Calcite

Aragonite is a polymorph of calcite. In hand sample, calcite can be distinguished by its rhombohedral cleavage and, if the sample is translucent enough, its double refraction. In thin section, calcite can be distinguished by its parallel extinction.

Physical Properties
Chemical formula	CaCO₃
Class	Carbonate
Crystal system	Hexagonal (rhombohedral)
Habit	Rhombohedral Hexagonal prisms Scalenohedra Tabular Others
Color	Colorless to white Gray Red Blue Yellow/orange Green Brown to black when impure
Hardness	3
Specific gravity	2.71
Cleavage	Perfect rhombohedral {101}
Fracture	Conchoidal
Luster	Vitreous
Transparency	Transparent to translucent
Streak	White
Optical Properties
PPL	Colorless, but may exhibit pale pastels High relief Visible polysynthetic “calcite” twinning in coarse grains
δ	0.172
XPL	Extremely high birefringence; colors may look pastel, washed out, or almost neon High relief Polysynthetic “calcite” twinning in coarse grains; fine grains appear as a sort of swirl of color
Twinning	Polysynthetic; “calcite” twinning
Extinction	Parallel
Special properties	Reacts with HCl (fizzes) Double refraction
	after Perkins, 382

Calcite in Hand Sample

Calcite
Four small hand samples
Several habits in one hand sample
Euhedral calcite crystals in a cobble of amygdaloidal basalt
Red and white calcite with green diopside
Calcite veins crosscutting faulted sedimentary strata
Calcite filling the gaps between segments of a stretched, broken belemnite fossil
Scanning electron micrograph of calcite
Another scanning electron micrograph of calcite

Calcite displaying double refraction.

The following two scanning electron micrographs come courtesy of Nik Deems.

Brief tangent: classroom chalk

If you’ve taken an intro Geology class (and I bet you have), you’ve probably heard that chalk is made up of the calcite tests of algae called coccolithophores. Well, sometimes it’s true. Behold this scanning electron micrograph of classroom chalk, and click the blue circle to see an intact coccolithosphere.

Some manufacturers make their chalk from a blend of dolomite and gypsum, seen in this scanning electron micrograph. Others make it of straight gypsum, seen in this scanning electron micrograph.