monoclinic Sulfates
Gypsum (Swallowtail Twin)
Formula CaSO4.2H2O
Crystal Structure
monoclinic[2/m]:{010}@1.0 + {100}@0.95 + {001}@0.7 + {110}@0.4 | twin(gypsum_swallow) monoclinic
2/m
{010} {100} {001} {110}
Quick Facts
Hardness
2.0
Specific Gravity
2.31
Refractive Index
1.52
Optical Character
Biaxial +
Physical Properties
- Crystal System
- monoclinic
- Hardness (Mohs)
- 2.0
- Specific Gravity
- 2.31
- Cleavage
- Perfect {010}, good {100} and {011}
- Fracture
- Conchoidal to splintery
- Lustre
- Vitreous to pearly (on cleavage surfaces)
Optical Properties
- Refractive Index
- 1.52
- Birefringence
- 0.009-0.010
- Optical Character
- Biaxial +
- Dispersion
- 0.033
- Pleochroism
- None
Characteristics
Colours
Colorless (selenite)WhiteGrayYellowBrownPinkOrange
Localities
Mexico (Naica - giant crystals)MoroccoUSA (Oklahoma, Utah)SpainEnglandAustralia
Crystal Forms
pinacoidprismtwin
Related Minerals
Other minerals in the monoclinic system
