Thermal and shock history of Almahata Sitta meteorite inferred from structure refinement of pyroxene and Mössbauer spectroscopy of Fe-Ni metal
Yassir A. Abdu, Abbasher M. Gismelseed, Atta G. Attaelmanan, Muawia H. Shaddad, Frank C. Hawthorne
MAPS
Version of Record online: 04 May 2023
“The crystal structures of orthopyroxene (En86.3Fs8.6Wo5.1, space group Pbca) and pigeonite (En81.7Fs8.8Wo9.5, space group P21/c) from the Almahata Sitta ureilite (fragment#051) have been refined to R1 indices of 3.10% and 2.53%, respectively, using single-crystal X-ray diffraction data. The unit formulas were calculated from electron microprobe analysis, and the occupancies at the M1 and M2 sites were refined for both pyroxenes from the single-crystal diffraction data. The results indicate a rather disordered intracrystalline Fe2+-Mg cation distribution over the M1 and M2 sites, with a closure temperature of 726(±55)°C for orthopyroxene and 704(±110)°C for pigeonite, suggesting fast cooling of these pyroxenes. The Mössbauer spectrum of the Fe-Ni metal particles of Almahata Sitta ureilite (fragment#051) is dominated by two overlapping magnetic sextets that are assigned to Fe atoms in Si-bearing kamacite, and arise from two different nearest-neighbor configurations of Fe* (=Fe+Ni) and Si atoms in the bcc structure of kamacite; (8F, 0Si) and (7Fe, 1Si). In addition, the spectrum shows weak absorption peaks that are attributed to the presence of small amounts of cohenite [(Fe,Ni)3C], schreibersite [(Fe,Ni)3P], and an Fe-oxide/hydroxide phase. The fast cooling of pyroxene to the closure temperature (after equilibration at ~1200°C) and the incorporation of Si in kamacite can be interpreted as due to a shock event that took place on the meteorite parent body, consistent with the proposed formation history of ureilites parent body where a fast cooling has occurred at a later stage of its formation.”
