A general equation for estimating Fe³⁺ concentrations in ferromagnesian silicates and oxides from microprobe analyses, using stoichiometric criteria

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;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.

Nb–Ta–(Ti–Sn) oxide mineral chemistry as tracer of rare-element granitic pegmatite fractionation in the Borborema Province, Northeastern Brazil

“…In the case of wodginite group minerals, the normalization was made recalculating Fe 2+ to Fe 3+ cations as much as necessary to complete the B+C sites to 9.000 (for easier comparison with other minerals, for a 24-oxygen formula instead of the 32-oxygen formula required for the unit cell). The effectiveness of the Fe 3+ correction procedure was checked by repeating some calculations using the method proposed by Droop (1987).…”

Geochemistry of the Rio Espinharas hybrid complex, northeastern Brazil

“…Augite crystals with calculated Fe 3 + (Droop, 1987) and Mg*=[100Mg/(Mg + Fe 2 + + Fe 3 + + Mn)] of 73 and 72 -76 are found in diorite and dioritic enclaves, respectively, whereas diopside (Morimoto et al, 1989) with Mg* of 71 -73, 59 -64 and 67 -73 occurs in quartz -monzonite, porphyritic quartzmonzonite and quartz -syenite, respectively. However, single-zoned crystals with a core of augite and a rim of diopside are found in the quartz -syenite.…”

Melilite-group minerals at Oldoinyo Lengai, Tanzania

“…The T1-site is mainly occupied by Al, Fe 2+ , and Mg, the X-site by Ca and Na. The estimation of Fe 2+ and Fe 3+ concentration was based on the procedure of Droop (1987). Melilite data are plotted in terms of the components Ca 2 Mg(Si 2 O 7 ), Ca 2 Fe(Si 2 O 7 ), and the sum of (CaNa)Al (Si 2 O 7 ) and (CaNa)Fe 3+ (Si 2 O 7 ) (Fig.…”