Plaster:
CaSO4˙2H2O
Crystal Chemistry:
Theoretical composition (wB%): CaO 32.5, SO3 46.6, H2O+ 20.9. The ingredients didn't change much. There are often mechanical admixtures such as clay and organic matter. Sometimes it contains impurities such as SiO2, Al2O3, Fe2O3, MgO, Na2O, CO2, and Cl.
Structure and shape:
Monoclinic, a0=0.568nm, b0=1.518nm, c0=0.629nm, β=118.23'; Z=4. The crystal structure is composed of [SO4]2-tetrahedron and Ca2+ connected to form a ∥(010) double layer, and the two layers are connected by H2O molecules. Its complete cleavage occurs in this direction. Ca has a coordination number of 8 and is linked to 6 O and 2 H2O molecules in the adjacent 4 [SO4] tetrahedra. H2O molecules are connected with O in [SO4] by hydrogen bonds, and water molecules are connected by molecular bonds.
Orthorhombic columnar crystals, C2h-2/m (L2PC). The crystals often develop into plates, and some are granular. Often simple shape: parallel double-sided b, p, orthorhombic column m, l, etc.; crystal plane and often longitudinal lines; sometimes lentil-shaped. Twins are common, one is Gary twin or dovetail twin with (100) as twin plane, and the other is Paris twin or arrowhead twin with (101) as twin face. The aggregates are mostly dense granular or fibrous. The fine-grained block is called alabaster; the fibrous aggregate is called fibrous gypsum. Rare rosette-like aggregates formed by lentil-like crystals. There are also soil-like and sheet-like aggregates.
