Orpiment, As2S3 (arsenic sulfide), is a rare mineral that usually forms with realgar. In fact, the two minerals are almost always together. Crystals of orpiment — Photo: Nessa Eull · CC0
Arsenic
A metalloid element (symbol As, atomic number 33) related to phosphorus and antimony — notorious as a poison, and historically an important ore element.
Intro adapted from Wikipedia (CC BY-SA).
Arsenic is an IMA-recognized mineral species in the native element class, with the chemical formula As.
It crystallizes in the trigonal crystal system (space group R-3m). It is a member of the Arsenic structural group.
Key properties: Mohs hardness 3.5; specific gravity 5.63–5.78; typically gray; Tin-white streak; Nearly metallic luster.
IMA symbol As, grandfathered status — known before the IMA’s founding.
Found in hydrothermal veins and deposits that contain other arsenic minerals; may be in Co–Ag sulfide veins.
It is found associated with Arsenolite, cinnabar, realgar, orpiment, stibnite, galena, sphalerite, pyrite, barite.
The name comes from the Latin arsenicum, earlier Greek arrenikos, or arsenikos, masculine, an allusion to its potent properties.
Collections: Rare & Remarkable, Collector Favorites.