How to identify Selenite

To correctly identify Selenite (Gypsum (fibrous 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 colorless and transparent, though impurities can impart shades of white, gray, yellow, brown, or pink.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to pearly, especially on cleavage surfaces. Fibrous varieties (satin spar) exhibit a silky luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth and cool to the touch. Can be easily scratched with a fingernail. Fibrous varieties feel silky.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect cleavage in one direction ({010}), yielding thin, flexible sheets. Good cleavage in two other directions ({100} and {111}), forming rhombic fragments.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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)

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

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