Selenite is a specific crystalline variety of the common sulfate mineral gypsum, recognized for its exceptional clarity and well-formed structure. The mineral takes its name from the Greek word selēnē, meaning “moon,” referencing the soft, pearly light reflection often seen on its surfaces. Selenite is widely distributed and known for its unique transparency, which allows light to pass through its crystals. It is distinct from other forms of gypsum due to the size and perfection of its crystals, which can grow remarkably large under stable geological conditions.
Fundamental Identity and Chemical Composition
Selenite is scientifically defined as hydrated calcium sulfate, with the precise chemical formula CaSO4 · 2H2O. This formula indicates that each molecule of calcium sulfate is bonded with two molecules of water, which are integral to the mineral’s crystalline structure. This water content chemically differentiates it from anhydrite (CaSO4), which is a water-free calcium sulfate mineral.
The mineral is a member of the monoclinic crystal system, meaning its structure is defined by three axes of unequal length, with one axis oblique to the other two. While all gypsum is chemically the same, “selenite” is reserved for the transparent, colorless, and often large, single-crystal forms. This distinguishes it from other gypsum varieties like alabaster (a fine-grained, massive form) and satin spar (which is fibrous and silky).
Distinct Physical Characteristics
Selenite’s most defining feature is its extreme softness, registering only a 2 on the Mohs scale of hardness. This low ranking means the mineral can be easily scratched by a copper penny or a fingernail. This softness contributes to its perfect cleavage, a property that allows the crystal to be split easily into thin, flexible sheets along specific structural planes.
Selenite’s high transparency is a notable physical trait, often appearing colorless and clear like glass in its purest state. When light strikes the surface, the crystals display a vitreous, or glass-like, luster, though cleavage surfaces can show a pearly sheen. Selenite crystals exhibit a variety of shapes, or habits, including tabular, prismatic, or bladed forms. Distinctive twin crystals, such as V-shaped “swallowtail” or “fishtail” twins, are frequently observed.
Geological Formation and Global Occurrence
Selenite is classified as an evaporite mineral, meaning it forms through the precipitation and crystallization of dissolved ions during the evaporation of water. The process begins in bodies of water, such as ancient seas or closed saline lakes, that are rich in calcium and sulfate ions. As the water evaporates, the concentration of these dissolved minerals increases until the solution becomes supersaturated, causing the calcium sulfate to crystallize.
The formation of exceptionally large selenite crystals requires highly stable and sustained environmental conditions. If the temperature, salinity, and chemical balance of the solution remain consistent over long geological timeframes, crystals can grow to enormous sizes. A notable example is the Cave of the Crystals in the Naica Mine, Mexico, where subterranean chambers contain some of the largest selenite crystals ever discovered, measuring up to 12 meters in length. Selenite deposits are found worldwide, with significant occurrences in Morocco, the United States, and Spain.
Practical Applications and Common Uses
The broader mineral family of gypsum, which includes selenite, has been utilized for millennia in various industrial and practical applications. When gypsum is heated, it loses some of its water content, resulting in the fine powder known as Plaster of Paris. This material rehydrates and hardens when mixed with water, making it fundamental in modern construction for wallboard, plaster, and stucco.
Historically, the exceptional transparency of large selenite sheets led to its use as a material for windows before the widespread availability of glass. Today, its distinctive aesthetic qualities and ease of carving make it a popular material for decorative items such as lamps, carved ornaments, and sculptures. In agriculture, ground gypsum is used as a soil conditioner to improve soil structure and provide calcium and sulfur nutrients to crops.