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

1,2Jörg Fritz,3Ansgar Greshake,4Mariana Klementova,5Richard Wirth,4Lukas Palatinus,6Reidar G. Trønnes,3,7,8Vera Assis Fernandes,9Ute Böttger,10Ludovic Ferrière
American Mineralogist 105, 1704–1711 Link to Article [http://www.minsocam.org/msa/ammin/toc/2020/Abstracts/AM105P1704.pdf]
1Zentrum für Rieskrater und Impaktforschung, Nördlingen, Vordere Gerbergasse 3, D-86720 Nördlingen,
Germany. ORCID 0000-0002-6333-4775
2Saalbau Weltraum Projekt, Liebigstraße 6, D-64646 Heppenheim, Germany
3Museum für Naturkunde Berlin, Invalidenstrasse 43, D-10115 Berlin, Germany. ORCID 0000-0001-6475-9751
4Institute of Physics of the Czech Academy of Science, v.v.i., Na Slovance 2, 182 21 Prague, Czech Republic. † ORCID 0000-0002-8987-8164
5Helmholtz-Zentrum Potsdam–Deutsches GeoForschungsZentrum, Sektion 3.5 Grenzflächen-Geochemie, Telegrafenberg, D-14473 Potsdam, Germany
6Natural History Museum and Centre for Earth Evolution and Dynamics (CEED), University of Oslo, N-0315 Oslo,
Norway. ORCID 0000-0002-4458-5624
7Department of Earth and Environmental Sciences, University of Manchester, Williamson Building, Oxford Road, M13 9PL Manchester, U.K.
ORCID 0000-0003-0848-9229
8Instituto Dom Luiz (IDL), Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016, Lisboa, Portugal
9Institut für Optische Sensorsysteme, Deutsches Zentrum für Luft und Raumfahrt Berlin, Rutherfordstrasse 2, D-12489 Berlin, Germany
10Natural History Museum, Burgring 7, A-1010 Vienna, Austria. ORCID 0000-0002-9082-6230
Copyright: The Mineralogical Society of America

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 three-dimensional electron diffraction (3D ED) reveals 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 (IMA) has approved this naturally formed CAS phase as the new mineral “donwilhelmsite” [CaAl4Si2O11], honoring the U.S. lunar 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 mm sized 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’s mantle. Donwilhelmsite forms in the Earth’s mantle during deep recycling of aluminous crustal materials, and is a key host for Al and Ca of subducted sediments in most of the transition zone and the uppermost lower mantle (460–700 km). Donwilhelmsite bridges the 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 expected to break down into minerals such as garnet, kyanite, and clinopyroxene. This process may trigger minor partial melting, releasing
a range of incompatible minor and trace elements and contributing to the enriched mantle (EM1 and EM2) components associated with subducted sedimentary lithologies.

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