Selenite is a translucent, crystalline variety of the common mineral gypsum, which is a hydrated calcium sulfate. This mineral is prized for its glass-like clarity, often appearing colorless or white, which is why its name derives from the Greek word for the moon. Selenite is remarkably soft, registering only a 2 on the Mohs scale of mineral hardness; a fingernail can easily scratch its surface. It exhibits perfect cleavage, allowing it to be split into thin, flexible sheets, and it commonly forms in prismatic, tabular, or fibrous crystal habits.
The Geological Process of Selenite Formation
Selenite primarily forms through sedimentary precipitation in evaporite environments. This process starts with a body of water, such as an ancient marine basin or saline lake, highly concentrated with dissolved calcium and sulfate ions. As the water slowly evaporates, the mineral content becomes oversaturated, forcing the calcium sulfate to precipitate out of the solution.
Crystallization occurs as the precipitate settles, often forming extensive layers of gypsum rock. The formation of clear, distinct selenite crystals, rather than massive gypsum, requires specific, stable conditions. These conditions include a slow, steady rate of evaporation and a lack of disruptive physical or chemical changes over extended periods.
The growth of selenite is often tied to the hydration of anhydrite, a waterless calcium sulfate mineral found in deep sedimentary layers. When groundwater contacts anhydrite, it rehydrates the compound, causing it to expand and crystallize into selenite. This action can occur near the surface in arid regions or deep underground. The resulting crystal habit varies, ranging from the clear, flat plates of true selenite to the fibrous, silky appearance of the variety known as Satin Spar.
Major Global Selenite Deposits
The most spectacular known occurrence of selenite is the Cave of the Crystals, located deep within the Naica Mine in Chihuahua, Mexico. This subterranean chamber, found 300 meters below the surface, holds the largest natural crystals ever discovered, with some selenite beams reaching lengths of up to 12 meters. These giant crystals grew under unique hydrothermal conditions, sustained by mineral-rich water heated by a magma chamber.
The crystals grew incredibly slowly over hundreds of thousands of years in a highly saturated solution. Although the Naica Mine is a scientific marvel, the extreme heat and humidity have made the cave inaccessible to the public. Some areas have been intentionally flooded to preserve the ongoing geological process.
Selenite is widely distributed across the globe in regions with historical evaporite basins. Morocco is a major commercial source, particularly for the fibrous Satin Spar variety extracted from large-scale desert mining operations. In the United States, significant deposits are found across the midwestern and western states.
Notable locations for selenite deposits include:
- Oklahoma, known for selenite crystals naturally embedded in clay.
- Utah, recognized for producing high-quality, transparent selenite plates.
- The Geode of Pulpí in Spain, which contains a giant geode lined with gypsum crystals.
- Various sites across Canada, Poland, and Australia.
Commercial Availability and Acquisition
Most people acquire selenite through commercial retailers like crystal shops and online vendors. The vast majority of the “selenite” sold, especially as polished wands, towers, or spheres, is actually the fibrous variety known as Satin Spar. True selenite is the rarer, transparent, plate-like crystal, which is less commonly shaped into commercial products due to its fragility.
Satin Spar is highly valued for its silky, cat’s-eye effect and its ease of carving, making it a popular material for decorative items and tools. The material is primarily sourced from large-scale mining operations, with a significant portion coming from Morocco. This concentration of commercial sourcing has raised concerns regarding ethical labor practices and environmental impact at extraction sites.
Consumers can find the mineral in several common forms. These include rough, uncut pieces, polished wands, charging plates, and carved bowls. Because selenite is soft and slightly water-soluble, it requires specific care, such as cleaning with a soft, dry cloth rather than water.