Selenite is a crystalline variety of the mineral gypsum, known for its often colorless and transparent appearance. This striking mineral is a form of hydrous calcium sulfate, meaning water molecules are structurally incorporated into its composition. Selenite crystals can grow to monumental sizes, providing a clear record of the specific and stable chemical and environmental conditions required for its formation. The growth process is primarily driven by the concentration of dissolved elements, resulting in some of the largest crystals found on Earth.
Necessary Chemical Components
The formation of selenite (CaSO4 · 2H2O) fundamentally requires three components: calcium ions, sulfate ions, and water. Calcium (Ca2+) ions are typically sourced from the weathering and dissolution of calcium-rich rocks, such as limestone. Sulfate (SO4 2-) ions often originate from the oxidation of sulfide minerals or are already present in high concentrations in ancient seawater and saline water bodies.
These ions must be dissolved in water to be available for the chemical reaction that creates the crystal structure. The presence of water is not only a transport mechanism but also a structural component, as two water molecules are directly incorporated into every unit of calcium sulfate. Therefore, the crystallization process depends on a solution containing a sufficient concentration of both the calcium and sulfate components.
The Primary Mechanism Evaporation and Precipitation
Selenite formation is a classic example of an evaporite process, where the concentration of dissolved solids increases as water is removed. As water evaporates from a reservoir, the remaining fluid becomes progressively more saturated with the dissolved calcium and sulfate ions. This increase in mineral content eventually leads to supersaturation, where the concentration exceeds the minerals’ solubility limit.
Once the supersaturation threshold is crossed, the ions begin to bond together, forming tiny crystalline structures called nuclei. This initiates the process of precipitation. Selenite, being a variety of gypsum, precipitates when this mineral concentration is sufficient. The rate of water loss is a determining factor in the quality of the resulting crystal.
Slow, uninterrupted evaporation allows the ions to attach to the growing crystal faces in a highly ordered manner. This stable growth environment yields the large, clear, and perfectly formed crystals characteristic of selenite. Conversely, rapid evaporation causes fast, disordered growth, resulting in smaller, fibrous, or opaque gypsum varieties.
Environments Where Selenite Crystals Grow
The conditions necessary for selenite growth—a mineral-rich solution and sustained evaporation—are most commonly met in arid environments known as evaporite basins. These include closed lakes, desert playas, and ancient seabeds where water is trapped and slowly recedes. The Great Salt Plains in Oklahoma is famous for its unique hourglass-shaped selenite crystals, which form just below the surface in saline groundwater.
Selenite can also form through secondary processes in environments not strictly defined by surface evaporation, such as caves and hydrothermal systems. In caves, calcium-rich groundwater dissolves existing rocks and precipitates selenite crystals on the walls as the water slowly evaporates into the air, forming delicate gypsum flowers. The most spectacular examples, such as the giant crystals in the Naica Mine in Mexico, formed in a unique hydrothermal setting. There, mineral-rich hot water remained at a stable temperature of approximately 58 degrees Celsius for half a million years, allowing the crystals to grow to lengths exceeding ten meters.