Tissintite-II in lunar meteorite Northwest Africa 13967: Implications for the high pressure/temperature mineralogy of the lunar regolith

Abstract

We studied lunar regolith breccia meteorite Northwest Africa (NWA) 13967 to explore its mineral and clast inventory with special focus on the ubiquitous occurrence of tissintite-II, a newly recognized, vacancy-rich high-pressure clinopyroxene with a feldspathic, Fe- and Mg-enriched composition. Lithic clasts in NWA 13967 indicate a provenance in the Feldspathic Highlands Terrane on the Moon. Most abundant are cumulate impact melt clasts (“poikilitic granulitic breccias”), granular impact melt rocks, vitric impact melt clasts including impact spherules, and anorthositic clasts, while basalt clasts are rare. The breccia groundmass is mostly fused to flow-textured, vesicular, crystallized impact melt that includes 1 μm corundum crystals and up to 5 μm tissintite-II near the contact with lithic clasts. Rare coesite occurs in moganite clasts entrained in the shock-melted groundmass and rimmed by tissintite-II. Petrographic features of NWA 13967 and its bulk rock chemical composition are most similar to the NWA 8046 clan of lunar meteorites, the largest known lunar meteorite. We discuss mineralogical and petrological characteristics of NWA 13967 to unravel chemical and structural changes of the lunar regolith during shock lithification, which may inform the ongoing exploration of the lunar surface.