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The Mineral orpiment

Bright Yellow Orpiment With Realgar

Orpiment is known for its strikingly bright yellow color, though it can also be bright orange or lustrous brown. The name Orpiment is derived from the Latin Auripigmentum, meaning gold pigment, in reference to its color and historical use as a golden-yellow pigment. Orpiment is often associated with bright red Realgar, and may form in strikingly colored and unique mineral combinations.

Orpiment is a photosensitive mineral and will eventually dull and develop a white powdery film upon prolonged exposure to light. Due to the instability of Orpiment, specimens should be stored enclosed and covered to prevent their exposure to light. Occasional exposure to look at a specimen will not cause damage; only prolonged or repeated exposure will cause deterioration.

Orpiment contains a significant amount of poisonous arsenic, and is itself somewhat toxic. Washing hands is recommended after handling Orpiment specimens, especially if powdery.

Chemical Formula

As2S3

Color

Bright yellow, orange-yellow, orange, orange-red, and brown

Crystal System

Monoclinic

Properties

Streak
Yellow
Hardness
1.5 - 2
Transparency
Transparent to opaque
Specific Gravity
3.4 - 3.5
Luster
Resinous to pearly.
Cleavage
1,1
Fracture
Uneven
Tenacity
Sectile and slightly flexible in thin flakes.

Crystal Habits

Individual Orpiment crystals are usually small prismatic or stubby, and often have chisel-shaped or triangular pyramidal terminations. Crystals are commonly in dense grouping of small crystals rather than individual crystals. Typical habits include micaceous, grainy, encrusting, in veins, and in foliated masses. Also drusy, botryoidal, in rosettes, in rounded balls, and in small crystals radiating from a central core. Crystals are usually striated horizontally and are are occasionally slightly rounded.

3D Crystal Atlas

Additional Information

Composition
Arsenic trisulfide
In Group
Sulfides; Simple Sulfides
Striking Features
Striking color, environment, and frequent association with Realgar.
Environment
In low-temperature hydrothermal veins and volcanic hot springs and fumaroles. Less commonly in arid borate deposits and metamorphic marble pockets.
Rock Type
Igneous, Sedimentary, Metamorphic

Uses

Orpiment is an ore of arsenic. It was historically used as a yellow pigment and an ancient decorative stone.

Noteworthy Localities

Relatively large and gemmy butterscotch Orpiment crystals come from the Jiepaiyu (Shimen) Mine, Hunan Province, China; and outstanding bright spiky crystal plates and drusy crystals from the El'brusskiy mine, near Mt. Elbrus, Russia. Good quality specimens have also come from the Zareh Shuran Mine, Takab, Iran; Allchar, Roszdan, Macedonia; and Lucéram, Alpes-Maritimes, France. Some of the best examples of Orpiment have come from Peru in Quiruvilca, La Libertad, in the form of bright gemmy orange-brown crystals. Also in Peru is the Palomo Mine, Huancavelica, which produced rounded, ball-like aggregates of this mineral.

In the U.S., a relatively new locality has produced some of the finest Orpiment specimens. This locality is the Twin Creeks Mine, Potosi District, Humboldt Co., Nevada, famous for its transparent butterscotch crystal groups. Bright yellow foliated Orpiment almost invariably associated with splotches of Realgar comes from Nevada at the Getchell Mine, Humboldt Co.; and from the White Cap Mine, Manhattan, Nye Co. Bright Orpiment crystals on a contrasting white Calcite matrix come from Mercur, Tooele Co, Utah.

Common Mineral Associations

Realgar, Calcite, Barite, Stibnite, Gypsum, Cinnabar

Distingushing Similar Minerals

Pararealgar - Occurs as a replacement of Realgar crystals; otherwise difficult to distinguish.
Sulfur - Lacks cleavage and has distinct odor.
Autunite - Forms in different crystal and environments, and often has a greenish tinge which is lacking in Orpiment.


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