RAMAN SPECTROSCOPY OF SHOCKED SILICATE FROM THE WETUMPKA IMPACT STRUCTURE: SEARCHING FOR MASKELYNITE AND COESITE

Meteorite impact structures exhibit various petrological features characteristic of shock metamorphism. These include planar deformation features (PDFs) in quartz (SiO₂) - characterized as closely spaced, parallel sets of crystallographically oriented amorphous planes within the mineral (~10–25 GPa), transformation of crystalline minerals into distinct high-pressure phases (~20–60 GPa, depending on target composition); and even complete amorphization of crystalline minerals at high shock pressures (~45–60 GPa). Feather features (PFs) are planar microstructures that, although similar in appearance to PDFs, form at lower shock pressures (~7–10 GPa) and are described as shear-induced, parallel-to-subparallel lamellae within quartz. The Wetumpka structure, ~5 km in diameter, located in Elmore County, Alabama, is proposed to have been formed by a shallow-marine meteorite impact during the Late Cretaceous. While PDFs and feather features have been confirmed in shocked quartz from drill-core samples, no high-pressure polymorph has yet been identified within the crater. This study focuses on the analysis of a quartz-feldspar augen extracted from the crater’s metamorphic basement. Our ongoing work involves searching for the presence of maskelynite - a diaplectic glass resulting from the amorphization of plagioclase [(Na,Ca)(Si,Al)₄O₈]. Coesite, a high-pressure phase of quartz, is also a target for the analysis. These minerals have been associated with other impact structures and serve as strong evidence of meteorite impact, given that they are typically stable only under the high static pressures of Earth's lower crust or mantle. Thin sections of the augen are prepared and analyzed using Raman spectroscopy (532 nm), targeting diagnostic spectral peaks of coesite (~520, ~465, and ~180 cm^-1) and maskelynite (~509, ~485, and ~1030 cm^-1). Collecting additional data regarding the presence of either mineral within the structure would offer valuable insight into the peak shock pressures generated by the impact, as well as refine our understanding of the distribution of shock levels within the crater.