Micas constitute an important group of rock forming minerals

Micas constitute an important group of rock forming minerals, as ferromagnesian silicates, next in importance to the amphiboles and pyroxenes. They form a link between felspar and felspathoids i.e., the light coloured constituents of the igneous rocks and the dark coloured minerals.

1. Atomic structure:

These are phyllosilicates which possess the Si₄Oio sheet structure.

2. Chemical composition:

These are hydrated alumina silicates of K., Na or Li, with Mg or Fe in darker members.

3. Classification:

Some classification of mica groups are as

(a) Dioctahedral group. Muscovite.

(b) Trioctahedral group. Phlogopite and Biotite.

According to other classifications, the composition of the micas are taken into account and they are as

(I) Muscovite group:

These are aluminosilicates of alkali metals without Mg or Fe; and are colourless. It includes members like:

i) Muscovite. K-aluminosilicate with (OH).

i) Paragonite. Na-aluminosilicate with (OH).

i) Lepidolite. K-Li-Al, silicate with (OH).

(II) Biotite group:

In addition to the alkali metals, they also contain Mg, Fe and are dark in colour. They contain the following members:

(i) Biotite. K, Mg, Fe, Al, silicate with (OH).

(ii) Phlogopite. K, Mg, Al, siiicatc with (OH).

(iii) Zinnwaldite. Li, Fe-Al, silicate with (OH).

4. Physical properties:

(a) They crystallise in monoclinic system but the forms are Pseudo-hexagonal.

(b) Moscovite. Colourless.

(c) Biotite. Dark coloured.

(d) Lusture. Pearly, splendant.

(e) Cleavage. Perfect basal cleavage.

(f) Hardness. 2 to 3.

(g) Sp. gravity. Low.

(h) Twinning. Rarely seen.

(i) Special characteristics. They possess unique combination of properties of elasticity, toughness, flexibility, transparency, resistance to heat and property of splittings into thin films.

5. Optical properties:

(i) Muscovites colourless to pale green, Biotites dark brown.

(ii) Coloured varieties are pleochroic.

(iii) Ref. index. High.

(iv) Bi-refringence. Strong, i.e., 0 033 to 0’059.

(v) Extinction. Parallel.

6 Varieties:

(i) Sericite. Secondary mica (muscovite).

(ii) Fuschsite. Cr-bearing mica.

(iii) Gilbertite. Fl-rich muscovite.

(iv) Roscoelite. Vanadium bearing muscovite.

(v) Damourite. Secondary muscovite by hydrothermal altera­tion of feldspar.

(vi) Lepidomelane. An iron-rich biotite.

(vii) Vermiculites. Altered biotite.

7. Occurrence:

Muscovite occurs in acid-igneous rocks. Biotite occurs in all igneous rocks, particularly the acid and inter­mediate ones.

In sedimentary rocks also they are present as an authigenic constituent.

In schists and gneisses (metamorphic rocks) micas are the usual constituents.

Important characteristics:

Micas show a peculiar optical property known as ‘birds eye structure’, which is noted as the extinc­tion is approached.

Percussion figures are also the characteristic features found in mica, when any pointed object is pressed against it.