Chromium influence on Mg-Al intracrystalline exchange in spinels and geothermometric implications

Flux-grown spinel crystals belonging to the MgAl2O4-MgCr2O4 spinel series were investigated to reveal the effects of Cr substituting for Al on cation distribution and their influence on Mg-Al intra-crystalline exchange. Samples were structurally and chemically characterized by single-crystal X-ray diffraction and electron microprobe, and cation distribution was obtained with a tested optimization model for site populations. The results evidenced that the contribution of the tetrahedral bond distance to the unit-cell parameter is smaller than that of the octahedral bond distance, which is driven by the substitution of Cr for Al. Moreover, the influence that Cr exerts on Mg-Al order-disorder intersite exchange is non-linear along the whole series.