How to identify Selenite

To correctly identify Selenite (Selenite (a variety of Gypsum)), 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, but can be white, gray, yellowish, or brownish due to impurities.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to pearly on cleavage surfaces, silky in the fibrous variety (satin spar).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth and cool to the touch. Can be fibrous in satin spar, or granular in massive gypsum. Selenite crystals are often tabular or prismatic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Often forms well-developed, tabular, prismatic, or acicular (needle-like) crystals. Can also occur as 'fishtail' twins, 'desert roses' (rosette aggregates), or 'gypsum flowers' (curved, branching aggregates).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in evaporite deposits, sedimentary beds, caves, and arid regions where saline waters have evaporated. Often associated with other evaporite minerals like halite and anhydrite, and sometimes with sulfur.

Key Facts

Hardness

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

Specific Gravity

2.31 - 2.32 g/cm³

Crystal System

Monoclinic

Color

Colorless, white, gray, yellowish, brownish (due to impurities)

Luster

Vitreous to pearly on cleavage surfaces; silky in fibrous varieties

Transparency

Transparent to translucent

Fracture

Uneven to conchoidal

Cleavage

Perfect in one direction ({010}), good in two others ({100} and {111})

Composition

Hydrous Calcium Sulfate (CaSO4·2H2O)

Physical characteristics

• Crystal Habit: Tabular, prismatic, acicular, massive, fibrous (satin spar), rosette (desert rose), twinned (fishtail)
• Cleavage Type: Perfect basal cleavage ({010}) yielding flexible sheets; good prismatic cleavage ({111}) and pinacoidal cleavage ({100})
• Fracture Type: Uneven to conchoidal
• Tenacity: Sectile (can be cut with a knife), flexible (thin sheets can be bent without breaking)
• Luster Type: Vitreous (glassy) to pearly; silky in fibrous varieties

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