Structure analysis and conditions of formation of akimotoite in the Tenham chondrite

1O.Tschauner,2C. Ma,3,4C. Prescher,3V. B. Prakapenka
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13009]
1Department of Geoscience, University of Nevada, Las Vegas, Nevada, USA
2Division of Geology and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
3Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois, USA
4Institut für Geologie und Mineralogie, Universität zu Köln, Köln, Germany
Published by arrangement with John Wiley & Sons

Akimotoite (Mg,Fe)SiO3 is one of the most common mineralogical indicators for high-level shock metamorphism in meteorites. First described 1997, its occurrence has been amply confirmed in a number of highly shocked chondrites. Yet, a thorough structure analysis of natural akimotoite has remained extant. Here we report accurate cell parameters, fractional atomic coordinates, and site occupancies for natural akimotoite from the holotype specimen based on synchrotron microdiffraction. The variation of unit cell shape and volume with Fe content define mixing volumes. Based on the mixing volume relation for akimotoite and hemleyite, we constrain the unit cell volume of endmember hemleyite to 273.8 ± 1.0 Å3. We show that mixing is nearly ideal for low Fe content but evolves to positive excess volume toward the Fe endmember. Based on this finding and the actual composition of akimotoite in Tenham, we show that this mineral has formed by solid–solid transformation prograde from enstatite, not by crystallization from melt.

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