HIGH PRESSURE CRYSTAL CHEMISTRY OF NORBERGITE
In an effort to extend the analysis of Kuribyashi, et al. (2008), the structure of a natural norbergite crystal was examined using single crystal x-ray diffraction analysis up to a pressure of 8.5 GPa. Pressure was increased from ambient to 2.5, 4.5, 6.5 and 8.5 GPa yielding R-values of 3.7, 6.5, 3.8, and 5.9%. Norbergite unit-cell parameters were refined by least-squares methods using 15-20 reflections. Between 4.5 and 6.5 GPa it was found that the edge-sharing octahedral chains, the principal feature of of the norbergite structure, buckle and dramatically distort. Pressure-volume data for the mineral also suggests a change in the compression mechanism around these pressures. Analysis of the structure with increasing pressure suggests the result of a discontinuous decrease in free volume. If this structural discontinuity plays a role in ilmenite exsolution, this result would be consistent with previous pressure estimates for UHPM around 3-4GPa.
Modeling of the energetics of norbergite compression using a simple electrostatic potential model reveal an instability in the norbergite structure at around 18 GPa. To further investigate this possibility, powder diffraction data was collected on the same norbergite sample at station 16-ID-B (HPCAT) at the Advanced Photon Source, Argonne National Laboratory. Diffraction patterns were collected at 5.6, 14.8, 18.3, and 20.0 GPa. Significant changes in the diffraction pattern are evident between 15 and 20 GPa, indicating the existence of a phase transition in norbergite at these pressures.