How to identify Gypsum

To correctly identify Gypsum (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

   Colorless, white, gray, yellow, red, brown. Often transparent to translucent.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to pearly, silky (satin spar).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Can be massive, granular, fibrous, or crystalline. Selenite is typically clear and crystalline, alabaster is fine-grained and massive, and satin spar is fibrous.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic crystals, often tabular, prismatic, or acicular. Can form 'swallowtail' twins. Also occurs as massive, granular, fibrous, or rosette-like aggregates ('desert rose').

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in one direction (pinacoidal), good in another, and distinct in a third, resulting in rhombic fragments. Selenite often shows thin, flexible sheets.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Evaporite deposits in arid or semi-arid regions, often associated with halite, anhydrite, and dolomite. Also found in hydrothermal veins, volcanic fumaroles, and as a weathering product of sulfide ores.

Key Facts

Hardness

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

Specific Gravity

2.31-2.33 g/cm³.

Crystal System

Monoclinic.

Color

Colorless, white, gray, yellow, red, brown.

Luster

Vitreous, pearly, silky.

Transparency

Transparent to translucent.

Fracture

Conchoidal to uneven.

Cleavage

Perfect on {010}, good on {100}, distinct on {011}.

Composition

Calcium sulfate dihydrate (CaSO4·2H2O).

Physical characteristics

• Crystal Habit: Tabular, prismatic, acicular, massive, granular, fibrous, rosette-like (desert rose), swallowtail twins.
• Cleavage Type: Perfect in one direction, good in another, distinct in a third.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Sectile (can be cut with a knife), flexible in thin sheets (selenite).
• Luster Type: Vitreous (glassy), pearly (on cleavage surfaces), silky (satin spar).

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