Mica Division - Mica Group, Muscovite, Lepidolite and Biotite
The micas form a series of complex silicates of aluminum with potassium and hydrogen, also often magnesium, ferrous iron, and in some varieties, sodium, lithium, ferric iron. More rarely manganese, chromium, barium, fluorine and titanium are present in small amounts. The composition of many of the micas is not definitely understood the formulas assigned to them are only approximate.
They crystallize in the monoclinic system but with an axial inclination of practically 90°, so that their monoclinic symmetry is not clearly seen. The crystals are usually tabular with prominent basal planes, and have either a diamond- or hexagonal-shaped outline with angles of 60° and 120°. The crystals, as a rule, therefore, appear to be either orthorhombic or hexagonal in their symmetry. They are all characterized by a very perfect basal cleavage.
They form an isomorphous series, and various gradations between the different members occur. Their isomorphism is further indicated by two members of the group frequently crystallizing together, with a parallel position, in the same crystal plate. Biotitic occurs crystallizing in this way with muscovite, and muscovite with lepidolite, ete.
The important member; of the group follow:
- Muscovite, H2Kal3(Si04)3.
- Lepidolite, KLi[Al.2(OH,F)]Al(Si03)3
- Biotite, (H,K)2(Mg,Fe)2Al2(Si04)3.
- Phogopite, H2Mga3Al(Si04)3?
- Lepidomelane, (H,K)2Fe3(Fe,Al)4(Si04)5?
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