Gypsum

Physical Properties
Chemical formula CaSO4∙2H2O
Class Sulfate
Crystal system Monoclinic
Habit Tabular
Elongate masses
Rosettes
Acicular splays
Massive
Granular
Color Colorless
White
Variable
Hardness 2
Specific gravity 2.32
Cleavage Perfect basal (010), good (100)
and {011}
Fracture Conchoidal
Luster Vitreous
Pearly
Transparency Transparent to translucent
Streak White
Optical Properties
PPL Colorless
Non-pleochroic
XPL Up to 1st order yellows
δ 0.009
after Perkins, 394-395

Gypsum in Hand Sample

Gypsum, var. selenite

Another selenite

Scanning electron micrograph of selenite

Gypsum, var. satinspar

Scanning electron micrograph of satinspar

Gypsum, var. alabaster

Scanning electron micrograph of gypsum from Zone 4 of the Calvert Formation

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 straight gypsum, as seen in this scanning electron micrograph.

3D Model

 

Gypsum in Thin Section

Gypsum, var. alabaster, plane polars

Gypsum, var. alabaster, crossed polars

Rock gypsum, plane polars

Rock gypsum, crossed polars

Gypsum, var. alabaster, plane polars
Gypsum, var. alabaster, plane polars

Gypsum, var. alabaster, crossed polars
Gypsum, var. alabaster, crossed polars

Gypsum, var. "rock", plane polars
Gypsum, var. “rock”, plane polars

Gypsum, var. "rock," crossed polars
Gypsum, var. “rock,” crossed polars

Further Reading

Gypsum at webmineral.com

Gypsum at mindat.org