Essay on the most important group of the rock-forming silicates

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This is the most important group of the rock-forming silicates. They constitute about 2/3rd of the igneous rocks.

Atomic structure Tektosilicates:

Chemical composition:

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They are aluminosilicates of potassium, sodium, calcium 3rd barium, and may be regarded as isomorphous mixtures of the four end members given below, of which the first three are common while the fourth is rare.

1. Orthociase and Microcline KAlSi308.

2. Albite NaAlSi308.

3. Anorthite CaAl2Si2O6.

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4. Celsian BaAl2Si2O6.

The three isomorphous series are:

(а) Anorthcclase series:

Between c-‘K-and a-‘Na’ feldspar.

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(b) Hyalophane series:

Between a-‘K’ feldspar and a-‘Ba’- feldspar with K>Na.

(c) Plagioclase series:

Between albite and anorthite:

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Physical Properties:

(i) Crystal form:

Orthociase is monoclinic. Microline and other plagioclases are triclinic.

(ii) Colour:

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Orthociase is flesh-red in colour. Microcline is green in colour and the colour of plagioclases ranges from white to gray.

(iii) Lusture:

Vitreous or pearly (play of colour is marked),

(iv) Cleavage:

2 sets-one parallel to (001) face and other to (010). The angle between the cleavages is 90° in case of orthoclase but less than 90° in other members.

(v) Hardness:

6 (six).

(vi) Sp. Gravity:

2.5 to 3, according to calcium content.

(vii) Twinning:

(a) (1) Carlsbad.-‘c’ axis is the twin axis and (010) the composition plane.

(2) Baveno:

(021) is the twin plane (i e., clinodome).

(3) Manebach:

(001) is the twin plane. These three are commonly found in orthoclase.

(b) Micrccline:

(1) Albite:

Twin plane (001), twin axis perpendicular to this.

(2) Pcricline law:

(010) composition plane, twin axis is the b-axis.

(c) Plagioclases:

They show all the above twinning types.

Microscopic characteristics:

1. Form:

Subhedral to anhedral.

2. Cleavage. 2 sets.

3. Br refringence. Weak (Ref. index=low), i.e., 0 009.

4. Twinning:

(i) Orthoclase: simple twinning.

(ii) Microcline: cross-hatched twinning,

(iii) Plagioclase: polysynthetic (or lamellar).

5. Extinction angle:

15 to 20°. Michael-Levy method is used to determine this.

Varieties:

(1) Sanidine:

A high temperature potassium feldspar (in volcanic rocks).

(2) Adularia:

Low-temperature orthoclase.

(3) Moon stone:

Opalescent adularia or albite.

(4) Aventurine:

Gem variety of albite.

(5) Amazon stone:

Bright green microcline.

Occurrence:

Alkali-feldspars, i.e., orthoclase, microcline and albite are common in acid igneous rocks like granite, grano diorite syenite etc. in sandstones and in gneisses (metamorphic rocks). Plagioclase occurs in intermediate to basic igneous rocks.

Important Features of Occurrence:

1. Perthite:

Intergrowth between orthoclase/microcline and albite or obligoclase. But albite patches found in microcline mass.

2. Antiperthite:

Microcline is found crystallised in predomi­nant albite (as ground mass).

3. Graphic:

Intergrowth between quartz and orthoclase feldspar.

4. Myrmekitic:

Intergrowth between quartz and plagioclase feldspar.

5. Zoning:

Variation of composition marked in the marginal region of feldspar-minerals in zones.

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