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Orpiment
Current inventory:  1 gem
 

Chemistry:  As₂S₃

Discovered in 1747;   IMA status: Valid (pre-IMA; Grandfathered).
Orpiment is named from the Latin words aurum meaning gold and pigmentum meaning pigment, in allusion to the vivid golden color.

Classification

Mineral Classification:

Sulfides

Strunz 8th Ed. ID:

2/F.02-70

Nickel-Strunz 10th Ed. ID:

2.FA.30

2 : SULFIDES and SULFOSALTS (sulfides, selenides, tellurides; arsenides, antimonides, bismuthides; sulfarsenites, sulfantimonites, sulfbismuthites, etc.)
F : Sulfides of arsenic, alkalies; sulfides with halide, oxide, hydroxide, H₂O
A : With As, (Sb), S

Related to:

Duranusite - Jeromite Series

Crystal Data

Crystallography:

Monoclinic - Prismatic

Crystal Habit:

Commonly in foliated columnar or fibrous aggregates, with cleavages as much as 60 cm across; may be reniform or botryoidal; also granular or powdery; rarely as prismatic crystals, to 10 cm. Twinning: On [100].

Twinning:

On [100]

Physical Properties

Cleavage:

[010] Perfect, [100] Imperfect; cleavage lamellae are flexible

Fracture:

Sectile (curved shavings)

Tenacity:

Brittle

Moh's Hardness:

1.5 - 2.0

Density:

3.49 - 3.56 (g/cm³)

Luminescence:

None

Radioactivity:

Not Radioactive

Health Warning:

CAUTION: Contains Arsenic, a poisonous element - always wash hands after handling. Avoid inhaling dust when handling or breaking. Never lick or ingest.

Optical Properties

Color:

Lemon Yellow, Brownish Yellow, Orange Yellow

Transparency:

Transparent to Translucent

Luster:

Pearly Barely observable because of strong internal reflections.

Refractive Index:

2.40 - 3.02  Biaxial ( - )

Birefringence:

0.620

Dispersion:

Very Strong (r > v)

Pleochroism:

Strong: white to gray white with reddish tint (reflected light)
Y=yellow, Z=greenish yellow (transmitted light)

Anisotropism:

Barely observable because of strong internal reflections.

Occurances

Geological Setting:

Occurs as a sublimation product in volcanic fumaroles, low temperature hydrothermal veins, hot springs and as a byproduct of the decay of another arsenic mineral, Realgar.

Common Associations:

Arsenic, Barite, Calcite, Gypsum, Realgar, Stibnite

Type Locality:

n/a

Year Discovered:

1747

View mineral photos:

Orpiment Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Orpiment is a mineral that is not uncommon in small amounts, but rare as fine specimens or large crystals. Orpiment is a very interesting mineral in that it has very high RI values, very high birefringence, very stong internal reflections, very strong dispersion, very high density, very strong pleochroism and is also anisotropic. Orpiment is also dangerous since it contains about 61% Arsenic. Arsenic and its compounds are poisonous. Because of this, and the fact that Orpiment has perfect cleavage, sectile fracture and very low Moh's hardness, it is not available as a faceted gemstone but may very rarely be found as a cabochon. Although handling is dangerous, Orpiments vivid colors do make for beautiful mineral specimens and a very rare cabochon for collectors.

Orpiment occurs in many locations worldwide but only a few produce fine specimens or large crystals. Crystals can be found at Mercur, Tooele County, Utah, USA; at the Getchell and Twin Creeks mines, Humboldt County, and the White Caps mine, Manhattan, Nye County, Nevada, USA; fine crystals from the Zarshuran gold deposit, north of Takab, northwestern Iran; large crystals from the Shimen mine, Hunan Province, China; and exceptional specimens from the Quiruvilca mine, La Libertad, Peru.
 

  
Orpiment gems for sale: