Figs. 1-2: Left: A small etched slice of St-Aubin. Note the gilt (metallic, brassy) lustre to margins of the lamellae of the Widmanstatten pattern, especially in the left-hand image. Note also angular schreibersite crystals on left and top margins. This mineral is a phosphide, ideal formula (Fe,Ni)3P, first identified in the Copiapo IAB iron. Right: Reverse face of slice. The margins of schreibersite, which appears to grow into the host metal, are very sharp.
Samples from Blaine Reed. The one shown above is 27.6 grams, maximum dimensions 35 x 35 x 4 mm. That in Figures 3-4 is 30.0 grams, 46 x 27 x 3 mm.
"Rock of the Month # 254, posted for August 2022" ---
The St-Aubin iron meteorite
was ploughed up in fields covering a strewnfield in the Champagne region of France, in 1968. Languishing in a barn, it was not identified as a meteorite until 2002. Additional pieces were found. The Meteoritical Bulletin reported TKW (total known weight) is 472 kg, in five individual masses. St. Aubin was identified as an ungrouped iron in Met.Bull. 87 (2003), and was later classified as a IIIAB iron (Met.Bull. 94, Meteoritics & Planetary Science 43, p.1571, 2008). For a very interesting iron, St-Aubin seems to figure in relatively few works, an exception being the beautifully-illustrated introduction to meteorites by Alain Carion (2009, p.54). Some work on chromite in St-Aubin is noted below. Nishimura et al. made an extensive study of rare gas abundances and isotopic compositions in St-Aubin, measuring He, Ne, Ar, Kr and Xe isotopes.
This IIIAB iron has reported bulk Ni of 11.5 wt.%, plus 0.4% Co and 0.2% P. In structural terms it is identified as a fine octahedrite. Quite nickel-rich, it also carries a substantial amount of phosphorus. Why is this? Well, it contains schreibersite crystals up to 6-10 cm long (contributing to the Ni and P contents) and some troilite (FeS). It also contains two phosphates: sarcopside, FeII3(PO4)2 and graftonite, (FeII,Mn,Ca)3(PO4)2. Haack et al. (1993) noted that the distribution of P is of particular interest, as it has a very high diffusion rate (2 orders of magnitude faster than Ni). At least 13 species of phosphates are known in IIIABs (Olsen et al., 1999). Four (sarcopside, graftonite, johnsomervilleite and galileiite) constitute the majority of occurrences The most common associate of phosphates in troilite nodules is chromite. Ion microprobe studies of Cr isotopes in some IIIAB irons suggest that the short-lived radionuclide 53Mn (half-life circa 3.7 million years) "was alive at the time of IIIAB iron formation" (Sugiura and Hoshino, 2003).
St-Aubin metal displays signs of shock, with Neumann lines and shock-hatched kamacite. The Nyaung iron (Ray et al., 2016) contains some large sulphide inclusions (troilite and daubreelite), and the metallography (including Neumann lines and polygonized structures) is consistent with a late shock event.
A remarkable feature of St-Aubin is the sporadic occurrence of large euhedral (pseudohexagonal) chromite crystals up to 30 mm in diameter, hosted in the metal bulk of the meteorite (Fehr and Carion, 2004; Kurat et al., 2005). The crystals are twinned on  according to the spinel law. The coarse chromite in St-Aubin is near-pure, the principal impurity being not common cations such as Mg or Al but V (0.73 wt.% V2O3). Chromite occurs in other irons (e.g., Fehr and Carion, 2004, microprobe data), such as Landes (IAB-MG), Bocaiuva (ungrouped iron) and Miles (IIE). It is also noted in a range of pallasites, such as Brahin and Brenham (e.g., Wasson et al., 1997).
Figs. 3-4: Left: A second small, etched slice of St-Aubin. Again, note the gilt tinge to margins of lamellae in the Widmanstatten pattern. Right: Reverse face of slice. In each photograph, note the pale angular phase growing along, parallel to, the kamacite plates: most probably troilite.
The IIIAB irons
Iron meteorites are readily distinguished from pretty much all terrestrial stones. Thus they are well-represented in collections. But they are not that common: the Meteoritical Bulletin, as of 17 June 2022, lists 69,733 characterized meteorites, of which 1,328 (1.90 percent) are irons. A total of 338 (25% of all irons) are classified as "IIIAB", the most abundant class of irons. In some collections, irons are rarer than the nominal two percent, or 1 in 50 finds and falls. The iron meteorite Shisr 043 was found in the south Oman desert, the first iron amongst >1,400 meteorites found in Oman (Al-Kathiri et al., 2006). Some of the IIIAB class are giants: the largest example is Cape York (Greenland, 58.2 tonnes), the third-largest the iconic Willamette (Oregon, 15.5 tonnes). Eleven in all have TKW of 1 tonne or more, while 24th and 25th in the list, by weight, are Boxhole (Northern Territory of Australia, at 500 kg) and St-Aubin.
IIIAB iron meteorites are magmatic irons, whose observed chemical variations are consistent with fractionation in a melt, analogous to many terrestrial silicate rocks that crystallized in magma chambers at different depths in the Earth's crust. IIIAB irons are the largest group of irons. They display wide chemical variation, including a 3-order-of-magnitude range in Ir contents, indicative of fractional crystallization of a metallic magma (Haack and Scott, 1993; Ulff-Moller, 1998; Wasson, 1999). Wasson observed that "because S is essentially insoluble in metal, the abundance of FeS is a measure of the fraction of trapped liquid". The parent magma of the IIIAB group may have had a modest bulk S content, about 2 wt.%. Wasson made a visual estimate of the FeS (and so S) content of irons by modal estimation of troilite abundance in sawn faces between 136 and 19,000 cm2 in area - the preferred minimum area is 200 cm2 - the largest face was on the Agpal mass of Cape York. Wasson and Richardson (2001) also compared the chemical trends in IIIAB irons with data on many IVA irons, of which the largest is Gibeon.
Troilite-dominated sulphide inclusions, formerly known as Reichenbach lamellae, may be rimmed by schreibersite and swathed by kamacite. These formed from residual sulphide-dominant melt after crystallization of the metal at higher temperatures (Brett and Henderson, 1967). These should not be confused with large platy schreibersites in irons, known as Brezina lamellae, distinct from the smaller crystallites known as rhabdites, both formed during the cooling of the host iron (Doan and Goldstein, 1969). See Wasson (1999, p.2886) for a fine photograph of an etched slice of the Buenaventura IIIAB iron with both linear and branching Brezina lamellae.
Yang et al. (2010) made a detailed comparison of some 34 pallasites with 6 IIIAB irons. In some meteorites, the classes seem juxtaposed, e.g., metal veins cutting classic pallasite textures in Seymchan. The chemical composition of metal in most main group pallasites is close to that in Ni-rich IIIAB irons. However, the IIIAB irons all cooled significantly faster than main group pallasites, such that most pallasites could not have formed at the core-mantle boundary of that body, up to now the most commonly cited mode of pallasite genesis. Furthermore, over 80% of IIIAB irons show shock features not seen in main group pallasites.
Al-Kathiri,A, Hofmann,BA, Gnos,E, Eugster,O, Welten,KC and Krahenbuhl,U (2006) Shisr 043 (IIIAB medium octahedrite): the first iron meteorite from the Oman desert. Meteoritics & Planetary Science 41, A217-230.
Brett,R and Henderson,EP (1967) The occurrence and origin of lamellar troilite in iron meteorites. Geochim. Cosmochim. Acta 31, 721-730
Carion,A (2009) Meteorites. Alain Carion, Paris, 3rd edition, translated from the French by Anne Black, 72pp.
Doan,AS and Goldstein,JI (1969). The formation of phosphides in iron meteorites. In "Meteorite Research" (Millman,PM editor), Springer Verlag, New York, 940pp., 763-779.
Fehr,KT and Carion,A (2004) Unusual large chromite crystals in the Saint Aubin iron meteorite. Meteoritics & Planetary Science 39, A139-141.
Haack,H and Scott,ERD (1993) Chemical fractionations in Group IIIAB iron meteorites: origin by dendritic crystallization of an asteroidal core. Geochim. Cosmochim. Acta 57, 3457-3472.
Haack,H, Scott,ERD, Rubio,GS, Gutierrez,DF, Lewis,CF, Wasson,JT, Brooks,RR, Guo,X, Ryan,DE and Holzbecher,J (1993) Systematic chemical variations in large IIIAB iron meteorites: clues to core crystallization. Lunar and Planetary Science 24, 593-594.
Kurat,G, Zinner,E and Varela,ME (2005) Trace element abundances in Saint-Aubin (UNGR iron) giant chromite and associated phases. Meteoritics & Planetary Science 40, A88.
Nishimura,C, Matsuda,J and Kural,G (2008) Noble gas content and isotope abundances in phases of the Saint-Aubin (UNGR) iron meteorite. Meteoritics & Planetary Science 43, 1333-1350.
Olsen,EJ, Kracher,A, Davis,AM, Steele,IM, Hutcheon,ID and Bunch,TE (1999) The phosphates of IIIAB iron meteorites. Meteoritics & Planetary Science 34, 285-300.
Ray,D, Ghosh,S and Murty,SVS (2015) Evidence of shock pressure above 600 kilobar and post-shock annealing in Nyaung IIIAB octahedrite. J.Geol.Soc.India 85, 153-162.
Sugiura,N and Hoshino,H (2003) Mn-Cr chronology of five IIIAB iron meteorites. Meteoritics & Planetary Science 38, 117-143.
Ulff-Moller,F (1998) Effects of liquid immiscibility on trace element fractionation in magmatic iron meteorites: a case study of group IIIAB. Meteoritics & Planetary Science 33, 207-220.
Wasson,JT (1999) Trapped melt in IIIAB irons: solid/liquid elemental partitioning during the fractionation of the IIIAB magma. Geochim. Cosmochim. Acta 63, 2875-2889.
Wasson,JT and Richardson,JW (2001) Fractionation trends among IVA iron meteorites: contrasts with IIIAB trends. Geochim. Cosmochim. Acta 65, 951-970.
Wasson,JT, Ulff-Moller,F, Lange,DE and Francis,CA (1997) Chromite and the fractionation of chromium in IIIAB iron meteorites and pallasites. Meteoritics & Planetary Science 32 no.4, supplement (Abs. 60th annual meeting, Hawaii), 137.
Yang,J, Goldstein,JI and Scott,ERD (2010) Main-group pallasites: thermal history, relationship to IIIAB irons, and origin. Geochim. Cosmochim. Acta 74, 4471-4492.
Graham Wilson, posted 17-18 June 2022, tweak of text on 19 June 2022.
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