Physical Properties
Chemical formula CaMg(CO3)2
Class Carbonate
Crystal system Hexagonal (rhombohedral)
Habit Typically rhombohedral
Steep rhombohedra
Color Pink
Hardness 3.5 to 4
Specific gravity 2.85
Cleavage Perfect rhombohedral {101}
Fracture Subconchoidal
Luster Vitreous
Transparency Transparent to translucent
Streak White
Optical Properties
PPL Colorless, but may exhibit
pale pastels
Visible polysynthetic “calcite”
twinning in coarse grains
XPL Extremely high birefringence;
colors may look pastel, washed out,
or almost neon
Polysynthetic “calcite” twinning” in coarse
grains; fine grains appear as a sort of swirl
of color
δ 0.179
Twinning Lamellar
Special properties Reacts with HCl (fizzes),
but only if powdered
after Perkins, 384-385

Dolomite in Hand Sample

Two small hand samples of dolomite
Differentially weathered calcite and dolomite
Scanning electron micrograph of dolomite

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. Sometimes it’s made of a blend of dolomite and gypsum, as seen in this scanning electron micrograph. Sometimes it’s just gypsum, as seen in this scanning electron micrograph.

Dolomite in Thin Section

Thin Section GigaPans

Dolomite in plane polars
Dolomite in crossed polars

Dolomite, PPL

Dolomite, XPL

Further Reading

Dolomite at
Dolomite at