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Selenite is the transparent, well-crystallized variety of the mineral gypsum. It is a hydrous calcium sulfate mineral, meaning it contains water molecules within its crystal structure. Selenite crystals are typically colorless and transparent, often exhibiting a pearly or vitreous luster. They are characterized by their perfect cleavage in one direction, allowing them to be easily split into thin, flexible sheets. The name 'selenite' comes from the Greek word 'selene', meaning moon, referring to the moon-like sheen or glow of some specimens. It is one of the softest minerals, with a Mohs hardness of 2, and can be scratched with a fingernail. Selenite can occur in various crystal habits, including tabular, prismatic, acicular (needle-like), and often forms large, impressive crystals.
How to Identify
- Color
- Typically colorless and transparent, though impurities can impart shades of white, gray, yellow, brown, or pink.
- Luster
- Vitreous (glassy) to pearly, especially on cleavage surfaces. Fibrous varieties (satin spar) exhibit a silky luster.
- Texture
- Smooth and cool to the touch. Can be easily scratched with a fingernail. Fibrous varieties feel silky.
- Crystal Form
- Often forms tabular, prismatic, or acicular crystals. Can also be massive, granular, or fibrous. Characteristic 'fishtail' twins are common. Large, clear crystals are characteristic of selenite.
- 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 arid regions, playa lakes, sabkhas, salt flats, caves, and as a secondary mineral in some ore deposits or volcanic fumaroles.
Key Facts
- Hardness: 2 (Mohs scale)
- Specific Gravity: 2.31-2.32
- Crystal System: Monoclinic
- Color: Colorless, white, gray, yellow, brown, pink (due to impurities)
- Luster: Vitreous to pearly on cleavage surfaces, silky in fibrous varieties
- Transparency: Transparent to translucent
- Fracture: Conchoidal to uneven
- Cleavage: Perfect on {010}, good on {100} and {111}
- Composition: Hydrous Calcium Sulfate (CaSO4·2H2O)
Quick Check
- Color: Colorless to white, transparent
- Luster: Vitreous to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Tabular, prismatic, acicular, massive, granular, fibrous, often twinned (e.g., 'fishtail' twins, 'ram's horn' twins, 'desert roses')
- Cleavage Type: Perfect basal cleavage ({010}), good prismatic cleavage ({100} and {111})
- Fracture Type: Conchoidal to uneven
- Tenacity: Sectile (can be cut with a knife), flexible in thin sheets
- Luster Type: Vitreous, pearly, silky
Formation
Selenite, a variety of gypsum, forms primarily through the evaporation of saline water bodies (lakes, lagoons, shallow seas). As water evaporates, the concentration of dissolved calcium sulfate increases, leading to the precipitation of gypsum. Selenite specifically refers to transparent, well-formed crystalline gypsum, often exhibiting distinct crystal habits. It can also form in hydrothermal veins, as a secondary mineral in sulfide ore deposits, or as a weathering product of other sulfate minerals.
Usage
Historically, gypsum has been used as a fertilizer and soil conditioner. Modern uses include plaster of Paris (for casts, sculptures, and construction), drywall (sheetrock), cement retarder, and as a flux in glassmaking. Selenite crystals, particularly large and clear specimens, are highly prized by mineral collectors and are also used in decorative arts and sometimes in metaphysical practices.
Age Distribution
Found in sedimentary deposits of various ages, from Precambrian to Cenozoic, wherever evaporitic conditions have occurred.
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 found, up to 12 meters long.
Great Salt Plains State Park, Oklahoma, USA
Known for its unique 'hourglass' selenite crystals, which visitors can dig for.
Coober Pedy, South Australia
While famous for opal, selenite is also found in the sedimentary layers here.
Bologna, Emilia-Romagna, Italy
Historically significant for its transparent selenite crystals, known as 'Pietra di Bologna'.
Various desert regions worldwide
Including parts of the Sahara, Atacama, and American Southwest, where evaporitic conditions lead to gypsum formation.
Finding Tips
Look for Evaporite Environments
Focus on areas that were once or are currently saline lakes, playas, or shallow marine basins in arid climates. Look for associated minerals like halite, anhydrite, and other sulfates.
Check Cave Systems
Selenite can form spectacular crystal growths in caves, often as 'gypsum flowers' or large, clear crystals, particularly in limestone caves.
Examine Sedimentary Layers
Selenite often occurs in distinct layers within sedimentary rock sequences, sometimes interbedded with shales, limestones, or sandstones.
Test Hardness
Its extreme softness (Mohs 2) is a key identifier. It can be easily scratched with a fingernail, which distinguishes it from harder minerals like quartz or calcite.
Observe Cleavage
The perfect cleavage into thin, flexible sheets is a very strong diagnostic feature for selenite.
Similar Rocks
Calcite
Calcite (CaCO3)
Also known as: Calcium Carbonate
Halite
Halite (NaCl)
Also known as: Rock Salt
Barite
Barite (BaSO4)
Also known as: Baryte
Scientific Classification
- Mineral Class
- Sulfates
- Group
- Gypsum Group
- Crystal System
- Monoclinic
- Chemical Formula
- CaSO4·2H2O
- Composition
- Calcium sulfate dihydrate
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