Mount Dooling is a rare IC iron (Buchwald, 1975, pp.849-850), a coarse octahedrite (Og) with grains from 1-2 mm to 10-20 mm, the latter equiaxial grains with Neumann bands, part recrystallized (see Fig. 2). It is an anomalous iron with 6.22% Ni, 0.46% Co, 0.27% P, 1.2 ppm Ir. The first substantial piece of the Mount Dooling iron was found in 1909 by a gold prospector, 8 km east of the eponymous topographic high. Under the microscope, the meteorite displays metal plus schreibersite, troilite and daubreelite. A shock event produced the Neumann bands in the metal, and melted troilite nodules to generate eutectic mixtures.

The IC irons (Scott, 1977) display a tight clustering of bulk Ni content (6-7 wt.%) and show regular correlations of major and minor elements, consistent with crystallization from a melt, as in other, larger groups such as the IIIAB irons. IC irons contain abundant cohenite and chromite, and cylindrical sulphide inclusions, but lack silicate and graphite inclusions in sulphides. However, the textures are extremely diverse, with great variation in generally very coarse grain size, from 2 to 25 cm or more, and in calculated cooling rates. The observed fractionations were plausibly generated by cooling of a metal melt, probably in a single melt pool. However, soon after solidification they may have been dispersed in an impact event, reaccreted, and thereafter cooled at different depths, hence the variable cooling rates.

A note on iron meteorite classification

Two excellent reviews of meteorite mineralogy, isotope chemistry, and the origin of most meteorites from asteroidal parent bodies, have been published in the past few months (Rubin and Ma, 2017; Greenwood et al., 2017). It appears that magmatic iron meteorites formed as little as 100,000 years after the formation of calcium-aluminium inclusions (refractory mineral aggregates, CAI, the earliest solids to form, by condensation, in the solar nebula). Due to decay of short-lived radionuclides such as ²⁶Al, these bodies soon melted and differentiated, allowing the fractionation of elements within a km-scale (and larger) "magma ocean". Most such early parent bodies were destroyed by impact events, asteroid 4 Vesta (thought to be the source of the HED achondrites) being an important exception. The heating and melting of such early-formed parent bodies, and the consequent cooling, elemental fractionation, crystallization and asteroidal core formation, gave rise to magmatic irons (the IC, IIAB, IIC, IID, IIF, IIG, IIIAB, IIIE, IIIF, IVA and IVB irons).

In contrast, later collisions between asteroids are thought to have generated impact melts, the source of other, "non-magmatic" classes, which commonly contain silicate inclusions. This usage of the term "non-magmatic" is unfortunate, since all irons surely passed through a molten stage, and on Earth we usually envision intrusions, from the largest batholiths down to cm-scale dykes, as being emplaced as magma. As noted above, the term is used in the context of irons to refer to those specimens whose inter-element systematics do not support an origin in a cooling, differentiating magma chamber. They have been envisaged as forming in a sulphur-rich asteroidal core, in a differentiating body disrupted by impact, or in a more localised impact on the surface of a parent body. These "non-magmatic irons" (primitive irons) plausibly formed as smaller-volume melts, within melt pools on their parent bodies (Wasson and Kallemeyn, 2002). These include the IAB complex and IIE irons (IAB, IIE, Udei Station grouplet, Pitts grouplet, and the sLL, sLM, sLH, sHL and sHH chemically-defined subgroups of the IAB complex: see below, and also the excellent reviews of Weisberg et al. (2006, pp.42-43) and Grady et al. (2014, pp.322-334).

The taxonomy of the IAB iron meteorites was reviewed by Wasson and Kallemeyn (2002). The IAB main group irons include some famous and/or large examples, such as Landes, Campo del Cielo , Nantan, Canyon Diablo, Youndegin, Cranbourne and Odessa. Five subgroups were defined on the basis of plots of select elements versus either gold or nickel, and are listed below with examples:

• Subgroup sLL (low Au, low Ni) includes Annaheim, Ogallala, Bischtube, Mazapil, Goose Lake, and Toluca .
• Subgroup sLM (low Au and medium Ni - formerly known as IIIC irons) includes Persimmon Creek, Mungindi, Edmonton (Kentucky) and Carlton.
• Subgroup sLH (low Au and high Ni - also known as the IIID irons) includes Dayton, Tazewell, Freda and Wedderburn.
• Subgroup sHL (high Au and low Ni) includes Sombrerete, Victoria West and Muzaffarpur.
• Subgroup sHH (high Au and high Ni, including the Gay Gulch trio) includes Garden Head, Gay Gulch and Mount Magnet.
• Nine other grouplets, pairs and sets of variously anomalous irons were also tabulated, e.g., the Udei Station and Pitts grouplets, the Mundrabilla duo (with Waterville), and the Twin City duo (with Santa Catharina). The single most important factor in the new classification was the plots of various elements against gold content. The IAB and IIICD irons cannot be separated by element-nickel plots, but element-gold plots achieve this end.

References

Bevan,AWR (2006) The Western Australian Museum meteorite collection. In `The History of Meteoritics and Key Meteorite Collections: Fireballs, Falls and Finds' (McCall,GJH, Bowden,AJ and Howarth,RJ editors), Geol.Soc. Spec.Publ. 256, 513pp., 305-323.

Buchwald,VF (1975) Handbook of Iron Meteorites. University of California Press, Berkeley, 3 volumes, 1418+8pp. Note: a collector's item for years now, the Handbook became available in a scanned digital edition in 2013. The 3 volumes were scanned at the University of Hawaii: all 1,426 pages, 2,124 figures, 8 appendices and a supplement. This unique reference carries detailed information on some 600 iron meteorites - see book announcement by Ed Scott in Meteoritics & Planetary Science 48 no.12, p.2608, 2013.

Grady,MM, Pratesi,G and Moggi-Cecchi,V (2014) Atlas of Meteorites. Cambridge University Press, 373pp.

Greenwood,RC, Burbine,TH, Miller,MF and Franchi,IA (2017) Melting and differentiation of early-formed asteroids: the perspective from high precision oxygen isotope studies. Chemie der Erde 77, 1-43.

Rubin,AE and Ma,C (2017) Meteoritic minerals and their origins. Chemie der Erde 77, 61pp., in press (2017).

Scott,ERD (1977) Composition, mineralogy and origin of group IC iron meteorites. Earth Planet.Sci.Letts. 37, 273-284.

Wasson,JT and Kallemeyn,GW (2002) The IAB iron-meteorite complex: a group, five subgroups, numerous grouplets, closely related, mainly formed by crystal segregation in rapidly cooling melts. Geochim.Cosmochim.Acta 66, 2445-2473.

Weisberg,MK, McCoy,TJ and Krot,AN (2006) Systematics and evaluation of meteorite classification. In `Meteorites and the Early Solar System II' (Lauretta,DS and McSween,HY editors), University of Arizona Press, Tucson, 943pp., 19-52.