"Rock of the Month # 119, posted for May 2011" ---

The NWA 5425 meteorite is classified as an H4 (S1, W4) ordinary chondrite meteorite. This material was acquired from Blaine Reid, January 2010. It was considered interesting as an H-chondrite with porosity that looked like an acapulcoite. According to the Meteoritical Bulletin database (see Weisberg et al., 2010), it was purchased in Erfoud, Morocco in 2006, but like many or most Northwest Africa meteorites, this information guarantees neither an exact nor a unique provenance. The official classification includes mean analytical estimates for olivine (Fa_17.3) and low-Ca pyroxene (Fs_16.2Wo₁) compositions.

The polished thin section provides additional information. Well-preserved chondrules are abundant, mostly 2 mm or smaller. Porphyritic olivine, barred olivine, very fine-grained and excentroradial pyroxene chondrules are all present. There is far more sulphide (troilite) than metal (kamacite), and evidence that this is due to extensive terrestrial oxidation of most of the metal in the meteorite. Some metal and troilite occur within the chondrules, but the bulk of these phases are in the granular matrix to the chondrules. One explanation of the evident porosity of the material could be that it is generated by extensive oxidation of kamacite. Terrestrial kamacite destruction can be compared to other H4 stones: it is slightly greater than in "Elm Creek (Manitoba)" [official name TBA) and much more advanced than in Wood Lake (an Ontario find).

Assorted chondrules and coarse olivine grains in NWA 5425 and (lower right) a view of the opaque phases, showing the matrix of a chondrule with yellowish troilite and minor relict metal (white) and extensive secondary iron oxides (grey). All photographs taken at nominal 50X magnification, long-axis field of view 1.7 mm. Photos in crossed-polarized transmitted light except (lower right) plane-polarized reflected light.

The 8.5 cm² thin section is consistent with the published classification, except that this slice seems less weathered than the type specimen, W3 rather than W4. By optical inspection, the section contains roughly 50% fine-grained groundmass (including some coarser olivine, likely in part disrupted chondrule material), 27% well-formed chondrules, 12% goethite, 10% troilite, 1% relict kamacite and trace chromite. Based on all available information, this meteorite is classified as H4 (S1, W3). Petrologic, shock and weathering classifications are summarized by Hutchison (2004).

Hutchison,R (2004) Meteorites: a Petrologic, Chemical and Isotopic Synthesis. Cambridge University Press, 506pp., pp.40-46.

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, pp.470-471.