High-temperature cystal chemistry of hortonolite

The crystal structure of a hortonolite (Fa₃₁) from lunar rock 12052 has been refined from intensity data collected at 24°,375° and 710°C. Significant ordering of Fe⁺² onM(l)was detected [K_D = 1.19(3)] and did not change over the temperature range studied. Mean M–O distances and octahedral volumes increase linearly from 24°–710°C whereas mean tetrahedral distances and volumes show no significant changes. Octahedral distortions increase differentially as a function of temperature with M(l) distortion increasing at a faster rate than M(2) distortion. A consideration of these distortions over the temperature range studied helps in understanding recent Mössbauer spectra of olivines as well as the observed relative enrichment of Fe²⁺ on the M(1) site in some olivines.

Structural refinements of another lunar olivine (Fa₁₈) from rock 12018 and a terrestrial metamorphic olivine (Fa₂₉) show them to be significantly ordered [K_D = l.l5(3)] and disordered [K_D = 1.02(4)], respectively. However, the X-ray site populations for 12018 olivine disclose considerably less ordering [K_D = 1.15(3)] than that [K_D = 1.75] determined by Virgo and Hafner(1972) using Mössbauer spectroscopy on a more Fe-rich olivine also from lunar rock 12018. This suggeststhat the earlier-crystallizing Mg-rich olivines are less ordered than the later-crystallizing Fe-rich olivines.