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Mineralogy & Crystallography

1.2. Definition of a Mineral A mineral is a naturally-occurring, homogeneous solid with a definite, but generally not fixed, chemical composition and an ordered atomic arrangement. It is usually formed by inorganic processes. Let's look at the five parts of this definition: 1.) "Naturally occurring" means that synthetic compounds not known to occur in nature cannot have a mineral name. However, it may occur anywhere, other planets, deep in the earth, as long as there exists a natural sample to describe. 2.) "Homogeneous solid" means that it must be chemically and physically homogeneous down to the basic repeat unit of the atoms. It will then have absolutely predictable physical properties (density, compressibility, index of refraction, etc.). This means that rocks such as granite or basalt are not minerals because they contain more than one compound. 3.) "Definite, but generally not fixed, composition" means that atoms, or groups of atoms must occur in specific ratios. For ionic crystals (i.e. most minerals) ratios of cations to anions will be constrained by charge balance, however, atoms of similar charge and ionic radius may substitute freely for one another; hence definite, but not fixed. 4.) "Ordered atomic arrangement" means crystalline. Crystalline materials are three-dimensional periodic arrays of precise geometric arrangement of atoms. Glasses such as obsidian, which are disordered solids, liquids (e.g., water, mercury), and gases (e.g., air) are not minerals. 5.) "Inorganic processes" means that crystalline organic compounds formed by organisms are generally not considered minerals. However, carbonate shells are minerals because they are identical to compounds formed by purely inorganic processes. An abbreviated definition of a mineral would be "a natural, crystalline phase". Chemists have a precise definition of a phase: A phase is that part of a system which is physically and chemically homogeneous within itself and is surrounded by a boundary such that it is mechanically separable from the rest of the system. The third part of our definition of a mineral leads us to a brief discussion of stoichiometry, the ratios in which different elements (atoms) occur in minerals. Because minerals are crystals, dissimilar elements must occur in fixed ratios to one another. However, complete free substitution of very similar elements (e.g., Mg+2 and Fe+2 which are very similar in charge (valence) and radius) is very common and usually results in a crystalline solution (solid solution). For example, the minerals forsterite (Mg2SiO4) and fayalite (Fe2SiO4) are members of the olivine group and have the same crystal structure, that is, the same geometric arrangement of atoms. Mg and Fe substitute freely for each other in this structure, and all compositions between the two extremes, forsterite and fayalite, may occur. However, Mg or Fe do not substitute for Si or O, so that the three components, Mg/Fe, Si and O always maintain the same 2 to 1 to 4 ratio because the ratio is fixed by the crystalline structure. These two minerals are called end-members of the olivine series and represent extremes or "pure" compositions. Because these two minerals have the same structure, they are called isomorphs and the series, an isomorphous series. In contrast to the isomorphous series, it is also common for a single compound (composition) to occur with different crystal structures. Each of these structures is then a different mineral and, in general, will be stable under different conditions of temperature and pressure. Different structural modifications of the same compound are called polymorphs. An example of polymorphism is the different minerals of SiO2 (silica); alpha-quartz, beta-quartz, tridymite, cristobalite, coesite, and stishovite. Although each of these has the same formula and composition, they are different minerals because they have different crystal structures. Each is stable under a different set of temperature and pressure conditions, and the presence of one of these in a rock may be used to infer the conditions of formation of a rock. Another familiar example of polymorphism is graphite and diamond, two different minerals with the same formula, C (carbon). Glasses (obsidian), liquids, and gases however, are not crystalline, and the elements in them may occur in any ratios, so they are not minerals. So in order for a natural compound to be a mineral, it must have a unique composition and structure. We will return in a few weeks to further discussion of stoichiometry and stability. The fourth part of our definition of a mineral, the part about the ordered atomic arrangement, leads us to a discussion of symmetry which will occupy our first few weeks. 1.3. Mineral Properties in Hand Specimen Learning to recognize hand specimens of approximately 100 of the most common rock-forming minerals is an important part of this course. This recognition is based on seven easily examined properties plus a few unique properties such as magnetism or radioactivity that are strong clues to a mineral's identity. These seven properties are: 1. Crystal form and habit (shape). 2. Luster and transparency 3. Color and streak. 4. Cleavage, fracture, and parting. 5. Tenacity 6. Density 7. Hardness 1.3.1. Crystal form and habit. Recognizing crystal forms (a crystal face plus its symmetry equivalents) in the various crystal systems is one of the reasons we spend some time in lab studying block models. The crystal faces developed on a specimen may arise either as a result of growth or of cleavage. In either case, they reflect the internal symmetry of the crystal structure that makes the mineral unique. The crystal faces commonly seen on quartz are growth faces and represent the slow est growing directions in the structure. Quartz grows rapidly along its c-axis (three-fold or trigonal symmetry axis) direction and so never shows faces perpendicular to this direction. On the other hand, calcite rhomb faces and mica plates are cleavages and represent the weakest chemical bonds in the structure. There is a complex terminology for crystal faces, but some obvious names for faces are prisms and pyramids. A prism is a face that is perpendicular to a major axis of the crystal, whereas a pyramid is one that is not perpendicular to any major axis. Crystals that commonly develop prism faces are said to have a prismatic or columnar habit. Crystals that grow in fine needles are acicular; crystals growing flat plates are tabular. Crystals forming radiating sprays of needles or fibers are stellate. Crystals forming parallel fibers are fibrous, and crystals forming branching, tree-like growths are dendritic. 1.3.2. Luster and transparency. The way a mineral transmits or reflects light is a diagnostic property. The transparency may be either opaque, translucent, or transparent. This reflectance property is called luster. Native metals and many sulfides are opaque and reflect most of the light hitting their surfaces and have a metallic luster. Other opaque or nearly opaque oxides may appear dull, or resinous. Transparent minerals with a high index of refraction such as diamond appear brilliant and are said to have an adamantine luster, whereas those with a lower index of refraction such as quartz or calcite appear glassy and are said to have a vitreous luster. 1.3.3. Color and streak. Color is fairly self-explanatory property describing the reflectance. Metallic minerals are either white, gray, or yellow. The presence of transition metals with unfilled electron shells (e.g. V, Cr, Mn, Fe, Co, Ni, and Cu) in oxide and silicate minerals causes them to be opaque or strongly colored so that the streak, the mark that they leave when scratched on a white ceramic tile, will also be strongly colored. 1.3.4. Cleavage, fracture, and parting. Because bonding is not of equal strength in all directions in most crystals, they will tend to break along crystallographic directions giving them a fracture property that reflects the underlying structure and is frequently diagnostic. A perfect cleavage results in regular flat faces resembling growth faces such as in mica, or calcite. A less well developed cleavage is said to be imperfect, or if very weak, a parting. If a fracture is irregular and results in a rough surface, it is hackly. If the irregular fracture propagates as a single surface resulting in a shiny surface as in glass, the fracture is said to be conchoidal. 1.3.5. Tenacity is the ability of a mineral to deform plastically under stress. Minerals may be brittle, that is, they do not deform, but rather fracture, under stress as do most silicates and oxides. They may be sectile, or be able to deform so that they can be cut with a knife. Or, they may be ductile and deform readily under stress as does gold. 1.3.6. Density is a well-defined physical property measured in g/cm3.a Most silicates of light element have densities in the range 2.6 to 3.5. Sulfides are typically 5 to 6. Iron metal about 8, lead about 13, gold about 19, and osmium, the densest substance, and a native element mineral, is 22. Density may be measured by measuring the volume, usually by displacing water in a graduated cylinder, and the mass. Specific gravity is very similar to density, but is a dimensionless quantity and is measured in a slightly different way. Specific gravity is measured by determining the weight in air (Wa) and the weight in water (Ww) and computing specific gravity from SG = Wa / (Wa-Ww). In practice this is done using a Jolly balance as we will see in lab. 1.3.7. Hardness is usually tested by seeing if some standard minerals are able to scratch others. A standard scale was developed by Friedrich Mohs in 1812 The standard minerals making up the Mohs scale of hardness are: 1. Talc 6. Orthoclase 2. Gypsum 7. Quartz 3. Calcite 8. Topaz 4. Fluorite 9. Corundum 5. Apatite 10 Diamond This scale is approximately linear up to corundum, but diamond is approximately 5 times harder than corundum. 1.3.8. Unique Properties. A few minerals may have easily tested unique properties that may greatly aid identification. For example, halite (NaCl) (common table salt) and sylvite (KCl) are very similar in most of their physical properties, but have a distinctly different taste on the tongue, with sylvite having a more bitter taste. Whereas it is not recommended that students routinely taste mineral specimens (some are toxic), taste can be used to distinguish between these two common minerals. Another unique property that can be used to distinguish between otherwise similar back opaque minerals is magnetism. For example, magnetite (Fe3O4), ilmenite (FeTiO3), and pyrolusite (MnO2) are all dense, black, opaque minerals which can easily be distinguished by testing the magnetism with a magnet. Magnetite is strongly magnetic and can be permanently magnetized to form a lodestone; ilmenite is weakly magnetic; and pyrolusite is not magnetic at all. 1.3.9. Other Properties . There are numerous other properties that are diagnostic of minerals, but which generally require more sophisticated devices to measure or detect. For example, minerals containing the elements U or Th are radioactive (although generally not dangerously so), and this radioactivity can be easily detected with a Geiger counter. Examples of radioactive minerals are uraninite (UO2), thorite (ThSiO4), and carnotite (K2(UO2)(VO4)2 rH2O). Some minerals may also be fluorescent under ultraviolet light, that is they absorb UV lighta and emit in the visible. (There is a display of fluorescent mineral on the first floor of the (old)Geology Building.) Other optical properties such as index of refraction and pleochroism (differential light absorption) require an optical microscope to measure and are the subject of a major section of this course. Electrical conductivity is an important physical property but requires an impedance bridge to measure. In general native metals are good conductors, sulfides of transition metals are semi-conductors, whereas most oxygen-bearing min erals (i.e., silicates, carbonates, oxides, etc.) are insulators. Additionally, quartz (SiO2) is piezoelectric (develops an electrical charge at opposite end under an applied mechanical stress); and tourmaline is pyroelectric (develops an electrical charge at opposite end under an applied thermal gradient). 1.4. Mineral Occurrences and Environments In addition to physical properties, one of the most diagnostic features of a mineral is the geologi cal environment in which it is occurs. Learning to recognize different types of geological environ ments can be thus be very helpful in recognizing the common minerals. For the purposes of aiding mineral identification, we have developed a very rough classification of geological environments, most of which can be visited locally. 1.4.1. Igneous Minerals. Minerals in igneous rocks must have high melting points and be able to co-exist with, or crystallize from, silicate melts at temperatures above 800 C. Igneous rocks can be generally classed according to their silica content with low-silica (<< 50 % SiO2) igneous rocks being termed basic or mafic, and high-silica igneous rocks being termed silicic or acidic. Basic igneous rocks (BIR) include basalts, dolerites, gabbros, kimberlites, and peridotites, and abundant minerals in such rocks include olivine, pyroxenes, Ca-feldspar (plagioclase), amphiboles, and biotite. The abundance of Fe in these rocks causes them to be dark-colored. Silicic igneous rocks (SIR) include granites, granodiorites, and rhyolites, and abundant minerals include quartz, muscovite, and alkali feldspars. These are commonly light-colored although color is not always diagnostic. In addition to basic and silicic igneous rocks, a third igneous mineral environment representing the final stages of igneous fractionation is called a pegmatite (PEG) which is typically very coarse-grained and simi lar in composition to silicic igneous rocks (i.e. high in silica). Elements that do not readily substitute into the abundant minerals are called incompatible elements, and these typically accumulate to form their own minerals in pegmatites. Minerals containing the incompatible elements, Li, Be, B, P, Rb, Sr, Y, Nb, rare earths, Cs, and Ta are typical and characteristic of pegmatites. 1.4.2. Metamorphic minerals. Minerals in metamorphic rocks have crystallized from other minerals rather than from melts and need not be stable to such high temperatures as igneous minerals. In a very general way, metamorphic environments may be classified as low-grade metamorphic (LGM) (temperatures of 60 to 400 C and pressures << .5 GPa (=15km depth) and high-grade meta morphic (HGM) (temperatures > 400 and/or pressures > .5GPa). Minerals characteristic of low- grade metamorphic environments include the zeolites, chlorites, and andalusite. Minerals character istic of high grade metamorphic environments include sillimanite, kyanite, staurolite, epidote, and amphiboles. 1.4.3. Sedimentary minerals. Minerals in sedimentary rocks are either stable in low-tempera ture hydrous environments (e.g. clays) or are high temperature minerals that are extremely resistant to chemical weathering (e.g. quartz). One can think of sedimentary minerals as exhibiting a range of solubilities so that the most insoluble minerals such as quartz gold, and diamond accumulate in the coarsest detrital sedimentary rocks, less resistant minerals such as feldspars, which weather to clays, accumulate in finer grained siltstones and mudstones, and the most soluble minerals such as calcite and halite (rock-salt) are chemically precipitated in evaporite deposits. Accordingly, I would classify sedimentary minerals into detrital sediments (DSD) and evaporites (EVP). Detrital sedimentary minerals include quartz, gold, diamond, apatite and other phosphates, calcite, and clays. Evaporite sedimentary minerals include calcite, gypsum, anhydrite, halite and sylvite, plus some of the borate minerals. 1.4.4. Hydrothermal minerals. The fourth major mineral environment is hydrothermal, minerals precipitated from hot aqueous solutions associated with emplacement of intrusive igneous rocks. This environment is commonly grouped with metamorphic environments, but the minerals that form by this process and the elements that they contain are so distinct from contact or regional metamorphic rocks that it us useful to consider them as a separate group. These may be sub-classi fied as high temperature hydrothermal (HTH), low temperature hydrothermal (LTH), and oxydized hydrothermal (OXH). Metals of the center and right-hand side of the periodic table (e.g. Cu, Zn, Sb, As, Pb, Sn, Cd, Hg, Ag) most commonly occur in sulfide minerals and are termed the chalcophile elements. Sulfides may occur in igneous and metamorphic rocks, but are most typically hydrothermal. High temperature hydrothermal minerals include gold, silver, tungstate minerals, chalcopyrite, bornite, the tellurides, and molybdenite. Low temperature hydrothermal minerals include barite, gold, cinnabar, pyrite, and cassiterite. Sulfide minerals are not stable in atmospheric oxygen and will weather by oxidation to form oxides, sulfates and carbonates of the chalcophile metals, and these minerals are characteristic of oxidized hydrothermal deposits. Such deposits are called gossans and are marked by yellow-red iron oxide stains on rock surfaces. These usually mark mineralized zones at depth and are very common in Colorado. 1.5. Classification of Minerals Minerals are classified on their chemistry, particularly on the anionic element or polyanionic group of elements that occur in the mineral. An anion is a negatively charge atom, and a polyanion is a strongly bound group of atoms consisting of a cation plus several anions (typically oxygen) that has a net negative charge. For example carbonate, (CO3) 2-, silicate, (SiO4)4- are common poly anions. This classification has been successful because minerals rarely contain more than one anion or polyanion, whereas they typically contain several different cations. 1.5.1. Native elements. The first group of minerals is the native elements, and as pure elements, these minerals contain no anion or polyanion. Native elements such as gold (Au), silver (Ag), copper (Cu), and platinum (Pt) are metals, graphite is a semi-metal, and diamond (C) is an insulator. 1.5.2. Sulfides. The sulfides contain sulfur (S) as the major "anion". Although sulfides should not be considered ionic, the sulfide minerals rarely contain oxygen, so these minerals form a chemically distinct group. Examples are pyrite (FeS2), sphalerite (ZnS), and galena (PbS). Minerals containing the elements As, Se, and Te as "anions" are also included in this group. 1.5.3. Halides. The halides contain the halogen elements (F, Cl, Br, and I) as the dominant anion. These minerals are ionically bonded and typically contain cations of alkali and alkaline earth ele ments (Na, K, and Ca). Familiar examples are halite (NaCl) (rock salt) and fluorite (CaF2). 1.5.4. Oxides. The oxide minerals contain various cations (not associated with a polyanion) and oxygen. Examples are hematite (Fe2O3) and magnetite (Fe3O4). 1.5.5. Hydroxides. These minerals contain the polyanion OH- as the dominant anionic species. Examples include brucite (Mg(OH)2) and gibbsite (Al(OH)3). 1.5.6. Carbonates. The carbonates contain CO32- as the dominant polyanion in which C4+ is sur rounded by three O2- anions in a planar triangular arrangement. A familiar example is calcite (CaCO3). Because NO3- shares this geometry, the nitrate minerals such as soda niter (nitratite) (NaNO3) are included in this group. 1.5.7. Sulfates. These minerals contain SO42- as the major polyanion in which S6+ is surrounded by four oxygen atoms in a tetrahedron. Note that this group is distinct from sulfides which contain no O. A familiar example is gypsum (CaSO4 2H2O). 1.5.8. Phosphates. The phosphates contain tetrahedral PO43- groups as the dominant polyanion. A common example is apatite (Ca5(PO4)3(OH)) a principal component of bones and teeth. The other trivalent tetrahedral polyanions, arsenate AsO43-, and vanadate VO43- are structurally and chemically similar and are included in this group. 1.5.9. Borates. The borates contain triangular BO33- or tetrahedral BO45-, and commonly both coordinations may occur in the same mineral. A common example is borax, (Na2BIII2BIV2O5(OH)4 8H2O). 1.5.6. Silicates. This group of minerals contains SiO44- as the dominant polyanion. In these minerals the Si4+ cation is always surrounded by 4 oxygens in the form of a tetrahedron. Because Si and O are the most abundant elements in the Earth, this is the largest group of minerals and is divided into subgroups based on the degree of polymerization of the SiO4 tetrahedra. Orthosilicates. These minerals contain isolated SiO44- polyanionic groups in which the oxygens of the polyanion are bound to one Si atom only, i.e., they are not polymerized. Examples are forsterite (Mg-olivine, Mg2SiO4), and pyrope (Mg-garnet, Mg3Al2Si3O12). Sorosilcates. These minerals contain double silicate tetrahedra in which one of the oxygens is shared with an adjacent tetrahedron, so that the polyanion has formula (Si2O7)6-. An example is epidote (Ca2Al2FeO(OH)SiO4 Si2O7), a mineral common in metamorphic rocks. Cyclosilicates. These minerals contain typically six-membered rings of silicate tetrahedra with formula. (Si6O17)10-. An example is tourmaline. Chain silicates. These minerals contain SiO4 polyhedra that are polymerized in one direction to form chains. They may be single chains, so that of the four oxygen coordinating the Si atom, two are shared with adjacent tetrahedra to form an infinite chain with formula (SiO3)2-. The single chain silicates include the pyroxene and pyroxenoid minerals which are common constituents of igneous rocks. Or they may form double chains with formula (Si4O11)8-, as in the amphibole minerals, which are common in metamorphic rocks. Sheet silicates. These minerals contain SiO 4 polyhedra that are polymerized in two dimensions to form sheets with formula (Si4O10)4-. Common examples are the micas in which the cleavage reflects the sheet structure of the mineral. Framework silicates. These minerals contain SiO4 polyhedra that are polymerized in three dimensions to form a framework with formula (SiO2) 0. Common examples are quartz (SiO2) and the feldspars (NaAlSi3O8) which are the most abundant minerals in the Earth's crust. In the feldspars Al3+ may substitute for Si4+ in the tetrahedra, and the resulting charge imbalance is compensated by an alkali cation (Na or K) in interstices in the framework.

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