Gypsum is a soft sulfate mineral commonly found in thick sedimentary deposits across the globe. Chemically known as hydrated calcium sulfate (\(text{CaSO}_4 cdot 2text{H}_2text{O}\)), its formation provides a geological record of past environmental conditions. Gypsum is classified as an evaporite mineral, meaning it forms from the precipitation of ions dissolved in water that has undergone significant concentration through evaporation.
Setting the Stage: The Source Materials and Water Chemistry
The formation of gypsum begins with the presence of dissolved calcium ions (\(text{Ca}^{2+}\)) and sulfate ions (\(text{SO}_4^{2-}\)). These ions are readily available in natural waters, particularly seawater, dissolved from the weathering of rocks. The concentration of these ions must reach a specific threshold before the mineral can begin to form.
This threshold is defined by saturation, the point at which the water can hold no more of the dissolved mineral. Additional water loss or ion input pushes the solution toward supersaturation, where the concentration exceeds equilibrium solubility. Because gypsum is moderately water-soluble, it requires a reasonably high ionic concentration to begin precipitating.
The Mechanics of Evaporite Formation
The primary mechanism for gypsum formation is the sustained loss of water through evaporation, which concentrates the dissolved ions into a brine. This reduction in water volume drives the solution past the point of saturation and into a state of supersaturation with respect to calcium sulfate. When evaporation reduces the original volume of seawater to about 19%, the conditions become favorable for calcium sulfate to begin precipitating.
Once supersaturation is achieved, the dissolved calcium and sulfate ions bond together to form the crystal lattice of gypsum (\(text{CaSO}_4 cdot 2text{H}_2text{O}\)). This process begins with nucleation, the initial formation of minute, stable crystal embryos, which then grow by adding more ions from the surrounding brine. Gypsum is one of the first salts to precipitate in the evaporite sequence, often forming after calcium carbonate but well before highly soluble salts like halite (table salt).
Geological Environments of Gypsum Deposition
The largest and most extensive deposits of gypsum are found in sedimentary environments where high evaporation rates are sustained over long periods. Ancient marine basins that were partially isolated from the open ocean often created the conditions for thick evaporite beds to form millions of years ago. In these restricted settings, the evaporation rate exceeded the water inflow, allowing for the massive accumulation of precipitated calcium sulfate.
Gypsum also forms in modern coastal and continental environments that experience arid or semi-arid climates. Coastal salt flats, known as sabkhas, see gypsum precipitate directly within the sediment as highly saline groundwater evaporates near the surface. Inland, temporary or permanent saline lakes, called playas, accumulate calcium and sulfate ions from inflowing rivers, and the subsequent drying leads to the deposition of gypsum layers. The resulting crystal form can vary, with large, clear crystals, known as selenite, forming when the crystallization process is slow.
Alternative Formation Pathways
While large-scale evaporation is the dominant process, gypsum can also form through other chemical reactions not related to surface water loss. One such pathway involves the oxidation of sulfide minerals, like pyrite (\(text{FeS}_2\)), which is common near mineral deposits or in mining waste. As pyrite reacts with oxygen and water, it generates sulfuric acid, and if calcium-rich water is present, the acid reacts with it to precipitate gypsum.
Another non-evaporitic method is hydrothermal deposition, where hot, mineral-laden fluids circulate through fractures in the Earth’s crust. As these fluids cool or mix with cooler groundwater near the surface, the solubility of gypsum decreases, causing the mineral to precipitate in veins or fissures. Gypsum can also form through the hydration of anhydrite (\(text{CaSO}_4\)), a related mineral that lacks water molecules; when anhydrite comes into contact with groundwater, it absorbs water and transforms into the hydrated gypsum structure.