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Improving Mindat.orgTschermakite- Mundarara Mine Tanzania

17th Dec 2012 21:17 UTCOlav Revheim Manager

The dark amphibole in the ruby/zoisite rocks from the Mundarara Mine is generally considered to be tschermakite. I have not been able to find any analytical support for this claim. On the contrary, this material has been analysed to be pargasite:

Martin Okrusch, Theodore E. Bunch(1973) Al-Rich Pargasite, American Mineralogist, Volume 58, pages 721-726, refers to an analysis performed by Bernard Leake: "The most Al-rich amphibole within this group, from a corundum-zoisite-amphibolite at Merkerstein( N.Arusha, Tanzania), is rather close to the theoretical pargasite composition: Si, 6.05; AlIV, 1.95; AlVI1.04; Na + K, 0.79, and Ca * Na + K, 2.60 (Leake,1971,Table1 )"


and

Cedric Simonet (2000): “Geology of Sapphire and Ruby Deposits -The example of the John Saul Ruby Mine, Mangare area, Southern Kenya” PhD Thesis, University of Nantes provides a very detailed account of the locality and he has microprobed 4 samples of the dark amphibole embedded in the green zoisite. All of them turned out to be pargasites, all quite similar to:


(Na0.680K0.060Ca0.030)(Ca1.870Mg0.120Mn0.010)(Mg2.900Fe0.860Al1.170Cr0.060Ti0.010)(Si5.950Al2.070O22)(OH)2.00


Even though P. M. GAME (1954): Zoisite-amphibolite with corundum from Tanganyika, MinMag/Volume_30 page 458f is more focused on optical data than the chemistry of the amphibole , he finds the amphibole to contain " about 45 molecular % edenite", which is quite compatible with a pargasite and not at all compatible with a tschermakite which ideally does not have an edenite component.


Does anyone have any analysis supporting the tschermakite ID of these amphiboles? or should the tschermakite entries be changed to pargasite?


Olav

18th Dec 2012 12:58 UTCAlfredo Petrov Manager

I was given some of those labelled "aluminotschermakite" rather than tschermakite; no idea which is correct. It would be quite hard to distinguish pargasite from aluminotschermakite unless one had a good analysis, with standards. The common standardless EDS that collectors get would be quite inadequate in this case. :-(

19th Dec 2012 20:55 UTCOlav Revheim Manager

Alfredo,


In the 2012 nomenclature, aluminotschermakite and tschermakite are merged and redefined in that the previous Ferritschermakite Ca2(Mg3Fe3+2)(Si6Al2)O22(OH)2, Tschermakite Ca2(Mg3Fe3+Al)(Si6Al2)O22(OH)2 and Aluminotschermakite Ca2(Mg3Al2)(Si6Al2)O22(OH)2 (+ the ferrous equivalents of these) are merged together to become Ferritschermakite Ca2(Mg3Fe3+2)(Si6Al2)O22 and Tschermakite Ca2(Mg3Al2)(Si6Al2)O22(OH)2 (+ the ferrous equivalents of these)


In addition the amphibole committee has moved the species defining position from the T position (i.e Si6Al2) to the C position (Mg3Al2). This seems like a cosmetic change, but for the majority of "tschermakites" the Al content in the T position is often 0,2-0,4 apfu higher than the sum (Al+Fe3++2Ti) now used to define these minerals. This means that many pre-2012 tschermakites now are considered magnsiohornblende-group minerals (sigh).


For the analysed amphiboles from the Mundarara Mine, the A(Na + K+2Ca) is > 0,5 apfu, so they are pargasites with a good margin. As can be seen by the calculated from Simonet above, the sum C (Al+Fe3++2Ti) in the C position is also below the 1,5 apfu required to be considered a tschermakite now. ( Based on the assumption that most of the Fe is ferrous, which is an assumption that seems to balance the formula quite well).


Based on the analytical data available combined with the new nomenclature, I think it would be unlikely that the main amphibole from here can be tschermakite, but I will happily reconsider if someone can show other information


:)-D


Olav

31st Dec 2012 22:48 UTCAlfredo Petrov Manager

Thanks, Olav! :)-D
 
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