RAMAN SPECTROSCOPY AND 2D IMAGING OF ULTRA HIGH-PRESSURE SIO2 POLYMORPHS

Hypervelocity impact events occur when an asteroid strikes Earth’ surface at solar system velocities and generates a shockwave. These extremely energetic are capable of inducing phase transformations in target rocks. In these impactites we can see high-pressure polymorphs otherwise not found on the Earth’s surface. In the SiO₂ system the commonly found quartz can transform into many polymorphs such as coesite, stishovite, and diaplectic glass. This transformation to coesite occurs at temperatures of approximately >700º C and pressures between 1-2.5 GPa. The formation of coesite is not exclusive to hypervelocity events and can occur in subduction zones and at depths of approximately 30-40 km beneath the surface, but is limited to very small grains usually as inclusions within garnets. Despite these polymorphs being chemically the same their inherit structures all vary greatly thus producing very different Raman spectra. Crystalline quartz exhibits a dominate peak at 464 Dcm^-1 with smaller peaks at 264 Dcm^-1 and 128 Dcm^-1. Crystalline coesite shows a dominant peak at 521 Dcm^-1 with smaller ones at 113, 173, 267, and 429 Dcm^-1. The diaplectic SiO₂ glass is characterized by a broad peak near 449 Dcm^-1 and a substantial drop-off in intensity at 494 Dcm^-1. Using Raman spectroscopy a variety of SiO₂ polymorphs have been observed at the Tenoumer Impact Crater, Mauritania: crystalline quartz, diaplectic quartz glass, and most notably coesite. The crystalline quartz and coesite aggregates occur inside the diaplectic quartz glass. The crystalline quartz, however, bears unusual optical properties that closely resemble coesite suggesting it could be a pseudomorph, having reverted from the higher-pressure phase. 2D Raman imaging is therefore a incredibly powerful tool for investigating these impactites.