How to identify Satin Spar

To correctly identify Satin Spar (Gypsum (variety: Satin Spar)), 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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Distinctive silky or pearly luster, often chatoyant.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Fibrous, with parallel, elongated crystals. Can feel smooth to slightly rough along the fibers.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

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

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect cleavage in one direction (parallel to the fiber length), and good cleavage in two other directions, though less apparent due to the fibrous habit.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Evaporitic sedimentary environments, often found in association with other evaporite minerals like halite and anhydrite. Occurs in shales, limestones, and sandstones as vein fillings or bedded deposits.

Key Facts

Hardness

2 on the Mohs scale

Specific Gravity

2.31-2.33

Crystal System

Monoclinic

Color

White, colorless, pale yellow, pink, or brown

Luster

Silky to pearly

Transparency

Translucent to opaque

Fracture

Uneven to splintery (across fibers)

Cleavage

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

Composition

Hydrous calcium sulfate (CaSO4·2H2O)

Physical characteristics

• Crystal Habit: Fibrous aggregates, often forming veins with fibers oriented perpendicular to the vein walls; acicular to columnar crystals.
• Cleavage Type: Perfect basal cleavage {010}, good prismatic cleavage {100} and {111}. The fibrous nature often makes the perfect cleavage less obvious than in selenite.
• Fracture Type: Uneven to splintery, especially when broken across the fibers.
• Tenacity: Sectile (can be cut with a knife), flexible but inelastic (bends but does not return to original shape).
• Luster Type: Silky to pearly, due to reflection from parallel fibers.

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