Iron, IIF, octahedrite
standby for del rio photo
Found 1965
29° 22' N., 100° 58' W.

A single iron meteorite weighing 3,346 g was found by a Mr. Mueller 4.5 miles west of Del Rio, Texas on a ranch owned by Mr. E. Daughtrey. Following a search of the area, five additional pieces were found, having a combined weight of 250 g.

Del Rio is a polycrystalline ataxite with distorted kamacite bandwidths of ~0.07 mm. These produce, in places, a finest-textured Thomson (Widmanstätten) structure in a mostly plessite field. Sub-mm-sized troilite inclusions occur throughout. The masses have been terrestrially weathered to the point where the fusion crust and 2–3 mm of the outer surface has been removed.

Resolution of this group is based on several factors including structure, which ranges from ataxitic to plessitic. The group has an unusually high Ge/Ga ratio, a high Ni content, a high Co and Cu content, and a low P content. The compositional trends are most consistent with igneous fractional crystallization of a core. The Ni content in Del Rio runs counter to accepted theory, having a positive correlation with the Thomson (Widmanstätten) structure bandwidth. This occurrence could be explained by either differences in the cooling rate, or the effects of bulk P content on bandwidth growth. Oxygen-isotopic data indicates that a relationship exists between the IIF irons, the Eagle Station pallasites, the CV carbonaceous chondrites, and several other meteorites, which suggests they probably formed in a similar region but perhaps on separate parent bodies. Based on the cooling rate of ~5°C/m.y. as determined from taenite profiles, the diameter of the IIF parent body was calculated to have been 50–200 km. See 'The Breakup of Antaeus' for additional details.

The IIF group contains only five members with a compositional gap dividing three low-Ni members (Monahans 1938, Dorofeevka, and Del Rio) from two high-Ni members (Corowa and Repeev Khutor). The photo above shows a 41.6 g slice of Del Rio previously in the Elbert A. King Collection.