Orpiment
Orpiment is arsenic sulfide, As2S3: lemon-yellow, hardness 1.5 to 2, density 3.49, flexible cleavage lamellae. 60.91% arsenic, the least of 10 IMA As–S rows.
Orpiment is arsenic sulfide, As2S3: a monoclinic, lemon-yellow to golden mineral, hardness 1.5 to 2, density 3.49, with a perfect {010} cleavage whose lamellae bend. It is 60.91% arsenic by mass, worked: with anorpiment, the lowest of the 10 IMA rows made only of arsenic and sulfur. Its sheet prints no light sensitivity, unlike realgar’s; its triclinic dimorph is anorpiment.
In short
- The species. The Handbook of Mineralogy orpiment sheet gives As2S3, monoclinic, foliated columnar or fibrous aggregates with cleavages up to 60 cm across, rarely prismatic crystals to 10 cm; perfect {010} cleavage with flexible lamellae; hardness 1.5 to 2; density 3.49; lemon-yellow to golden, streak pale lemon-yellow, resinous to pearly.
- The arsenic–sulfur ladder. A script over the September 2026 IMA-CNMNC Master List finds 10 rows whose formula holds only As and S. Worked on standard atomic weights they run from duranusite, As4S, 90.34% arsenic, to orpiment and anorpiment, As2S3, 60.91%. Of the ten, those two carry the most sulfur, 1.5 atoms per arsenic.
- Documented failure: the balance. A 20.00 g orpiment reads 14.27 g in water; pararealgar reads 14.32 g, alacránite 14.17 g and realgar 14.38 g. The weights sit within 0.21 g. Streak, cleavage and the flexible lamellae separate them; anorpiment, the same formula in crusts of crystals to 0.2 mm, needs X-ray work.
- Light: what the sheets do and do not say. The realgar sheet records disintegration in light, with orpiment among the products. The orpiment sheet prints no light statement. The NatSCA leaflet on the care and conservation of geological specimens names the arsenic sulphide group as most susceptible, with realgar as its example; it does not name orpiment.
- Arsenic. 60.91% by mass. This page gives no handling guidance; the regulatory entries are quoted on is orpiment toxic. This is not health, safety or legal advice.
| Species | IMA formula, status | S per As | As by mass | Colour (sheet) | Streak (sheet) | Hardness | Density | 20.00 g in water |
|---|---|---|---|---|---|---|---|---|
| Duranusite | As4S; A, 1973-003, France | 0.25 | 90.34% | Red; opaque, metallic | n.p. | n.d. (VHN 58) | n.d. | - |
| Dimorphite | As4S3; G, 1849 | 0.75 | 75.70% | Yellow-orange | n.p. | 1.5 | 3.58 | 14.41 g |
| Paradimorphite | As4S3; A, 2020-101, Italy | 0.75 | 75.70% | No sheet | No sheet | - | - | - |
| Realgar | AsS; G, 1747 | 1 | 70.03% | Red to yellow-orange | Red-orange to red | 1.5–2 | 3.56 | 14.38 g |
| Pararealgar | As4S4; A, 1980-034, Canada | 1 | 70.03% | Bright yellow when powdery | Bright yellow | 1–1.5 | 3.52 | 14.32 g |
| Bonazziite | As4S4; A, 2013-141, Kyrgyzstan | 1 | 70.03% | No sheet | No sheet | - | - | - |
| Alacránite | As8S9; Rn, 1985-033, Russia | 1.125 | 67.50% | Orange; orange-yellow in transmitted light | Yellow-orange | 1.5 | 3.43 | 14.17 g |
| Uzonite | As4S5; A, 1984-027, Russia | 1.25 | 65.15% | Yellow | n.p. | about 1.5 | 3.385 | 14.09 g |
| Orpiment | As2S3; G, year ? | 1.5 | 60.91% | Lemon-yellow to golden or brownish yellow | Pale lemon-yellow | 1.5–2 | 3.49 | 14.27 g |
| Anorpiment | As2S3; A, 2011-014, Peru | 1.5 | 60.91% | Greenish-yellow | Yellow | 1.5 | 3.33 | 13.99 g |
What is orpiment? The arsenic sulfide As2S3
Orpiment is As2S3, a soft yellow arsenic sulfide grandfathered on the IMA list, whose sheet describes it as foliated, sectile and flexible on its cleavage. The Handbook of Mineralogy orpiment sheet says it is stated to be very near As2S3, with no analysis printed; worked, that is 60.91% arsenic and 39.09% sulfur, the same figure the realgar page gives. Monoclinic, point group 2/m, space group P21/n; commonly in foliated columnar or fibrous aggregates with cleavages as much as 60 cm across, also reniform, botryoidal, granular or powdery, rarely as prismatic crystals to 10 cm; twinned on {100}.
Physical and optical. Cleavage perfect on {010}, imperfect on {100}; the cleavage lamellae are flexible; sectile; hardness 1.5 to 2; density 3.49 measured, 3.48 calculated. Transparent; lemon-yellow to golden or brownish yellow; streak pale lemon-yellow; lustre resinous, pearly on the cleavage surface. Biaxial (−), α 2.4, β 2.81, γ 3.02 in lithium light, 2V 76°, strong dispersion r > v.
How it forms. The Occurrence field reads: in low-temperature hydrothermal veins, hot springs and fumaroles; also commonly as an alteration product of arsenic minerals, especially realgar. The association line is stibnite, realgar, arsenic, calcite, barite and gypsum; stibnite and baryte have their own pages. The sheet derives the name from the Latin auripigmentum, golden paint, in allusion to the colour.
Orpiment and the other arsenic sulfides: the As–S ladder
The arsenic–sulfur ladder: of the 10 IMA rows made only of arsenic and sulfur, orpiment and anorpiment have the most sulfur and the least arsenic, 60.91%. Method: each line of the September 2026 list was split on runs of two or more spaces and kept if the formula field contains no element but As and S; a second, token-based scan of the raw lines returns the same ten. In order of falling arsenic: duranusite As4S, 90.34%; dimorphite and paradimorphite As4S3, 75.70%; realgar AsS, pararealgar and bonazziite As4S4, 70.03%; alacránite As8S9, 67.50%; uzonite As4S5, 65.15%; orpiment and anorpiment As2S3, 60.91%.
Three formulas held by two or three species. AsS/As4S4 is shared by realgar, pararealgar and bonazziite; As4S3 by dimorphite and paradimorphite; As2S3 by orpiment and anorpiment. The anorpiment sheet says dimorphous with orpiment, triclinic, drusy crusts of wedge-shaped crystals to 0.2 mm, perfect and easy {001} cleavage, hardness 1.5, density 3.33, greenish-yellow with a yellow streak, and names one locality, the Palomo mine, Castrovirreyna Province, Huancavelica, Peru. Its name alludes to the triclinic, or anorthic, symmetry.
Documented failure: weight does not sort the yellow ones. Worked as 20 − 20/D for a 20.00 g piece: orpiment 14.27 g, pararealgar 14.32 g, alacránite 14.17 g, uzonite 14.09 g, anorpiment 13.99 g, realgar 14.38 g. Six readings inside 0.39 g, and orpiment within 0.05 g of pararealgar. What does separate them is on the sheets: streak (pale lemon-yellow against pararealgar’s bright yellow and alacránite’s yellow-orange), habit (pararealgar is a fine powder) and orpiment’s perfect cleavage with flexible lamellae. Methods: how to use a streak plate and how to measure specific gravity at home.
Does orpiment change in light? What the sheets print
The orpiment sheet prints no light behaviour; the realgar sheet does, and names orpiment as one of realgar’s products. A script search of the orpiment sheet for light, exposure and disintegrat returns nothing outside its optical-data lines, and a ligature-aware search for fluorescence and phosphorescence returns nothing, while the same search finds fluorescence on the hemimorphite and willemite sheets. The realgar sheet reads: disintegrates on long exposure to light to a powder composed of pararealgar or arsenolite and orpiment. The orpiment sheet agrees from the other side: commonly an alteration product of arsenic minerals, especially realgar.
What the conservation leaflet says. The NatSCA leaflet on the care and conservation of geological specimens states that light will irreversibly damage some mineral species, that the arsenic sulphide group of minerals are the most susceptible, including realgar, and that susceptible species should be stored in light-proof containers. Orpiment is an arsenic sulphide, but the leaflet names realgar and not orpiment, and gives no rate. This page does not transfer realgar’s pararealgar story to orpiment. The light-sensitive species are discussed on which minerals fade in light.
Magnetism. USGS Open-File Report 99-529, Table 1 ran 1 orpiment sample: above 1.70 amp in both columns, which the report calls diamagnetic. Realgar, 3 samples, behaved the same.
Where orpiment is found
The sheet calls orpiment not uncommon in small amounts but rare in fine specimens. Its Distribution field names, in the USA, crystallised material from Mercur, Tooele Co., Utah; in Nevada the Getchell mine, Humboldt Co., the White Caps mine, Manhattan, Nye Co., and fine examples from the Twin Creeks mine, Humboldt Co. Tajov, Slovakia; Křeševo, Bosnia-Herzegovina; Alšar (Allchar), near Rošden, Macedonia; fine crystals from the Zarshuran gold deposit, 35 km north of Takab, Iran; Jelamerk, Turkey. In Russia, Loukhoumi in the Caucasus and Men-Kyule, Sakha; Racha Luyumi, Georgia; large crystals from the Shimen mine, Hunan, China; and exceptional specimens from the Quiruvilca mine, La Libertad, Peru.
No British locality is printed, and none is added. The X-ray powder pattern on the sheet is from Baia Sprie (Felsőbánya), Romania. Several of these localities also appear on the realgar sheet, including Getchell, Mercur, Alšar, Men-Kyule and Shimen.
Arsenic. Orpiment is 60.91% arsenic by mass. This page gives no handling guidance; the HSE and COSHH entries are quoted on is orpiment toxic, and is realgar safe to handle covers the realgar case. This is not health, safety or legal advice. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.