Anhydrite (orthorhombic) Ca[SO₄]

	Colour: colourless Pleochroism: non pleochroic Habit: tabular, prismatic or square shaped cross section, grained aggregates Relief: positive Cleavage, fracture: perfect (100) and (010), good (001)
	Isotropic or anisotropic: anisotropic Extinction: straight Interference colours: 2nd/3rd order

Gypsum (monoclinic) Ca[SO₄] x 2H₂O

	Colour: colourless Pleochroism: non pleochroic Habit: massive, tabular crystals Relief: low negative Cleavage, fracture: perfect (010) Twinning: common (110) called swallow-tail
	Isotropic or anisotropic: anisotropic Extinction: straight Interference colours: 1st order, grey, yellow