How to identify Selenite

To correctly identify Selenite (Gypsum variety (CaSO4·2H2O)), 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 to white, sometimes with tints of yellow, brown, or gray due to impurities.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to pearly on cleavage surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, often cool to the touch. Can be easily scratched with a fingernail.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Often found as well-formed, transparent, monoclinic crystals. Common habits include tabular, prismatic, bladed, or acicular. Can also form 'fishtail' twins or 'desert roses'.

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 sedimentary basins, salt flats, playas, caves, and as a secondary mineral in hydrothermal veins or altered volcanic rocks.

Key Facts

Hardness

2 on the Mohs scale (can be scratched by a fingernail)

Specific Gravity

2.31 - 2.32 g/cm³

Crystal System

Monoclinic

Color

Colorless, white, sometimes tinted yellow, brown, or gray

Luster

Vitreous to pearly

Transparency

Transparent to translucent

Fracture

Uneven to conchoidal

Cleavage

Perfect on {010}, good on {100} and {111}

Composition

Hydrous calcium sulfate (CaSO4·2H2O)

Physical characteristics

• Crystal Habit: Tabular, prismatic, bladed, acicular, massive, granular, fibrous (satin spar), rosette (desert rose), often twinned (fishtail twins).
• Cleavage Type: Perfect basal cleavage (010) yielding flexible sheets; good prismatic cleavage (100) and (111) forming rhombic fragments.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Sectile (can be cut with a knife), flexible in thin sheets, but not elastic.
• Luster Type: Vitreous on crystal faces, pearly on cleavage surfaces.

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