Mineral Species: Orpiment

Chemical Formula: As2S3

Chemical Name: Arsenic Sulphide

Mineral Class: Sulphide

Mineral Notes: Orpiment is a rare mineral that usually forms from the transformation of Realgar. Both almost always occur together. Crystals of Orpiment are extremely rare as it usually forms masses and crusts. These are sometimes transparent and have a gemmy appearance. The yellow colour is very distinctive and is rarely confused with other minerals. Orpiment is derived from the Latin auripigmentum, or 'golden pigment'. Its use as a dye or pigment, however, is limited by its instability. Over time, Orpiment colour will deteriorate, a process accelerated by exposure to light.

Description: A very rich encustation of golden-yellow Orpiment on a grey matrix. Whilst the majority is a thick yellow crust, numerous well-formed crystals are also present. The largest is 5mm across, very bright and lustrous. It shows excellent psudo-orthorhombic form. Numerous others are also present in a nearby indentation of the matrix.

Location: Potosi District, Humboldt County, Nevada, USA.