Cation distribution and cooling rates of Cr-substituted Mg-Al spinel from the Olkhon metamorphic complex, Russia

Abstract

A crystal chemical study was carried out on seven Mg-Al rich and Cr-bearing spinels from a calciphyre of the Olkhon metamorphic complex (western Lake Baikal, Russia) in order to: a) verify the structural effects related to the progressive substitution of Cr 3+ , the main substituent cation in this suite pertaining to the MgAl 2 O 4 - MgCr 2 O 4 binary join; b) estimate the closure temperature of the exchange reaction involving Mg and Al in tetrahedral (T) and octahedral (M) sites. This will be a first step towards evaluation of the cooling rate of the host rock. Chemical composition is Mg (Al 2-p , Cr p ) O 4 with 0.03 ≤ p ≤ 0.23 afu. With increasing p, the cell edge a increases from 8.090 to 8.117 A, whereas the oxygen positional parameter u is almost constant. As u is related to the (M-O)/(T-O) ratio, Cr substituting for Al in the M site causes an increase not only of the M-O bond distance, but also that of the tetrahedral T-O. The observed T-O increase is due to Mg ordering into the T site, with increasing Cr content. Cation distributions where used to estimate the closure temperature of the exchange reaction applying two available models. Both estimates are rather consistent for very low Cr content, but substantial disagreement is observed for Cr-rich samples. The inversion trend of the studied spinel suite extrapolated to Cr= 0, enables us to calculate the closure temperature of the MgAl 2 O 4 end-member with the same cooling rate. This suggests that kinetic results from cooling experiments on MgAl 2 O 4 may be applied to cation distributions measured in samples along the MgAl 2 O 4 - MgCr 2 O 4 binary join.