Selenite is often mistaken for a type of rock, likely because it is frequently encountered in large, solid masses. However, Selenite is not classified as a rock; it is a specific crystalline variety of the common mineral gypsum. This distinction is crucial in geology: minerals possess a defined chemical composition and crystal structure, while rocks are aggregates of one or more minerals. Selenite’s identity as a mineral explains its unique formation process and distinct physical characteristics.
Selenite’s True Mineral Identity
Selenite is the transparent, colorless, and well-formed crystal habit of the mineral gypsum. Gypsum is a soft sulfate mineral composed of hydrated calcium sulfate (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)). The presence of two water molecules within its structure is what distinguishes it from anhydrite (\(\text{CaSO}_4\)), the water-free form. Selenite crystallizes in the monoclinic crystal system. It is this crystalline structure that allows Selenite to form the spectacular, often enormous, transparent crystals for which it is known.
The name Selenite originates from the Greek word for “moon,” a reference to the mineral’s gentle, pearly light reflection. While all Selenite is gypsum, the term Selenite is reserved exclusively for the clear, glassy, and well-crystallized specimens. Its single, uniform chemical composition confirms its classification as a mineral, even when it forms large masses.
How Selenite Forms in Nature
Selenite is classified as an evaporite mineral, meaning its formation is linked to the evaporation of ancient, mineral-rich bodies of water. The process begins with water bodies, such as shallow seas, lagoons, or closed basins, that are highly concentrated with dissolved calcium and sulfate ions. In arid climates, intense evaporation of this water causes the concentration of these dissolved minerals to increase.
As the water continues to evaporate, the solution becomes supersaturated, forcing the dissolved calcium sulfate to precipitate. This precipitation leads to the crystallization of gypsum, including the transparent Selenite variety. Selenite crystals grow slowly and uninterrupted within the fine-grained sediments, often clay beds, or attached to a base rock. This geological environment explains why Selenite is frequently found in association with sedimentary rock layers, such as limestone or shale, which are characteristic of ancient lakebeds or marine deposits.
Unique Physical Characteristics
The physical properties of Selenite make it distinct and easily identifiable. Selenite is remarkably soft, registering a 2 on the Mohs scale of mineral hardness. This means it can be easily scratched, even with a fingernail, a consequence of the weak bonds between its hydrated calcium sulfate layers.
Another defining characteristic is its perfect cleavage in one direction, allowing it to be split readily into thin, flexible, and often transparent sheets. Selenite exhibits a vitreous, or glassy, luster, though cleavage surfaces may display a pearly sheen. The transparent nature of pure Selenite allows it to transmit light effectively; some fibrous forms even display fiber-optic qualities. Selenite crystals are also known to form unique shapes, such as the V-shaped “swallow tail” twins or “fish-tail” twins, which are characteristic of its crystal habit.
Varieties and Practical Uses
Selenite is one of several crystalline varieties of the mineral gypsum, each with a distinct appearance. While Selenite is defined by its transparency and large, prismatic crystal structure, the variety known as Satin Spar is characterized by its fibrous, parallel growth habit. This fibrous structure gives Satin Spar a silky, pearly luster and can cause it to exhibit a cat’s-eye effect, known as chatoyancy. A third variety, the Desert Rose, forms rosette-like clusters when Selenite crystals grow in sandy environments, trapping sand grains between the “petals” and resulting in a beige or brown color.
The mineral gypsum, including its Selenite form, has a wide range of practical applications. Industrially, gypsum is ground and heated to produce Plaster of Paris, a material used extensively in the production of cement, drywall, and plaster for construction. It is also used as a soil conditioner and fertilizer in agriculture, providing calcium and sulfur to crops. As Selenite, its uses tend toward the decorative and spiritual; its transparency and soft glow make it a popular material for carving into ornaments, lamps, and decorative bowls.