Anorpiment

Chemical formula: As₃+₂S₂-₃

Orpiment is an arsenic sulfide, forming characteristic, lemon-yellow, flexible lamellae and aggregates with a resinous or pearly luster.

## Characteristics Orpiment, also known as orpiment, is an arsenic sulfide, valued for its intense, lemon-yellow to golden-yellow color. It most often occurs as lamellar, scaly, or radial aggregates, as well as in the form of crusts and earthy masses. Well-formed crystals are rare and appear as small, prismatic columns. A characteristic feature of its thin lamellae is flexibility. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 1.5-2, meaning it can be scratched with a fingernail. Orpiment lamellae are flexible but not elastic – they do not return to their original shape after bending. It has a relatively high density for a non-metallic mineral, approximately 3.49 g/cm³. The luster is resinous on fresh fracture surfaces and pearly on cleavage planes. ## Colors and Varieties Orpiment is an idiochromatic mineral, whose color – from light yellow, through lemon, to orange-yellow – is inextricably linked to its chemical composition. No color or commercial varieties are distinguished. Under sunlight, it tends to fade and slowly decompose on the surface, forming white, powdery arsenolite (As₂O₃). ## History and Name The name "orpiment" comes from the Latin words *aurum* (gold) and *pigmentum* (pigment), referring to its golden color and historical use as a pigment. It was used as early as ancient Egypt, Greece, and Rome for producing paints, cosmetics, and as a medicinal agent, despite its toxicity. As a mineral, it was described by Wallerius in 1747. ## Uses Historically, orpiment was one of the most important yellow pigments in painting, known as "king's yellow." Due to the toxicity of arsenic and its instability, it has been almost entirely replaced by more modern, synthetic dyes. Today, it has little industrial significance, being a minor ore of arsenic. However, it is a valued and sought-after collector's stone.

Properties

Mohs hardness: 1.5-2
Luster: Resinous, Pearly
Streak: Light yellow
Density: 3.49
Cleavage: Perfect on {010}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Orpiment is relatively easy to identify based on its characteristic, bright, lemon-yellow color, very low hardness (can be scratched with a fingernail), and perfect cleavage in one direction, which reveals lamellae with a pearly luster. The flexibility of these lamellae is also an important diagnostic feature. It often emits a faint, garlic-like odor, especially when rubbed or struck. ## Distinguishing from similar minerals - **Native sulfur**: Has a similar yellow color but is harder (2-2.5), more brittle (does not form flexible lamellae), and has a different crystal habit. - **Greenockite**: Is much rarer, harder (3-3.5), and has a higher density. It usually forms only thin coatings on sphalerite. - **Autunite/Torbernite (uranium minerals)**: May have a similar color and lamellar structure, but are strongly radioactive, which can be easily checked with a Geiger counter. ## Crystal forms Crystals are rare, usually small, with a short-prismatic or tabular habit. Most often, it forms lamellar, scaly, radial aggregates, as well as compact earthy masses, crusts, and veins. Aggregates often show a layered or reniform structure.

Geological environment

## Genesis Orpiment is a mineral of low-temperature hydrothermal processes. It forms in ore veins, often near hot springs and volcanic fumaroles, where it precipitates from solutions rich in arsenic and sulfur. It can also form as a sublimation product from volcanic gases and as a result of the alteration of other arsenic minerals, especially realgar, with which it often co-occurs. ## Mineral associations It is most often accompanied by realgar (AsS), whose intense red color contrasts with the yellow of orpiment. It also co-occurs with other arsenic sulfides and sulfosalts (e.g., getchellite), stibnite, marcasite, pyrite, as well as with calcite, barite, and quartz. ## Localities Significant deposits and beautiful collector's specimens come from: - **Peru**: Quiruvilca Mine, La Libertad region – source of large, bright yellow aggregates with high luster. - **China**: Hunan Province, especially the Jiepaiyu Mine in Shimen – known for the world's largest and best-formed orpiment crystals, often in association with calcite. - **Russia**: Elbrussky arsenic mine in the North Caucasus. - **USA**: Mines in Humboldt and Eureka counties in Nevada (e.g., Getchell Mine, Twin Creeks Mine) – classic localities, from which magnificent specimens originate. - **Romania**: Baia Sprie and Cavnic.

Rarity

Not very common

For collectors

## Quality criteria The most highly valued orpiment specimens are characterized by an intense, saturated, lemon-yellow color without orange or brown hues. Large, well-formed, transparent crystals are desirable, though extremely rare. Rich aggregates with a strong, resinous luster are also highly prized, especially those contrasting with a white matrix (e.g., with calcite or quartz) or with red realgar. The specimen should not show signs of decomposition (white coatings) caused by light exposure. ## Market prices Orpiment prices vary. Small, several-centimeter fragments of aggregates can be purchased for a few tens of Polish zlotys. Better quality specimens, with intense color and good luster, palm-sized, cost from 200 to 800 PLN. Top-class world specimens, especially large, well-formed crystals from China or aesthetic compositions from Peru, can reach prices of several to over ten thousand PLN on the collector's market. ## Popular localities Collectors most seek specimens from two localities: the Jiepaiyu Mine in China, due to its unique, large crystals, and the Quiruvilca Mine in Peru, which yields exceptionally bright and lustrous aggregates. Classic specimens from Nevada (USA) are also valued in historical collections.

Care and storage

## Cleaning Orpiment specimens should only be dry-cleaned, using a soft brush or compressed air to remove dust. Contact with water is absolutely inadvisable, as it can initiate chemical processes leading to the mineral's decomposition. Due to the high toxicity of arsenic, all care activities should be performed with gloves and caution, to avoid inhaling dust. ## What to avoid Orpiment is very sensitive to sunlight (UV radiation), which causes its slow decomposition into white, powdery arsenic oxide (arsenolite) and loss of color. Exposure to light, high temperatures, and all chemicals should be avoided. The mineral is extremely soft and brittle, susceptible to scratches and mechanical damage. ## Storage Specimens should be stored in closed, airtight containers or display cases, in a dark and dry place. It is best to keep it away from other minerals to avoid contamination with arsenic dust. Always place a clear label with a warning about toxicity. After each contact with the mineral, hands should be thoroughly washed.

Sources

IMA Mineral List (CNMNC)