G. Artioli et al., HIGH-TEMPERATURE FE-MG CATION PARTITIONING IN OLIVINE - IN-SITU SINGLE-CRYSTAL NEUTRON-DIFFRACTION STUDY, The American mineralogist, 80(1-2), 1995, pp. 197-200
The intracrystalline partitioning of Fe and Mg in the octahedral sites
of olivine is known to be dependent on temperature, pressure, and com
position. Interpretations of the temperature effect on the partitionin
g have been mostly based on heating and quenching experiments, which s
eem to indicate that Fe2+ preferentially orders into the M1 site with
increasing temperature. The present single-crystal neutron diffraction
experiments yield the first in-situ high-temperature structure refine
ments above 900 degrees C and clearly indicate an ordering reversal ab
ove this temperature. Three data sets collected at 960, 1030, and 1060
degrees C show a remarkable progressive decrease in the K-D parameter
with temperature, whereas the data at 880 degrees C are consistent wi
th a slight preference of Fe2+ for M1, as reported in the literature.
The effect is tentatively interpreted on the basis of competing contri
butions of configurational and vibrational entropy at high temperature
, and it is expected to have profound implications for the thermodynam
ic modeling of olivine in the Earth's mantle and in planetary processe
s.