Stoichiometry-based estimates of ferric iron in calcic, sodic-calcic and sodic amphiboles: A comparison of various methods
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| Natura: | Artículo científico |
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Academia Brasileira de Ciências
2005
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| author | Guilherme A.R. Gualda |
| author_facet | Guilherme A.R. Gualda |
| contents | Stoichiometry-based estimates of ferric iron in calcic, sodic-calcic and sodic amphiboles: A comparison of various methods Guilherme A.R. Gualda Silvio R.F. Vlach Multidisciplinaria (Ciencias Naturales y Exactas) EPMA amphiboles ferric iron stoichiometry An important drawback of the electron microprobe is its inability to quantify Fe3+/Fe2+ ratios in routinework. Although these ratios can be calculated, there is no unique criterion that can be applied toall amphiboles.Using a large data set of calcic, sodic-calcic, and sodic amphibole analysis from A-type granites and syenitesfrom 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. Maximumestimates selected most frequently are: 13 cations excluding Ca, Na, and K (13eCNK 66%); sum of Si andAl equal to 8 (8SiAl 17%); 15 cations excluding K (15eK 8%). These selections are appropriate basedon crystallochemical considerations. Minimum estimates are mostly all iron as Fe2+ (all Fe2 71%), andare 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 werecompared. 13eCNK and maximum estimates are precise and accurate (concordance correlation coefficient rc ≈ 0.85). As expected, averages yield poor estimates (rc = 0.56).We recommend, thus, that maximum estimates be used for calcic, sodic-calcic, and sodic amphiboles. 2005 artículo científico 0001-3765 https://www.redalyc.org/articulo.oa?id=32777312 en http://www.redalyc.org/revista.oa?id=327 Anais da Academia Brasileira de Ciências application/pdf Academia Brasileira de Ciências Anais da Academia Brasileira de Ciências (Brasil) Num.3 Vol.77 |
| format | Artículo científico |
| id | redalyc_32777312 |
| institution | Redalyc |
| language | en |
| publishDate | 2005 |
| publisher | Academia Brasileira de Ciências |
| spellingShingle | Stoichiometry-based estimates of ferric iron in calcic, sodic-calcic and sodic amphiboles: A comparison of various methods Guilherme A.R. Gualda Multidisciplinaria (Ciencias Naturales y Exactas) EPMA amphiboles ferric iron stoichiometry Stoichiometry-based estimates of ferric iron in calcic, sodic-calcic and sodic amphiboles: A comparison of various methods Guilherme A.R. Gualda Silvio R.F. Vlach Multidisciplinaria (Ciencias Naturales y Exactas) EPMA amphiboles ferric iron stoichiometry An important drawback of the electron microprobe is its inability to quantify Fe3+/Fe2+ ratios in routinework. Although these ratios can be calculated, there is no unique criterion that can be applied toall amphiboles.Using a large data set of calcic, sodic-calcic, and sodic amphibole analysis from A-type granites and syenitesfrom 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. Maximumestimates selected most frequently are: 13 cations excluding Ca, Na, and K (13eCNK 66%); sum of Si andAl equal to 8 (8SiAl 17%); 15 cations excluding K (15eK 8%). These selections are appropriate basedon crystallochemical considerations. Minimum estimates are mostly all iron as Fe2+ (all Fe2 71%), andare 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 werecompared. 13eCNK and maximum estimates are precise and accurate (concordance correlation coefficient rc ≈ 0.85). As expected, averages yield poor estimates (rc = 0.56).We recommend, thus, that maximum estimates be used for calcic, sodic-calcic, and sodic amphiboles. 2005 artículo científico 0001-3765 https://www.redalyc.org/articulo.oa?id=32777312 en http://www.redalyc.org/revista.oa?id=327 Anais da Academia Brasileira de Ciências application/pdf Academia Brasileira de Ciências Anais da Academia Brasileira de Ciências (Brasil) Num.3 Vol.77 |
| title | Stoichiometry-based estimates of ferric iron in calcic, sodic-calcic and sodic amphiboles: A comparison of various methods |
| topic | Multidisciplinaria (Ciencias Naturales y Exactas) EPMA amphiboles ferric iron stoichiometry |
| url | https://www.redalyc.org/articulo.oa?id=32777312 |