How to identify Orpiment

To correctly identify Orpiment (Arsenic Sulfide (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

   Bright lemon-yellow to golden-yellow, sometimes orange-yellow. Can be translucent to transparent.

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 massive. Can also be columnar or reniform. Individual crystals are rare and typically small.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic. Crystals are typically short prismatic, tabular, orthorhombic-like, but often occur as aggregates of flexible, thin sheets or scales. Rarely forms distinct crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect on {010}, allowing it to be easily split into flexible sheets. Imperfect on {100} and {110}.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Low-temperature hydrothermal veins, hot spring deposits, fumaroles, and as a secondary mineral in altered arsenic-rich sulfide deposits. Often found in association with realgar, stibnite, cinnabar, and native arsenic.

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, orange-yellow

Luster

Pearly on cleavage surfaces, resinous to greasy on other surfaces

Transparency

Translucent to transparent

Fracture

Conchoidal to uneven

Cleavage

Perfect on {010}, good on {100}, distinct on {110}

Composition

Arsenic Sulfide (As2S3)

Physical characteristics

• Crystal Habit: Foliated, lamellar, columnar, massive, reniform. Rarely as distinct, short prismatic crystals.
• Cleavage Type: Perfect basal cleavage, allowing it to be split into flexible sheets.
• Fracture Type: Conchoidal to uneven, but rarely observed due to perfect cleavage.
• Tenacity: Sectile (can be cut with a knife), flexible in thin lamellae.
• Luster Type: Pearly on cleavage surfaces, resinous to greasy on other surfaces.

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