Selenite

Description

Selenite is a transparent, colorless to white, crystalline variety of the mineral gypsum. It is characterized by its excellent cleavage, allowing it to be split into thin, flexible sheets. The name 'selenite' comes from the Greek word 'selene,' meaning 'moon,' referring to its soft, moon-like glow or translucence. It typically forms in well-developed, often large, monoclinic crystals, which can be prismatic, tabular, or acicular. Selenite is known for its low hardness and solubility in water, making it a delicate mineral.

How to Identify

Color

Typically colorless to white, sometimes with tints of yellow, brown, or gray due to impurities.

Luster

Vitreous (glassy) to pearly on cleavage surfaces.

Texture

Smooth, often cool to the touch. Can be easily scratched with a fingernail.

Crystal Form

Often found as well-formed, transparent, monoclinic crystals. Common habits include tabular, prismatic, bladed, or acicular. Can also form 'fishtail' twins or 'desert roses'.

Cleavage

Perfect cleavage in one direction (010), yielding thin, flexible sheets; good cleavage in two other directions (100 and 111), forming rhombic fragments.

Geological Environment

Evaporite deposits in sedimentary basins, salt flats, playas, caves, and as a secondary mineral in hydrothermal veins or altered volcanic rocks.

Key Facts

• Hardness: 2 on the Mohs scale (can be scratched by a fingernail)
• Specific Gravity: 2.31 - 2.32 g/cm³
• Crystal System: Monoclinic
• Color: Colorless, white, sometimes tinted yellow, brown, or gray
• Luster: Vitreous to pearly
• Transparency: Transparent to translucent
• Fracture: Uneven to conchoidal
• Cleavage: Perfect on {010}, good on {100} and {111}
• Composition: Hydrous calcium sulfate (CaSO4·2H2O)

Quick Check

• Color: Colorless to white
• Luster: Vitreous to pearly
• Streak: White

Physical Characteristics

• Crystal Habit: Tabular, prismatic, bladed, acicular, massive, granular, fibrous (satin spar), rosette (desert rose), often twinned (fishtail twins).
• Cleavage Type: Perfect basal cleavage (010) yielding flexible sheets; good prismatic cleavage (100) and (111) forming rhombic fragments.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Sectile (can be cut with a knife), flexible in thin sheets, but not elastic.
• Luster Type: Vitreous on crystal faces, pearly on cleavage surfaces.

Formation

Selenite forms primarily through the evaporation of saline waters (e.g., ancient seas, salt lakes, playas). As water evaporates, the concentration of dissolved calcium sulfate increases, leading to the precipitation of gypsum. Selenite specifically refers to the well-crystallized, often transparent, monoclinic form of gypsum. It can also form from the alteration of other calcium-bearing minerals in the presence of sulfate-rich solutions, or as a secondary mineral in caves and mines.

Usage

While gypsum (of which selenite is a variety) has extensive industrial uses (e.g., plaster of Paris, drywall, fertilizer), selenite itself is primarily valued for its aesthetic qualities. It is used in ornamental carvings, decorative pieces, and as a collector's mineral. Its optical properties (transparency, cleavage) have also been explored for niche applications.

Age Distribution

Found in sedimentary deposits of various ages, from Precambrian to Cenozoic, wherever evaporitic conditions occurred.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Sulfates

Group

Gypsum Group

Crystal System

Monoclinic

Chemical Formula

CaSO4·2H2O

Composition

Calcium sulfate dihydrate

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