Gypsum is a soft sulfate mineral with the chemical formula CaSO₄·2H₂O, consisting of hydrated calcium sulfate, and exhibiting a Mohs hardness of 2, making it one of the softer common minerals.[1][2] It typically appears white or colorless in massive form but can form transparent crystals known as selenite, fibrous satin spar, or fine-grained alabaster varieties.[2] Gypsum primarily forms through the evaporation of seawater or saline lake waters in sedimentary environments, resulting in extensive evaporite deposits worldwide, often interbedded with limestone, shale, or halite.[3] Economically, it is a vital resource mined globally for manufacturing plaster of Paris—produced by calcining gypsum to remove water—drywall panels, Portland cement retarders, and agricultural soil amendments that supply calcium and sulfur while improving soil structure in sodic or compacted soils.[4][5] The United States produces substantial quantities, supporting industries that output billions of square feet of gypsum-based building materials annually.[6]

Gypsum possesses the molecular formula CaSO₄·2H₂O, comprising one calcium cation, one sulfate anion, and two water molecules of crystallization, which constitute approximately 20.9% of its mass.[7][5] The dihydrate structure stabilizes the mineral under standard environmental conditions, as the water molecules form hydrogen bonds that integrate into the lattice, preventing spontaneous dehydration at ambient temperatures below 50–60 °C.[8][9]