How to identify Satin Spar Gypsum

To correctly identify Satin Spar Gypsum (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

   Typically white, colorless, or pale shades of yellow, pink, or brown due to impurities.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Distinctive silky, pearly, or satiny luster, especially on fractured surfaces parallel to the fibers.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Fibrous, with a smooth, almost slippery feel when rubbed parallel to the fibers. Can be splintery when broken across the fibers.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Aggregates of parallel, elongated, acicular (needle-like) crystals, often forming veins or layers.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect cleavage in one direction (parallel to the length of the fibers), producing thin, flexible sheets. Good cleavage in two other directions, but less prominent than selenite.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Evaporite deposits, often found in arid regions where ancient seas or lakes have evaporated. Also occurs as secondary mineralization in sedimentary rocks, shales, limestones, and clays, filling fractures and voids.

Key Facts

Hardness

2 on the Mohs scale

Specific Gravity

2.31 - 2.32

Crystal System

Monoclinic

Color

White, colorless, pale yellow, pink, brown

Luster

Silky, pearly, satiny

Transparency

Translucent to opaque

Fracture

Uneven, splintery across fibers

Cleavage

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

Composition

Hydrous calcium sulfate (CaSO4·2H2O)

Physical characteristics

• Crystal Habit: Fibrous aggregates, acicular (needle-like) crystals, often parallel or radiating.
• Cleavage Type: Perfect basal cleavage (010), allowing it to be split into thin, flexible sheets. Two other good cleavages.
• Fracture Type: Uneven to splintery, especially when broken perpendicular to the fibers.
• Tenacity: Sectile (can be cut with a knife), flexible in thin sheets, but not elastic.
• Luster Type: Silky, pearly, or satiny, due to the reflection of light off the parallel fibrous structure.

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