X-RAY DIFFRACTION CHARACTERISTICS OF VARIABLY SHOCKED, STRAINED, AND MICROCRYSTALLINE QUARTZ IN MIXTURES: IMPLICATIONS FOR IMPACT PETROLOGY AND CONSTRUCTION AGGREGATES

We have developed novel procedures to interpret the shock state of quartz using powder X-ray diffractometry. Quartz samples from confirmed impact craters and ejecta deposits were ground to -40 μm in a ZrO₂ mortar, and diffraction spectra were collected with a Bruker D8 Advance. Rietveld-refined profiles were used to calculate mean coherent scattering domain (CSD) lengths for each indexed crystallographic plane.

We have found that plotting 1/domain length vs. 2θ° is a convenient way to display our results. Unshocked samples have larger CSDs and smaller differences in size between planes. Power law curves fit to plots for these materials approximate expected instrumental line-broadening, with CSD size decreasing with increasing 2θ°. Conversely, power law fits for shocked samples show a systematic decrease in CSD size with increasing petrographic shock level. For monomineralic samples collected or separated from known shocked rocks and sediments, our procedures seem to provide a fast and accurate companion to traditional petrographic methods of shock barometry. Shocked quartz data also show large variations in CSD sizes between crystallographic planes. Disordering associated with rhombohedral forms lags behind other planes producing patterns distinct from quartz containing tectonic strain.

Quartz that has experienced directional, high-strain-rate shear, along with poorly crystalline materials such as opaline chert and metamict samples, have provided a challenge to our ultimate goal of having a diagnostic tool for confirming and studying impact ejecta in mixed sediments. To address this problem we are developing a database of natural and experimental mixtures. Our effort is aided by access to several large homogeneously shocked pegmatitic quartz crystals from the Wetumpka crater. Quartz from different shock levels can be mixed with varying concentrations of endogenically deformed material, variably crystalline silica, and other phases. Thus far, our results suggest that we can determine contributions from different types of quartz, and our accuracy should improve as the database grows. The crystallinity of silica phases in construction aggregates also is an important predictor of deleterious reactivity in concrete; this database is providing useful data for aggregate quality control evaluations.