HIGH-PRESSURE DEFORMATION OF ANDESINE FELDSPAR: A COMPARISON OF STATIC AND DYNAMIC EXPERIMENTS

Under high-pressure conditions, feldspar minerals transform to an amorphous phase as solid-state glass. This is seen in terrestrial impact materials, highly shocked meteorites, and shocked Apollo lunar samples. Despite many years of study, the specific details of how this transformation occurs (e.g., P-T-t conditions and specific deformation mechanism) remain unclear. Here we present initial results of a micro-Raman spectroscopic comparison of shock experiments and Diamond Anvil Cell (DAC) static compression experiments on andesine feldspar.

Both shock and DAC experiments show a similar pattern of structural deformation with increasing pressure. This manifests itself as a decrease in intensity of characteristic vibrational modes, specifically a decrease in intensity of the cation mode at 479 cm^-1, merging of modes at 479 and 507 cm^-1, and eventual loss of all modes except the Si-O vibrational mode near 492 and 583 cm^-1.

Although the progression of deformation is similar in DAC and shock experiments, there are three key differences:

1) Pressures at which andesine transforms to a fully amorphous phase occur between 12 and 13 GPa in DAC experiments compared to between 28 and 29 GPa during shock experiments.

This may be explained by effects of shock experiments, which used single shock waves to achieve peak pressures and may estimate the pressures required to deform crystal structures differently than static pressures from DAC experiments.

2) Positions of vibrational modes increase with pressure during DAC experiments (at least up to the currently available 18 GPa) but not in shock experiments. This likely reflects measuring samples in the DAC while under compression whereas shock experiments were measured post-shock under ambient conditions.

3) Structural ordering returns upon decompression during DAC experiments, compared to shocked samples, which retain their disordered nature permanently after shock experiments. This remains unclear in detail but likely reflects the kinetic effects of the shock event.

Ongoing work will expand the DAC pressures to 60 GPa for a more complete comparison to experimentally shocked sample pressures. We will also compare experimentally shocked and statically compressed bytownite and albite feldspars to quantify the effects of composition on observed variations in Raman spectra.