Mesosiderite, group 2C
standby for northwest africa 1827 photo
Purchased June 2002
no coordinates recorded

A complete, fusion-crusted, stony-iron meteorite, weighing 877 g, was purchased in Risani, Morocco on behalf of an American collector. A polished thin section from NWA 1827 was prepared for analysis at the Department of Earth and Space Sciences, University of Washington (A. Irving and S. Kuehner). Optical petrography, energy dispersive electron microprobe analyses, and quantitative microprobe analyses were utilized in the initial analysis of this meteorite. Comparative studies of NWA 1827 with NWA 1879 and other independently owned masses (NWA 1882, 1912, 1951, 1982, 3055, and 1645) show that close similarities exist between them, and all of these masses, totaling at least 26.4 kg, are presumed to be paired (Bunch et al., 2004). Continued research was conducted by the Bunch et al. (2014) on these specimens and a large number of mesosiderite samples previously considered to represent a separate fall. It was eventually determined that all of these mesosiderites represent a single strewn field (totaling at least 80 kg) comprising mesosiderites of differing subgroups (see also NWA 1878).

The general appearance of NWA 1827 was found to be similar to that of a mesosiderite, exhibiting an equilibrated, igneous texture, and consisting primarily of metal and abundant orthopyroxene, along with minor troilite, chromite and merrillite, and rare anorthite. However, NWA 1827 was inconsistent with a classification as a mesosiderite for several reasons. It has a lower metal content than other mesosiderites (~10 vol% vs. ~50% [~17–90 wt%]), and the metal has a lower Ni content than typical mesosiderites (5 wt.% vs. 7.5–10%). The Fs content of the largest orthopyroxene grains is bronzitic (Fs16.2), with that of the average-sized orthopyroxene grains being somewhat higher (Fs22.5–Fs27.3). This Fs range is significantly lower than that of typical mesosiderites (Fs20–Fs40); however, orthopyroxenes in this range are found in diogenites. The plagioclase content (~2%) is also lower than that of other mesosiderites, with the exception of the single 2C member, RKPA79015; again, anorthitic plagioclase in this abundance is found in some diogenites. The FeO/MnO ratio of the orthopyroxene grains in NWA 1827 (29.7–34.8) is higher than that of most mesosiderites, but do fall in the range of diogenites. Finally, NWA 1827 lacks olivine and chromite, common constituents in mesosiderites.

Based on a preliminary analysis, it was proposed that the data best fit a classification as a recrystallized, metal-rich diogenite, which was intruded by an exotic metallic body and annealed within a deep regolith. However, further analysis conducted at the Northern Arizona University (T. Bunch) of a larger area of this heterogeneous mesosiderite revealed sparse eucrite and diogenite inclusions, and has led to the conclusion that NWA 1827 is most similar to a mesosiderite of the 2C subgroup, the second such mesosiderite after RKPA79015.

The difficulty in determining that NWA 1827 is actually a mesosiderite was similarly felt by Clark and Mason (1982) in their attempt to classify RKPA79015 (Haack et al., 1996). They found that the amount of tetrataenite present in meteorites with comparable bulk Ni concentrations increases with decreasing cooling rate from 500 to 250°C. Therefore, tetrataenite rims on taenite grains are much wider in mesosiderites (typically 10–20 µm) than in chondrites (2–5 µm) and iron meteorites (less than or equal to 2 µm) that have comparable bulk Ni concentrations (Clark and Scott, 1980; Hassanzadeh et al., 1990). Clark and Mason were able to classify RKPA79015 as a mesosiderite on the basis of this feature. The photo of NWA 1827 shown above is an 11.94 g end section. Excellent photos of a large section of NWA 1827 can be seen at