Selenite Mineral Overview
- Mineral name: Selenite (variety of gypsum)
- Mineral class: Sulfate
- Chemical formula: CaSO₄·2H₂O
- Crystal system: Monoclinic
- Mohs hardness: 2
- Color range: Colorless, white, pale gray
- Luster: Vitreous to pearly
- Streak: White
- Cleavage: Perfect in one direction
- Fracture: Uneven
- Specific gravity: Approximately 2.3
- Transparency: Transparent to translucent
- Common locations: Mexico, United States, Morocco, Australia
- Uses: Decorative objects, industrial gypsum source, collector mineral
- Similar minerals: Gypsum (other varieties), calcite, satin spar
Naming and Classification
Selenite is a transparent, crystalline variety of gypsum, a calcium sulfate mineral. The name comes from the Greek selene, meaning “moon,” referring to its soft glow and translucency.
It is one of several forms of gypsum, which also include:
- Alabaster: Fine-grained, massive form
- Satin spar: Fibrous, silky variety
All share the same chemical composition but differ in crystal habit and appearance.
Physical and Optical Characteristics
Selenite typically forms elongated, tabular, or blade-like crystals, often transparent and well-defined. Some crystals can grow to very large sizes, especially in certain environments.
Its most notable features include:
- High transparency, often allowing light to pass through easily
- Smooth, flat cleavage surfaces
- A soft, sometimes slightly pearly sheen
With a Mohs hardness of 2, selenite is very soft and can be scratched with a fingernail. It also exhibits perfect cleavage, meaning it splits easily into thin sheets.
These sheets can sometimes be flexible but will break rather than bend permanently.
Crystal Forms and Notable Varieties
Selenite can occur in several distinctive forms:
- Clear, plate-like crystals
- “Desert roses”: Rosette-shaped clusters with sand inclusions
- Large transparent crystals: Found in certain cave environments
One of the most well-known occurrences is the giant selenite crystals found in caves, where crystals can reach several meters in length.
Chemical Composition and Structure
Selenite is composed of:
- Calcium (Ca)
- Sulfate groups (SO₄)
- Water (H₂O)
It is a hydrated mineral, with water molecules integrated into its crystal structure. This contributes to:
- Its softness
- Its ability to cleave easily
- Its stability under surface conditions
Formation and Geological Occurrence
Selenite forms in evaporite environments, where water rich in dissolved minerals evaporates and leaves behind crystals.
Typical formation settings include:
- Dry lake beds and salt flats
- Evaporating inland seas
- Sedimentary basins
It can also form in:
- Caves, where mineral-rich water slowly deposits crystals
- Low-temperature hydrothermal environments
Associated minerals include:
- Halite
- Anhydrite
- Calcite
These minerals are typical of evaporite systems.
Common Locations
Selenite is found worldwide, with notable occurrences in:
- Mexico: Naica Mine (famous for giant crystals)
- United States: Oklahoma, Utah
- Morocco: Desert deposits
- Australia: Evaporite basins
The Naica crystals in Mexico are among the largest known natural crystals of any mineral.
Uses and Practical Significance
Selenite is used primarily for:
- Decorative objects: Carvings, lamps, and ornamental pieces
- Collector specimens: Especially large or clear crystals
- Industrial gypsum source: Used in plaster, drywall, and cement
Due to its softness, it is not suitable for most jewelry applications.
Similar and Related Minerals
Selenite can be confused with other light-colored, transparent minerals:
- Calcite: Harder and reacts with acid
- Quartz: Much harder (Mohs 7) and lacks perfect cleavage
- Satin spar (gypsum): Fibrous rather than clear and crystalline
Cleavage and softness are key distinguishing features.
Identification Notes for Collectors
- Look for transparent, blade-like crystals
- Check very low hardness (scratches easily)
- Observe perfect cleavage into thin sheets
- Note association with evaporite environments
- Handle carefully due to softness and fragility
Selenite is a transparent variety of gypsum, recognized for its clarity, softness, and formation in evaporite environments, and is widely appreciated for both its natural crystal forms and decorative uses.