Selenite is a variety of the mineral gypsum, a hydrous calcium sulfate (CaSO₄·2H₂O). It often appears as transparent to translucent crystals, ranging from colorless to white, though impurities can impart other hues like yellow, brown, or blue. This mineral is notably soft, registering a 2 on the Mohs scale of hardness, meaning it can be easily scratched. Its name, derived from the Greek word for “moon,” refers to its often pearly luster, which can resemble moonlight. Selenite is valued for its aesthetic qualities and is often used in decorative items.
Major Selenite Deposits Across the United States
The United States boasts numerous significant selenite deposits, each offering unique crystal formations. Oklahoma’s Great Salt Plains National Wildlife Refuge is home to unique “hourglass selenite” crystals. These crystals form within the Permian Flowerpot Formation, a gypsum unit.
New Mexico features extensive selenite occurrences, particularly within the Tularosa Basin. White Sands National Park is home to the world’s largest gypsum dune field, where selenite crystals break down into sand, forming the vast white dunes. Nearby, Lake Lucero contains beds of impure, brownish selenite crystals, some reportedly up to three feet long, which precipitate as water evaporates. Additionally, large transparent selenite crystals, sometimes a foot in length, have been discovered at the Grand Quivira ruins in Torrance County.
Arizona is recognized for its “Desert Rose” selenite formations, particularly in arid regions like the St. David area and Sierrita Mountains. These rose-like clusters of bladed crystals include embedded sand grains. Another notable site is Glitter Mountain, near the Utah border, which has commercial selenite mining operations accessible to the public for a few months each year.
Other states also contribute to the nation’s selenite landscape. Utah’s Great Salt Lake region yields translucent to clear selenite crystals, often found on its shores. California’s Mojave Desert, near Barstow, contains selenite crystals in ancient lake sediments, and Arkansas’s Ouachita Mountains also have deposits within Late Mississippian and Pennsylvanian strata.
The Geology Behind Selenite Formation
Selenite, a variety of gypsum, forms primarily through a geological process known as sedimentary precipitation in evaporite deposits. This process begins with water bodies, such as ancient seas or saline lakes, containing dissolved calcium and sulfate ions. In arid or semi-arid climates, these waters evaporate, leading to an increasing concentration of these dissolved minerals.
As evaporation continues, the water becomes supersaturated, meaning it can no longer hold the dissolved minerals in solution. At this point, calcium and sulfate ions begin to combine and form tiny selenite crystals, acting as nucleation sites. The crystals then grow as more ions precipitate out of the evaporating water. This often occurs slowly, allowing for the development of distinct, elongated crystal structures.
Over time, these selenite crystals settle and accumulate, forming thick layers or beds. The specific conditions during crystallization, such as the presence of impurities or sand particles, influence the final crystal habit and color. For example, trapped sand can create unique patterns, while different mineral inclusions can impart various hues. This geological history explains why selenite deposits are often found in regions that were once ancient seabeds or desert environments.
Iconic Selenite Structures and Sites
The United States is home to remarkable selenite formations, showcasing the mineral’s diverse crystal habits. Among the most unique are the “hourglass selenite” crystals found exclusively at the Great Salt Plains National Wildlife Refuge in Jet, Oklahoma. These transparent gypsum crystals encapsulate sand and clay particles in a distinct hourglass pattern, a result of trapped iron oxide during their rapid growth in saline conditions. The hourglass shape forms because loose particles are incorporated only at the crystal’s ends as it grows, pushing aside particles along its sides. This rare formation has even been designated as Oklahoma’s official state crystal.
Another formation is the “Desert Rose.” These intricate clusters of bladed selenite crystals form a rosette pattern, resembling a blooming rose, often with embedded sand grains. The “petals” are flattened crystals that fan out in radiating clusters. They are found in arid regions.