3. Mineral chemistry and chondrite class refinement: the general procedure is to determine the composition of olivine (Mason, 1963, 1967), low-Ca pyroxene (generally orthopyroxene) and, according to preference, any other minerals (my third choice is kamacite). Results of wavelength-dispersive electron microprobe analysis of 28 points are summarized below. These analyses, employing reference materials, were part of a larger "bundle" of work that was devoted largely to meteorites and fulgurites. The carbon-coated thin section was analysed on a Cameca SX-50 instrument (University of Toronto Dept. of Earth Sciences, with thanks to lab manager Dr Liu Yanan).

• Olivine (n=5), Fa_{24.78 ± 0.65} (2σ), FeO/MnO 48.1. Apparently quite homogeneous, even more so than the opx (see below).
• Orthopyroxene (n=11), Of_{21.34 ± 1.30} (2σ), FeO/MnO 28.7. This composition is bronzite. The estimated mean CaO contents of olivine (0.10 wt.%) and of bronzite (1.07 wt.%) are in the expected ranges for these minerals.
• Kamacite (n=9) averages 10.6 wt.% Ni (very variable, 7.4-14.3%) and 0.592 ± 0.085 wt.% (2σ) Co (tightly clustered, range 0.54-0.67%). Trace of P noted in the metal, but no detectable S nor Cr.
• Troilite (n=2) contains trace Cr, which is not unusual for this meteoritic sulphide.
• Goethite (n=1) rich in Ni (6.8 wt.%) and Co (0.3%), clearly derived from metal by terrestrial weathering.

4. Shock state: the overall dark appearance, mosaic extinction of olivine, and irregular extinction in coarse (1.0 mm) olivine (Fig. 4) are consistent with appreciable shock in the source region of this meteorite's parent body. The darkening of the rock is due in large part to dispersion of minute (0.001-0.05 mm) blebs of troilite through the groundmass, and into the coarser silicates (Fig. 5). A proportion of carbonaceous matter might also be present, but the troilite seems to be the cause of the darkening (whence "black chondrite"), and organic matter content is probably low. Opaque shock veins and large melt pockets were not noted, but the common clouds of micron-scale sulphide droplets are consistent with incipient melting and a moderate, S4 shock state (Stoffler et al., 1991). Note one limitation of this analysis, which in a sense is luxurious, with access to 3 thin sections. A great many stony meteorites are breccias, composed of clasts (fragments) surrounded by a very variable proportion of fine-grained matrix. A slice through another part of the UNWA might reveal more in the way of melt veins and pockets, and then the classification might grade into S5.

5. Weathering grade: the material is quite fresh, and this plus the lack of well-developed "desert varnish" on the fusion crust may indicate a relatively short terrestrial residence time. Some iron oxyhydroxide (goethite) is however present on margins of metal grains and a weathering grade of W1 is assigned (where W0 is pristine and W6 is thoroughly altered: Wlotzka, 1993).

6. Bulk magnetic susceptibility: three faces of the sample were measured 4 times each using an SM-30 meter with a 50-mm coil. The measurements were made on the main mass, which is circa 9.0 x 8.5 x 6.0 cm in size. The mean of 12 readings is 166x10^-3 SI units, which with an assumed density of 3.3 g/cc yields a corrected log value of 4.92 by the methodology of Gattacceca et al. (2004; see also Rochette et al., 2003, 2008, 2009a,b; Macke et al., 2010, 2011a,b), consistent with an L chondrite, or one of the more magnetic classes of carbonaceous chondrite. However, basic petrographic observations quickly rule out CI, CK, CB, CR and CO classes (see, e.g., Hutchison, 2004; Norton, 2002; Norton and Chitwood, 2008).

7. Conclusions: visual inspection ([1], above, and Figs. 1-3) indicated a chondrite of some kind. The mineralogy [2] and degree of preservation of the chondrules suggest a petrologic grade of 4. The composition of olivine and orthopyroxene [3] indicate an L chondrite. Mineralogy and the relative homogeneity of the rock-forming silicates, and absence of coarse recrystallized feldspar, are consistent with an L4 stone, absent features strongly supportive of a rarer carbonaceous chondrite (see [2], also [6]). Further support for an L stone comes from metal chemistry [3], specifically the Co content* (Rubin, 1990). Shock [4] and weathering [5] states are estimated at S4 and W1, given the mosaic texture and planar fractures in olivine, clouds of minute dispersed troilite blebs in the groundmass, and the very limited occurrence of goethite on kamacite margins. The bulk magnetism of the material [6] is consistent with an L chondrite.

In conclusion, the meteorite is classified as an ordinary chondrite, L4 (S4, W1). The TKW (total known weight) of this meteorite (this batch of NWA material) is the 1263.2 grams seen in Figure 1. A small additional amount, perhaps 17 grams in 3 sawn chips, was dispersed in 2010 prior to the sequential purchase in 2010-2011 of the material described here.

Given the lack of documentation, it is of interest to see whether any closely similar NWA chondrites are listed in the Meteoritical Bulletin database, in other words, whether this is part of a known find or is a similar meteorite, possibly a "paired" find that may be part of the same fall. As of 31 March 2014, the Meteoritical Bulletin lists a total of 7,648 NWA meteorite names, of which 5,120 have been validated by adequate research and documentation. No less than 1,663 of the validated NWA are L chondrites, which is not surprising, given that L chondrites comprise some 35% of all known meteorites, second in abundance only to the H chondrites.

This classification (first posted 01 April 2014, no joke intended!) was approved in 2019 (see note under Fig. 1), and the name NWA 12807 was given to this meteorite.

* Footnote: At the same time a few analyses were made of the H4 (S1, W3-4) chondrite NWA 5425, previously featured here. The results were as follows:

• Olivine (n=5), Fa_{18.8 ± 4.1} (2σ).
• Kamacite (n=8) averages 6.67 wt.% Ni (uniform, 6.61-6.74%) and 0.408 ± 0.025 wt.% (2σ) Co (tightly clustered, range 0.395-0.425%).

A brief scanning electron microscope examination of uncoated polished thin section 2010-1, courtesy of Dr Giovanni di Prisco, provided some confirmation of the petrographic and microprobe results (Fig. 6). The backscattered electron imagery and energy spectra revealed the following: olivine and bronzite, kamacite and troilite, and an aluminosilicate glass (with minor Na, Mg, K, Ca, Fe) between silicates in both barred olivine and radiating pyroxene chondrules. No taenite was noted in the kamacite. The metal oxidizes in situ into a nickeliferous goethite. Chondrules may be mantled by troilite. Chromite was noted, up to 60 microns in diameter. A 430x45-micron Ca phosphate grain (apatite or whitlockite?) was plated on the margin of a barred olivine chondrule.