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Brachina meteorite, Brachina, South Flinders Ranges, Flinders Ranges, South Australia, Australia

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Latitude & Longitude (WGS84): 31° 18' South , 138° 23' East
Latitude & Longitude (decimal): -31.30000,138.38333
Non-native locality type:Meteorite

Brachina, 1974 find; 202.85 g, 2 stones
Prototype for Brachina-like primitive achondrites

Two small olivine-rich stones found in May 1974 attracted very interested attention as the meteorite’s overall mineralogy was similar in certain respects to the Chassigny meteorite, a Martian dunite. Brachina is not a Martian meteorite - for starters oxygen isotope rations and Fe/Mn ratios in silicates rule out that possibility. However, meteoritic dunites and wehrlites (olivine-rich, but not monomineralic like dunites) are always interesting because as apparent plutonic rocks their excavation from an original homeworld asteroid would presumably require an energetic and penetrating impact. In time, several other meteorites have been found and today Brachina is the designated prototype for the ‘Brachinites’ — a small group of primitive achondritic meteorites. Brachina’s most obvious textural characteristic — its ‘achondritic’ signifier — are its predominantly equiangular olivine crystals clearly indicative of crystallization and equilibration from an igneous melt. However, its overall chemistry is roughly chondritic, indicating that it has not experienced the drastic differentiation of large and small terrestrial worlds — hence its designation as a ‘primitive’ achondrite. Brachina is ~80 vol% nearly homogeneous olivine, 10 vol% plagioclase, 5% augite, 3% sulfide (mostly troilite) with minor amounts of chlorapatite, chromite and glass. Trace amounts of pentlandite, orthopyroxene, and Fe-Ni Metal (51% Ni) are also found. Olivine, augite, and trace orthopyroxene are all magnesian in composition. The plagioclase is almost entirely oligoclase, but anorthitic glass is found. To be sure, trace constituents in olivine crystals and, even more diagnostic, minor and trace constituents in the interstitial plagioclase, augite, chromite, phosphates, sulfides, and Fe-Ni metal indicate that it has experienced episodes sufficient to both distinguish it from other achondrites and to remove significant amounts of plagioclase.

Brachina is more oxidized and apparently less ‘primitive’ than the Acapulcoite-Lodranite group of primitive achondrites and some have argued that Brachina should be considered a ‘differentiated meteorite’ but these are to some extents matters of degree and taste. More problematic at the present time is that ‘membership’ in the Brachinite group tends to be a somewhat unstable designation as several meteorites have been classified as Brachinites and then ‘removed’ from the class. Oxygen isotope, silicate FeO/MnO, and bulk Ga/Al ratios have all been considered - but how much weight should be given to these and other issues related to class boundaries are presently unresolved.

Not so controversial is the evidence that Brachina is a very ancient rock. Excess Xe-129 and Cr-53 plus fission track data indicate original aggregation ~ 4.6 billion years ago. Other isotope pairs used for radiometric dating (K-Ar , Sm-Nd, Rb-Sr) are disturbed and have been further complicated by terrestrial weathering. However, a strong signal in the K-Ar studies suggest an apparent event 4.1 billion years ago which may have resulted from a catastrophic world-disrupting impact.

Mineral List

6 valid minerals.

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Graham, A. L. [ed.] (1978) Meteoritical Bulletin, no. 55: Meteoritics 13 () :327-352. (Sept 1978)

Nehru, C. E., Prinz, M., Delaney, J. S., Dreibus, G., Palme, H. & Wanke, H. (1983) Brachina: a new type of meteorite, not a chassignite. Lunar and Planetary Science XIV: 552-553 (Mar 1983)

Nehru, C. E., Prinz, M., Delaney, J. S., Dreibus, G., Palme, H., Spettel, B. & Waenke, H. (1983) Brachina - a new type of meteorite, not a chassignite. Journal of Geophysical Research, Supplement. vol. 88: pp. B237-B244. (Nov 1983)

Mittlefehldt, D. W., McCoy, T. J., Goodrich, C. A. & Kracher, A. (1998). Non-chondritic meteorites from asteroidal bodies. In: Planetary Materials (Papike, Jay J., Editor): Chapter 4, 195 pages. Mineralogical Society of America: Washington, DC, USA. (1998)

Grady, M. M. (2000) Catalogue of Meteorites (5/e); Cambridge University Press: Cambridge, New York, Oakleigh, Madrid, Capetown. 689 pages.

Mittlefehldt, D. W., Bogard, D. D., Berkley, J. L., & Garrison, D. H. (2003) Brachinites: Igneous rocks from a differentiated asteroid. Meteoritics &Planetary Science 38 (11): 1601-1625. (Nov 2003)

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