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Identity HelpMystery silver mineral

23rd Aug 2019 03:51 UTCKyle Beucke

09814880015665279534189.jpg
Hello,

The mineral in question is brittle (tested it with a steel pin; it isn't sectile at all), black, with a black streak (no evidence of any reddish color to powder). The crystal in the photo (field of view ~ 5mm) is flattened and has triangular striations.  It is sitting in an open pocket in a quartz vein.

This is from a silver mining area that has produced miargyrite and pyrargyrite.  

Spectroscopic analysis of the vein sample this came from showed abundant silver but no copper, lead, or zinc.

An EDS analysis of this imaged crystal is posted below.

Any ideas?

Thanks!

Kyle Beucke

23rd Aug 2019 03:53 UTCKyle Beucke

06630670015665287367683.png
Here is the EDS analysis of the crystal.  

23rd Aug 2019 03:59 UTCKyle Beucke

07604140015665291177343.jpg
Here is another crystal that is next to the analyzed one.  It is also black (including streak) and brittle.  Photo is not great, sorry.  FOV 2 mm.

29th Aug 2019 16:17 UTCUwe Kolitsch Manager

The EDS spectrum may be that of a thin acanthite surface layer (or was a freshly broken surface analysed?).

31st Aug 2019 02:42 UTCKyle Beucke

Uwe,

Thanks!  A mixture (via replacement?) of a silver sulfosalt and acanthite did cross my mind, but analysis of an acanthite coating over a sulfosalt makes more sense (it would allow for the bulk of the crystal to be brittle).  I am going to have the analysis done again.

Cheers

Kyle

31st Aug 2019 17:31 UTCFrank Keutsch Expert

I have seen polybasite/pearceite having triangular striated faces.

Frank

2nd Sep 2019 16:29 UTCKyle Beucke

Frank,

Thanks.  Polybasite would have been my guess but the lack of copper (spectroscopic analysis of a rich fragment and EDS of this crystal) doesn't support that.  It could be a pseudomorph I suppose.  I am going to have this crystal analyzed again making sure a fresh fracture is analyzed.  I will also pursue additional analyses.  

Thanks,

Kyle

26th Sep 2019 20:49 UTCKyle Beucke

02338000015695270714854.png

Here is another EDS analysis of a fragment of this crystal, focusing on a fresh fracture surface.  No copper and possibly no antimony either.  I am thinking some kind of weird intergrowth, possibly a replacement of an earlier sulfosalt by acanthite.  The material is brittle and certainly does not behave like pure acanthite (I have flattened many a grain of acanthite under the scope).  Strange!  Soon I hope to have some backscatter images from this same rock with lots of the brittle black mystery mineral and might be able to figure this out.


Kyle

12th Oct 2019 19:19 UTCKyle Beucke

A microprobe analysis was done on a piece of this vein sample and the results were very interesting.  Some things relevant to figuring out what this crystal might be:

-The silver-bearing minerals did not show any apparent supergene textures.  There is nothing that looks like supergene acanthite replacing anything.

-No copper (or lead or zinc) was found in any of the metallic phases analyzed.  No evidence whatsoever for polybasite, etc.

-The only arsenic-bearing minerals appear to be pyrargyrite with minor As and what I am about to describe next.

-One of the common silver minerals appears to be a Ag-Au-As-Se-S mineral with a composition of approximately Ag: 73, As: 4, Se: 3, S: 14, Au: 6.  Gold varies quite a bit down to less than one percent.  The backscatter images of the analyzed grains do not show obvious replacement textures, and the relatively consistent composition suggests this is a single mineral.

I did a little searching for minerals with this composition and all I came up with are some (unnamed?) from the Broken Hills deposit in New Zealand.  

Kyle

12th Oct 2019 19:52 UTCUwe Kolitsch Manager

Here is another EDS analysis of a fragment of this crystal, focusing on a fresh fracture surface.
 Since it's brittle, it could be billingsleyite.

12th Oct 2019 19:53 UTCKevin Conroy Expert

Where is this one from?

12th Oct 2019 20:14 UTCKyle Beucke

Alpine County, California
 
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