tetrahedral sheets. This gives rise to the monoclinic symmetry of most micas. Polytypism results from changing the stacking and staggering of micas: because of the three-fold symmetry of Si2O5 sheets, there are three alternate directions that octahedra can be staggered.

Class Discussisons:

Survey of Hydrous Phyllosilicates

Structure of Clay Minerals and resulting uses - expandable structure - perfect basal cleavage, low hardness - fine grain size

Chemistry of Clay Minerals and resulting uses: - aluminum-rich clay minerals - bonding with polar molecules - serpentine producing reactions - kaolinite producing reactions

Geologic Occurrences of select clay and serpentine group minerals. - Crysotile-Antigorite - Vermiculite - Kaolinite


Day 21


These pages are under construction, and contian class notes....

Phyllosilicates (p. 498-524)

Serpentine Group
Antigorite Mg3Si2O5(OH)4 Monoclinic
Chrysotile Mg3Si2O5(OH)4 Monoclinic

Clay Group
Kaolinite Al2Si2O5(OH)4 Triclinic
Pyrophyllite Al2Si4O10(OH)2 Monoclinic
Talc Mg3Si4O10(OH)2 Monoclinic
Glauconite (K,Na,Ca)0.5-1(Fe,Al,Fe,Mg)2(Si,Al)O10nH2O Monoclinic

Chlorite Group
Chlorite (Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6 Monoclinic

Mica Group
Muscovite KAl2 (AlSi3O10)(OH) 2 Monoclinic
Paragonite NaAl2(AlSi3O10)(OH) 2 Monoclinic
Margerite CaAl2(Al2Si2O10)(OH) 2 Monoclinic
Lepidolite K(Li,Al)2-3(AlSi3O10)(OH)2 Monoclinic
Phlogopite KMg3(AlSi3O10)(OH)2 Monoclinic
Biotite K(Fe,Mg)3(AlSi3O10)(OH)2 Monoclinic

Structural and Chemical Development of the Micas

The name of the group is derived from Greek phylon, or leaf, as in phyllo pastry. Structure consists of an infinite sheet of SiO4 tetrahedra, in which three of the oxygen are shared with adjacent tetrahedra (down 1 from the four of tectosilicates) and the apical oxygen of the sheets have a net charge of -1; sharing of oxygen leads to a 2:5 ratio of Si:O in the tetrahedral sheets, and thus the formula for the sheets if (Si2O5)-2. Each sheet, if undistorted, has a 6-fold symmetry.

Most phyllosilicates are hydroxyl bearing, with the (OH)-1 group located in the center and the same height of the rings of unshared apical oxygens. When ions, external to the (Si2O5OH)3- sheets, are bonded to the sheets they coordinate with two oxygen and the single (OH)-; the size of the triangle formed is close to that of an ideal XO6 octahedron (with X commonly Fe or Mg - more discussion of the consequences of misfit will follow). This means that each tetrahedral (Si2O5OH)3- sheet will coordiante with a sheet of regular octahedra, in which each octahedron is tilted onto one of it's triangular sides. When such tetrahedral and octrahedral sheets are joined , the general geometry of the kaolinite and antigorite structures are formed. (An 'open-faced' t-o sandwhich).

The phyllosilicates are divided into two major groups: trioctahedral and dioctahedral. The trioctahedral micas have divalent cations (mainly Fe+2 or Mg+2) in the octrahedral sites and each octahedral site is occupied. Each oxygen of the trioctahedral micas is surrounded by and coordinated to three cations in adjacent, filled octahedral sites. For the dioctahderal micas, trivalent cations in the octahedral sheets (generally Al, Fe+3, or Cr+3) are also bound to three oxygen or (OH)- groups, but to maintain charge balance one third of the octahedral sites are left empty (as for the corundum-ilmenite-hematite structures), thus each oxygen or (OH)- group has cations in two adjoining octahedra.

Brucite, Mg(OH)2, consists of two (OH)- planes between which Mg octahedra are coordinated. The sheets of the brucite structure can be noted as Mg3(OH)6. If we replace two of the (OH)- on one side with the apical oxygen of an Si2O5 sheet, then we obtain Mg3Si2O5(OH)4, which is the formula and structure for the trioctahedral mica antigorite. In short, the antigorite and chrysotile structures are built from one tetrahedral sheet and one octahedral sheet, giving them t-o layers that are electrically neutral and held together by van der Waals bonds. The equivalent structure of dioctahedral micas is kaolinite, Al2Si2O5(OH)4 (built from Gibbsite, Al(OH)3, note the missing cation to maintain charge balance).

Trioctahedral Micas Oct. Brucite Layer Tet. Si-O layer Antigorite 3Mg(OH)2 + (Si2O5) -2 = Mg3Si2O5(OH)4 + 2(OH)-

Dioctahedral Micas Oct. Gibbsite Layer Tet. Si-O layer Kaolinite 2Al(OH)3 + (Si2O5) -2 = Al2Si2O5(OH)4 + 2(OH)-

If we bind Si2O5 sheets to both sides of the octahedral layers we can derive additional phyllosilicates which each have t-o-t layers: as for talc, Mg3Si4O10(OH)2, and phyrohyllite, Al2Si4O10(OH)2. In each case it can begin with either brucite or gibbsite, and replace two (OH)- with two apical oxygen of Si2O5 groups.

If some Al+3 substitutes for Si+4 in the tetrahedral sheets, then we can carry the evolution of phyllosilicates. This substitution causes a charge to be located along the surfaces of the t-o-t layers. An ideal substitution of Al for Si occurs when one of every four Si is replaced by Al: this results in a sufficient charge to bind a cation in 12-fold coordination between the bases of SiO5 sheets. This binding of t-o-t layers increases hardness, etc., relative to other micas.

Phlogopite KMg3(AlSi3O10)(OH)2 Monoclinic
Muscovite KAl2 (AlSi3O10)(OH) 2 Monoclinic

If half the Si are replaced by Al, then two charges per t-o-t layer become available. Then such ions as Ba and Ca may enter between and bind the t-o-t layers. The ionic bonds are stronger, hardness is increased, forming the "brittle micas"

Margerite CaAl2(Al2Si2O10)(OH)2 Monoclinic

Addional members of the micas can be developed by considering the layers of their structures as different "minerals": Chlorite can be considered as two layers of talc , Mg3Si4O10(OH)2, separated by a layer of brucite.

Mismatch between the octahedral and tetrahedral layers causes distortion of structures. This misfit is accommodated by bending the larger octahedral layers around the tetrahedral layers.

Note also that the octahedral sheets are staggered relative to the tetrahedral sheets. This gives rise to the monoclinic symmetry of most micas. Polytypism results from changing the stacking and staggering of micas: because of the three-fold symmetry of Si2O5 sheets, there are three alternate directions that octahedra can be staggered.

Class Discussisons:

Survey of Hydrous Phyllosilicates

Structure of Clay Minerals and resulting uses - expandable structure - perfect basal cleavage, low hardness - fine grain size

Chemistry of Clay Minerals and resulting uses: - aluminum-rich clay minerals - bonding with polar molecules - serpentine producing reactions - kaolinite producing reactions

Geologic Occurrences of select clay and serpentine group minerals. - Crysotile-Antigorite - Vermiculite - Kaolinite


Days 22


These pages are under construction, and contian class notes....

INOSILICATES

Pyroxenes
Single-chain inosilicates with Si:O ratio of 1:3 Pyroxenes are major rock forming minerals of earth's crust and mantle, also of lunar rocks.

Si:O in chain in 1:3 ratio, orthopyroxenes are XSiO3 and clinopyroxenes are XYSi2O6 - major component of igneous rocks, classifications of many plutonic rocks are based on pyroxenes - appearance of pyroxene in most metamorphic rocks is part of the granulite facies - sodic pyroxenes (jadeite, omphacite) are indicative of high pressure

Orthopyroxenes
Enstatite MgSiO3 Orthorhombic
Ferrosilite FeSiO3 Orthorhombic
*Bronzite (En70-90) and hypersthene, (En50-70) are common names for solid-solutions in the enstatite-ferrosilite series.

Clinopyroxenes
Diopside CaMgSi2O6 Monoclinic
Hedenbergite CaFeSi2O6 Monoclinic
Augite (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 Monoclinic
Pigeonite Ca0.25(Mg,Fe) 1.75Si2O6 Monoclinic
Aegirine NaFeSi2O6 Monoclinic
Jadeite NaAlSi2O6 Monoclinic
*Omphacite is a high-pressure solid solution between augite and jadeite.

Pyroxenoids
Wollastonite CaSiO3 Triclinic

Amphiboles
Double-chain inosilicates with a Si:O ratio of 4:11, and (OH)

Orthorhombic Amphiboles
Anthophyllite-Gedrite (Mg,Fe) 7Si8O22(OH)2 Orthorhombic

Monoclinic Amphiboles (Clino-amphiboles)
Cummingtonite-Grunerite (Fe,Mg) 7Si8O22(OH)2 Monoclinic
Tremolite-Actinolite Ca2(Mg,Fe) 5Si8O22(OH)2 Monoclinic
Hornblende (Ca,Na,K)2-3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH)2 Monoclinic
Glaucophane Na2Mg3Al2Si8O22(OH)2 Monoclinic
Riebekite Na2Fe3Fe2Si8O22(OH)2 Monoclinic

Mineral Formula Calculation - Examples of Ortho- and Clinopyroxenes

Wt. % Oxides Molec. Wt. Oxides Molecular Porp. Atomic Prop. Sum Molec Prop. = MPF

Calculation of Mineral Formulas for Hydrous Minerals.

Plotting of compositions (wt %, Molecular %, atomic %) in triangular diagrams - extent of solid solution - coexistence of minerals in a specific rock type


Days 23


These pages are under construction, and contian class notes....

INOSILICATES

Pyroxenes
Single-chain inosilicates with Si:O ratio of 1:3 Pyroxenes are major rock forming minerals of earth's crust and mantle, also of lunar rocks.

Si:O in chain in 1:3 ratio, orthopyroxenes are XSiO3 and clinopyroxenes are XYSi2O6 - major component of igneous rocks, classifications of many plutonic rocks are based on pyroxenes - appearance of pyroxene in most metamorphic rocks is part of the granulite facies - sodic pyroxenes (jadeite, omphacite) are indicative of high pressure

Orthopyroxenes
Enstatite MgSiO3 Orthorhombic
Ferrosilite FeSiO3 Orthorhombic
*Bronzite (En70-90) and hypersthene, (En50-70) are common names for solid-solutions in the enstatite-ferrosilite series.

Clinopyroxenes
Diopside CaMgSi2O6 Monoclinic
Hedenbergite CaFeSi2O6 Monoclinic
Augite (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 Monoclinic
Pigeonite Ca0.25(Mg,Fe) 1.75Si2O6 Monoclinic
Aegirine NaFeSi2O6 Monoclinic
Jadeite NaAlSi2O6 Monoclinic
*Omphacite is a high-pressure solid solution between augite and jadeite.

Pyroxenoids
Wollastonite CaSiO3 Triclinic

Amphiboles
Double-chain inosilicates with a Si:O ratio of 4:11, and (OH)

Orthorhombic Amphiboles
Anthophyllite-Gedrite (Mg,Fe) 7Si8O22(OH)2 Orthorhombic

Monoclinic Amphiboles (Clino-amphiboles)
Cummingtonite-Grunerite (Fe,Mg) 7Si8O22(OH)2 Monoclinic
Tremolite-Actinolite Ca2(Mg,Fe) 5Si8O22(OH)2 Monoclinic
Hornblende (Ca,Na,K)2-3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH)2 Monoclinic
Glaucophane Na2Mg3Al2Si8O22(OH)2 Monoclinic
Riebekite Na2Fe3Fe2Si8O22(OH)2 Monoclinic

Mineral Formula Calculation - Examples of Ortho- and Clinopyroxenes

Wt. % Oxides Molec. Wt. Oxides Molecular Porp. Atomic Prop. Sum Molec Prop. = MPF

Calculation of Mineral Formulas for Hydrous Minerals.

Plotting of compositions (wt %, Molecular %, atomic %) in triangular diagrams - extent of solid solution - coexistence of minerals in a specific rock type


Day 24


These pages are under construction, and contian class notes....

INOSILICATES

Pyroxenes
Single-chain inosilicates with Si:O ratio of 1:3 Pyroxenes are major rock forming minerals of earth's crust and mantle, also of lunar rocks.

Si:O in chain in 1:3 ratio, orthopyroxenes are XSiO3 and clinopyroxenes are XYSi2O6 - major component of igneous rocks, classifications of many plutonic rocks are based on pyroxenes - appearance of pyroxene in most metamorphic rocks is part of the granulite facies - sodic pyroxenes (jadeite, omphacite) are indicative of high pressure

Orthopyroxenes
Enstatite MgSiO3 Orthorhombic
Ferrosilite FeSiO3 Orthorhombic
*Bronzite (En70-90) and hypersthene, (En50-70) are common names for solid-solutions in the enstatite-ferrosilite series.

Clinopyroxenes
Diopside CaMgSi2O6 Monoclinic
Hedenbergite CaFeSi2O6 Monoclinic
Augite (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 Monoclinic
Pigeonite Ca0.25(Mg,Fe) 1.75Si2O6 Monoclinic
Aegirine NaFeSi2O6 Monoclinic
Jadeite NaAlSi2O6 Monoclinic
*Omphacite is a high-pressure solid solution between augite and jadeite.

Pyroxenoids
Wollastonite CaSiO3 Triclinic

Amphiboles
Double-chain inosilicates with a Si:O ratio of 4:11, and (OH)

Orthorhombic Amphiboles
Anthophyllite-Gedrite (Mg,Fe) 7Si8O22(OH)2 Orthorhombic

Monoclinic Amphiboles (Clino-amphiboles)
Cummingtonite-Grunerite (Fe,Mg) 7Si8O22(OH)2 Monoclinic
Tremolite-Actinolite Ca2(Mg,Fe) 5Si8O22(OH)2 Monoclinic
Hornblende (Ca,Na,K)2-3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH)2 Monoclinic
Glaucophane Na2Mg3Al2Si8O22(OH)2 Monoclinic
Riebekite Na2Fe3Fe2Si8O22(OH)2 Monoclinic

Mineral Formula Calculation - Examples of Ortho- and Clinopyroxenes

Wt. % Oxides Molec. Wt. Oxides Molecular Porp. Atomic Prop. Sum Molec Prop. = MPF

Calculation of Mineral Formulas for Hydrous Minerals.

Plotting of compositions (wt %, Molecular %, atomic %) in triangular diagrams - extent of solid solution - coexistence of minerals in a specific rock type


Day 25


These pages are under construction, and contian class notes....

Mineral Formula Calculation - Examples of Ortho- and Clinopyroxenes

Wt. % Oxides Molec. Wt. Oxides Molecular Porp. Atomic Prop. Sum Molec Prop. = MPF

Calculation of Mineral Formulas for Hydrous Minerals.

Plotting of compositions (wt %, Molecular %, atomic %) in triangular diagrams - extent of solid solution - coexistence of minerals in a specific rock type


Day 26


These pages are under construction, and contian class notes....

CYCLOSILICATES

Cyclosilicates Ring silicates: Si:O ratio of 1:3. Beryl, Tourmaline, are primary examples; corderite has rings of silica tetrahedra bridged by aluminum tetrahedra

Beryl Be3Al2Si6O18 Hexagonal
Tourmaline (Na,Ca)(Li,Mg,Al)(Al,Fe,Mn) 6(BO3)3(Si6O18)(OH)4 Hexagonal
Cordierite (Mg,Fe)2Al4Si5O18nH2O Orthorhombic
*Cordierite may also be considered as a tectosilicate; cordierite is pseudohexagonal, and the high-temperature polymorph is truely hexagonal.

SOROSILICATES

Sorosilicates Si:O ratio of 2:7. The epidote group minerals; bowtie arrangements of silica tetrahedra

Zoisite Ca2Al3O(SiO4)(Si2O7)(OH) Orthorhombic
Clinozoisite Ca2Al3O(SiO4)(Si2O7)(OH) Monoclinic
Epidote Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH) Monoclinic
Allanite (Ca,Ce) 2(Fe,Fe)Al2O(SiO4)(Si2O7)(OH) Monoclinic


Day 27


These pages are under construction, and contian class notes....

NESOSILICATES

Olivine Group
Forsterite Mg2SiO4 Orthorhombic
Fayalite Fe2SiO4 Orthorhombic

Olivine Fo-Fa Transformation of Mg-rich olivine to spinel is thought to be responsible for increase in seismic velocities at top of earths transition zone (e.g., p. 376). The total cation charge for oilvines and spinels is the same, and there is a solid solution series for spinel analogous to olivine. Olivine with spinel-structure has been foundin meteorites. Spinel is about 12% denser than olivine, for the same composition. Spinel MgAl2O4 Monoclinic Mg2SiIVO4 = Mg2SiIVO4 (b-spinel, see pg. 216-217).

Olivine has a distorted hexagonal closest-packed array of anions in which one eighth of the tetrahedral and one half of the octahedral interstices are occupied by cations; the spinel structure is the cubic closest packing analog of olivine.

forsterite+water=serpentine+brucite forsterite+quartz=enstatite dolomite+quartz=forsterite+calcite+CO2 fayalite is restricted to metamorphosed iron-rich lithologies

Garnet Group

pyralspite garnets
Pyrope Mg3Al2(SiO4)3 Isometric
Almandine Fe3Al2(SiO4) 3 Isometric
Spessartine Mn3Al2(SiO4)3 Isometric
ugrandite garnets
Grossular Ca3Al2(SiO4) 3 Isometric
Uvarovite Ca3Cr2(SiO4)3 Isometric
Andradite Ca3Fe2(SiO4)3 Isometric

General Formula
Structure and Symmetry
External growth forms
Cation substitutions
Garnet compositional changes as a function of P and T
Garnet occurrences in igneous, metamorphic, and sedimentary environments

Aluminum Silicates

Kyanite Al2SiO5 Triclinic
Sillimanite Al2SiO5 Orthorhombic
Andalusite Al2SiO5 Orthorhombic

Aluminum Silicate polymorphs
Phase diagrams
structural considerations of als occurrences
map of als occurrences in new england
tectonic reconstructions of new england
influence of heat flow and tectonic setting on als occurrence

Other Nesosilicates of Particular Importance

Chloritoid (Fe,Mg)2Al4O2(SiO4)2(OH)4 Monoclinic
- alternating brucite-like and corundum-like octahedral layers that are bridged by isolated SiO4 tetrahedra

Staurolite Fe2Al9O6(SiO4)4(O,OH)2 Monoclinic
- monoclinic, commonly as pseudo-orthorhombic, twinned crystals. Medium to high grade metamorphic rocks. - kyanite-like portions of unit cell

Titanite ('Sphene') CaTiSiO5 Monoclinic
- common Ti-bearing accessory. Monoclinic, commonly in wedge shapped crystals.

Topaz Al2SiO4(F,OH)2 Orthorhombic

Zircon ZrSiO4 Isometric


Day 29


Aluminum Silicates

Kyanite Al2SiO5 Triclinic
Sillimanite Al2SiO5 Orthorhombic
Andalusite Al2SiO5 Orthorhombic

Aluminum Silicate polymorphs
Phase diagrams
structural considerations of als occurrences
map of als occurrences in new england
tectonic reconstructions of new england
influence of heat flow and tectonic setting on als occurrence

Other Nesosilicates of Particular Importance

Chloritoid (Fe,Mg)2Al4O2(SiO4)2(OH)4 Monoclinic
- alternating brucite-like and corundum-like octahedral layers that are bridged by isolated SiO4 tetrahedra

Staurolite Fe2Al9O6(SiO4)4(O,OH)2 Monoclinic
- monoclinic, commonly as pseudo-orthorhombic, twinned crystals. Medium to high grade metamorphic rocks. - kyanite-like portions of unit cell

Titanite ('Sphene') CaTiSiO5 Monoclinic
- common Ti-bearing accessory. Monoclinic, commonly in wedge shapped crystals.

Topaz Al2SiO4(F,OH)2 Orthorhombic

Zircon ZrSiO4 Isometric