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Identity HelpBlack coating on kesterite + mushistonite from Pingwu

11th Apr 2021 04:02 UTCLuís Martins 🌟

08523990017061986298662.jpg
I've recently acquired a specimen of kesterite covered with mushistonite from Pingwu ( https://www.mindat.org/loc-5416.html ). 
The back of the specimen shows a black coating over the mushistonite that does not come off with a gentle mechanical scrub. Photo attached, showing the specimen (4cm wide).
Any ideas on what this is? I cannot seen anything similar in the photos from this locality.

11th Apr 2021 04:03 UTCLuís Martins 🌟

05226390017061986315311.jpg
The front of the specimen, for reference

12th Apr 2021 05:02 UTCPavel Kartashov Manager

03559680017061986331644.jpg
According to my analytical data such black films are fine mixtures of chalcocite group minerals and acanthite.
By the way, and this mushistonite quite able to be vismirnovite.

12th Apr 2021 05:34 UTCLuís Martins 🌟

Pavel, thank you very much for the information, very interesting.
What test would be able to distinguish between mushistonite and vismirnovite? Would EDS suffice?

12th Apr 2021 14:23 UTCPavel Kartashov Manager

Only EDS spectrum without quantitative measurement isn't enough. As you may to see in the caption to the spectrum above, vismirnovite from Pingwu contains all Zn, Cu and Fe, which changes in wide range. It is difficult to judge, who from them predominate, by spectrum (with exception of cases, when some of them absents).
 
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