Name/Title
OrpimentEntry/Object ID
2018.11.5Description
Crystal System: Monoclinic System
Description: Orpiment is a common monoclinic arsenic sulfide mineral. It melts at 300 °C to 325 °C. Optically it is biaxial (-) with refractive indices of a = 2.4, b = 2.81, g = 3.02.
Realgar most commonly occurs as a low-temperature hydrothermal vein mineral associated with other arsenic and antimony minerals. It also occurs as volcanic sublimations and in hot spring deposits. It occurs in association with orpiment, arsenolite, calcite and barite.[1]
It is found with lead, silver and gold ores in Hungary, Bohemia and Saxony. In the US it occurs notably in Mercur, Utah; Manhattan, Nevada and in the geyser deposits of Yellowstone National Park.[4]
It is commonly held that after a long period of exposure to light realgar changes form to a yellow powder known as pararealgar (ß-As4S4). It was once thought that this powder was the yellow sulfide orpiment, but has been recently shown to be a distinct chemical compound.
Realgar and orpiment are very similar minerals. They are both arsenic sulfides and members of the monoclinic crystal system. They form in the same geological environments and can be closely associated in the same deposits. They have similar physical properties and similar histories of use by man. Because of these similarities, we decided to describe realgar and orpiment in a single article.
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.
Realgar and orpiment are both toxic minerals, and contact with them should be avoided.
Common Name: Orpiment
Chemistry: As2S3, Arsenic Sulfide
Group Name: Sulffides / Sulfosalts
Location: El Brusskiy MIne, Northern Caucasus region of Russia
Description: Thick white matrix base the orpiment crystal are the golden layer on top
PHYSICAL CHARACTERISTICS:
Color is orange-yellow to yellow.
Luster is resinous to pearly
Transparency crystals are translucent to transparent.
Crystal System Monoclinic; 2/m
Crystal Habit: is usually foliated or earthy masses and crusts, also fiberous and as small tabular crystals that appear orthorhombic.
Cleavage is perfect in one direction producing flexible, non-elastic flakes.
Fracture is flaky.
Hardness is 1.5-2.
Specific Gravity is 3.5
Streak is yellow
Associated Minerals realgar, calcite, stibnite, barite and gypsum.
Other Characteristics: orpiment is unstable in light; specimens should be stored in complete darkness. Also usually has a distinct odor similar to sulfur, but is due to the arsenic.
Best Field Indicators are crystal habit, cleavage, odor and color
Fracture: Uneven
Hardness: 2 Gypsum
Luster: Resinous
Specific Gravity: 3.49
Streak: Pale lemon-yellowCollection
UnknownDimensions
Height
1 inWidth
6 inDepth
5 inLength
5 inWeight
34.4 oz
