How to identify Orpiment

To correctly identify Orpiment (As2S3), 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

   Lemon-yellow to golden-yellow, sometimes brownish-yellow. The color is a primary diagnostic feature.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Pearly on cleavage surfaces, resinous to greasy on other surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often foliated, lamellar, or columnar, giving it a layered or fibrous appearance. Can also be granular or massive.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic. Crystals are typically short prismatic, tabular, or acicular, but well-formed crystals are rare. More commonly found as foliated or reniform masses.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect on {010}, allowing it to be easily split into flexible, non-elastic sheets.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Low-temperature hydrothermal veins, hot springs, volcanic fumaroles, and as an alteration product of other arsenic minerals (e.g., realgar, arsenopyrite) in ore deposits. Often found in association with realgar.

Key Facts

Hardness

1.5 - 2 on the Mohs scale (very soft)

Specific Gravity

3.49 (relatively dense)

Crystal System

Monoclinic

Color

Lemon-yellow, golden-yellow, brownish-yellow

Luster

Pearly on cleavage surfaces, resinous to greasy on others

Transparency

Transparent to translucent

Fracture

Uneven to conchoidal

Cleavage

Perfect on {010}

Composition

Arsenic sulfide (As2S3)

Physical characteristics

• Crystal Habit: Foliated, lamellar, columnar, reniform, granular, massive; rarely as short prismatic or tabular crystals.
• Cleavage Type: Perfect basal cleavage on {010}, yielding flexible, non-elastic sheets.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Sectile (can be cut with a knife), flexible but not elastic.
• Luster Type: Pearly on cleavage surfaces, resinous to greasy on other surfaces.

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