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

Abstract

An important drawback of the electron microprobe is its inability to quantify Fe 3+ /Fe 2+ 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, we assess 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 Fe 2+ (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.

Phase-equilibrium geobarometers for silicic rocks based on rhyolite-MELTS—Part 3: Application to the Peach Spring Tuff (Arizona–California–Nevada, USA)

“…1, S 1). One of the crystals (KPST01K2_2) shows some intra-crystal differences in the temperature-sensitive edenite substitution when using the Holland and Blundy (1994) formula recalculation method ( Figure S 1), but it is important to note that cation site allocations are highly sensitive to the formula recalculation method used, and errors propagate such that Na contents of the A-site are particularly prone to inaccuracy (e.g., Gualda and Vlach 2005). …”

Geology, petrography and geochemistry of the A-type granites from the Morro Redondo Complex (PR-SC), southern Brazil, Graciosa Province

“…were estimated with a variant of the method by Schumacher (1997), as discussed in Gualda and Vlach (2005). Clinopyroxene was classified according to Morimoto et al (1988).…”

Melanite-bearing nepheline syenite fragments and 40Ar/39Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas Alkaline Massif (MG/SP), and implications

“…Matrix effect corrections and data reduction were performed with the PRZ/Armstrong scheme. Cationic proportions, structural formulae and Fe 3+ /Fe 2+ partitions were computed using the MinCal software (e.g., Gualda & Vlach 2005) and considering 32, 24, 22 and 6 O in the formula unit of feldspar, garnet, mica and pyroxene, respectively. Garnet end members were recast following Locock (2008).…”