PALMAS DE MONTE ALTO

Iron

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ACHONDRITE

MEDIUM OCTAHEDRITE

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IIIAB

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BRAZIL - BA

1954

Iron Meteorite - IIIAB Medium Octahedrite.

PETROGRAFIA:

The piece is covered by a fusion crust of brown color with signs of intense oxidation, mainly in the mentioned protuberance, in which the crust has been entirely removed. The meteorite exhibits numerous rounded or elongated depressions, called regmalites, whose dimensions vary from 20 to 100 mm in diameter. At the end of the protuberance there is an oval-shaped hole measuring 33 × 24 mm that runs through the entire metallic mass. There is another circular hole 47 mm in diameter located near the end that has the protuberance. This second hole does not cross the entire mass like the first. Both were probably originally filled with the troilite mineral (FeS), which vaporized during the passage of the meteorite through the Earth`s atmosphere, in view of its low melting point (1,188ºC) in relation to that of the Fe-Ni alloy, which melts at 1,500ºC , and this temperature may vary slightly depending on the Ni concentration. The internal surface of the meteorite observed after sawing a small slice had a metallic gray color similar to that of steel, with no signs of oxidation on the internal part of the main body. A polished section of the Palmas de Monte Alto meteorite was treated with nital to attack the Fe-Ni alloy components of the meteorite. The treatment revealed a well-defined Widmanstätten pattern, with kamacite lamellae with an average width of 0.95 ± 0.15 mm, without orientation correction and length-to-width ratio of around 15 times. Evaluating the polished slice under the petrographic microscope, in reflected light, there are numerous of Neumann lines and hatched kamacite, which are indicative of structural changes resulting from shock while the meteor wandered through space. Bands of kamacite with texture of sub-grains decorated with phosphides are also present, as well as taenite and plessite - mixture of taenite with kamacite, - and represent between 25 and 35% of the examined area, presenting themselves in martensitic, black, net and "comb" forms. Through microscopic examinations, in addition to the metallic phases Fe-Ni - kamacite, taenite and plessite -, inclusions of chromite (FeCr2 O4), troilite (FeS), and schreibersite [(Fe, Ni) 3 P]. Three distinct varieties of kamacite have been identified: decorated with phosphide sub-grains, with crosshatched sipes and with ε-structure. The latter type shows the effects of high intensity shocks. Source: De Carvalho et al. 2018.

GEOQUÍMICA:

According to the determinations using EDS / MEV in eight crystals, the Ni content in kamacite in the Palmas de Monte Alto meteorite varies from 5.8 to 7.3%, in accordance with the values found for kamacite of the members of the IIIAB group ( 5,5% < Ni > 7,5%; Scott and Wasson, 1975). In taenite and plessite, the content of this element rises to an average of 22.4% (ranging from 15,3 < Ni < 31,9%), with some contents slightly below the lower limit of the average IIIAB iron meteorites (25% < Ni < 50%). These analyzes identified the presence of the mineral phase roaldite (Fe, Ni)4N, which develops as plaques in the kamacite phase in the vicinity of the filled shaft. Schreibersite (Fe, Ni)3P, identified in the petrographic analysis, was also analyzed by SEM / EDS and EPMA, revealing average contents of the elements Fe (57.2%), Ni (27.0%) and P (14, 7%). The meteorite was analyzed by INAA at the University of California at Los Angeles (UCLA), which obtained the following concentrations: Ni (9.40%), Ga (22 ppm), Co (0.54%), Ir (0.70 ppm), Au (1.70 ppm) and As (16 ppm). Source: De Carvalho et al. 2018.

CLASSIFICAÇÃO:

Chemical and petrographic analyses show that, depending on the width of the kamacite bands present in the Widmanstätten pattern and nickel content found, this rock represents a medium octahedrite of the IIIAB group. Source: De Carvalho et al. 2018.

CLASSIFICADORES:

M. E. Zucolotto

HISTÓRIA:

In an article for the Revista do Observatório, geologist Orville Derby (1888a, 1888b, 1888c) mentioned a statement received by the National Museum about a probable iron meteorite similar to Bendegó, found in the Monte Alto mountain range, in southwest Bahia. The occurrence was never investigated or proven until, in 2007, professor of paleontology Douglas Riff, from the State University of Southwest Bahia (UESB), on a field mission, had his attention awakened by reports of the existence of a mass of iron stored at the Municipal School Marcelino Neves, in the Palmas de Monte Alto city. Riff noticed many similarities between the iron mass that was presented to him and the Bendegó meteorite exhibited at the National Museum, in Rio de Janeiro, leading him to communicate the fact to Elizabeth Zucolotto, who, immediately, volunteered to travel to Bahia to go to the city of Palmas de Monte Alto accompanied by Wilton Carvalho, in order to collect samples and gather information about the finding. The field mission took place in the week of July 2 to 5, 2008, with the fact that it was an iron meteorite that could be correlated to the one mentioned by Derby (1888a, 1888b, 1888c). The meteorite was found by the farmer Francisco da Cruz when extracting sap from mangabeiras at the top of the Monte Alto mountain range, at a point close to the coordinates 14º22&#39;08 ”South and 43º01’02” West. The exact location of the finding could not be georeferenced, given that the meteorite discoverer did not have physical conditions, in 2008, to go to the site, passing information to a son about the area where he found the iron mass. Carvalho was taken by Francisco&#39;s son to the indicated location, as it was an extensive area with many native mango trees, making it impossible to determine which of them Francisco was extracting sap when he saw the meteorite. According to Francisco&#39;s account, his attention was aroused by an irregularly shaped “stone” “similar to an old model sewing machine”. Francisco hit the “stone” with the tool he used to bleed the mangabeira, producing a metallic sound characteristic of iron blows against iron. He then tried to move the “stone” and failed, due to its large weight. Despite not knowing exactly when this occurred, Francisco said he found the meteorite when he was a boy, before getting married. Based on the marriage certificate, Francisco da Cruz was born in 1924 and married in 1955. Thus, the year of the find was before 1955 and, if we consider a 20 year old boy, the estimated period of the find would be between 1940 and 1954. The probability of the finding occurring in the 1940s is reinforced by other comments from former residents of the city of Palmas de Monte Alto, who estimate the time when the “stone” arrived in the municipality in about 60 years. To remove the 97 kg iron mass, Francisco had the help of two other men, who transported the meteorite to the city of Palmas de Monte Alto using a strong blanket to improvise a net, in which the piece was deposited and suspended by a long, sturdy pole, supported on the shoulders. For many years, the meteorite remained under the custody of the Municipal School Marcelino Neves and, since 2008, has been under the custody of the city hall, deposited with the Secretary of Education. Source: De Carvalho et al. 2018.

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