Ferric iron in chrome-bearing spinels: implications for microprobe correction procedures

[Marco E. Ciriotti]

Prossima pubblicazione.

Referenza:
▪ Rollinson, H. & Adetunji, J. (2023): Ferric iron in chrome-bearing spinels: implications for microprobe correction procedures. Mineralogical Magazine, 87, (in press).

Abstract:
We investigated the mineral chemistry of a suite of 361 chrome-bearing spinels from spinel peridotites, ophiolitic mantle chromitites and from layered igneous intrusions in which the Fe2+ / Fe3+ ratio has been determined by Mössbauer spectroscopy. We explore the crystalchemical controls on the distribution of Fe3+ and on mineral stoichiometry in the light of electron-probe correction procedures for estimating Fe3+ in spinel. We find that chrome-bearing spinels can be subdivided into three groups – a Cr-Al rich group, a high-Fe group and a highly oxidised group. Spinels of the Cr-Al group are found in spinel peridotites and ophiolitic mantle chromitites. They are normal spinels with a low Fe3+ content and compositions that are close to stoichiometric. Spinels in the high-Fe group are found in layered igneous intrusions. They are also normal spinels but have higher concentrations of Fe3+ on the octahedrally coordinated site than is found in the Cr-Al group. Stoichiometric calculations tend to overestimate the Fe3+ content and their compositions do not conform to the MgO-cr# correlation found in the Cr-Al group. Spinels in the highly oxidised group are found in layered intrusions and ophiolitic mantle chromitites. They have (Fe3+/Fe)Möss > 0.4 and high Cr (cr# > 0.5) but relatively low Fe2+ (fe# 0.24-0.56). Stoichiometric calculations tend to underestimate the Fe3+ content. They represent normal spinels in which tetrahedrally coordinated Fe2+ has been oxidised to Fe3+. Our data show that the more Cr- and Fe-rich spinels are sufficiently different in their crystal chemistry from the more aluminous spinels to indicate that the EPMA correction procedures developed for Fe3+ in aluminous spinels based upon the Cr/Al ratio, and used in oxythermobarometry are inappropriate for more Cr-rich and Fe-rich compositions.

[Marco E. Ciriotti]

Pubblicazione effettuata.

Referenza:
▪ Rollinson, H. & Adetunji, J. (2023): Ferric iron in chrome-bearing spinels: implications for microprobe correction procedures. Mineralogical Magazine, 87, 702-710.