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Identity HelpDouble Rainbow Mine South Dakota Purchased at a Show

19th Apr 2024 14:41 UTCKim A. Strange

Hello mindat community. I picked up several interesting specimens from the Double Rainbow Mine (Galena, South Dakota) at a show last weekend. I was not familiar with this locality before coming across these specimens. This piece is labeled galena with sphalerite but there are also brassy flat crystal groups on the galena which I think may be marcasite. The marcasite? seem encrusted with additional sphalerite somewhat hiding their brassy coloration. I'm looking for your thoughts on; 1) the likelihood of the flat brassy crystals being marcasite and if not marcasite, what are the likely suspects? Pyrrhotite may be another possibility but the very thin crystal form is not characteristic of this locality. 2) given that the form of the galena is not "typical" for the locality, whether the galena could be pseudomorphs after pyrrhotite. The specimen has a layer of massive pyrite at the base, then the tiny dark black sphalerite crystals, then the galena and then the brassy crystals. It measures 60mm by 40mm by 30mm. Thank you for your thoughts. Additional photos in replies.

Questions Answered
Can it scratch glass? : No
Can you scratch it with your fingernail? : No
Is it light/heavy for the size? : Heavy

19th Apr 2024 14:42 UTCKim A. Strange

close-up photo of brassy crystals

19th Apr 2024 14:43 UTCKim A. Strange

Another photo of the brassy crystals

19th Apr 2024 14:48 UTCKevin Conroy Manager

I'm not familiar with this mine, but I know a nice galena specimen when I see one. A beauty!

19th Apr 2024 15:07 UTCWayne Corwin

I ditto Kevin's comment ! ! !

19th Apr 2024 20:43 UTCEric D. Fritzsch 🌟

The Double Rainbow Mine was an underground mine operated by Homestake for a short period of time in the 1960's I believe. The mine flooded immediately after pulling out pumps and abandoning it. I use to collect on the dumps but those have been removed too. The platy mineral is most like pyrrhotite, which is fairly common at this location. Nice piece.

24th Apr 2024 22:00 UTCKim A. Strange

Arsenopyrite

Double Rainbow Mine, Galena, Galena Mining District, Lawrence County, South Dakota, USA

Thank you Eric (and Wayne and Kevin as well). I especially value the thoughts of collectors who have been to and collected at a given locality. I did find one photo in mindat (see https://www.mindat.org/photo-1298691.html) that was/is on display at the SD School of Mines Geology Museum that shows the thin flat crystal form similar to what is on the galena above. Photo 1298691 is identified as arsenopyrite ps. pyrrhotite - there are multiple mineral forms present on this piece other than the flat crystal form in question. Due to the brassy color of the flat crystals on the galena above, I'm warming up to the idea that it may be pyrrhotite rather than marcasite. Thanks again for your thoughts.

25th Apr 2024 16:20 UTCUwe Kolitsch Manager

Due to the brassy color of the flat crystals on the galena above, I'm warming up to the idea that it may be pyrrhotite rather than marcasite.

Or a (common) pseudomorph of pyrite and/or marcasite after platy pyrrhotite.

Magnetism could be checked.

25th Apr 2024 19:25 UTCAlfredo Petrov Manager

Pyrrhotite alters to pyrite very commonly, so such pseudomorphs are not unusual. Replacement of pyrrhotite by arsenopyrite is less common.

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