Arsenopyrite

Arsenopyrite is an iron arsenic sulfide with the formula FeAsS, one of the most widespread minerals containing the element arsenic. It crystallizes in the monoclinic system, historically mistaken for orthorhombic due to its near-perfect pseudo-orthorhombic symmetry, and typically forms sharply striated prismatic crystals with wedge-shaped terminations, silver-white to steel-gray in color with a metallic luster and black streak. With a Mohs hardness of 5.5–6 and a notably high specific gravity around 6.1, specimens have real heft in hand. Crystals often twin on {101} and {100}, sometimes forming attractive star-shaped trillings, and the mineral gives off a characteristic garlic odor when struck. As the principal ore of arsenic and a common host for invisible gold in orogenic gold deposits, arsenopyrite has long been mined alongside precious metals, and its cobalt-bearing variety, danaite, grades toward the related species glaucodot.

The world's most celebrated locality for fine arsenopyrite is Panasqueira in Portugal, whose tin-tungsten veins produce lustrous, sharply striated silvery crystals perched on snow-white quartz alongside honey-colored siderite and brilliant green fluorapatite. Classic European occurrences include Freiberg in Saxony, Germany — historically mined as mispickel — plus the Himmelfahrt Mine and Zug locality nearby, the Stari Trg mine at Trepča in Kosovo/Serbia, numerous tin and copper mines across Cornwall and Devonshire in England, and the Swedish deposits of Sala, Tunaberg, Stollberg, Nordmark, and Boliden. Exceptional crystals also come from Asia, notably the Yaogangxian Mine in Hunan and the Dachang district in China, prized for complex wheel-and-cog-like crystals approaching 10 cm, and the Obira Mine in Japan. Further important sources include the historic gold-bearing veins of the Deloro district in Ontario, Canada, the silver mines of Potosí in Bolivia, the Santa Eulalia district in Chihuahua, Mexico, and Dalnegorsk in Russia.

Collectors prize arsenopyrite for its sculptural geometry and mirror-bright metallic sheen, which can rival pyrite or stibnite, as well as for the striking contrasts it forms with associated minerals such as quartz, fluorite, siderite, cassiterite, and fluorapatite. It is a staple of serious sulfide and old European vein collections, valued equally for crystalline perfection and for the mining history attached to classic localities like Freiberg, Panasqueira, and Trepča. Because good crystals from key districts can reach several centimeters while true museum-caliber matrix groups remain genuinely scarce, arsenopyrite offers something for every level of mineral collector, from affordable thumbnails to important cabinet specimens.