Discovery of tetragonal almandine, (Fe,Mg,Ca,Na)_3(Al,Si,Mg)_2Si_3O_(12), a new high-pressure mineral in Shergotty

Abstract

Introduction: During a nanomineralogy investigation of the Shergotty meteorite, we have identified a new shock-induced high-pressure silicate, majoritic almandine with a tetragonal I4_1/ɑ structure, in an impact melt pocket. Field-emission scanning electron microscope, electron back-scatter diffraction, electron microprobe and synchrotron diffraction were used to characterize its chemical composition, structure, and associated phases. Tetragonal majorite (Mg_3(SiMg)Si_3O_(12)) with the I4_1/ɑ structure was synthesized [e.g., 1] and found later in the Tenham chondrite [2,3]. Tetragonal almandine-pyrope phase (TAPP) with the I-42d structure was reported to occur as inclusions in lowermantle diamonds [4]. We present here the first occurrence of tetragonal almandine with the I41/a structure in a shocked meteorite from Mars as a new high-pressure phase.