CALCITE

General Formula:
CaCO3
Sample: M-24 System:
Hexagonal (trigonal)
Calcite
Irregular calcite crystals within a marble. Note the rhombohedral cleavage displayed by the grain just left of center.
Field of View = 2.7 mm, plane light
Calcite
Extreme inteference colours of calcite and the presence of twinning in the top right grain.
Field of View = 2.7 mm, crossed polars
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Optical Properties

Colour
Pleochroism
colourless
non pelochroic
Form variety of habits, but usually coinsist of scalenohedron and rhombohedron combinations. In most rocks calcite forms anhedral grains or grain aggregates
Relief
RI
moderate negative to high positive, marked change with stage rotation
nw = 1.658
ne = 1.486
Cleavage perfect rhombohedral cleavage, angle between cleavages 74°57'
Birefringence
Interference Colours
0.172
extreme, creamy high order colours
Twinning lamellar twins parallel to one edge of the cleavage rhomb or along the long diagonal of the rhomb
Interference Figure
Optic Sign
2V
uniaxial
negative
Optic Orientation extinction is inclined or symmetrical to cleavage traces, the fast ray is parallel to the short diagonal of the rhombohedral faces
Composition dominantly CaCO3, but substitution of Mg, Fe, Mn, or Zn and minor Sr and Ba Alteration altered to dolomite during diagenesis, calcite is soluble in natural waters and may be removed by solution
Occurrence common and widespread as a major mineral in limestones, and an accessory in igneous, metamorphic and sedimentary rocks Distinguishing Features cleavage, variable relief, extreme interference colours


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