Biotite
Color |
Pale green, light brown, red brown |
Pleochroism |
Strongly pleochroic, light brown/green -dark brown/green |
Shape |
Hexagonal plates |
Relief |
Moderate |
Cleavage |
Distinct cleavage, 1set |
Inclusion |
Colourless, apatite |
Alteration |
Present, becomes chlorite |
Cross- nicol |
Anisotropic |
Interference colour |
Strong, ranges upto 2nd order red |
Extinction position |
Parallel extinction |
Twinning |
Not observable |
Diagnostic property |
Col, pleo, Clea, int. Col |
Muscovite
Color |
Colorless |
Pleochroism |
Non-pleochroic, may be pale green |
Shape |
Tabular crystals, lamellar, flaky |
Relief |
Moderate |
Cleavage |
1 set, perfect |
Inclusion |
Present |
Alteration |
Present |
Under X- Nicol |
Anisotropic |
Int. Color |
Upto 2nd order, yellow or red |
Extinction position |
Parallel extinction |
Twinning |
Hard to detect |
Diag. Prop. |
Shape, cleavage, ex. Pos. |
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Calcite
Color |
Colorless, sometimes cloudy |
Pleochroism |
Non pleochroic |
Shape |
Fine to coarse grained, subhedral to euhedral |
Relief |
High to low, variable |
Cleavage |
3sets of perfect, rhombohedral |
Inclusion |
Present |
Alteration |
Absent |
Cross nicol |
Anisotropic |
Int. Col |
High 3,4 or 5th order |
Ex. position |
Symmetrical to cleavage |
Twinning |
Prominent |
Diag. Prop |
Col, Clea, rel, twink, int.col |
Olivine
Color |
Colorless/ faint green |
Pleochroism |
Non-pleochroic |
Shape |
Rounded/ anhedral |
Relief |
High |
Cleavage |
Absent |
Inclusion |
Absent |
Alteration |
Present |
Cross- nicol |
Anisotropic |
Int. Color |
Bright colours of 2nd and 3rd order |
Extinction position |
Parallel to cleavage & crystal outlines |
Twinning |
Rare and poorly developed |
Diag. Prop. |
Col, cleav, ext. Pos & twinning |
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Hornblende
Color |
Shades of yellow, green or brown |
Pleochroism |
Strongly pleochroic, yellowish green to dark brown |
Shape |
Prismatic xtal, 6sided(basal section) |
Relief |
Moderate to high |
Cleavage |
Two distinct cleavage |
Inclusion |
Present |
Alteration |
Absent |
Cross- nicol |
Anisotropic |
Int.color |
2nd order |
Extinction position |
Oblique section, b/w 12°-20° |
Twinning |
Simple twins |
Diag. Prop. |
Pleo, cleavage, ex.position |
Plagioclase
Color |
Colorless |
Pleochroism |
Non-pleochroic |
Shape |
Lath shape, Subhedral to anhedral |
Relief |
Low |
Cleavage |
Present |
Inclusion |
Absent |
Alteration |
Present |
Cross- nicol |
Anisotropic |
Int. Color |
1st order, light grey to yellowish grey |
Extinction position |
Oblique section |
Twinning |
Polysynthetic, Albite twin law |
Diag. Prop. |
Shape, rel, cleav, twin |
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Microcline
Colour |
Colorless, sometimes cloudy |
Pleochroism |
Non-pleochroic |
Shape |
Lath shape, Subhedral to anhedral plates |
Relief |
Low |
Cleavage |
Perfect |
Under cross nicol |
Anisotropic |
Int. Color |
1st order grey |
Ext. Position |
Oblique, < varies upto 15° |
Twinning |
Cross hatched, or grid twinning |
Diag. Prop. |
Col, relief, shape and twinning |
Quartz
Color |
Colorless |
Pleochroism |
Non-pleochroic |
Shape |
Subhedral to euhedral, Trigonal |
Relief |
Very low |
Cleavage |
Absent |
Inclusion |
Present |
Alteration |
Absent |
Cross- nicol |
Anisotropic |
Interference colour |
1st order, white and grey |
Extinction position |
Undulose extinction |
Twinning |
Not visible |
Diag. Prop. |
Relief, cleavage, ex.pos., int.col.......it shows all -ve optical character |
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