| General Formula: CaCO3 |
Sample: M-24 | System: Hexagonal (trigonal) |
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| 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 |
