The Mica Group of minerals|
The micas are an important group of minerals. They represent the classic phyllosilicate mineral and are usually the first minerals to be thought of from this subclass of the Silicates Class. Micas are significant rock forming minerals being found in all three rock types: igneous, metamorphic and sedimentary. Because thin flakes of mica are generally flexible and brittle, it is surprising how resistant and durable mica crystals can be in withstanding high temperatures and pressures in metamorphic regimes as well as the punishment of erosional environments. The term "mica" is so familiar to the general public that it is often considered a mineral in itself. Of course it is actually a group of minerals and most people who are knowledgeable about minerals know the three most common mica minerals: muscovite, biotite, and lepidolite and perhaps a few of the less common micas glauconite, paragonite, phlogopite and zinnwaldite. The Mica Group is actually a rather large group of minerals with over 30 members.
Being true phyllosilicates, all micas are thus composed of sheets of silicate tetrahedrons. The silicate sheets are composed of interconnected six membered rings. These rings are responsible for the micas typical six sided pseudohexagonal symmetry, in actuality they are only monoclinic or triclinic. Each tetrahedron in the rings shares three of their oxygens with three other tetrahedrons and all the tetrahedrons in a given sheet point their unshared oxygen in the same direction. The structure of micas is stacked like a building with several different layers. Two tetrahedral layers (T) with their tetrahedral points pointing toward each other, sandwich small metal ions such as aluminum in an octahedral layer (O). This tetrahedral-octahedral-tetrahedral (TOT) sandwich is stacked with layers of large cations such as potassium or calcium. This cation layer is known as the interlayer (i) because it is between the TOT sandwich layers. This i layer, is needed to balance the formula due to the substitution of the +3 charged aluminum for +4 charged silicons in the T layers. The whole structure can then be illustrated with the following sequence of layers: ...iTOTiTOTiTOTiTOTi...
The Mica Group minerals are closely associated with the clay minerals. The clays have a similar structure but include brucite and gibbsite layers between their silicate layers. However, often crystals will be intergrown with mica and clay layers forming a composite crystal; as in a few layers of pure biotite and then a few layers of pure vermiculite, then biotite, etc, etc. This type of arrangement in a single crystal is making identification and classification of these minerals extremely complicated and confusing. The mica minerals muscovite, glauconite and illite are often considered clays due to their clay like properties.
The general formula for the micas is AB2-3(X, Si)4O10(O, F, OH)2. The tetrahedral layers by themselves have a formula of (X, Si)2O5. In most micas the A ion is usually potassium but can also be sodium, calcium, barium, cesium and/or ammonium. These ions occupy positions in the interlayer i discussed above. The B ion can be either aluminum, lithium, iron, zinc, chromium, vanadium, titanium, manganese and/or magnesium. These ions occupy positions in the octahedral layers O. The X ion is usually aluminum but can also be beryllium, boron and/or iron (+3) and they sit in the center of the tetrahedrons substituting for silicons by up to 50%.
There are three major divisions within the Mica Group; The True Micas, The Brittle Micas and the a new division called The Interlayer-deficient Micas. The True Micas have a majority of singularly charged ions in the A position (ions such as potassium and sodium). The Brittle Micas have a majority of doubly charged ions in the A position (ions such as calcium or barium). The Interlayer-deficient Micas, which used to be called the Hydromicas, have fewer i ions than other micas, hence the name. The three divisions can further be divided into dioctahedral and trioctahedral groups. The B ions occupy octahedrally coordinated sites, bonded to six oxygens and two of the extra non-tetrahedral anions (hydroxide, fluorine and/or extra oxygen ions). Dioctahedral micas have two (or less than 2.5) B ions in their formulas, whereas trioctahedral micas have three (or at least 2.5 or more) B ions in their formulas.