Micas and Other Phyllosilicates
Micas and other Phyllosilicates
Thomas A. Loomis
Dakota Matrix Minerals
The eight most abundant elements which constitute 98% of Earth’s crust are: oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. These same elements combine to form a majority of the minerals in the mica group. Given this fact, you might conclude that mica should be found in great abundance worldwide. But two other minerals require most of the same elements to form. These minerals are quartz and feldspar, which constitute about 95% of the Earth’s crust. Although mica group minerals are found worldwide in geologically diverse environments, it also requires hydrogen to form. Even though hydrogen is the most common element in the universe, it constitutes only 0.14% of the Earth’s crust. The hydrogen requirement limits mica formation and therefore requires specific but diverse geological environments.
Micas are a complex group of no less than 45 individual minerals. The most abundant, muscovite and biotite, are common rock-forming minerals. These micas are found in plutonic igneous rocks with feldspar and quartz and are localized in pegmatites. Although volumetrically minor, micas are concentrated in pegmatites relative to their host rocks. Large sheets and "books" of muscovite mica are known to occur in pegmatites with dimensions measured in the feet or meters. They are also found in extrusive igneous rocks but not in concentrated quantities. Large scale metamorphism also creates micas which can occur in a broad range of rocks such as gneiss, slates, phyllites, and marbles. Mica in metamorphic rock typically occur as minute crystalline grains on the order of less than 1mm. Sedimentary rock also contain mica minerals although only as an accessory mineral and usually in minute crystal grains such as illite clay and pelletized glauconite. On a smaller scale within the Earth’s crust, hydrothermal alteration and other mineral producing environments produce micas and other mica-like minerals from the phyllosilicate class of which the mica minerals are a member.
Phyllosilicates, as its root name implies, are silicate minerals with a phyllon or leaf-like texture or habit. This texture is a manifestation of its crystal chemistry, which is formed by negatively charged SiO4 (silica) tetrahedrons networked in very well ordered sheets layered with positively charged ions of aluminum, iron, magnesium and hydroxyl, fluorine and/or chloride. Simple combinations of these layers with complex and highly variable ionic substitutions create the large class of minerals sometimes called the “sheet or layered silicates” but formally known as the phyllosilicates. Mica group minerals, are phyllosilicates composed of two silica tetrahedral (T) layers which sandwich a layer of octahedrally (M) coordinated cations (Al, Fe, Mg). Cations are “coordinated” when they are surrounded by oxygen anions. Depending upon how large a cation is, determines its coordination number. Silica requires four oxygen atoms and forms a tetrahedron and thus has a coordination number of four. The larger aluminum, iron and magnesium ions require six oxygen atoms and form an octahedron (coordination of six). These octahedrons form the M layer and the tetrahedrons form the T layer. The stacked order in mica group minerals are layers T – M – T. The layers impart a mineral property said to be micaceous or foliated, which is a fundamental characteristic of almost all phyllosilicate minerals but is especially pronounced in the micas. Micas in hand specimen can be pried apart with a knife into thin, flat plates. This mineral characteristic or habit is called micaceous. Other phyllosilicate minerals like talc or pyrophyllite are so soft they can be scratched with your fingernail which is a result of thin, wavy laminae said to be foliated. Foliated habit and the micaceous habits are rather distinct and is a result of the crystal chemistry of phyllosilicate minerals. Foliated minerals are another discussion.
Mica with a hardness of 2.5 has to be pried apart while talc falls apart with a scratch from your fingernail. They are both phyllosilicates but uniquely different. Why? Because there is a third type of layer which mica has that talc does not. This third layer, called the interlayer (I), allow mica minerals to maintain their sheeted structure and increase their hardness. In micas the silicon cation in the tetrahedral layer is invariably replaced by aluminum, which creates a charge imbalance. It is balanced when large cations calcium, sodium, and/or potassium neutralizes the crystal structure upon forming the interlayer. The interlayer cations sit between two tetrahedral layers so the overall structure of mica is basically [T- M –T - I- T- M –T].
Depending on your source and application, the phyllosilicates can be broken down into several groups:
- The Mica group
- The Clay & Vermiculite group
- The Serpentine group
- Talc-Pyrophyllite group
- The Chlorite group
Classification of these groups are based upon many criterion but mainly layering order, the nature and composition of the octahedral layer, and the presence or absence of the interlayer. All of the members have characteristic layered structure as mentioned above, with perfect basal cleavage (right angles to the c-axis), which yield thin laminae or plates with relative ease. Further, most of them belong to the monoclinic crystal system but with pseudohexagonal symmetry. As hydrogen must be present to form micas and other phyllosilicates they yield water upon ignition. Micas will yield about 4-5% water, while chlorites yield even more at 10-13% due to trapped water and not structural water (Ford, 1949). Sinkankas (1964) notes that in all phyllosilicates the crystals tend to form into elongated prisms but also tabular to wafer-like crystals. An interesting optical aspect of sheet silicates is that light can be transmitted through the prism more easily than thin cleavages due to the optical properties of the sheet structure.
Mica group minerals are grouped according to their interlayer cations. The interlayer (I) composition will determine if the micas are classified as brittle, ordinary or true, and interlayer deficient. These groups are further broken down by the octahedral layer’s composition. The structural formula unit of mica includes room for three octahedrally coordinated vacancies in the M layer to be filled with cations. Depending upon the valence state of a cation will determine if two or three of the vacancies are filled.
Rieder et al 1998, defined a simplified formula for the mica group:
I M2-3 [ ]1-0 T4 O10 A2
Where I the interlayer is commonly Cs, K, Na, NH4, Rb, Ba, Ca
M, the octahedral layer, includes 2 to 3 cations which may include Li, Fe (di or trivalent), Mg, Mn, Zn, Al, Cr, V, Ti (or combinations thereof). If two octahedrons are filled the mica is dioctahedral. If three are filled, the mica is trioctahedral.
[ ] is a vacancy in the octahedral layer if only two octahedrons are filled as indicated above (the mica is dioctahedral).
T is commonly Be, Al, B, Fe (trivalent), Si (or combinations thereof).
A is commonly Cl, F, OH, O (oxy-micas), S (or combinations thereof).
Note the above simplified formula includes 10 atoms of oxygen (O) and four T atoms (cations). All phyllosilicates formulas will include a ratio of 2:5 (silica to oxygen). The reader is referred to Rieder et al 1998 for the caveats to this formula.
The octahedral (M) layer of the above formula must charge balance the opposing tetrahedral (T) layers, which sandwich the M layer. The two opposing T layers have an excess charge of (-6). This includes four partially shared oxygen anions each carrying a charge of (-1). These oxygens are termed apical and are at the "apex" of the silicon tetrahedrons. One T layer points up and the other points down. The apical oxygens carry a collective charge of (-4). The remaining negative charge comes from two hydroxyl (OH-) units, or a combination thereof with F-, Cl- or in the rare case S-, sulfur in the crystal structure of anandite. These anions conveniently fit at the center of the two dimensional, hexagonal rings created by the silicon tetrahedrons of the T layer. Thus, typically there are two to three M layer cations satisfying the charges of the T layers. If the valence state of the cation is (+2), all three octahedral vacancies will be filled and the charges will balance, and the mica is termed trioctahedral. Accordingly, if the cation charge is (+3), only two octahedral sites will be filled since the net positive charge is (+6) and the mica is termed dioctahedral.
True or Ordinary mica usually includes the K and but must contain >50% monovalent cations in the interlayer (I). The True micas contain flexible and elastic sheets. Brittle micas are just that, brittle, and usually break when their sheets are cleaved apart. Brittle micas usually contain Ca or Ba but must contain >50% divalent cations. The Interlayer-deficient micas exhibit <85% and 0.6 positive charge.
A breakdown of the micas can be found here in Rieder et al 1998, Nomenclature of the Micas, published by the Canadian Mineralogist.
An abbreviated listing of the Micas is included here adapted from Rieder et al, 1998.
Ordinary or True Micas
Citations and a short Bibliography:
Berry, L. G. and Mason, B. 1959 Mineralogy: Concepts Descriptions Determinations
Deer, F.R.S., Howie, R. A., Zussman,J. 1966 An Introduction to the Rock – Forming Minerals
Dyar, M.D., Gunter, M. E., Tasa, D. 2008, Mineralogical Society of America
Ford, W. E. 1949 Dana’s Textbook of Mineralogy 4th Ed.
Lauf, R. J. 2008 Collector’s Guide to the Mica Group by Schiffer.
Rieder, M. et al Nomenclature of Micas, Canadian Mineralogist v.36 (1998).
Roberts, W. L. and Rapp Jr. G. R. 1974 Encyclopedia of Mineralogy
Sinkankas, J. 1964 Mineralogy for Amateurs
Wenk, H-R and Bulakh 2004 Cambridge University Press