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Mineralogical ClassificationNew amphiboles from China ?

12th Mar 2014 22:44 UTCPeter Haas

The following paper has analytical data for several amphiboles from a mafic-ultramafic complex in Qinghai Province:

Huiwen Liu, Xueping Wang, Ji Shao, Kesi Lu, Fanglian Tian, and Shuya Xiong (2014): Rock characteristics of Niubiziliang mafic-ultramafic complex. Mineral Deposits 33(1), 87-103 (in Chinese with English abstract).

Free full paper download (the download link is at the end of the second line from the top): http://www.kcdz.ac.cn/ch/reader/view_abstract.aspx?file_no=20140106&flag=1

The data are presented in a table on page 95. Columns 3-5 and 10 (referred to in the paper as "钛角闪石" = titanium amphibole) correspond to the following idealized compositions (columns 1 & 2 are tschermakite group species; columns 7, 8 and 11 are pargasite group species; note that OH and F have not been determined, so the authors stick to group names here):

Columns 3, 5 and 10:

(Na) (Ca₂) <(Mg,Fe²⁺)₄(Ti,Al,Cr)> (Al₂Si₆O₂₂) W₂

Column 4:
<> (Ca₂) <(Mg,Fe²⁺)₄(Ti,Al,Cr)> (Al₂Si₆O₂₂) W₂

Unless I'm missing something, such compositions have not yet been reported.

19th Mar 2014 18:41 UTCOlav Revheim Manager

Peter,

Kaersutite was redefined in the 2012 amphibole nomenclature ( see link) from NaCa₂(Mg₄Ti) (Al₂Si₆O₂₂) O(OH) to NaCa₂(Mg₃AlTi) (Al₂Si₆O₂₂) O₂, with Ti>0,5 apfu and O> 1 apfu as the defining criteria. There are no requirement in the definition of kearsutite for (Mg,Fe²⁺)<3,5 or for (Al,Fe³⁺,Ti) >1,5 apfu to satisfy the ^C(Mg₃AlTi) part of the folmula. It is also said in the definition that if the O content is not known, it can be assumed to be minimum 2Ti.

The pre-2012 formula is quite close to the analyses in columns 3.5 and 10 in the article you linked to and of the 19 kaersutite analyses listed in Deer,Howie and Zussmann (1997): Rock forming minerals 2nd ed., only 6 meets the (Al,Fe³⁺,Ti) >1,5 apfu criteria suggested by the formula. It shall be said that for a handful of the analysis the Fe²⁺/Fe³⁺ ratio was not determined so the number of 6 may not be very accurate. Nevertheless, kaersutites meeting the general formula (Na) (Ca2) <(Mg,Fe2+)4(Ti,Al,Cr)> (Al2Si6O22) W2 is not rare.

Compositions like the ones listed in columns 3,5 and 10 would only be considered new species if (OH)>^WO (assuming neglectable F content). This is not to be expected since the Ti content in amphiboles generally is assigned to the substitution TiO₂<->Mg(OH)₂. This may not always be true though.

The analysis presented in column 4,<> (Ca2) <(Mg,Fe2+)4(Ti,Al,Cr)> (Al2Si6O22) W2 is unusual as the A position is vacant in combination with Ti>0,5 apfu. This might be a new species, which would be rather interesting as the formula would not be balanced with the ^WO dominance assumed in the nomenclature report for Ti>0,5apfu amphiboles.

Also be aware that tschermakite was redefined in 2012, so that the analyses presented in columns 1 and 2 are now considered magnesio-hornblendes rather than tschermakites.

Olav

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