NORTHWEST AFRICA 5312

Mesosiderite, group C (reclassified in MetBull 103)
(Diogenite [olivine–orthopyroxenite] in MetBull 95)
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Purchased 2007
no coordinates recorded

A single stone weighing 354 g was purchased by F. Kuntz from in Morocco. Sample were submitted for analysis and classification to the Northern Arizona University (T. Bunch and J. Wittke). Initial analysis of NWA 5312 led to the conclusion that it was consistent with the newly characterized olivine diogenite group. However, in accordance with the new terminology proposed in a revision to the diogenite classification scheme utilizing an IUGS-based system, NWA 5312 was considered to represent the olivine–orthopyroxenite subgroup of diogenites (Beck and McSween, 2010; Wittke et al., 2011). This diogenite subgroup includes GRA 98108, ALHA77256, LEW 88679, and NWA 5405.

A comparative study conducted by Bunch et al. (2004) of a number of independently owned masses (NWA 1827, NWA 1879, NWA 1882, 1912, 1951, 1982, 3055, and 1645) demonstrated that close similarities exist among them, and that all of these masses, totaling at least 26.4 kg, were presumed to be paired. Continued research was conducted by Bunch et al. (2014) on these specimens and a large number of mesosiderite samples previously considered to represent a separate fall. It was ultimately determined that all of these mesosiderites represent a single strewn field with a total weight of at least 80 kg comprising mesosiderites of differing groups (see also NWA 1827).

Northwest Africa 5312 is composed of 46 vol% orthopyroxene, 24 vol% olivine, and 18 vol% troilite, along with lesser amounts of merrillite, chromite, and taenite (Irving et al., 2009). Oxygen isotopic ratios obtained for NWA 5312 overlap those of other HED members (D. Rumble III, Carnegie Institution).

Based on in-depth mineralogical and textural studies of numerous samples of this massive mesosiderite (e.g., NWA 1817, 1827, 1878, 1879, 1882, 1912, 1951, 1979, 1982, 2042, 3055, 5312), Bunch et al. (2014) proposed an alternate petrogenetic history for the mesosiderites. Because they did not find any eucritic inclusions in any of the fragments from this mesosiderite, and the O- and Cr-isotopic compositions of the orthopyroxene and plagioclase in this mesosiderite are virtually identical to those of diogenites (see diagram below), they developed a scenario in which a small, previously cooled iron-rich asteroid collided with a still warm (~800°C), previously differentiated diogenitic asteroid at a low angle and relatively low velocity (<6 km/s). The frictional shear forces created in this oblique impact produced small rotating molten metal spherules which mechanically incorporated diogenite fragments, resulting in a minimally-shocked, fragmental stony-iron layer of debris.

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Diagram credit: Bunch et al., 45th LPSC, #2554 (2014)

Because of the relatively small size of this meteorite, and similarly the related mesosiderite NWA 1827, the general appearance resembles that of a metal-rich diogenite as initially classified. The photo of NWA 5312 shown above and below is a 5.26 g partial slice.

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