Gypsum

Gypsum is an evaporitic mineral par excellence, formed from the precipitation of marine salt solutions in arid and lagoonal environments. It crystallizes in the monoclinic system with tabular or fibrous habit (selenite variety for transparent and colorless crystals, alabaster for massive and compact form). Its hardness of 2 on the Mohs scale makes it extremely friable, a characteristic that made it prized since antiquity: the Egyptians used it as a binder in monumental constructions, Renaissance artisans for stuccos and decorations, and today it remains fundamental in modern construction as a component of drywall. Perfect cleavage along the (10) plane produces extremely thin, nearly transparent lamellae. It is often associated with halite, sylvite, and other evaporitic minerals in stratified deposits.

Crystallographic properties: Monoclinic system, space group C2/c, lattice parameters a = 5.68 Å, b = 15.20 Å, c = 6.29 Å, β = 113.8°. Crystal habit variable from tabular (selenite) to fibrous-radiating (alabaster). Perfect cleavage along (10), good along ((100)) and ((101)). Physical properties: Hardness 2 Mohs, density 2.32 g/cm³, vitreous-silky luster, transparent to translucent, colorless or weakly colored (yellow, pink, gray from traces of Fe, Mn, C). Optical properties: Birefringent, refractive indices nα = 1.520, nβ = 1.523, nγ = 1.530; biaxial positive, 2V angle ≈ 58°. No pleochroism. Spectroscopy: Characteristic IR absorption bands at 3406, 3305 cm⁻¹ (O-H stretching), 1685 cm⁻¹ (SO₄²⁻). Raman: peaks at 1008, 1136 cm⁻¹ (SO₄ stretching). Thermal analysis: Progressive water loss with complete dehydration at 190 °C (formation of hemihydrate CaSO₄·½H₂O) and at 250 °C (anhydrite CaSO₄). Mineralogical associations: Halite, sylvite, carnallite, anhydrite, dolomite, calcite in evaporitic successions; sometimes with sulfides (pyrite, galena) in oxidation environments.