We investigated the compositions 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 crystal-chemical controls on the distribution of Fe3+ and on mineral stoichiometry with regard to 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 which have higher concentrations of Fe3+ on the octahedrally coordinated site than is found for 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+/ΣFe)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 spinels with greater Cr and Fe are sufficiently different in their crystal chemistry from the aluminous spinels to indicate that the EPMA correction procedures developed for Fe3+ in aluminous spinels on the basis of the Cr/Al ratio, and used in oxy-thermobarometry, are inappropriate for Cr-rich and Fe-rich compositions.

我们研究了来自尖晶石橄榄岩、蛇绿地幔铬铁矿和层状火成岩的一组 361 颗含铬尖晶石的成分，其中 Fe 2+ /Fe 3+比率已通过穆斯堡尔光谱法测定。我们探索了晶体化学对 Fe 3+分布和矿物化学计量的控制，以及用于估计尖晶石中Fe 3+的电子探针校正程序。我们发现含铬尖晶石可分为三类：富 Cr-Al 类；高铁基团；和高度氧化的基团。Cr-Al 族尖晶石存在于尖晶石橄榄岩和蛇绿地幔铬铁矿中。它们是正常的尖晶石，Fe 3+含量低且成分接近化学计量。高铁族尖晶石存在于层状火成岩侵入体中。它们也是普通尖晶石，其八面体配位位点上的 Fe 3+浓度高于Cr-Al 族尖晶石。化学计量计算往往会高估 Fe 3+含量，并且它们的成分不符合 Cr-Al 族中发现的 MgO-cr# 相关性。高度氧化的尖晶石存在于层状侵入体和蛇绿地幔铬铁矿中。它们具有 (Fe 3+ /ΣFe) Möss > 0.4 和高 Cr (cr# > 0.5)，但 Fe 2+相对较低(fe# 0.24–0.56)。化学计量计算往往会低估Fe 3+含量。它们代表正常尖晶石，其中四面体配位的 Fe 2+已被氧化为 Fe 3+。

我们的数据表明，具有较高 Cr 和 Fe 的尖晶石在晶体化学上与铝尖晶石有很大不同，这表明 EPMA 校正程序是根据 Cr/Al 比率针对铝尖晶石中的 Fe 3+ 开发的，并用于氧- 热压分析法不适用于富含 Cr 和富含 Fe 的成分。