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Improving Mindat.orgFreibergite?

17th Sep 2016 18:02 UTCReiner Mielke Expert

http://www.mindat.org/photo-772550.html You know this is not tetrahedrite because?

17th Sep 2016 23:17 UTCKyle Beucke 🌟

A few more I was wondering about:

http://www.mindat.org/photo-701392.html

http://www.mindat.org/photo-412095.html

http://www.mindat.org/photo-759677.html

Not trying to cast doubt on anyone's knowledge/ability, but without an analysis, I have to be skeptical. It appears that freibergite requires Ag>Cu. I had two tetrahedrite grains from this locality analyzed with EDS and they were both apparently silver-bearing tetrahedrite, not freibergite.

Kyle

17th Sep 2016 23:50 UTCRalph S Bottrill 🌟 Manager

Probably most freibergite, tetrahedrite, tennantite etc is just a visual ID and should be labelled tetrahedrite group unless there is a chemical analysis.

18th Sep 2016 02:15 UTCFrank Keutsch Expert

I agree with Ralph.

Just to clarify, according to the sulfosalt report "Varieties with Ag below 4 a.p.f.u. should be called Ag rich tetrahedrite." This means usually 4 out of 12 metal (non As/Sb/Bi/Te) atoms. However, the report also states that work is needed.

I have analyzed many samples and true freibergite is not very common in my experience. Ag-rich tetrahedrite is much more common. Without analysis it is much safer to call the latter and not freibergite...

Streak can give an indication to Zn vs. Fe content, as the high Zn varieties have a red/brown streak and the high Fe ones black but distinguishing tennantite from tetrahedrite is visually not possible, I think.

Frank

18th Sep 2016 20:39 UTCRob Woodside 🌟 Manager

Thanks Frank. Sometime time ago you sent me an email about these and I thought it was in the database, but I now see it is not. So I'll get on it. Thanks! With 2 metal positions there is probably more room for names. With As-Sb series, often the As are blacker and the Sb greyer, but visual ID is a mugs game.