The mica group of minerals crystallizes in the monoclinic system. They are phyllosilicates with their general formula as – X2Y4-6Z8O20[OH,F]4
Where, X = K, Na, Ba, Ca Y = Al, Mg, Fe+2, Li, Cr, Mn, Zn Z = Si, Al, Fe+3
- They are hydrated alumino-silicates of K, Na, Li and Mg or Fe as darker members.
- Mica minerals are generally found in all felsic rocks and makes an important part in pegmatites.
- They are highly conductor of heat and bad conductor of electricity.
STRUCTURE
Structurally micas can be divided into the two groups –
- The Di-octahedral sheet silicates where each O or OH ion is surrounded by 2 tri-valent cations, usually Al+3 [muscovite]
- The tri-octahedral sheet silicates where each O or OH ion is surrounded by 3 di-valent cations, like Mg+2, Fe+2
These sheets are jointed by the cations like K be wan der vaals bond giving a basal cleavage.
important minerals –
- Phlogopite – KMg3Si3AlO10[OH]2
- Biotite – K[Fe, Mg]3Si3AlO10[OH]2
- Muscovite – KAl2Si3AlO10[OH]2
- Lapidolite – KLi2AlSi3AlO10[OH]2
- Paragonite -NaAl2Si3AlO10[OH]2
- Variety –
- Fuschsite is Cr- bearing mica
- Roscoelite is Vanadium bearing muscovite
- Vermiculite is altered biotite
physical and optical properties
physical properties
- color – white for muscovite and brown for biotite
- form – tabular
- hardness – 2-3
- specific gravity – low
- lusture – vitreous
- special property – elasticity, transparency
optical properties
- color- muscovite is colorless, biotite is brown
- RI- high
- Birefringence – strong
- extinction – parallel
Mode of occurrence –
- muscovite occurs in acid igneous rocks while biotite can ocur in all igneous rocks
- they are also found in sedimentary rocks
- they are vary common in the schists and gneises.
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