Gypsum is a chemical sedimentary rock, formed through the precipitation of minerals from an aqueous solution. It is composed of hydrated calcium sulfate, which includes water molecules within its crystal structure. This origin places gypsum into the subcategory of sedimentary rocks known as evaporites.
Gypsum’s Classification as an Evaporite
Chemical sedimentary rocks are distinct from clastic sedimentary rocks, which are formed from the fragments of pre-existing rocks. Chemical rocks like gypsum result from the inorganic precipitation of dissolved ions carried in water. When this precipitation occurs due to the evaporation of a water body, the resulting deposit is termed an evaporite. Gypsum is one of the most common minerals formed through this process, alongside halite, or rock salt.
Evaporites are defined by a formation environment where the concentration of dissolved solids becomes so high that they crystallize out of the solution. This process occurs in arid regions where the rate of water evaporation significantly exceeds the rate of water inflow. The resulting rock layers are found in thick beds, highlighting that gypsum is a product of changing water chemistry rather than the accumulation of weathered particles.
The Mechanism of Evaporite Formation
The precipitation of gypsum requires an arid climate and a restricted body of water, such as a lagoon or an inland sea basin. As the water evaporates, the remaining brine becomes progressively more concentrated with dissolved ions, including calcium and sulfate. Minerals precipitate sequentially as the salinity increases, according to their solubility.
Carbonates, such as calcite, precipitate first because they have the lowest solubility. Gypsum crystallizes next, once the brine becomes saturated with calcium sulfate. This occurs before the precipitation of halite, which requires much higher concentration levels. The crystallization process incorporates water molecules from the brine into the mineral’s structure.
Identifying Physical Characteristics
Gypsum is easily identified due to its low hardness. On the Mohs scale, gypsum ranks at a value of 2, meaning it can be scratched with a fingernail. This softness helps distinguish it from other minerals found in similar environments.
While pure gypsum is colorless and transparent, it commonly appears white or gray in its massive rock form due to impurities, and may also have tints of yellow, pink, or brown. Gypsum has several recognizable crystalline varieties, including selenite (clear crystals), satin spar (fibrous with a silky luster), and alabaster (a massive, fine-grained form often used for sculpting).