The NWA 1242 Mesosiderite

--- a stony-iron meteorite from Libya, north Africa

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Above: a polished slice of the metal-rich meteorite. This face displays metal nodules up to 10x5 mm in section, as well as angular, mm- to cm-scale, very
fine-grained lithic clasts, which in turn contain extremely fine-grained metal grains. Visually, the slice displays roughly 40% metal and 60% of the
dull grey, silicate-dominated matrix. Sample 3206, 121 grams, 10x7x0.5 cm in size, from Allan Langheinrich.
A similar slice is shown in Norton and Chitwood (2008).

"Rock of the Month #145, posted for July 2013" ---

Mesosiderites are a class of stony-iron meteorite, albeit less in the public view than the more visually- arresting, olivine-rich pallasites. Currently (01 June 2013) the Meteoritical Bulletin lists 45,763 authenticated meteorites, including 1,070 iron meteorites (2.34% of all meteorites); 187 mesosiderites (0.41%) and 95 pallasites (0.21%).

According to the Meteoritical Bulletin (original listing: Russell et al., 2002), NWA 1242 was discovered near the village of Gillio, Libya in 1985, by oil-exploration personnel. Two pieces were found, total mass circa 7 kg. After 13 years of service as bookends, the meteorite was sold to a meteorite dealer in 1998. The meteorite was originally referred to as Sahara 85001. Analysis of the material revealed metal nodules up to 19 mm wide, in a mixture of pyroxenes (orthopyroxene, pigeonite and augite), calcic plagioclase feldspar (anorthite), chromite, pyrrhotite, the nickel-iron alloys kamacite (5.6 wt.% Ni) and taenite (42 wt.% Ni), silica and phosphate.

Mesosiderites are polymict breccias composed of metal and silicate -dominated clasts in a granular matrix. This admixture of disparate components, not uncommon in meteorites, is suggestive of a role for violent impact events in their origin (e.g., Scott et al., 2001). The parent body or bodies are as yet unknown, though these metal-rich samples are unlikely to come from an intact, major asteroid such as Vesta, the generally-recognized source of the HED achondrite clan (Rubin, 2013).

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Above: a polished slice of a second mesosiderite, NWA 2932, photo courtesy of Blaine Reed. This sample is a 115.6-gram slice showing pronounced clumping of metal within a silicate matrix, an igneous texture suggestive of partial immiscibility of coexisting metal and silicate melts. Meteorite reportedly purchased in Erfoud, Morocco in 2005, and paired with a larger mesosiderite find, NWA 2923.


Norton,OR and Chitwood,LA (2008) Field Guide to Meteors and Meteorites. Springer-Verlag London Limited, 287pp., pages 172- 173.

Rubin,A (2013) Are eucrites and mesosiderites from the same parent body? Meteorite 19 no.1, 33-34.

Russell,SS, Zipfel,J, Grossman,JN and Grady,MM (2002) The Meteoritical Bulletin, No.86, 2002 July. Meteoritics & Planetary Science 37 no.7, supplement, A157-184.

Scott,ERD, Haack,H and Love,SG (2001) Formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid. Meteoritics & Planetary Science 36, 869-881.

Graham Wilson, 01-02 June 2013, updated with NWA 2932, 06 May 2014, minor edit 22 December 2014

Visit the Turnstone Meteorite Index

or the broader "Rock of the Month" Archives!

Compare this mesosiderite with the Springwater and Seymchan pallasites.

See also the remarkable mesosiderite-like "meteorwrong" which is the
native iron-rich Putorana basalt from Siberia

The 76th Annual Meeting of the Meteoritical Society took place July 29 to August 02 2013 in
Edmonton, Alberta, Canada.