NWA 5731, an LL3.2 unequilibrated ordinary chondrite

- meteorite mineralogy and textures

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Figure 1. The thin section, crowded with chondrules, seen in plain transmitted light. The section is some 22x16 mm in size, circa 3.8 cm2 in area. The specimen is crowded with well-preserved chondrules, some 60 percent of the volume of the rock, which are well-defined against the fine-grained matrix. Section from Jeffrey Rowell, February 2010.

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Figure 2. Two photomicrographs of chondrules. Left: barred olivine chondrule with thin rim. Right: A large, coarse-grained olivine-bronzite chondrule. Nominal magnification 50X, long-axis field of view 1.7 mm, in (left) plain and (right) crossed-polarized transmitted light, showing details of chondrules.

"Rock of the Month #162, posted for December 2014" ---

The NWA 5731 meteorite

The NWA 5731 was a small find, perhaps the size of a clementine, weighing just 241 grams. It was purchased in Erfoud, Morocco in December 2008. It is a striking example of what is called an unequilibrated ordinary chondrite. That is, the intensity and duration of metamorphic conditions in the meteorite's source region, within the asteroidal parent body of LL chondrites, were inadequate to bring about recrystallization and homogenization of the principal rock-forming minerals, olivine and orthopyroxene (or, more properly, low-Ca pyroxene). According to the "onion-shell" model of ordinary chondrite parent bodies, this meteorite was probably derived from a shallow layer, where conditions were cooler and pressure lower than nearer the centre of the body. Olivine composition is especially varied (Fa0.7-62.9) and pyroxene rather less so (Of1.5-21.8: Weisberg et al., 2010). NWA 5731 displays a wide range of chondrule types (see the matched pair of images in Brandstatter et al., 2013, pp.106-107). A comparable chondrite is NWA 487, an L/LL3.2 with a black matrix and many chondrules (Killgore et al., 2002, pp.116-117).

Unequilibrated chondrites are uncommon, and the Meteoritical Bulletin (on 02 November 2014) lists just 21 LL3.2 stones. Only seven of these are >1 kg in size. Six are in the 1-4 kg range, while Krymka, a 1946 fall in Ukraine, is the undisputed champion at 50 kg TKW. Textural complexities are especially well-preserved in chondrites of low petrologic grade, e.g., Y-790448 (Okazaki and Nakamura, 2006). As is the case with some carbonaceous chondrites, presolar grains such as diamond may be found (Russell et al., 1996).


Brandstatter,F, Ferriere,L and Koeberl,C (2013) Meteoriten - Meteorites: Zeitzeugen der Entstehung des Sonnensystems / Witnesses of the Origin of the Solar System. Verlag des Naturhistorisches Museum, Vienna, 270pp. (in Engl. and in Ger.).

Killgore,K, Killgore,M and Killgore,E (2002) Southwest Meteorite Collection, a Pictorial Catalog. Southwest Meteorite Press, 201pp.

Okazaki,R and Nakamura,T (2006) Microchondrules within a troilite-rich rim around a chondrule in the Yamato-790448 LL3 chondrite. Meteoritics & Planetary Science 41, A135.

Russell,SS, Arden,JW and Pillinger,CT (1996) A carbon and nitrogen isotope study of diamond from primitive chondrites. Meteoritics & Planetary Science 31, 343-355.

Weisberg,MK, Smith,C, Benedix,G, Herd,CDK, Righter,K, Haack,H, Yamaguchi,A, Chennaoui Aoudjehane,H and Grossman,JN (2010) The Meteoritical Bulletin, No.97. Meteoritics & Planetary Science 45, 449-493.

Graham Wilson, 01 October 2014, 02 November 2014.

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