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 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 indicate that a relationship exists among the IIF irons, the Eagle Station pallasites, the CV carbonaceous chondrites, and other diverse meteorites, which suggests they probably all formed in a similar nebula region. However, the difference in trace element abundances between IIF irons and ES pallasites indicates they likely formed on multiple parent bodies (Kracher et al., 1980). In a geochemical analysis of the IIF irons and the Eagle Station pallasites, Hilton et al. (2020) found that the ES pallasites are significantly enriched in Ni and depleted in Ge and Ga compared to the IIF irons. They concluded that the parent body of the ES pallasites was more oxidized than that of the IIF irons, which supports the conclusion of Kracher et al. (1980). Based on the cooling rate of ~5°C/m.y. as determined from taenite profiles, the diameter of the IIF parent body was 50–200 km.

The IIF group contains eight members: Balambala, Corowa, Del Rio, Dorofeevka, Monahans (1938), NWA 6932, Purmela, and Repeev Khutor. Hilton et al. (2020) noted that the the Ge abundance in Purmela is considerably lower than that in all other IIF irons and therefore may derive from a separate compositionally and isotopically similar parent body (see diagram below). Zhang et al. (2022) determined that only Del Rio, Dorofeevka, NWA 6932, and Repeev Khutor represent a fractionation trend, and they recommend that the other four group members be reclassified as anomalous IIF irons. Zhang et al. (2022) used fractional crystallization modeling to determine that the initial bulk composition of the IIF metallic core contained 5 (±1) wt% S and 0.65 (±0.05) wt% P, and that the four normal group members mentioned above represent ≤61% crystallization of the parental core liquid. The photo above shows a 41.6 g slice of Del Rio, previously in the Elbert A. King Collection.

standby for iif ge vs. ga diagram
Diagram credit: Hilton et al., MAPS, vol. 55, #12, p. 2571 (2020)
'Crystallization histories of the group IIF iron meteorites and Eagle Station pallasites'