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Selenite is the crystalline variety of the mineral gypsum, characterized by its often transparent to translucent appearance and well-formed crystals. It is a hydrated calcium sulfate mineral, meaning it contains water molecules within its crystal structure. Selenite is known for its perfect cleavage, allowing it to be easily split into thin, flexible sheets. It is one of the softest minerals, with a Mohs hardness of 2, making it easily scratched by a fingernail. Its name is derived from the Greek word 'selene', meaning moon, referring to its moon-like sheen and transparency. Selenite can occur in various crystal habits, including tabular, prismatic, acicular, and often forms large, impressive crystals.
How to Identify
- Color
- Typically colorless and transparent, but can be white, gray, yellowish, or brownish due to impurities.
- Luster
- Vitreous (glassy) to pearly on cleavage surfaces, silky in fibrous varieties (satin spar).
- Texture
- Smooth and cool to the touch. Can feel slightly greasy in some massive forms.
- Crystal Form
- Commonly forms tabular, prismatic, or acicular (needle-like) crystals. Can also occur as 'desert roses' (rosette-like aggregates), 'gypsum flowers' (delicate, branching forms), or massive, granular, and fibrous aggregates.
- Cleavage
- Perfect cleavage in one direction (010), yielding thin, flexible sheets; good cleavage in two other directions (100 and 011), forming rhombic fragments.
- Geological Environment
- Evaporite deposits (ancient lakebeds, playas, sabkhas), sedimentary rocks (shales, limestones), caves (speleothems), hydrothermal veins, and as an alteration product of anhydrite.
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, gray, yellowish, brownish (due to impurities)
- Luster: Vitreous to pearly (on cleavage surfaces), silky (satin spar)
- Transparency: Transparent to translucent
- Fracture: Uneven to conchoidal
- Cleavage: Perfect in one direction (010), good in two others (100 and 011)
- Composition: Calcium sulfate dihydrate (CaSO4·2H2O)
Quick Check
- Color: Colorless to white, sometimes yellowish or brownish
- Luster: Vitreous to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Tabular, prismatic, acicular, massive, fibrous, granular, rosette-like (desert rose), branching (gypsum flower)
- Cleavage Type: Perfect basal cleavage (010), good prismatic cleavage (100 and 011)
- Fracture Type: Uneven to conchoidal
- Tenacity: Sectile (can be cut with a knife), flexible in thin sheets
- Luster Type: Vitreous (glassy) to pearly, silky (satin spar variety)
Formation
Selenite is a variety of gypsum (CaSO4·2H2O) that forms primarily through the evaporation of saline water bodies, such as ancient seas, lakes, and playas. It can also form as a secondary mineral in caves, hydrothermal veins, and as an alteration product of anhydrite (CaSO4). The presence of water is crucial for its formation, as it is a hydrated mineral. It precipitates from supersaturated solutions, often in arid or semi-arid environments where evaporation rates are high.
Usage
While selenite itself is not typically used industrially due to its softness and solubility, its parent mineral gypsum is extensively used. Gypsum is a primary component in plaster of Paris, drywall (sheetrock), cement, and as a soil conditioner in agriculture. Selenite, particularly in its clear, well-formed crystal habits, is highly valued by mineral collectors and is used in decorative items, carvings, and as a source of calcium and sulfur in some niche applications. Its optical properties (transparency) have also led to its use in some historical optical instruments.
Age Distribution
Found in geological formations ranging from the Precambrian to the Cenozoic, often associated with evaporite deposits.
Where to Find
Naica Mine, Chihuahua, Mexico
Famous for its 'Cave of Crystals' (Cueva de los Cristales), containing some of the largest selenite crystals ever discovered, reaching up to 12 meters in length.
Great Salt Plains State Park, Oklahoma, USA
Known for its unique 'hourglass' selenite crystals, where sand inclusions form an hourglass shape within the crystal.
Utah, USA
Various locations, particularly in desert regions, yield selenite crystals, including 'desert roses'.
Australia
Significant deposits of gypsum, including selenite, are found in various arid regions.
Spain
Historical and contemporary sources of gypsum and selenite, including large crystals.
Finding Tips
Look in Evaporite Environments
Selenite is commonly found in areas where ancient seas or lakes have evaporated, leaving behind sulfate-rich sediments. Look for dry lakebeds, playas, and sedimentary rock outcrops in arid regions.
Check for Associated Minerals
Selenite often occurs with other evaporite minerals like halite (rock salt), anhydrite, and various clays. The presence of these minerals can indicate a favorable environment for selenite.
Observe Crystal Habits
Selenite can form distinct crystal habits. Look for transparent, tabular crystals, fibrous 'satin spar' veins, or 'desert roses' in sandy soils. Its softness (scratchable with a fingernail) is a key identifier.
Handle with Care
Due to its softness and perfect cleavage, selenite crystals can be fragile. Handle specimens gently to avoid scratching or breaking them. Avoid prolonged exposure to water, as it is slightly soluble.
Similar Rocks
Anhydrite
Calcium Sulfate
Also known as: Anhydrous Gypsum
Calcite
Calcium Carbonate
Also known as: Calcium Carbonate
Halite
Sodium Chloride
Also known as: Rock Salt
Scientific Classification
- Mineral Class
- Sulfates
- Group
- Gypsum Group
- Crystal System
- Monoclinic
- Chemical Formula
- CaSO4·2H2O
- Composition
- Calcium sulfate dihydrate (CaO 32.57%, SO3 46.50%, H2O 20.93%)
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