Selenite, a variety of gypsum, is a hydrous calcium sulfate, meaning it contains water molecules within its structure. It is often sought for its luminous appearance and various forms. This article will guide you through the primary characteristics and simple tests to help you identify this unique mineral.
Key Physical Characteristics
Selenite commonly appears clear to milky white, often transparent or translucent. While typically colorless, impurities can sometimes tint it with pale shades of yellow, orange, brown, green, or blue. The mineral exhibits a vitreous, or glassy, luster; cleavage surfaces can display a pearly sheen, and the fibrous variety known as satin spar often has a silky appearance.
Selenite’s crystal habits are diverse, frequently forming as tabular, prismatic, or columnar crystals. Distinctive formations include “fishtail” or “swallowtail” twins, where two crystals intergrow in a specific orientation. Fibrous varieties, such as satin spar, typically show long, parallel striations. When touched, selenite usually feels smooth, sometimes possessing a slightly waxy or greasy texture.
Simple Home Tests
Selenite’s softness, measured on the Mohs scale, is a key identification method. With a Mohs hardness of 2, it is soft enough to be easily scratched by a fingernail (hardness 2-2.5). To perform this scratch test, gently attempt to scratch an inconspicuous area with your fingernail; a selenite specimen will yield a visible mark.
Selenite exhibits perfect cleavage in one direction, a property that allows it to split readily into thin, flexible sheets or splinters. This characteristic can be observed if a piece is chipped or broken, revealing delicate, parallel layers. In some fibrous varieties, particularly satin spar, a unique optical phenomenon called chatoyancy, or the “cat’s eye” effect, can be observed. This effect creates a linear band of light that shifts as the mineral is moved.
Distinguishing Selenite from Similar Minerals
Distinguishing selenite from other minerals often comes down to differences in hardness and crystal structure. Quartz can sometimes be mistaken for clear selenite due to its transparency. However, quartz is significantly harder, with a Mohs hardness of 7, meaning it cannot be scratched by a fingernail. Quartz also forms in a hexagonal crystal structure, which differs from selenite’s monoclinic system.
Calcite, another mineral that can resemble selenite, is slightly harder than selenite, registering a Mohs hardness of 3. While a fingernail might leave a faint mark on calcite, it will not scratch it as easily as selenite. Calcite also exhibits rhombohedral cleavage, resulting in blocky, rhombus-shaped fragments when broken, contrasting with selenite’s thin, flexible sheets. Selenite is a transparent variety of the broader mineral gypsum, which also includes opaque or massive forms like alabaster.