Iron, IAB complex, sHH subgroup
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Found 1916
28° 2' S., 117° 58' E.

Two fragments constituting a single sickle-shaped mass having a combined weight of 16.5 kg were found in Western Australia, about 10 km east of Mount Magnet (Buchwald, 1975). The mass exhibits significant terrestrial weathering with no evidence of fusion crust or heat-affected zone. Mount Magnet is classified structurally as a plessitic octahedrite (Opl) with kamacite needles of a few µm in size. This iron was initially classified as geochemically anomalous, but has since been included in the high-Au, high-Ni subgroup (sHH) of the IAB complex (Wasson and Kallemeyn, 2002).

In a study involving Mo and W isotope compositions for the IAB complex, Worsham et al. (2017) determined that the Mo isotopic composition of the sHL and sHH subgroups are identical, but differ from that of the sLL, sLM, and sLH subgroups. Moreover, they calculated the formation ages of these irons based on W isotopes, and determined that the sHL and sHH subgroups formed significantly earlier than the MG and sLM subgroup. From this study they conclude that the MG and the sLL, sLM, and sLH subgroups formed in distinct impact-melt pools on a common asteroid, whereas the sHL and sHH subgroups formed by fractional crystallization associated with core formation on one or more separate parent bodies. An analysis of siderophile elements in IAB irons conducted by Worsham et al. (2017) revealed that Mount Magnet has a more fractionated HSE pattern compared to the other two sHH samples in the study, Kofa and ALHA80104, both of which have nearly identical HSE patterns and which are somewhat similar to two late-crystallized IIIB irons (see diagram below, where Grant is dashed line and Chupaderos is dotted line). Based on this analysis, they suggest that Mount Magnet may have formed from a distinct parental melt.

standby for hse abundances diagram
Diagram credit: Worsham et al., GCA, vol. 188, p. 268 (2016)
'Siderophile element systematics of IAB complex iron meteorites: New insights into the formation of an enigmatic group'

The interior structure of Mount Magnet is ataxitic, containing numerous ribbons of schreibersite, small grains and clusters of silicates, and monocrystalline grains of troilite—each of these inclusions are often rimmed by kamacite. Both small pockets and skeletal crystals of schreibersite are also present, the former usually surrounded by wide kamacite rims. To learn more about the relationships within the IAB complex and among other iron chemical groups, click here. The photo shown above is an 11.32 g partial slice of Mount Magnet.