"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).

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.