hydrated calcium sulfate. Sometimes recommended as a soil conditioner or a source of calcium.
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk chalk. Gypsum also crystallizes as translucent crystals of selenite. It forms as an evaporite mineral and as a hydration product of anhydrite. The Mohs scale of mineral hardness defines gypsum as hardness value 2 based on scratch hardness comparison.
| Gypsum | |
|---|---|
 | |
| General | |
| Category | Sulfate minerals |
| Formula (repeating unit) | CaSO4·2H2O |
| IMA symbol | Gp |
| Strunz classification | 7.CD.40 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) H-M symbol: (2/m) |
| Space group | Monoclinic Space group: I2/a |
| Unit cell | a = 5.679(5), b = 15.202(14) c = 6.522(6) Å; β = 118.43°; Z = 4 |
| Identification | |
| Color | Colorless (in transmitted light) to white; often tinged other hues due to impurities; may be yellow, tan, blue, pink, dark brown, reddish brown or gray |
| Crystal habit | Massive, flat. Elongated and generally prismatic crystals |
| Twinning | Very common on {110} |
| Cleavage | Perfect on {010}, distinct on {100} |
| Fracture | Conchoidal on {100}, splintery parallel to [001] |
| Tenacity | Flexible, inelastic |
| Mohs scale hardness | 1.5–2 (defining mineral for 2) |
| Luster | Vitreous to silky, pearly, or waxy |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.31–2.33 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.519–1.521 nβ = 1.522–1.523 nγ = 1.529–1.530 |
| Birefringence | δ = 0.010 |
| Pleochroism | None |
| 2V angle | 58° |
| Fusibility | 5 |
| Solubility | Hot, dilute HCl |
| References | |
| Major varieties | |
| Satin spar | Pearly, fibrous masses |
| Selenite | Transparent and bladed crystals |
| Alabaster | Fine-grained, slightly colored |
Fine-grained white or lightly tinted forms of gypsum known as alabaster have been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire, and the Nottingham alabasters of Medieval England.