This is from http://www.minerals.net/mineral/calcite.aspx
I didn't make this up, folks. Calcite is not
an isometric mineral, rather it is a hexagonal mineral with (normally) rhombohedral cleavage. (Bass - YOU are a better mineralogist than I ever thought to be. Fur shame, fur shame !.)
Chemical Formula CaCO3
Composition Calcium carbonate, sometimes with impurities of iron, magnesium, or manganese, and occasionally zinc and cobalt.
Variable Formula (Ca,Fe,Mg,Mn,Zn,Co)CO3
Color Colorless, white, yellow, brown, orange, pink, red, purple, blue, green, gray, black. May also be multicolored or banded.
Hardness 3Crystal System Hexagonal
and Aggregates Occurs in a great variety of shapes, with the most common forms asrhombohedral and scalenohedral crystals. Crystals may be tabular, acicular, prismatic, flaky, and needle-like. May occur as bundles of scalenohedrons, intergrown rhombohedrons, hair-like masses of acicular crystals, grainy, stalactitic, fibrous, massive, and earthy. Scalenohedral twinning is common.
Transparency Transparent to opaque
Specific Gravity 2.7
Cleavage 1,3 - rhombohedral
Fracture Conchoidal. Rarely observed due to the perfect cleavage.
Other ID Marks 1) Commonly fluorescent; specimens from different localities fluoresce different colors. Some Calcite is also phosphorescent.
2) Transparent crystals exhibit strong double refraction.
3) May be thermoluminescent.
Complex Tests 1) Effervescent in hydrochloric acid and most other acids.
2) Calcite that doesn't fluoresce usually becomes fluorescent upon heating.
In Group Carbonates; Calcite Group
Striking Features Hardness, cleavage, fluorescence, and effervescence with hydrochloric acid.
Environment Calcite is a constituent of all mineral environment, including sedimentary, igneous, and metamorphic.