How to identify Selenite

To correctly identify Selenite (CaSO4 H2O), 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 fibrous varieties (satin spar).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth and cool to the touch. Can feel slightly greasy in some massive forms.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Commonly forms tabular, prismatic, or acicular (needle-like) crystals. Can also occur as 'desert roses' (rosette-like aggregates), 'gypsum flowers' (delicate, branching forms), or massive, granular, and fibrous aggregates.

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 011), forming rhombic fragments.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Evaporite deposits (ancient lakebeds, playas, sabkhas), sedimentary rocks (shales, limestones), caves (speleothems), hydrothermal veins, and as an alteration product of anhydrite.

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, gray, yellowish, brownish (due to impurities)

Luster

Vitreous to pearly (on cleavage surfaces), silky (satin spar)

Transparency

Transparent to translucent

Fracture

Uneven to conchoidal

Cleavage

Perfect in one direction (010), good in two others (100 and 011)

Composition

Calcium sulfate dihydrate (CaSO4·2H2O)

Physical characteristics

• Crystal Habit: Tabular, prismatic, acicular, massive, fibrous, granular, rosette-like (desert rose), branching (gypsum flower)
• Cleavage Type: Perfect basal cleavage (010), good prismatic cleavage (100 and 011)
• Fracture Type: Uneven to conchoidal
• Tenacity: Sectile (can be cut with a knife), flexible in thin sheets
• Luster Type: Vitreous (glassy) to pearly, silky (satin spar variety)

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