Cat's Eye Selenite

Description

Cat's Eye Selenite is a fibrous variety of the mineral gypsum (hydrated calcium sulfate). Its most distinctive feature is chatoyancy, a 'cat's eye' optical effect, which appears as a silky, movable band of light across the surface when the mineral is cut en cabochon or viewed under direct light. This effect is caused by the reflection of light from the parallel fibrous inclusions or structures within the mineral. It typically forms as elongated, parallel crystals, often in veins or layers. It is very soft, easily scratched, and can be cleaved into thin, flexible sheets.

How to Identify

Color

Typically white, colorless, or pale yellow/brown. Can sometimes be tinted by impurities.

Luster

Silky to pearly, especially on fibrous surfaces, contributing to the chatoyancy.

Texture

Fibrous, often appearing as parallel, elongated crystals. Can feel smooth or slightly rough along the fibers.

Crystal Form

Usually occurs as aggregates of parallel, acicular (needle-like) or fibrous crystals, forming veins or masses. Individual crystals are monoclinic.

Cleavage

Perfect cleavage in one direction (010), allowing it to be split into thin, flexible sheets. Good cleavage in two other directions (100 and 011).

Geological Environment

Found in evaporite deposits, sedimentary beds, caves, and as a secondary mineral in hydrothermal veins or altered volcanic rocks. It forms in arid or semi-arid regions where water bodies evaporate, concentrating dissolved sulfates, or where sulfate-rich waters interact with calcium-rich rocks.

Key Facts

• Hardness: 2 on the Mohs scale
• Specific Gravity: 2.31-2.33
• Crystal System: Monoclinic
• Color: Colorless, white, pale yellow, pale brown
• Luster: Vitreous to pearly on cleavage surfaces, silky on fibrous surfaces
• Transparency: Transparent to translucent
• Fracture: Uneven to conchoidal (rarely observed due to perfect cleavage)
• Cleavage: Perfect on {010}, good on {100} and {011}
• Composition: Hydrated calcium sulfate (CaSO4·2H2O)

Quick Check

• Color: White, colorless, pale yellow/brown
• Luster: Silky to pearly, chatoyant
• Streak: White

Physical Characteristics

• Crystal Habit: Fibrous, acicular, columnar, often forming parallel aggregates in veins or masses. Can also occur as tabular or bladed crystals.
• Cleavage Type: Perfect basal cleavage (010), allowing it to be split into thin, flexible sheets. Good prismatic cleavage (100) and (011).
• Fracture Type: Uneven to conchoidal, but rarely observed due to prominent cleavage.
• Tenacity: Sectile (can be cut with a knife), flexible in thin sheets, but not elastic.
• Luster Type: Silky to pearly on fibrous surfaces, vitreous on other crystal faces.

Formation

Cat's Eye Selenite, a fibrous variety of gypsum, forms primarily through the evaporation of saline waters (e.g., ancient seas, playa lakes, sabkhas) or as a secondary mineral from the alteration of calcium-bearing rocks by sulfate-rich solutions. The fibrous habit, which gives rise to the cat's eye effect (chatoyancy), typically develops when gypsum crystallizes in confined spaces, such as veins or seams, where crystal growth is preferentially oriented along a single direction. This directional growth results in elongated, parallel fibers.

Usage

Primarily used as an ornamental stone, for carving, and in metaphysical practices due to its unique optical properties and soft, translucent appearance. It is not typically used in industrial applications due to its softness and water solubility, unlike massive gypsum which is used in plaster and drywall.

Age Distribution

Can form in geological environments ranging from recent to ancient, wherever evaporitic conditions or hydrothermal alteration of calcium-rich rocks occur.

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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