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
Habit Rhombohedral
Hexagonal prisms
Color Colorless to white
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

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.

Calcite in Thin Section

Thin Section GigaPans

Calcite in marble, plane polars
Calcite in marble, crossed polars
Sandstone with calcite cement, plane polars
Sandstone with calcite cement, crossed polars
Huge euhedral calcite, plane polars
Huge euhedral calcite, crossed polars

Calcite in marble, PPL

Calcite in marble, XPL

Sandstone with calcite cement, PPL

Sandstone with calcite cement, XPL

Huge euhedral calcite, PPL

Huge euhedral calcite, XPL. Your friendly author is pretty sure calcite informed the design aesthetic of the mid- to late 1980s.

Fine-grained calcite vein, PPL

Fine-grained calcite vein, XPL

Further Reading

Calcite at
Calcite at