BRAZIL - GO
Iron Meteorite - IIAB Coarsest Octahedrite.
The etched sections display a beautiful mix of a very thick Widmanstätten pattern and apparently equiaxial kamacite grains. The kamacite lamellae are very short and bulky and have a width of 5 ± 2 mm. The oriented lamellae structure is not pronounced because significant amounts of kamacite were developed around numerous eutectic schreibersite-metal-troilite before the Widmanstätten pattern could grow. Eutectic, including kamacite, cover - on average - 30% of available sections. The interfaces between the swathing and the lamellar kamacite areas are ragged because some late grain boundary adjustment has taken place the grain boundaries. It is always the swathing kamacite that has expanded at the expense of the widmanstiitten pattern. The kamacite has well-developed sub-borders, decorated by 1-2 µm thick rhabdites . Neumann lines are common, some of them being decorated by rows of 1-2 µm thick rhabdites. The hardness is 175±8, decreasing to 150±5 in the nickel- and phosphorus-depleted a-zones around schreibersite. Taenite and plessite cover less than I% by area, mostly situated in the grain boundaries but occasionally completely embedded within one uniformly oriented swathing kamacite grain. The comb plessite attains sizes of 3 x 2 mm and it exhibits pearlitic, spheroidized and martensitic areas. Schreibersite is the essential component of the eutectics where it forms 1-5 mm wide, winding ribbons with an occasional crystallographic facet. Its hardness is 925±25. Chromite is locally present as 0.5-1 mm wide euhedral cubes with rounded faces. Schreibersite is further common as 20-200µm wide grain boundary veinlets. Rhabdites are very common, perhaps mairliy as 0.5-2µm thick prisms in the kamacite interior. The Neumann lines are narrow, 0.5-1 µm, in the rhabdite-loaded areas but are as wide as 10µm in the rhabdite-free zones near schreibersite. Troilite forms long subparallel stringers and irregular plates through the whole mass. The exposed surface of the main mass in Rio de Janeiro (Vidal 1931: 20) clearly shows that the sulfide is oriented parallel to the topside and underside of the main mass, thus suggesting a mild hot working with compression and some shearing. The texture suggests that the mass was deformed during slight movements of the overburden on the parent asteroid. Source: Buchwald (1975).
6.60% Ni, 0.47% Co, 0.9% P, 48 ppm Ga, 110 ppm Ge, 0.010 ppm lr. Source: Buchwald (1975).
Not reported by the Meteoritical Bulletin Database. According to Buchwald (1975), the meteorite was studied by Meen (1939), Perry (1944), Curvello (1950a), Henderson (1965), Vilcsek & Wanke (1963) and Chang & Wanke (1969).
According to Vidal (1931) the first vague information about this meteorite reached Rio de Janeiro's Mineralogical Museum in 1922, but because of the unsettled situation in the country, no action was taken. A fragment was exhibited in 1922 at the International Exposition in Rio which celebrated Brazil's centennial and it was awarded a bronze medal. The specimen was sold to the United States and is probably identical to the 4.5 kg individual which was acquired in 1925 by the Field Museum in Chicago. Six slices were cut from it in the Smithsonian Institution in 1926, so the remaining mass now weighs 2.5 kg (exchange of letters between Farrington and Merrill, Accession Number87821 ; Horback&Olsen 1965 : 291). The main mass of 1,890 kg had evidently been well known to the local population for many years when it was excavated and transported to Rio de Janeiro's National Museum in 1928. The mass was lightly buried in micaceous schists in a small ravine, 20 km north-northwest of Santa Luzia (presently called Luziania) in the Federal District. Vidal (1931) gave a map and a colorful discription of the conveyance on a primitive carriage drawn by 18 oxen and assisted by 30 men. He quoted various incidents to indicate that the mass had fallen in June 1919, and this information is still retained in some catalogs (Horback & Olsen 1965: 291 ). However, the corrosion is so thorough that the terrestrial age must be counted in tens of thousands of years. Vidal provided two figures of the exterior. In 1936 Ward's Natural Science Establishment imported two fragments of 18.4 and 9 kg which were acquired by the Smithsonian Institution (Nos. 1275 and 1219). They had been collected at the Upper Santa Maria River, 24 km from Santa Luzia, and had been found on the surface only one kilometer apart (Accession Numbers 143643 and 146466 in the Smithsonian Institution). This locality is only 2 or 3 km east of the main mass as may be seen by examining the original map sketch by Vidal (1931). Finally, Curvello (1950a) described a 1.64 kg individual which had been found on the surface only 100 m from the large mass. He gave a photograph· of the exterior and two photomicrographs. The shower has thus provided a large main mass and four small fragments scattered around it up to a few kilometers distance. Of course, more may be present but covered by soil or unreported.
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