orpiment

n.
Arsenic trisulfide, As₂S₃, a yellow mineral used as a pigment.

[Middle English, from Old French, from Latin auripigmentum : aurum, gold + pigmentum, pigment; see pigment.]

A mineral having composition As₂S₃. Crystals are small, tabular, and rarely distinct; the mineral occurs more commonly in foliated or columnar masses. The hardness is 1.5–2 (Mohs scale) and the specific gravity is 3.49. The luster is resinous and pearly on the cleavage surface; the color is lemon yellow. Orpiment is found in Romania, Peru, Japan, and Russia. In the United States it occurs at Mercer, Utah; Manhattan, Nevada; and in deposits from geyser waters in Yellowstone National Park. See also Arsenic.

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orpiment

An arsenic sulfur compound; used in paints as a yellow pigment.

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As
Monoclinic -- prismatic

Environment

Nearly always associated with realgar in low-temperature veins, occasionally alone.

Crystal description

Almost always well crystallized, though often in compact masses characterized by micaceous yellow flakes. Free-growing crystals relatively rare; usually they form as crystalline crusts, with individuals being difficult to orient. Elongated rodlike crystals are found in Hunan, China.

Physical properties

Orange-yellow to yellow to brown and greenish black on crystal surfaces other than the flaky cleavage direction; tends to blacken on dumps. Luster resinous to pearly; hardness 1Ɖ-2; specific gravity 3.4-3.5; streak micaceous golden flakes; cleavage perfect micaceous, side pinacoidal. Sectile; flexible but inelastic cleavage flakes; translucent to transparent.

Composition

Arsenic trisulfide (61% As, 39% S).

Tests

Same as for realgar (above).

Distinguishing characteristics

Unlikely to be taken for any other mineral. Strikingly brilliant cleavage faces distinguish it from sulfur with no need of blowpipe tests. An almost inescapable association with realgar and stibnite eliminates often fluorescent yellow minerals such as uranium compounds (autunite) and greenockite.

Occurrence

In Russia and Macedonia, orpiment sometimes forms large micalike cleavable masses. Good crystals from Rumania and Luceram (s. France), and unusually large isolated ones at Mercur, Utah. Rich masses, mixed with realgar, lie all about the dumps at the Getchell gold mine, Nevada; their weather-affected surfaces look dark greenish yellow, while the crumbling realgar dust is orange. Fine brownish resinous crystal crusts have been found in Peru, in Iran at Valilo, and at Ahar, in Azerbaijan. Yellow hairs in calcite crevices at Manhattan, Nye Co., Nevada, long thought to be orpiment, are now identified as a newer mineral, also found in Japan, named wakabayashilite (As,Sb) ₁₁ S ₁₈ .

Remarks

Less light sensitive than realgar, though old specimens do deteriorate. In collections and light orpiment tends to dull and develop a white film, presumably a response to exposure and oxidation. The usual color of the front and top crystal faces is a brownish orange, very different from the golden color and high luster of a fresh cleavage surface. The red and yellow arsenic sulfide mixture is very decorative, but the poisonous character of the oxide coupled with its tendency to disintegrate make it unsatisfactory, and perhaps even unwise, for interior design use, though it is widely sold for such uses.

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A sulfide ore containing arsenic.

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