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Identity HelpTetrahedrite?

18th Aug 2019 21:17 BSTKyle Beucke

06638820015661592735956.jpg
Greetings,

Does tetrahedrite seem reasonable for the attached EDS spectrum?  The sample had the appearance of tetrahedrite.

Thanks,

Kyle

18th Aug 2019 23:59 BSTFrank K. Mazdab Manager

Hi Kyle,

The spectrum is consistent with a tetrahedrite group mineral.  Although there are other minerals with similar chemistries, since you further indicated the sample had the appearance of tetrahedrite (I assume you based that on crystal morphology?), tetrahedrite is a reasonable assumption.  It's perhaps a bit unusual to not have any notable Fe in it (yours clearly seems to be Zn >> Fe), but that's not unheard of. The significant Ag and small Hg amounts are similarly consistent with the "garbage can" nature of the tetrahedrite group.  It appears your sample will likely end up as an example of the recently re-named tetrahedrite-(Zn) end-member, but you'll probably want some quantitative data to ensure the notable Ag content isn't sufficient to inadvertently pop you into another species (I don't think it will be, but quantitative data is certainly always a good thing).

19th Aug 2019 04:16 BSTKyle Beucke

Thanks Frank!  The sample did not have euhedral crystals, but it was black, had an irregular fracture surface (no good cleavage), and was relatively hard (when scratching it with a steel pin under the scope, it felt closer to tetrahedrite than, say, a silver sulfosalt).

Kyle 

19th Aug 2019 04:41 BSTAlfredo Petrov Manager

Kyle, does it have a reddish streak color, or show red internal reflections under the microscope with a strong light? I noticed that on a Zn-rich Fe-poor tennantite once, but am not sure whether that‘s more an effect of the Zn-Fe ratio, the Sb-As ratio, or some other factor.

19th Aug 2019 04:47 BSTFrank K. Mazdab Manager

agreed it's definitely not a silver sulfosalt (way too much Cu), but there are other copper sulfosalts with Cu:Sb:S ratios close to tetrahedrite that *potentially* could be candidates (although likely differing a bit in physical properties).  But the minor elements, particular Ag and Zn, seem very consistent with tetrahedrite (particularly specifically with a Ag±Hg-bearing tetrahedrite-(Zn)), so I do agree that tetrahedrite is a very reasonable and plausible identification.

19th Aug 2019 04:49 BSTKyle Beucke

Alfredo, funny you mentioned that!  Yes, there is a distinct dark red color to the powder.  The sample is associated with somewhat oxidized pyrite, so there are reddish/orange iron oxides in the sample as well.  Therefore, I wasn't too confident in streak color.  But maybe it really is reddish after all!

Kyle

19th Aug 2019 15:27 BSTRichard Gunter Expert

Hi Kyle:

Tennantite from the Kidd Creek Bornite Zone has distinct red internal reflections under a binocular microscope. The Tennantite from there is Se rich but has almost no iron. I have not powdered a sample to see its streak but that may be reddish as well.

19th Aug 2019 19:33 BSTAlfredo Petrov Manager

I think low Fe is the necessary factor for the red. With Fe it is black.
 
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