Selenite is a mineral recognized for its striking, often transparent appearance and distinct crystalline structure. Its name comes from the Greek word for the moon, referencing its soft, pearly luster. Selenite forms some of the largest known crystals on Earth, captivating researchers and collectors. Understanding its classification and chemical building blocks explains these unique characteristics.
Selenite’s Place in the Gypsum Family
Selenite is not a unique mineral species but rather a specific crystalline variety of the common mineral gypsum. All forms of gypsum share the same underlying chemistry, but they differ significantly in their crystal habit, or the external shape and appearance of the crystals. Selenite is defined by its characteristic transparency and well-formed, often tabular or bladed crystals.
This variety is distinct from other members of the gypsum family that share the same chemical formula. For instance, Satin Spar is a fibrous variety, characterized by a silky, pearly sheen caused by tightly packed, parallel crystal fibers. Alabaster, another form of gypsum, is a massive, fine-grained material often used for carving because of its dense structure.
The Desert Rose variety also falls under the gypsum umbrella, forming rosette-like clusters when crystals grow in semi-arid, sandy environments. While these forms are sometimes collectively referred to as “selenite,” the term technically denotes the clear, single-crystal form of the mineral. Selenite’s defining features are based on its specific crystallization pattern, not a unique chemical composition.
The Specific Chemical Makeup
The fundamental chemical composition of selenite is calcium sulfate dihydrate, represented by the formula CaSO4 · 2H2O. This formula breaks down into two main components: calcium sulfate (CaSO4) and two molecules of water (2H2O). The presence of the water molecules defines the mineral’s characteristics.
The water molecules are integrated directly into the crystal lattice, meaning they are chemically bonded rather than simply trapped within the structure. This embedded water is responsible for selenite’s low ranking on the Mohs scale of hardness, registering only a 2. This softness explains why the mineral can be easily scratched, even with a fingernail.
The layered arrangement of the chemical components and the hydrating water molecules also dictate the mineral’s perfect cleavage. The weak bonds between the layers of the calcium sulfate and the water molecules allow the crystals to split readily into thin, parallel sheets. This structural detail is why selenite specimens often appear as clear, flat, glass-like blades.
Geological Origin and Formation
Selenite is classified as an evaporite mineral, meaning it forms through the precipitation of salts left behind by the evaporation of water. This process typically occurs in sedimentary environments such as ancient seas, saline lakes, or closed inland basins. As water bodies recede, the concentration of dissolved minerals increases until the solution becomes oversaturated.
Once oversaturation is reached, the calcium sulfate begins to precipitate out of the solution and crystallize. For selenite to form with its characteristic transparency and large crystal size, the environment must remain exceptionally stable for extended periods. Fluctuations in temperature, pressure, or chemical concentration can interrupt the slow, continuous growth necessary for massive crystals.
The world’s largest selenite crystals, for example, grew in deep caves where constant, warm, mineral-rich groundwater provided the ideal, undisturbed conditions for millennia. These geological settings allow the crystal lattice to organize slowly and perfectly, resulting in the clear, flawless structure that distinguishes selenite from other, more opaque forms of gypsum. The process is a slow record of a former aqueous environment.