Selenite is a captivating natural formation known for its unique appearance. Its name, derived from the Greek word for “moon,” alludes to its often moon-like glow. This transparent to translucent material appears delicate yet possesses distinct qualities.
Selenite: Mineral or Rock?
Selenite is a transparent variety of gypsum. Minerals are naturally occurring, inorganic solids characterized by a definite chemical composition and an ordered atomic structure. This internal arrangement creates a crystalline structure unique to each mineral. Selenite’s chemical formula is CaSO₄·2H₂O, indicating it is composed of calcium, sulfur, oxygen, and two molecules of water.
Rocks, in contrast, are aggregates of one or more minerals or mineraloids, which lack a crystalline structure. While a rock is classified by its formation process, a mineral is defined by its consistent chemical makeup and internal atomic arrangement. Therefore, despite its often large, rock-like appearance, selenite is scientifically categorized as a mineral.
Distinctive Characteristics
Selenite crystals are typically transparent to translucent, often appearing colorless or white. Impurities can impart pale tints. These crystals exhibit a vitreous, or glass-like, luster, with some surfaces displaying a pearly sheen. The fibrous satin spar variety of gypsum is known for its silky luster.
A defining feature of selenite is its softness, registering at a 2 on the Mohs scale of hardness. It can be easily scratched with a fingernail. Selenite also possesses perfect cleavage in one direction, allowing it to split readily into thin sheets or blades. Common crystal habits include tabular or prismatic forms, and it can create distinctive “fishtail” or “swallowtail” twin crystals.
Formation and Occurrence
Selenite forms through the evaporation of saline water bodies, such as ancient seas or salt lakes, in arid environments. As water evaporates, dissolved calcium and sulfate ions precipitate, leading to gypsum crystallization. This process results in selenite deposits within evaporite rock formations. Selenite can also originate as a secondary mineral, forming from the alteration of other calcium-bearing minerals, or within hydrothermal veins.
Impressive selenite crystals are found in the Naica Mine in Chihuahua, Mexico. This location hosts massive crystals, with some specimens reaching lengths of up to 12 meters and weighing 12 tons. These giant crystals developed within a hot, humid cave system, where stable conditions and mineral-rich water, heated by an underlying magma chamber, allowed for their growth over hundreds of thousands of years.