Overview
Aurostibite is a rare gold antimonide, AuSb2, and one of the few minerals in which gold is chemically combined rather than present as native metal. It belongs to the pyrite group and was first described in 1952 from gold deposits in Canada. Because it carries gold, aurostibite is of genuine interest to economic geologists studying how gold behaves in antimony-rich, sulphur-poor ores, but it is too rare and too inconspicuous to be a mainstream collector or jewellery material. It is encountered mainly as microscopic grains within gold ore.
Composition & structure
Aurostibite is a simple compound of gold and antimony with the formula AuSb2. It crystallises in the isometric (cubic) system and adopts the pyrite structure type, meaning its gold and antimony atoms are arranged like the iron and sulphur atoms in pyrite, with antimony taking the role that sulphur plays in FeS2. This places aurostibite among the antimonide members of the pyrite group, a small and unusual set of minerals. Its high antimony and gold content give it a notably high density of close to 10 g/cm³.
| Formula | AuSb2 |
| Crystal system | Isometric (cubic), pyrite group |
| Mohs hardness | About 3 |
| Lustre | Metallic |
| Colour | White to grey, often with a bornite-like tarnish |
| Type locality | Yellowknife area, Northwest Territories, Canada |
Formation & occurrence
Aurostibite forms in hydrothermal gold-quartz veins, specifically in settings that are rich in antimony but deficient in sulphur. Under those conditions gold can combine with antimony rather than crystallising as native gold or being locked in sulphides. At the Canadian type localities it occurs in gold ores alongside native gold, stibnite, sulphosalts such as freibergite and jamesonite, and carbonate gangue. Its presence is generally a signal of late, antimony-enriched stages of gold mineralisation.
Identification & similar species
Aurostibite is opaque and metallic, appearing white to grey and commonly tarnishing to iridescent, bornite-like hues. It is soft for a metallic mineral, with a hardness near 3, and very dense. Because it occurs as tiny grains intergrown with gold and other ore minerals, it is rarely identified by eye; reflected-light microscopy and chemical analysis are normally required. It can be confused with other grey antimony-bearing ore minerals and with tarnished sulphides, so its distinctive gold-plus-antimony chemistry and cubic pyrite-type structure are the reliable diagnostics.
Notable localities & collecting
The classic occurrences are the gold mines of the Yellowknife district in the Northwest Territories and the Larder Lake area of Ontario, Canada, where aurostibite was first recognised. It has since been reported from scattered antimony-rich gold deposits elsewhere, but always in small amounts. For collectors, aurostibite is essentially a micromount and reference species rather than a showpiece, and genuine identified specimens are uncommon. Any sample should be backed by analytical confirmation given how easily it is mistaken for other ore minerals.