Abstract

Electron microprobe analysis of chromite in seven “equilibrated” chondrites shows that, within a given meteorite, chromite grains are remarkably similar in composition; no zoning occurs. From meteorite to meteorite, however, the mineral has the narrow compositional range (weight percents) Cr2O3 54.6–57.0, A12O3 5.2–6.3, TiO2 2.08–2.98, FeO 30.5–33.9, MgO 1.54–2.48, MnO 0.65–1.05, V2O3 0.65–0.77, which corresponds to the average structural formula (Fe6.93Mg0.87 Mn0.20)(Cr12.64AI1.90 V0.16 Fe0.74 Ti0.61)O32. These chondritic chromites are higher in iron than most terrestrial examples, and approach the theoretical composition (FeCr2O4) of chromite. Titanium contents are also higher than the average for terrestrial chromite, and several considerations indicate that the element is in the structure of the mineral and not present as rutile or ilmenite inclusions. Aluminum is, on the other hand, deficient compared to most terrestrial chromites, and this is probably due to presence of plagioclase in chondrites, most of the aluminum being taken up by the feldspar.

Although the range in composition of chromite in the chondrites studied is slight, amounts of oxides present in the mineral do appear to show a relation to the chemistry of the meteorites: chromite from the H-group chondrites tends to be higher in chromium and aluminum than L- or LL-group chromites; and chromite in H-group chondrites is lower in titanium than L- or LL group chromites. Iron and magnesium show, respectively, an increase and a decrease in amount as one passes from the H- to L- to LL-group chromites. Because this iron and magnesium variation has the same trend as found in the olivines and pyroxenes in H-, L-, and LL-group chrondites, it is suggested that, in the examples studied, chromite is in equilibrium with the ferromagnesian silicates, a situation which may be due to (1) equilibration during contemporaneous crystallization of the iron-magnesium silicates and the chromite or (2) equilibration during contemporaneous metamorphism of silicates and chromite.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.