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Selenite is a transparent, colorless to white crystalline variety of the mineral gypsum. It is characterized by its excellent cleavage, allowing it 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 a hydrous calcium sulfate mineral, meaning it contains water molecules within its crystal structure. Selenite is known for forming large, often perfectly clear crystals, sometimes exhibiting twinning (e.g., fishtail selenite) or fibrous habits (satin spar) or rosette-like aggregates (desert rose).
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 the fibrous variety (satin spar).
- Texture
- Smooth and cool to the touch. Can be fibrous in satin spar, or granular in massive gypsum. Selenite crystals are often tabular or prismatic.
- Crystal Form
- Often forms well-developed, tabular, prismatic, or acicular (needle-like) crystals. Can also occur as 'fishtail' twins, 'desert roses' (rosette aggregates), or 'gypsum flowers' (curved, branching aggregates).
- Cleavage
- Perfect in one direction ({010}), yielding thin, flexible sheets; good in two other directions ({100} and {111}), forming rhombic fragments.
- Geological Environment
- Found in evaporite deposits, sedimentary beds, caves, and arid regions where saline waters have evaporated. Often associated with other evaporite minerals like halite and anhydrite, and sometimes with sulfur.
Key Facts
- Hardness: 2 on the Mohs scale (can be scratched with 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 in fibrous varieties
- Transparency: Transparent to translucent
- Fracture: Uneven to conchoidal
- Cleavage: Perfect in one direction ({010}), good in two others ({100} and {111})
- Composition: Hydrous Calcium Sulfate (CaSO4·2H2O)
Quick Check
- Color: Colorless to white (can be tinted by impurities)
- Luster: Vitreous to pearly (silky in satin spar)
- Streak: White
Physical Characteristics
- Crystal Habit: Tabular, prismatic, acicular, massive, fibrous (satin spar), rosette (desert rose), twinned (fishtail)
- Cleavage Type: Perfect basal cleavage ({010}) yielding flexible sheets; good prismatic cleavage ({111}) and pinacoidal cleavage ({100})
- Fracture Type: Uneven to conchoidal
- Tenacity: Sectile (can be cut with a knife), flexible (thin sheets can be bent without breaking)
- Luster Type: Vitreous (glassy) to pearly; silky in fibrous varieties
Formation
Selenite forms through the evaporation of saline water bodies (lakes, seas, lagoons) or from groundwater rich in calcium and sulfate ions. It precipitates as calcium sulfate dihydrate. The specific crystal habits (e.g., large, clear crystals of selenite, fibrous satin spar, or rosette-like desert roses) depend on the rate of evaporation, temperature, presence of impurities, and available space for crystal growth.
Usage
Historically, selenite was used as a window pane material due to its transparency (e.g., 'lapis specularis' in ancient Rome). Today, its primary uses are as an ornamental stone, for metaphysical and spiritual practices, and as a source of gypsum for industrial applications (e.g., plaster of Paris, drywall, fertilizer), though less commonly than massive gypsum. Its aesthetic appeal makes it popular for carvings and decorative items.
Age Distribution
Selenite can form in various geological periods, from ancient evaporite deposits to modern-day arid environments. Its formation is not tied to a specific geological age but rather to specific environmental conditions.
Where to Find
Naica Mine, Chihuahua, Mexico
Famous for its 'Cave of the Crystals' (Cueva de los Cristales), containing some of the largest selenite crystals ever discovered, reaching up to 12 meters (39 feet) in length.
Oklahoma, USA
Known for its 'Great Salt Plains' where 'hourglass selenite' crystals form, characterized by sand inclusions that create an hourglass shape within the crystal.
Utah, USA
Various locations produce selenite, including 'desert roses' and clear crystals.
Australia
Significant deposits of selenite and other gypsum varieties are found, particularly in arid and semi-arid regions.
Spain
Historically important source of 'lapis specularis' (transparent selenite) used by the Romans. Modern deposits are also found.
Morocco
A major source of 'desert rose' selenite formations.
Finding Tips
Look in Evaporite Deposits
Selenite is common in areas where ancient seas or lakes have evaporated, leaving behind sulfate-rich sediments. Look for exposures of shale, limestone, or sandstone in arid regions.
Check Cave Environments
Caves can provide stable conditions for the growth of large, clear selenite crystals, especially in areas with gypsum-rich bedrock and circulating groundwater.
Identify Associated Minerals
Selenite often occurs with other evaporite minerals such as halite (rock salt), anhydrite, and sometimes native sulfur. The presence of these minerals can indicate a favorable environment for selenite.
Test Hardness
Selenite is very soft (Mohs hardness of 2). It can be easily scratched with a fingernail, which helps distinguish it from harder minerals like quartz or calcite.
Observe Cleavage
Its perfect cleavage into thin, flexible sheets is a key diagnostic feature. Calcite, while also having good cleavage, produces rhombic fragments that are not flexible.
Consider Crystal Habit
Recognizing the distinct crystal forms like tabular crystals, 'fishtail' twins, 'desert roses,' or fibrous 'satin spar' can aid in identification.
Similar Rocks
Calcite
Calcite (CaCO3)
Also known as: Calcium Carbonate
Quartz
Quartz (SiO2)
Also known as: Rock Crystal
Muscovite
Muscovite (KAl2(AlSi3O10)(OH)2)
Also known as: White Mica
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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