Selenite

Selenite is a calcium sulfate mineral that is the crystallized transparent variety of Gypsum. Satin Spar is the silky and fibrous variety of Gypsum that is abundant and commonly mislabeled as Selenite. Both of these can be found crystallized in the form of masses, acicular, granular, tabular, and prismatic shards. It was first discovered in 1747 by J. G.

Wallerius and named after the Greek word meaning “moon” in reference to its blue and pale colors. The formation of selenite begins with the presence of calcium and sulfate ions in the surrounding environment. These ions are typically derived from the weathering of rocks containing calcium and sulfur compounds. It commonly forms in areas where there is a high concentration of dissolved minerals in water bodies. In arid or semi-arid regions, such as desert environments or ancient seabeds, water often becomes trapped in shallow basins or depressions.

As the water evaporates under the influence of heat and dry climate, the concentration of dissolved minerals increases. As evaporation continues, the concentration of calcium and sulfate ions in the remaining water becomes so high that it exceeds their solubility limit. This results in supersaturation, where the water is unable to hold the dissolved minerals in solution. With the onset of supersaturation, the excess calcium and sulfate ions start to come together and form tiny crystals. These crystals act as nucleation sites for further crystal growth.

As the supersaturated water continues to evaporate, the selenite crystals grow by the addition of more calcium and sulfate ions onto their surfaces. The growth process is often slow, allowing the crystals to develop intricate and elongated fibrous or columnar structures. Over time, the selenite crystals settle and accumulate on the bottom of the water body or on existing sediments. The combination of crystal growth and sedimentation leads to the formation of thick layers or beds of selenite. With the burial of the sediments over geologic time, the selenite crystals undergo diagenesis, which involves processes like compaction and cementation.

This solidifies the sedimentary layers and transforms the loose crystals into a cohesive rock known as gypsum. In some cases, tectonic forces and geological processes uplift the sedimentary rocks containing selenite to the Earth’s surface. Erosion and weathering then expose the selenite-bearing rocks, making them accessible for collection or mining. It’s important to note that selenite can also form as a secondary mineral through the alteration of other calcium-bearing minerals in the presence of sulfate-rich fluids. However, the most common and abundant occurrence of selenite is in evaporite deposits formed through the process described.

Selenite is soluble in water, meaning it can dissolve when exposed to moisture or immersed in water. This solubility is due to the presence of calcium and sulfate ions, which can easily dissociate in water. Satin Spar: Satin Spar is a fibrous variety of selenite characterized by its silky and satin-like appearance. It often exhibits a fibrous texture with parallel or radiating fibers that give it a silky sheen. Ram’s Horn: A rare variety of Satin Spar