Stoichiometry-based estimates of ferric iron in calcic, sodic-calcic and sodic amphiboles: a comparison of various methods

Anais da Academia Brasileira de Ciências

Guilherme A.r. Gualda¹  Silvio R.f. Vlach¹ 

[1] ,Universidade de São Paulo Instituto de Geociências Departamento de Mineralogia e GeotectônicaSão Paulo SP ,Brasil

关键词: amphiboles;    stoichiometry;    ferric iron;    EPMA;    anfibólios;    estequiometria;    ferro férrico;    EPMA;   

DOI  :  10.1590/S0001-37652005000300012

来源: SciELO

【 摘 要 】

An important drawback of the electron microprobe is its inability to quantify Fe3+/Fe2+ ratios in routine work. Although these ratios can be calculated, there is no unique criterion that can be applied to all amphiboles. Using a large data set of calcic, sodic-calcic, and sodic amphibole analysis from A-type granites and syenites from southern Brazil, weassess the choices made by the method of Schumacher (1997, Canadian Mineralogist, 35: 238-246), which uses the average between selected maximum and minimum estimates. Maximum estimates selected most frequently are: 13 cations excluding Ca, Na, and K (13eCNK - 66%); sum of Si and Al equal to 8 (8SiAl - 17%); 15 cations excluding K (15eK - 8%). These selections are appropriate based on crystallochemical considerations. Minimum estimates are mostly all iron as Fe2+ (all Fe2 - 71%), and are clearly inadequate. Hence, maximum estimates should better approximate the actual values. To test this, complete analyses were selected from the literature, and calculated and measured values were compared. 13eCNK and maximum estimates are precise and accurate (concordance correlation coefficient- r c » 0.85). As expected, averages yield poor estimates (r c = 0.56). We recommend, thus, that maximum estimates be used for calcic, sodic-calcic, and sodic amphiboles.

【 授权许可】

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