Shock experiments on pre-heated α- and β-quartz: I. Optical and density data

Abstract

Discs of single crystal quartz, unheated, and pre-heated to 275°C and 540°C (i.e., α-quartz) and 630°C (i.e., β-quartz) were experimentally shocked to pressures ranging from 20 to 40 GPa, with the shock front propagating parallel to either (10 1 0) or (0001). Refractive indices, density and the orientation of planar deformation features (PDFs) were determined on the recovered quartz samples. Refractive indices of pre-heated quartz are unaffected up to 25 GPa but density starts to decrease slightly up to this pressure. Above 25 GPa, pre-heating causes drastic variations: Refractive indices and birefringence of quartz shocked at ambient temperature decrease continuously, until complete isotropization is reached at 35 GPa. In quartz shocked at 630°C, refractivity drops discontinuously in the interval from 25 to 26 GPa, and complete transformation to diaplectic glass is reached at 26 GPa. Density follows the trends demonstrated by the optical parameters, with higher pre-shock temperatures yielding lower density at a given shock pressure. These results indicate that the threshold pressure for the onset of transformation to diaplectic quartz glass is largely temperature-invariant, lying at 25 GPa, whereas the pressure limit for complete transformation decreases with increasing pre-shock temperature from ≈ 35 to ≈ 26 GPa. Quartz shocked parallel to (0001) always has a higher density and refractivity than that shocked parallel to (10 1 0), indicating a significant influence of the structural anisotropy. This is also evident from the distribution of PDF orientations. Pressures ⩾ 25 GPa cause, in quartz shocked parallel to (10 1 0), PDFs that are predominantly oriented parallel to {10 1 2}, while quartz shocked to the same pressures but parallel to (0001) contains almost exclusively PDFs parallel to {10 1 3}. PDF orientations in quartz shocked at ambient temperature parallel to (10 1 0) show the following characteristics: (i) The frequency of {10 1 3} is high at 20 GPa, declines with increasing pressure, and finally is totally absent at 32 GPa, (ii) {10 1 3} begins to occur at 25 GPa, increases, and finally is the only remaining orientation at 32 GPa, and (iii) {10 1 2} and {11 2 2} are only present at 20 and 25 GPa. In contrast, quartz shocked at 630°C parallel to (10 1 0) displays a broad PDF distribution with indistinct maxima, and {10 1 3} is totally absent. The small structural difference between α- and β-quartz, which is reflected in the PDF distribution, should allow, in principle, evaluation of the pre-shock temperature in naturally impacted metamorphic rocks. The results substantiate that formation of shock effects in quartz is dependent on (i) shock pressure, (ii) pre-shock temperature and (iii) shock wave direction. So far, the latter two parameters have not been considered in the application of experimental results to impact sites.