PONTES E LACERDA
BRASIL - MT
Iron Meteorite - IIIAB Medium Octahedrite.
(Maria Elizabeth Zucolotto, MN/UFRJ) The examined slice is a 9 cm2 endpiece with no fusion crust and heat affected zone. It shows exfoliation along corroded Widmanstatten planes, typical of long terrestrial exposure. Etched sections display a normal, medium Widmanstatten structure with 1.05±0.15 mm bandwith kamacite. Some subboundaries are decorated with tiny rhabdites. In a heavy etching the kamacite shows abundant cross-hatched pattern. Taenite and plessite are common and found in about 60% of the slice area, mainly as comb and net plessite. Martensite and black etching duplex fields are also common at the edges. Neumann bands are weakly developed. Schreibersite (to 500 μm) was found in the interiors and borders of kamacite bands. Rhabdites are very common as 1-5 μm thick prisms. No troilite was observed in the examined section, although there were two holes possibly as lost inclusions.
(J. T. Wasson UCLA) INAA, 8.1 wt% Ni; 0.49 wt% Co; 103 ppm Cr; 181 ppm Cu; 20.8 ppm Ga; 5.43 ppm As; 4.84 ppm Ir; 0.738 ppm Au. Composition of major phases (A. Tosi, IGEO-UFRJ) kamacite (Ni 7.2±0.2 %, Co 0.65±0.15%, P 0.02%).
This meteorite can be compared to other IIIAB iron having Au in the range 0.686 to 0.770 ppm. No similar IIIAB has been found in South America. The only one that has a similar composition is Hot Springs, but even it is resolvable in terms of As and probably also Au.
Wasson J. T., Zucolotto M. E. and Tosi A. A.
The meteorite was found by a prospector, Mr. M. Braga, while he was searching for gold with a metal detector. The finder sent samples to Andre Moutinho who performed preliminary tests that confirmed the samples have a meteoritic origin. Samples were then provided to Dr. Zucolotto for a complete analysis and classification.
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