|Cleavage||Perfect basal (010), good (100)
|Transparency||Transparent to translucent|
|XPL||Up to 1st order yellows|
|after Perkins, 394-395|
Gypsum in Hand Sample
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.