High-Pressure Crystal Chemistry of Norbergite

Humite intergrowths in olivine have been found in several important ultrahigh pressure (UHP) metamorphic terranes, and have been considered, according to some models, responsible for ilmenite exsolution during metamorphism. Norbergite, the simplest member of the humite group, possesses structural features found in the more complicated members of the series. In this work, we sought to uncover any possible structural discontinuity in the compression of norbergite that may assist in the exsolution process at depth in the higher members of the humite group. The primary goal was to observe the response of the polyhedral network as a whole during compression. 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%. Norbergiteunit-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 that this is the result of a discontinuous decrease in free volume between the polyhedral frameworks. The similarities in structural features between norbergite and the more complex humites further suggests that they will respond similarly to compression. 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 the 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 natural 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, indicating the existence of a phase transition in norbergite at these pressures. Kuribyashi, T., et al. (2008) Synchrotron x-ray analysis of the norbergite structure at high pressure up to 8.2 GPa. Physics and Chemistry of Minerals, 35: 559-568.