PMG pallasite meteorite

A pallasite belonging to the main chemical group. Metal compositions in all PMG are closely related to those in evolved IIIAB irons, and are generally consistent with their formation in the IIIAB parent asteroid. The O-isotope composition of their olivine (forsterite) is a main parameter to separate the main-group pallasites (PMG) and Eagle-Station pallasites (PES). The mean PMG ∆¹⁷O (=δ¹⁷O-0.52·δ¹⁸O) value is -0.28 ± 0.06‰. The mean PES ∆¹⁷O value is much lower, ≈-4.6‰.

The major mineral assemblages of most PMG are quite simple, consisting of troilite and schreibersite in addition to olivine (forsterite), kamacite and taenite; olivine Fa values range from 11 to 18 mol%, with >80% of the values in the range 11.6–13.5 mol% (Buseck and Goldstein, 1969). Zaisho (Fa18.3) has the highest Fa among PMG (Buseck and Clark, 1984). In addition, several characteristic minor minerals have been reported. Chromite is relatively abundant; some grains have euhedral shapes and seem to have crystallised from a melt (Buseck, 1977). A few chromites are quite massive; a large (10-cm) chromite-olivine assemblage in Brenham was described in detail by Wasson et al. (1999). Phosphates are minor phases in all PMG (common only in Springwater); the identified phosphates are farringtonite, stanfieldite, and merrillite (Buseck and Holdsworth, 1977). Phosphoran olivine is rare in most pallasites but minor amounts may be ubiquitous in the more oxidized PMG (Buseck, 1977; Buseck and Clark, 1984). Low-Ca pyroxene is a rare accessory phase (Buseck, 1977).

Some PMG are assigned to an anomalous subset (PMG-an) because of their anomalous properties (deviant metal compositions or appreciably higher olivine Fa contents).