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;