"Rock of the Month #188, posted for February 2017" ---

The brachinites, a rare class of olivine-rich achondrites,

are represented by just 41 confirmed examples, out of more than 55,000 classified meteorites (Meteoritical Bulletin, as of 26 January 2017). These rarities, less than one per thousand meteorites, include 29 NWA examples, 6 from Australia, 4 from Antarctica, and one each from Oman and Western Sahara. Only eight exceed 1 kg in TKW (total known weight), with the largest being NWA 7904 (6330 grams) and NWA 7828 (4770 grams: now at the Royal Ontario Museum). Twenty of the others are smaller than Brachina (see below), TKW in the range 4 to 200 grams.

The table shows very similar mineral chemistry of the silicates olivine (mole proportion of fayalite, Fa_27-36) and orthopyroxene (mole proportion of (ortho)ferrosilite, Of/Fs) in these meteorites. Fayalite and ferrosilite are the iron-rich end-member molecules of, respectively, the olivine and orthopyroxene solid solution series. This is not, in itself, sufficient to be sure that the two unclassified meteorites (labelled *) are both brachinites like NWA 10637.

The small (203-gram) meteorite Brachina was found in South Australia in 1974. The two pieces together would have been about the size of a large plum: not terribly imposing. Olivine-rich, it was identified as a chassignite, one of a vanishingly-rare class of what would soon be recognized as dunites (olivine-dominant, metal-poor, ultramafic igneous rocks) from Mars. Nehru et al. (1983) examined Brachina, a a fine-grained igneous rock, composed of unshocked grains 0.1-0.3 or at most up to 0.9 mm in size. They estimated that the Brachina material they examined contained an estimated 79.1% Fa₃₀ olivine, 1.6% orthopyroxene, 5.4% clinopyroxene, 9.7% plagioclase feldspar, 0.5% chlorapatite, 0.5% chromite, 2.9% troilite, 0.3% pentlandite, and a trace of taenite. The detailed chemistry suggested a new class of meteorite, a classification accepted in due course (Wlotzka, 1992). Only one prior find, Eagles Nest (1960), was subsequently recognized as a brachinite. From 1960 to 1999 just ten small examples were identifed: six from Australia and four from Antarctica, all smaller than 400 grams. The rarity of brachinites and similar olivine-rich achondrites explains why the MINLIB bibliographic database currently has just 44 records flagged for brachinites, 1983-2016, in 87,437 records, 31 January 2017. Being such a small and (relatively) recently-recognized group, brachinites receive short summaries in some of the most useful general references on meteorites (e.g., McSween, 1999; Hutchison, 2004; Lauretta and Killgore, 2004; Krot et al., 2005; Grady et al., 2014) and surveys of achondrites (Mittlefehldt et al., 1998; Mittlefehldt, 2005). The physical properties of achondrites, including some brachinites, have been compiled by, e.g., Rochette et al. (2009) and Macke et al. (2011).

Figure 2. Inverted-tone false-colour image of the 19x15 mm slice of NWA 6292 in the covered thin section. This gives an impression of the texture, with predominant finely granular olivine (black) and interstitial minor and accessory minerals. Hyde et al. (2014) examined brachinite NWA 4872, and determined that, in addition to the primary mineralogy, weathering products include marcasite, hematite, magnetite and Fe phosphate. The thin section of NWA 6292 is dominated by (visual estimate) circa 87 vol.% olivine, as generally unstrained, equant grains 0.1-1.2 mm in size. The section also displays 1% equant, opaque (?) chromite, 2% pyroxenes, and 10% secondary Fe oxyhydroxides, ascribed to terrestrial weathering along silicate grain boundaries and fractures.

Research on Brachinites and similar Achondrites

Burbine et al. (1996) noted the scarcity of olivine-dominated mantle material in meteorite collections. It has long been argued as to whether our collections reflect accurately the "demographics" of the complete population of the asteroid belt (Meibom and Clark, 1999). Real or apparent rarities such as the brachinites may represent evidence for the existence of metal-free olivine-dominated asteroids. The textures and mineralogy of brachinites are consistent with formation as igneous cumulates, some with indications of orientation of olivine crystals during magmatic flow (Mittlefehldt et al., 2003; Irving et al., 2005). Some have been identified as orthocumulates or heteradcumulates, implying modes of crystallization documented in terrestrial magma chambers. 120-degree triple junctions between grains are quite common in brachinites and some of the other small groups of ultramafic achondrites, suggesting that some or most of them have undergone recrystallization. Trace elements in bulk and mineral grains in `brachinites are distinct from those of acapulcoite- lodranite clan meteorites, a suite of high-grade metamorphic rocks and anatectic residues'. Brachina, an unshocked example, has excess ¹²⁹Xe attributable to the presence of radionuclide ¹²⁹I at the time of formation. It seems that brachinites formed early, within a few million years of chondrite formation.

Figure 3. Two photomicrographs of NWA 6292 in crossed-polarized, transmitted light. Long-axis field of view 1.7 mm. As noted in Figure 2, the primary mineralogy is largely olivine plus minor clinopyroxene, with abundant, opaque secondary iron oxyhydroxides due to terrestrial weathering.

Selected References, in chronological order

Nehru,CE, Prinz,M, Delaney,JS, Dreibus,G, Palme,H and Wanke,H (1983) Brachina: a new type of meteorite, not a chassignite. Lunar and Planetary Science 14, 552-553.

Wlotzka,F (1992) The Meteoritical Bulletin, No.73. Meteoritics 27, 477-483 (1992).

Burbine,TH, Meibom,A and Binzel,RP (1996) Mantle material in the main belt: battered to bits? Meteoritics & Planetary Science 31, 607-620.

Mittlefehldt,DW, McCoy,TJ, Goodrich,CA and Kracher,A (1998) Non-chondritic meteorites from asteroidal bodies. In `Planetary Materials' (Papike,JJ editor), Min.Soc.Amer. Reviews in Mineralogy 36, chapter 4, 195pp.

McSween,HY (1999) Meteorites and their Parent Bodies. Cambridge University Press, 2nd edition, 310pp.

Meibom,A and Clark,BE (1999) Evidence for the insignificance of ordinary chondritic material in the asteroid belt. Meteoritics & Planetary Science 34, 7-24.

Mittlefehldt,DW, Bogard,DD, Berkley,JL and Garrison,DH (2003) Brachinites: igneous rocks from a differentiated asteroid. Meteoritics & Planetary Science 38, 1601-1625.

Hutchison,R (2004) Meteorites: a Petrologic, Chemical and Isotopic Synthesis. Cambridge University Press, 506pp. (2004).

Lauretta,DS and Killgore,M (2004) A Color Atlas of Meteorites in Thin Section. Golden Retriever Publications / Southwest Meteorite Press, 301pp.

Irving,AJ, Kuehner,SM and Rumble,D (2005) Brachinite NWA 3151 and (?) brachinite NWA 595. Meteoritics & Planetary Science 40, A73.

Krot,AN, Keil,K, Goodrich,CA, Scott,ERD and Weisberg,MK (2005) Classification of meteorites. In `Meteorites, Comets, and Planets' (Davis,AM editor). Treatise on Geochemistry volume 1 (Holland,HD and Turekian,KK editors), Elsevier- Pergamon, Oxford, 737pp., 83-128.

Mittlefehldt,DW (2005) Achondrites. In `Meteorites, Comets, and Planets' (Davis,AM editor). Treatise on Geochemistry volume 1 (Holland,HD and Turekian,KK editors), Elsevier- Pergamon, Oxford, 737pp., 291-324.

Rochette,P, Gattacceca,J, Bourot-Denise,M, Consolmagno,G, Folco,L, Kohout,T, Pesonen,L and Sagnotti,L (2009) Magnetic classification of stony meteorites: 3. Achondrites. Meteoritics & Planetary Science 44, 405-427.

Macke,RJ, Britt,DT and Consolmagno,GJ (2011) Density, porosity, and magnetic susceptibility of achondritic meteorites. Meteoritics & Planetary Science 46, 311-326.

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

Hyde,BC, Day,JMD, Tait,KT, Ash,RD, Holdsworth,DW and Moser,DE (2014) Characterization of weathering and hetereogeneous mineral phase distribution in brachinite Northwest Africa 4872. Meteoritics & Planetary Science 49, 1141-1156.

Graham Wilson, 27-31 January 2017, updated 15 March 2017 (microscopy to follow)

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