¹Chi Ma,²Oliver Tschauner,³John G. Spray,⁴Zhongxu Pan
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70129]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
²Department of Geoscience, University of Nevada, Las Vegas, Nevada, USA
³Planetary and Space Science Centre, University of New Brunswick, Fredericton, New Brunswick, Canada
⁴School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui, China
Published by arrangement with John Wiley & Sons

We report a previously unknown aluminosilicate mineral, (Si0.91□0.09)Σ1.00(Al1.46□0.54)Σ2.00O4 with a vacancy-stabilized spinel-type structure (henceforth “SiAl-spinel”). This novel aluminosilicate occurs with coesite, stishovite, and majoritic garnet in a shock melt vein in metaquartzite from the outer collar of the Vredefort Dome, the eroded central uplift of the Vredefort impact structure of South Africa. Formation conditions for this new high-pressure, high-temperature phase are around 10 GPa and 1400°C, upon release from peak shock conditions. Based on its composition and formation conditions, this new high-pressure, high-temperature phase is predicted to be a common occurrence in terrestrial impactites and in subducted slabs.

^1,2Lidia Pittarello,³Valeria De Santis,³Laura Carone,⁴Giovanni Pratesi,⁵Mauro Gemmi,⁵Paola Parlanti,⁶Andreas Steiger-Thirsfeld,⁷Alessandro Di Michele,³Gabriele Giuli
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70134]
¹Naturhistorisches Museum, Mineralogisch-Petrographische Abteilung, Vienna, Austria
²Department of Lithospheric Research, University of Vienna, Vienna, Austria
³Geology Divison, School of Science and Technology, University of Camerino, Camerino, Italy
⁴Department of Earth Sciences, University of Firenze, Firenze, Italy
⁵Electron Crystallography, Istituto Italiano di Tecnologia, Pontedera, Italy
⁶Technische Universit€at Wien, University Service Center for Transmission Electron Microscopy, Vienna, Austria
⁷Department of Physics and Geology, University of Perugia, Perugia, Italy
Published by arrangement with John Wiley & Sons

The occurrence of ringwoodite in shocked L6 ordinary chondrites has been frequently reported, mostly within shock veins. Only recently, ringwoodite has also been found in a fragment from the Alfianello meteorite, occurring as rim or core of olivine clasts in impact melt pockets, in lamellae crosscutting olivine grains in the host rock, and in fine-grained aggregates in association with wadsleyite. In all cases, ringwoodite shows a higher Fe/Mg ratio than the original olivine, whereas wadsleyite shows a lower Fe/Mg ratio than the original olivine. Detailed TEM studies of the occurrence of both high-pressure polymorphs allow the identification of the most likely formation process, explaining the coexistence of these polymorphs. The lack of any crystallographic relationships but the complementary Fe/Mg ratios supports formation of the assemblage through fractional crystallization from impact melt under high (shock) pressure conditions. However, the other occurrences of ringwoodite reported in the same sample, traditionally interpreted as resulting from solid-state transformation, emphasize the heterogeneity of distribution of shock effects and shock-induced processes recorded in a single meteorite.