"Rock of the Month # 41, posted November 2004" ---

NWA 1463 represents an extremely rare class of meteorite: the known mass of this find is 1001 grams (Russell et al., 2003). This meteorite was donated to the Royal Ontario Museum by collector Dr. David Gregory, certified by the Canadian Cultural Properties Review Board, and officially received as a donation to the Royal Ontario Museum in Toronto, accession number M49055, on 02 April 2004. There are now 6 pieces comprising the 1.001 kg mass. It is thought to be the largest mass of winonaite held by a public institution. Remarkably, this rare, chondrule-bearing sample was originally suspected of being an H5 ordinary chondrite. Happily, its unique nature came to light, and this rare snapshot of solar-system history is now preserved for further scientific enquiry!

Figure 1, above, is a scanned image of the polished thin section of sample "3a", viewed in transmitted light. The flat margin at the left is an exterior margin, darkened by a thin fusion crust. This face is 23 mm wide. The slice presents a cross-section of roughly 5.1 cm². A number of the larger chondrules can be seen, as well as blebs of nickel-iron alloy. Two of the larger chondrules, roughly 2 mm in diameter, are a rounded, barred example composed largely of tabular enstatite (upper right quadrant) and an unusual flattened, fine-grained olivine-rich chondrule (near lower-left corner).

NWA 1463, a recent find in the vast and necessarily nebulous region designated "Northwest Africa", has been classified as a winonaite (Benedix et al., 2003). A brief survey of the illustrated thin section was undertaken in the summer of 2002 in order to characterize the material for museum purposes (Wilson, 2002). At the time Dr. Gregory's keen eye had rescued the sample from obscurity, but it was tentatively identified as a piece of the acapulcoite NWA 725. The resulting petrographic data were consistent with an identification of acapulcoite, but were inconclusive. More detailed work elsewhere, including microprobe analysis of the minerals and oxygen isotope analysis, confirmed that it is a fine example of a similar group of achondrites, the winonaites. Both these classes of stony meteorite are notable in being "primitive achondrites", representing one or more chondritic parent bodies in their earliest stages of melting and igneous differentiation. Together with the lodranites (coarser, more extensively recrystallized cousins of the acapulcoites) they can be said to be the oldest igneous rocks in the solar system. Examples of both acapulcoites and winonaites are notable in retaining broadly chondritic chemistry, particularly with regard to major elements, and some contain relict chondrules in excellent stages of preservation.

NWA 1463 is now recognized as the most primitive known winonaite, a mixture of orthopyroxene, magnesian olivine (Fa_3.2), plagioclase feldspar, clinopyroxene, whitlockite, chromite, phosphide, troilite and abundant Ni-Fe metal (Benedix et al., 2003).

When examined in 2002, the sample was a set of three pieces, roughly 1.0 kg in weight, of an achondrite said to be NWA 725. The hand specimen displays abundant metal, traces of chondrules on sawn surfaces, and a thin fusion crust. In thin section the meteorite is dominated by magnesian orthopyroxene and olivine, abundant sodic plagioclase feldspar and kamacite. Also identified by their optical properties were troilite, secondary Fe oxide (goethite, after sulphide and metal) and clinopyroxene (chromian diopside), as well as accessory taenite and chromite (Wilson, 2002). The meteorite is granular and rather fine-grained, with most grains 50-250 microns in diameter, and only kamacite exceeding 400 microns. Most remarkably, the chondrules are well-preserved, up to 2 mm in diameter, and quite common (at least 24 were recognized in a 5 cm³ thin section). Troilite is remobilized and partially oxidized in the fusion crust, which is at most 600 microns (0.6 mm) thick.

The winonaites are quite similar to silicate inclusions in some IAB iron meteorites, such as Caddo County (Mittlefehldt et al., 1998, pp.4-22 to 4-32) and thus seem to have shared a common parent body. Winonaites and the silicate inclusions in IABs have a complement of essential minerals similar to ordinary chondrites, with the addition of accessory minerals formed under reducing conditions more typical of enstatite chondrites, such as schreibersite, daubreelite and alabandite (Hutchison, 2004, pp.253-256). The primitive achondrites are as interesting as they are rare: in the summer of 2002, a literature search uncovered just 15 acapulcoites, 14 lodranites and 11 winonaites.

References

Benedix,GK, McCoy,TJ and Lauretta,DS (2003) Is NWA 1463 the most primitive winonaite? Meteoritics & Planetary Science 38, A70.

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

McCoy,TJ, Mittlefehldt,DW and Wilson,L (2004) Asteroid differentiation. (This is a modern, detailed review which emphasises asteroids and the processes involved in their evolution, as opposed to the ultimate products from which so much of the evidence is derived - meteorites). Available at: . http://ares.jsc.nasa.gov/mittlefehldt/AsteroidDifferentiation.pdf, 43pp.

Mittlefehldt,DW, McCoy,TJ, Goodrich,CA and Kracher,A (1998) Non-chondritic meteorites from asteroidal bodies. In `Planetary Materials' (Papike,JJ editor), Min.Soc.Amer. Reviews in Mineralogy 36, chapter 4, 195pp.

Russell,SS, Zipfel,J, Folco,L, Jones,R, Grady,MM, McCoy,T and Grossman,JN (2003) The Meteoritical Bulletin, No.87, 2003 July. Meteoritics & Planetary Science 38, A189-248.

Wilson,GC (19 July 2002) Mineralogy, chemistry, classification and pairing of a 1-kg trove of the primitive achondrite Northwest Africa 725. IsoTrace Laboratory, University of Toronto, 26pp. [the sample later found to be NWA 1463].