Mineralogy and petrography of the Almahata Sitta ureilite

Abstract

Abstract-We performed a battery of analyses on 17 samples of the Almahata Sitta meteorite, identifying three main lithologies and several minor ones present as clasts. The main lithologies are (1) a pyroxene-dominated, very porous, highly reduced lithology, (2) a pyroxene-dominated compact lithology, and (3) an olivine-dominated compact lithology. Although it seems possible that all three lithologies grade smoothly into each other at the kg-scale, at the g-scale this is not apparent. The meteorite is a polymict ureilite, with some intriguing features including exceptionally variable porosity and pyroxene composition. Although augite is locally present in Almahata Sitta, it is a minor phase in most (but not all) samples we have observed. Low-calcium pyroxene (<5 mole% wollastonite) is more abundant than compositionally defined pigeonite; however, we found that even the low-Ca pyroxene in Almahata Sitta has the monoclinic pigeonite crystal structure, and thus is properly termed pigeonite. As the major pyroxene in Almahata Sitta is pigeonite, and the abundance of pigeonite is generally greater than that of olivine, this meteorite might be called a pigeonite-olivine ureilite, rather than the conventional olivine-pigeonite ureilite group. The wide variability of lithologies in Almahata Sitta reveals a complex history, including asteroidal igneous crystallization, impact disruption, reheating and partial vaporization, high-temperature reduction and carbon burning, and re-agglomeration.

Stable isotopes and the noncarbonaceous derivation of ureilites, in common with nearly all differentiated planetary materials

“…Those 17 comprise seven EL chondrites, three impactmelted EL chondrites, one EH chondrite, three impactmelted EH chondrites, two H chondrites, and one sample of a chondrite that is ungrouped but shows affinity with R chondrites. Zolensky et al (2010) found four (additional?) chondritic pieces, comprising one each of L, H, EL and EH.…”

The mid‐infrared transmission spectra of multiple stones from the Almahata Sitta meteorite

“…During the course of our studies it was determined that stone #25 is not a ureilite, but is instead an H5 ordinary chondrite (Zolensky et al. 2010).…”

Internal structure and physical properties of the Asteroid 2008 TC3 inferred from a study of the Almahata Sitta meteorites

“…Most of the recovered meteorites are classified as ureilites (Jenniskens et al, 2009). However, a significant number of pieces of various chondritic compositions have been found within the recovered meteorites which most likely also originated from the Asteroid 2008 TC 3 impact (Bischoff et al, 2010;Kohout et al, 2010;Shaddad et al, 2010;Zolensky et al, 2010) revealing significant compositional heterogeneity of the 2008 TC 3 asteroid.…”