NWA 1840

Enstatite achondrite, ungrouped
standby for northwest africa 1840 photo
Purchased August 2003
no coordinates recorded A single, partially-crusted, achondrite weighing 132.8 g was purchased from a Moroccan dealer in Erfoud, Morocco. The stone was subsequently sold to American collector N. Oakes. A portion of the meteorite was sent to Northern Arizona University for classification (T. Bunch and J. Wittke), and it was determined that NWA 1840 is a brecciated, enstatite orthopyroxenite shocked to stage S3/4 and weathered to grade W2/3, the weathering mostly limited to those ares that border fractures. The meteorite exhibits tiny parallel compression fractures throughout the mass (see photo below).

This achondrite has modal mineral contents quite unlike those of any other aubrite, with the exception of the anomalous aubrite Shallowater. The following table compares some of the mineral modes of NWA 1840 to those of Shallowater and other selected aubrites:

Comparison of Modal Abundances for NWA 1840 and some aubrites
enstat plag anorth diops forster Si-kam troil Cr/troil oldham graph
NWA 1840 92.0 2.0 42.0 ? 4.0 2.0 10.2–14.7 none <1.0
Shallowater 79–81 2.5–2.9 0.4–17.6 none 2.9–4.7 3.3–3.7 2.9–7.1 3.4–5.5 trace abund
Norton County 84.5 1.0 ? 2.7 10.0 0.3 1.0 abund
Cumber. Falls 94.0 0.7 ? 0.9 1.5 0.7 1.1

Portions excerpted from Planetary Materials, Chapter 4, Mittlefehldt et al., 1998 (from Watters and Prinz, 1979)

The table reveals that many similarities exist between NWA 1840 and Shallowater, and that there are significant differences in the compositions of both NWA 1840 and Shallowater compared to most other aubrites. NWA 1840 and Shallowater both contain a large Si-kamacite component, a large plagioclase component, and are depleted in diopside and sulfides. However, unlike NWA 1840, Shallowater is unbrecciated (similar only to E-achondrite Mt. Egerton) and has a unique composition of primarily coarse-grained orthoenstatite (ordered orthopyroxene) crystals (~80 vol%). Also unlike NWA 1840, Shallowater contains a component of xenolithic material (~20 vol%) which represents the melt-entrapped remnants of a solid impactor. This xenolithic material is present in interstices and as inclusions in the orthoenstatite, and is composed (vol%) of weathered opaques (8), FeNi-metal (3.3), troilite (2.9), forsterite (2.9), plagioclase (2.5), low-Ca clinoenstatite (1), and schreibersite (0.4), along with traces of niningerite and oldhamite (Keil, et al., 1989).

Previous petrologic, mineralogic, and geochemical investigations relating to Shallowater, the aubrites, and the EH and EL chondrites have established that these meteorite groups probably represent four distinct planetesimals that formed in a similar region of the Solar System ~4.566 b.y. ago. Since the formation of Shallowater occurred as a result of the breakup and reassembly of its parent body, related lithologies should be found; perhaps NWA 1840 is such a related lithology. Continued analyses of NWA 1840 will be needed to establish whether it is genetically related to Shallowater or whether NWA 1840 represents a fifth enstatite parent body. The photo shown above is a 1.08 g specimen of NWA 1840, while that below is the main mass exhibiting parallel compression fractures. standby for nwa 1840 photo
Photo courtesy of Nelson Oakes—Meteorites–R–Us

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