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Met Name

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Thin Sections

  Eucrite-mmict

TKW 2.5 kg. Observed fall 30 June 1957, Minas Gerais, Brazil.

It was originally intended for this to appear with yesterday's MPOD but it proved to be impossible for technically unexplainable reasons.

From the MetBul:

The fireball passed northwestward and accompanied with a noise like the reverberation of thunder has been observed. This phenomenon has been marked in the radius about 160 km. At the end point of the trajectory the fireball was broken up and disappea­red at the height about 10 or 12 km above the earth’s surface. The meteorite was found near the predicted point of the fall, in the village Ibitira. It lay in a hole in the ground about 25 cm deep and 20 cm in diameter. The meteorite has a brilliant black crust, and a light brown (interior).

 

Steve writes:

Ibitira – the earliest-recovered and best known of the vesicular meteorites – is an anomalous, unbrecciated basaltic monomict eucrite that fell 78 years to the day after Nogoya, and only a few thousand miles north in Minas Gerais, Brazil. At around 5:15 PM local time, eyewitnesses saw a fireball pass northwestward and heard accompanying noises described as comparable to the reverberation of thunder, with the fireball going dark approximately 10 km above the ground. An amateur astronomer who also witnessed the fall teamed with the César Lattes Center for Astronomical Studies to send letters to all city halls in the region, asking if they’d talk with witnesses to discern the direction in which the bolide was seen to travel. Using the feedback then received, they were they able to estimate the fall’s endpoint as the Martinho Campos area, near the small village of Ibitira.

While in nearby Belo Horizonte, a team from the center learned of a boy in the region who’d heard what he described as the sound of thunder followed by a whistle like that of a passing bullet – a noise sufficient to send nearby cattle running from the pasture over the hill into the valley. Then moments later, he claimed to have heard the sound of something hitting the ground, but he didn’t know where. An initial search of the area by the team was unsuccessful, so they returned to Belo Horizonte and organized a caravan for another search a few days later on August 3rd. But in the interim, they learned of a farmer who, while gathering firewood, had already found a mostly fusion-crusted and flight-marked 2.5 kg mass in what he described as a 20 cm wide, 25 cm deep hole in the ground. He thought the stone to be strange but didn’t recognize it as a meteorite, so he gave it to a local pharmacist. In fact, a subsequent analysis by the Technological Institute of Belo Horizonte gave its full petrological description but surprisingly didn’t classify it as a meteorite. That recognition came later in December 1957 when it was published in the Meteoritical Bulletin no. 6 (though no report was made regarding who submitted the classification information). No other pieces of the meteorite have since been found (the site of the fall is a savannah that’s hard to access and explore, making subsequent searches difficult).

Eucrites are the most common of the HED meteorites, which scientists believe mostly originated on asteroid 4 Vesta (the fourth asteroid ever discovered, having been first recognized by German astronomer Heinrich Wilhelm Olbers in March, 1807). Eucrites are thought to have formed on the surface of that asteroid as molten magma cooled. Besides its unusual open vugs (which in Ibitira previously contained free-growing crystals, but in other meteorites, when present, are usually just atmospheric ablation holes), there are other indications that Ibitira – like Pasamonte – originated on a different parent body: its plagioclase is Ca-dominated anorthite (most eucrites contain Na-rich albitic plagioclase); its sparse iron is nickel poor (less than one percent – unusual for meteoritic iron); and its oxygen isotope and Fe/Mn ratios are atypical of other eucrites. In fact, Ibitira is currently believed to represent a fifth and totally different parent body, where it formed by rapid cooling of a magma flow between two and twenty meters thick, possibly as a horizontal layer of intruded rock between two older layers. (There is some work that suggests Agoult may be related to Ibitira due to elemental, chemical, and textural similarities, but Ibitira appears to be slightly older.)

I don’t have any actual pieces of Ibitira but I do have a thin section, which I’ve scanned both in xpol and white light (Photos 1 and 2 respectively). Comparing them, one can easily see Ibitira’s many vugs and vesicles – five to seven percent by volume of the original stone – which is unusual in meteorites given that most don’t crystallize from melts (in fact, since vugs and vesicles are so uncommon in meteorites, many will say that one way to identify meteor "wrongs" is by the presence of these kinds of voids). Interestingly, their density varies throughout the meteorite, sometimes changing quite quickly between dense and sparse. And while Ibitira also has occasional flecks of metal, the dark areas seen in the white light thin section (Photo 2) are actually opaque minerals (in Ibitira, these include chromite, ilmenite, and iron titanium dioxide). Photo 3 shows one atypical feature of this thin section, and Photos 4 and 5 show two different areas with vugs – one obviously in xpol and one in white light. And for those that would like to pan around and zoom in and out at whatever areas they choose, the xpol and white light Gigapan links are here and here, respectively.