Antlerite is a green copper sulphate mineral, with the chemical formula Cu3(SO4)(OH)4. Its colour ranges in shade from light green to emerald green to black, and is translucent with a pale green streak. It has a hardness of 3-3.5. Antlerite occurs in weathered zones of carbonate-poor copper deposits in arid areas. It was originally thought to be fairly rare, but was discovered to be the main ore at the Chuquicamata mine in Chile and Antler mine in Arizona, USA, the latter of which lends the mineral its name. Antlerite is soluble in dilute sulphuric acid; however it can be distinguished from similarly coloured malachite because it does not dissolve in hydrochloric acid (malachite does). It may also be confused with brochantite, with which it is closely related. However, antlerite is less common.
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Image: Antlerite from Ingadanais Mines, Castelo Branco, Portugal, by Leon Hupperichs, via Wikimedia Commons. Picture width: 3mm.