estimacion de Fe3 Droop, 1987

estimacion de Fe3 Droop, 1987 - A general equation for...

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A general equation for estimating Fe 3+ concentrations in ferromagnesian silicates and oxides from microprobe analyses, using stoichiometric criteria G. T. R. DROOP Department of Geology, University of Manchester, Oxford Road, Manchester M13 9PL Abstract A simple general equation is presented for estimating the Fe 3 § concentrations in ferromagnesian oxide and silicate minerals from microprobe analyses. The equation has been derived using stoichiometric criteria assuming that iron is the only element present with variable valency and that oxygen is the only anion. In general, the number of Fe 3 + ions per X oxygens in the mineral formula, F, is given by; F = 2X(1 - T/S) where T is the ideal number of cations per formula unit, and S is the observed cation total per X oxygens calculated assuming all iron to be Fe 2 § Minerals for which this equation is appropriate include pyralspite and ugrandite garnet, aluminate spinel, magnetite, pyroxene, sapphirine and ilmenite. The equation cannot be used for minerals with cation vacancies (e.g. micas, maghemite) unless, as in the case of amphiboles, the number of ions of a subset of elements in the formula can be fixed. Variants of the above equation are presented for some of the numerous published schemes for the recalculation of amphibole formulae. The equation is also inappropriate for minerals showing SP += 4H § substitution (e.g. staurolite, hydrogarnet), minerals containing an unknown proportion of an unanalysed element other than oxygen (e.g. boron-bearing kornerupine) and minerals containing two or more elements with variable valency. K~YwoRos: Fe~+/Fe a+ estimation, microprobe analyses, iron-bearing minerals. Introduction I T has long been known from wet chemical analysis, and more recently from M6ssbauer spectroscopy, that many iron-bearing oxide and silicate minerals contain appreciable quantities of both Fe 2 + and Fe 3+. Unfortunately, the most commonly used technique nowadays for analysing minerals, electron-probe microanalysis, cannot detect the two oxidation states of iron separately. Conse- quently Fe 2 +/Fe 3+ ratios in minerals analysed in this way have to be estimated by indirect means, i.e. by computation after the analysis has been per- formed. The problem of estimating FeZ+/Fe 3+ ratios in minerals from microprobe analyses has received much attention, particularly with respect to pyro- xenes (e.g. Cawthorn and Collerson, 1974; Brown and Bradshaw, 1979; Carpenter, 1979). However, most published Fe3+-recalculation schemes are mineral-specific and usually applicable only to certain ranges of composition (e.g. metamorphic sodic pyroxenes: Carpenter, 1979). For petrolo- gists, who commonly need to analyse several coexisting ferromagnesian phases in each rock, the implementation of such diverse schemes can be cumbersome, and a generally applicable method would be more convenient. In this paper, I derive a simple general equation for estimating the Fe 3+ content of oxides and silicates from microprobe analyses. Although variations of this method have been published for individual minerals (e.g. pyroxenes: Robinson, 1980;
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This note was uploaded on 03/19/2012 for the course GEOLOGY 6 taught by Professor Rm during the Spring '12 term at UNAM MX.

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estimacion de Fe3 Droop, 1987 - A general equation for...

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