I am a second year student at Queen's University in a medial program of Geology and Chemistry. In the introductory class to minerology we learn how the atomic structure of minerals and crystals effect the shape of the crystals and where thier planes of weakness, called cleavage, lie. The following is a brief summary of the chemistry-related content of that course intended for chemistry students considering broadening their horizons into the field of geology.
Major types of atomic arrangements in mineralsNenosilicates are the smallest units and consist of isolated SiO4 tetrahedra bound together by ionic forces between the contained cation and the anions of surrounding tetrahedra.
There are a number of common minerals that are primarily nenosilicates including:
Follow the links to see a sample of each mineral. All jpgs taken from The Mineral Gallery.
The differences from mineral to mineral lie in atomic substitution within the nesosilicate (most often Al³+ instead of Si³+) and the arrangements of the tetrahedra along with octahedra and polyhedra which accomidate a cenral cation of their own uncluding Mg²+, Fe²+, Fe³+, Mn²+, Ca²+, and Cr³+.
A garnet structure showing tetrahedra in blue, octahedra in purple and filler cations in light blue
Inosilicates are chains of tetrahedra that link together by sharing oxygen. The Si:O ratio is 1:3. There can be single-chain minerals or double-chain minerals. In the double-chain minerals two chains are stuck together by every second tetrahedra in one chain sharing an oxygen with every second tetrahedra in the other chain. The Si:O ratio of a double chain inosilicate is 4:11. The single-chain minerals are called Pyroxenes (the most common example, Diopside) and the double chain minerals are Amphiboles (it's most common member, Hornblende). Another important difference between the two is that Amphiboles have OH- inlcuded in their structure.
As with nenosilicates (and indeed all of the structural groups) the differences between minerals of the same group lie in slightly different structure and Si substitutions inlcuding Fe²+, Mg²+, Ca²+, and Na¹+.
This is the single chain structure of pyroxene.
Phyllosilicates are minerals composed of sheets of tetrahedral units and octahedral units, each sheet held together by sharing oxygens and sheets held to other sheets by the weaker ionic forces between them. As a result, most minerals of this variety are soft and platey and have one prominent cleavage.
Differences between minerals of this group depend on Si substiutions but structural differences play a larger role in this group. The simplest structural unit is a sheet of octahedra linked together by sharing oxygens.
When a sheet of tetrahedra shares the the oxygens on one side of the octahedral sheet soft, clayey minerals like Kaolinite result.
If a second sheet of tetrahedra is bonded to the other side of the octahedras it is a t-o-t structure (tetrahedra, octahedra, tetrahedra). This is again a soft powdery mineral: talc (talcum powder) is a common example.
Now, if two t-o-t structures are bonded together by cations between them the platey minerals are formed. A common example is muscovite which is soft, lustrous, and transparent and is used as glitter.
If the intersructural cations are replaced by brucute sheets (sheets of octahedra with OH- groups on the outsides) another common mineral, chlorite, results. Chlorite is still platey but not as perfect as muscovite.
The structure of muscovite where Si is grey, O is red and, purple and brown are interstructural cations.
Tectosilicates are massive frameworks of SiO4 tetrahedra where each O²- is shared with a neighboring tetrahedra so the overall Si:O ratio is 1:2. Common examples of tectosilicates are quartz and all it's varieties (e.g. chert, flint, rose quartz, smokey quartz, amathyst, opal) and Feldspars like Potassium feldspar and labradorite (pictured in feldspar link).
The structures in this group are generally the same and the main differences amoung tectosilicate minerals is in the inclusions of various cations like K¹+, Na¹+, and Ca²+.
This is an image of the quartz tetrahedra arranged in it's crystal lattice.
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Structural images from
