Donwilhelmsite, [CaAl4Si2O11], a new lunar high-pressure Ca-Al-silicate with relevance for subducted terrestrial sediments

We report on the occurrence of a new high-pressure Ca-Al-silicate in localized shock melt pockets found in the feldspatic lunar meteorite Oued Awlitis 001 and discuss the implications of our discovery. The new mineral crystallized as tiny, micrometer sized, acicular grains in shock-melt pockets of roughly anorthitic bulk composition. Transmission electron microscopy based 3 dimension electron diffraction (3D ED) revealed that the CaAl4Si2O11 crystals are identical to the
calcium aluminum silicate, CAS, phase first reported from static pressure experiments. The new mineral has a hexagonal structure, with a space group of P63/mmc and lattice parameters of a = 5.42(1) Å; c = 12.70(3) Å; V = 323(4) Å3; Z = 2. This is the first time 3D ED was applied to structure determination of an extraterrestrial mineral. The International Mineralogical Association has approved this naturally formed CAS phase as the new mineral “donwilhelmsite” [CaAl4Si2O11], honoring the lunar US geologist Don E. Wilhelms. On the Moon, donwilhelmsite can form from the primordial feldspathic crust during impact cratering events. In the feldspatic lunar meteorite Oued Awlitis 001 needles of donwilhelmsite crystallized in ~200 μm in size shock melt pockets of anorthositic-like chemical composition. These melt pockets quenched within milliseconds during declining shock pressures. Shock melt pockets in meteorites serve as natural crucibles mimicking the conditions expected in the Earth’ mantle. Donwilhelmsite forms in the Earths’ mantle during deep recycling of aluminous crustal materials, where it is a key host for Al and Ca of subducted
sediments in most of the transition zone and in the uppermost lower mantle (460-700 km). Donwilhelmsite bridges the mineralogical gap between kyanite and the Ca-component of clinopyroxene at low pressures and the Al-rich Ca-ferrite phase and Ca-perovskite at high pressures. In ascending buoyant mantle plumes, at about 460 km depth, donwilhelmsite is expectedto break down into minerals such as garnet, kyanite, and clinopyroxene. This may trigger minor
3 partial melting releasing a range of incompatible minor and trace elements contributing to the enriched mantle (EM1 and EM2) components associated with subducted sedimentary lithologies.