This is a really neat specimen. It is a group of acanthite crystals (pseudomorphs after argentite), featuring a large 2.2 cm parallel-growth crystal at the top. With magnification, one can see that the surface of the forward-facing face (as I would display this piece) of the large crystal hosts many small wires and knobs of native silver. Sweet!
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About These Acanthites
Well-crystallized acanthite has never been common and most of the known localities for crystals are historic, making it difficult, and usually expensive, to obtain fine specimens. The finds at Imiter (Africa’s largest silver mine) in recent years have placed it among the world’s best localities for crystallized acanthite. Although these crystals formed as argentite, argentite is not stable below 179C, at which point it changes to monoclinic acanthite. The specimens are acanthite pseudomorphs after argentite, retaining the isometric crystal form of the original argentite crystals. I assembled this lot of acanthites over a period of about two years. Most of these specimens, from the 2008 find, remained in Morocco until 2013.