Anhydrite

CaSO₄

Non-silicate

Crystal system: orthorhombic (+)

Colour in hand specimen: white or colourless when pure 
Cleavage: two at 90° 
Hardness: 3-3.5

Colour in plane polarised light: colourless 
Pleochroism: none 
Relief: low 
Birefringence: ~0.04; 3^rd order blue 
Extinction: straight 
Twinning: simple or repeated