Structure and formation of the San Cristobal meteorite, other IB irons and group IIICD

Abstract

San Cristobal is an unusual group IB ataxite with 25 per cent Ni, composed of taenite grains 2–3 cm in diameter and silicate-troilite-graphite nodules concentrated on the grain boundaries. Silicate compositions are typical of group IAB: olivine Fa 3.3, orthopyroxene Fs 6.9 and feldspar Ab 88. Plagioclase shows peristerite unmixing, previously unrecorded in meteorites, and occasional K-rich feldspar grains have an unusual antiperthite exsolution. Brianite Na 2CaMg(PO 4) 2 and haxonite (Fe, Ni) 23C 6 are common in nodules and matrix, respectively, while cohenite is rare. Part of the matrix contains a pearlitic kamacite precipitate instead of the usual oriented platelets. San Cristobal has extreme concentrations of many elements; e.g. the highest published Ag, Cu, In and Sb contents and the lowest Mo and Pt in irons. These data and the mineralogy show that San Cristobal has many characteristics of both groups IB and IIID, but that it fits group IB trends better. Ratios of refractory element abundances to those in Cl chondrites (both normalized to Ni) decrease through IB from l in IA to 0.03 in San Cristobal, but the other siderophilic elements have a small range of abundance ratios, 0.5–2, throughout IAB. We suggest that IB grains either formed in a part of the solar nebula where refractories had been previously removed, or else failed to equilibrate with a refractory-rich, high-temperature condensate. After condensation of the volatiles, Fe was partially removed, perhaps by oxidation. Group IIICD seems to have experienced similar fractionations. Unlike other iron meteorite groups, neither IAB nor IIICD appears to have been fully molten.