Liebermannite, KAlSi3O8-Hollandite, a New High-Pressure Mineral Formed By Impact on Mars: An Integrated SEM-EPMA-Synchrotron Diffraction Investigation

Abstract:

The combination of SEM-EBSD-EDS, EPMA, and synchrotron micro-diffraction mapping is developing into a powerful tool for the characterization of micron-scale phases in rocks, including shock metamorphic phase assemblages in meteorites. During a nanomineralogy investigation of the Zagami meteorite, which is a heavily shocked Martian basaltic shergottite that fell at Zagami, Nigeria in 1962, we identified the new shock-induced high-pressure mineral liebermannite (KAlSi₃O₈ with a I4/m hollandite-type structure). The Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association approved both the mineral and its name. The name honors Robert C. Liebermann, a geophysicist at Stony Brook University, for his fundamental contributions to high-pressure mineral physics research. Liebermannite is the K-analog of lingunite (NaAlSi₃O₈ with a I4/m hollandite-type structure). It has an empirical formula for the type material of (K_0.76Na_0.14Ca_0.02)Al_1.03Si_3.00O₈, based on 8 O atoms pfu, and it occurs as aggregates of crystals up to 15 microns across. The Zagami liebermannite contacts one or more of augite (phenocrysts), plagioclase-composition glass, a silica phase (likely stishovite), ilmenite, baddeleyite, tuite, and merrillite. In one occurrence, lingunite is also present and presents sharp contacts with the liebermannite. Liebermannite formed in Zagami during a shock event that transformed a precursor alkali-feldspar to a high-pressure form. Liebermannite is stable between ~10 and 20-30 GPa; it melts congruently at high temperature and transforms to an I2/m phase at higher pressures. Liebermannite is, therefore, a potentially important indicator of shock conditions in Martian meteorites. Liebermanite may also occur in subducted sediments within the Earth’s upper mantle and may, therefore, be an important factor in the thermal evolution of the Earth in its role as a repository of ⁴⁰K.