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519.4 grams.   Iron, IAB-ung

TKW 24 MT. Observed fall: no. Found 1911, Western Australia.



   

Steve writes:

Before offering a description of Mundrabilla’s rich history, I wanted to apologize for the post-processing liberties I took with these photos when trying to better show the details of the metal. I have real problems photographing irons, and the incandescent lighting I used this time gave the entire meteorite a rusted look. In correcting that, I also ended up subduing the colors of the troilite. Rather than pull the slice out of the safe to properly reshoot it with better lighting, I’ll simply be lazy and note that the troilite’s color is more accurately represented in other MPOD posts on Mundrabilla. My principal intent with this post was to provide an excuse for offering additional information about this interesting meteorite.

In 1911, a single 112 gram iron meteorite was found by Mr. H. Kent in the Premier Downs area of Australia’s Nullarbor Plain. Initially named Premier Downs I, the meteorite was followed later that same year by another small 116 gram individual also found by Mr. Kent about 13 km west from the Premier Downs I find. Not surprisingly, it was called Premier Downs II and like its predecessor is a medium octahedrite iron. In 1914, Simpson and Bowley suggested that both meteorites were part of the same fall, and four years after that a 99 gram iron meteorite was found in the same area and named Premier Downs III.

Almost a half century passed before another small 108 gram iron with similar characteristics to the Premier Downs trio was found near Loongana Station by a Mr. Harrison sometime around 1962. In 1965, McCall and DeLaeter suggested its possible pairing with the previous Premier Downs samples. That same year, Mr. W. A. Crowle found 94.1 gram, 45.0 gram, and 38.8 gram meteorites 16 km north of Mundrabilla Siding that were also presumably related.

Separately, rumors of a large meteorite in the Nullarbor Plain had been circulating since as far back as 1944. In 1963, prospector T. Dimer described the meteorite as being “as big as a motor car” and claimed he could locate it. But it wasn’t until April of 1966 that geologists R. B. Wilson and A. M. Cooney – after a pair of previously unsuccessful expeditions – located two large masses just north of the Transcontinental Railway. The geologists discovered these masses protruding from very slight depressions in the clay soil approximately 180 meters apart and surrounded by a large number of small iron fragments. The larger mass weighed 9980 grams and the smaller one 5440 grams; they were later named, respectively, Mundrabilla I and Mundrabilla II.

Mundrabilla I was the largest meteorite yet found in Australia. It had a crude hemispherical shape and its nose was partially buried in the soil at an angle of approximately 60°. Evidence of fragmentation from a larger mass was suggested by a sharp, angular vertical face that matched a similar sharp face on Mundrabilla II. A preliminary study of the impact site suggested the meteorite came from the west at relatively low velocity and high angle.

In 1967, another small iron (66.5 grams) was found by Mr. W. H. Butler and named Loongana Station West. Then in 1970, McCall and Cleverly suggested the Mundrabilla meteorites were actually closely related to the Loongana Station and Premier Downs meteorites and had fragmented from the same mass during its atmospheric ablation. Today, all masses are considered part of the Mundrabilla fall. In all, some 22 tons of fragments of this ancient meteorite shower have been recovered across a 100 km strewn field, with more continuing to be found. But despite two more large pieces discovered in 1979 about 20 km east of the 1966 finds, Mundrabilla I is still the largest piece and is currently conserved at the Western Australia Museum in Perth.

As an aside, many meteorites from separate falls – mostly stones (including two howardites and a CV2 carbonaceous chondrite) – have been found on the Nullabor Plain, often as widely scattered individuals or in small groups. Many were recovered by the Carlisle family, who were rabbit trappers during the 1960's and 1970's. Without actual analysis, it’s difficult to determine whether all nickel-iron meteorites sold as Mundrabilla are actually from that fall, as smaller pieces aren’t large enough to exhibit some of the meteorite’s unique features seen in larger slices.

In particular, Mundrabilla contains a large fraction of troilite inclusions with graphite and schreibersite as accessory minerals. The darker troilite is estimated to constitute between 25% and 35% of the meteorite, with parallel precipitates of duabreelite (the iron chromium sulfide FeCr2S4). Mundrabilla also contains an impressive number of reduced extraterrestrial compounds. Today it’s classified as an ungrouped silicated medium octahedrite falling within the broader IAB group. This group is often viewed as consisting of many sub-groups, with the Mundrabilla and Waterville meteorites forming the "Mundrabilla duo" or "Mundrabilla grouplet" (a “grouplet” being a group of meteorites with less than 5 members).

To add to Mundrabilla’s significance, in 2002 NASA’s Marshall Space Flight Center studied a 45 kg Mundrabilla sample on loan from the Smithsonian Institution’s National Museum of Natural History. Working with Dr. Donald Gillies, Kennedy Space Center’s Pete Engel processed five hundred millimeter-wide computed tomography scans of that sample to non-destructively study the unaltered internal structure of the meteorite. In this way, the scientists could study how billion year-old metal alloys slowly cooled in a low gravity environment – an experiment not possible to replicate on Earth or during any of our lifetimes. As Dr. Gillies explained:

Most meteorites are solid chunks of metal, surrounded by a rocky surface. This one is a combination of materials (iron-nickel and iron-sulfide) that became solid at different rates in cooling over millions of years. It offers an amazing opportunity for understanding fundamentals of alloy formation.

The goal of this work was in part to help design advanced materials that could be used for future spacecraft, improved jet aircraft, and even alloys for cars or other domestic uses.

A decade and a half later, scientists at the University of California in San Diego and the Brookhaven Laboratory in New York, having had little luck searching for naturally occurring terrestrial superconducting materials, studied 15 meteorites using magnetic field modulated microwave spectroscopy and found that two of them – Mundrabilla and the Antarctic Graves Nunataks (GRA) 95205 – contained superconductive grains, which researchers argued in cold environments could affect planetary formation, the shapes and origins of magnetic fields, and the motion of charged particles in space, to name a few. Of note is the fact that Mundrabilla is an iron-sulfide-rich metal meteorite that formed after melting in an asteroidal core, while GRA 95205 is a surficial stone ureilite that underwent heavy shocks during its formation. Though these meteorites are obviously from significantly different classes, the superconducting grains in each were principally an indium and tin alloy (with the possible addition of lead in Mundrabilla’s), suggesting to the researchers that such grains likely exist in other meteorites as well.

The 2.5 kg piece of Mundrabilla at south Australia’s Adelaide Museum was taken from the second largest mass, which in 1973 was sent to the Max Planck Institute for Physics in Heidelberg, where Paul Ramdohr cut slices for subsequent distribution (it was one of the first uses of the now more common wire saw). One of those slices was first delivered to the Max Planck Institute in Mainz, then subsequently sent to Frankfurt’s Senckenberg Museum where German meteorite dealer Moritz Karl was later allowed to cut off pieces that included the part-slice in this post. At 519.4 grams, it’s large enough to demonstrate the remarkable characteristics of this unusual meteorite.

Photos 1 and 2 show the etched and unetched sides, respectively, along with a metric metal ruler to give an idea of scale. Photos 3 and 4 display blown-up areas of both sides that better illustrate the characteristics of the metal alloys in this meteorite. One can also see at the edge of this slice remnants of the highly oxidized surface of the original mass. Some of the larger Mundrabilla finds were surrounded by a shale crust, pieces of which are occasionally found available for purchase.