How to identify Cat's Eye Selenite

To correctly identify Cat's Eye Selenite (Selenite (fibrous gypsum variety)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

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

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

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

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

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

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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)

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.

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