Cation distribution and cooling rates of Cr-substituted Mg-Al spinel from the Olkhon metamorphic complex, Russia
Cation distribution and cooling rates of Cr-substituted Mg-Al spinel from the Olkhon metamorphic complex, Russia (in Special issue dedicated to the memory of Luciano Ungaretti, Maria Chiara Domeneghetti (prefacer))
European Journal of Mineralogy (April 2003) 15 (2): 435-441
A crystal chemical study is reported on seven Mg-Al-rich and Cr-bearing spinels from a calciphyre of the Olkhon metamorphic complex, W Lake Baikal, Russia. Their chemical composition is Mg(Al (sub 2-p) Cr (sub p) )O (sub 4) with 0.03 < or = p < or = 0.23 pfu. With increasing P the cell edge a increases from 8.090 to 8.117 Aa, 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 in the M-O distance but also in the tetrahedral T-O distance. The inversion trend of this spinel suite extrapolated to Cr = 0 allows calculation of the closure T of the MgAl (sub 2) O (sub 4) end-member with the same cooling rate, suggesting that kinetic results from cooling experiments on MgAl (sub 2) O (sub 4) may be applied to cation distribution in samples along the MgAl (sub 2) O (sub 4) -MgCr (sub 2) O (sub 4) binary join.
