From Sierra Gorda area, Atacama desert, Chile. I'm merely a geophysicist, please help! ;)
this post was submitted on 29 Jun 2023
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According to Mindat, this is the Lomas Bayas disctrict, and the most prominent mine appears to be the Fortuna de Cobre, in a copper porphyry deposit. I would normally reach for malachite first, but it is not listed as found at any of the mines, so your main options are brochantite or (surprise!) atacamite.
Malachite can be eliminated for sure by testing with vinegar; it will effervesce slightly. Between brochantite and atacamite, there is considerable overlap in color, hardness, etc. You will have to get a lens and look for some crystals where the shape is distinct enough to determine if they are monoclinic (brochantite) or orthorhombic (atacamite).
You're absolutely right that it is copper porphyry ore (from an abandoned historical artisanal mine). However, I don't have the sample and cannot perform the tests beyond that point. Just wanted to kick-start the thread :)
I would have called it malachite, maybe it effervesces due to some carbonate in the rock? Whip out your portable XRD!
Yeah, almost certainly malachite. :)
Oh, a pop quiz! I will try to post one too from a computer later.
This came from Pilar, New Mexico, USA.
Are those little tourmalines or something like that? It's definitely got some fabric to it too.
There is tourmaline in the same unit (I've seen some schorl), but these are...
piemontite!