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  Aubrite

TKW 1.1 tons (1000 kg). Observed fall February 18, 1948, near Norton, Kansas, USA.


   

Steve writes:

Norton County is another February fall, but I didn’t get this done by the 18th for its anniversary.

Photos 1 and 2 show a 35.2 gram, 43 x 40 x 20 mm piece without fusion crust.

In my 7/4/2017 MPOD submission, I demonstrated Norton County’s fluorescent properties with a 1.3 gram fragment; but its small size suffered from a reflected purplish-blue component of the 405 nm UV source that made its naturally yellowish glow appear magenta in long exposures. Understandably, there were some questions about Norton County’s actual fluorescence, so Photos 3 and 4 offer two additional views of this larger piece, each alternating between white and long-wave UV illumination (a lower wavelength 365 nm light source combined with a narrow-band optical bandpass filter eliminated the visible purplish-blue component, thus better demonstrating the meteorite’s intrinsically yellow fluorescence).

I also hadn’t noticed any Norton County thin section pictures in prior MPOD submissions. Photo 5 shows a thin section slide in xpol, and Photos 6 through 9 capture a few magnified areas I found interesting. The entire thin section can be viewed here:

Norton County Aubrite Meteorite Xpol Thin Section (gigapan.com)

[As a reminder, click on the diagonal arrows to the right of the image to go full screen, then use the mouse and its scroll wheel to zoom in and out and pan around.]

Finally, I’ve always been intrigued by the variation in Norton County’s tan-colored fusion crust. Some wonderful examples can be seen in prior Norton County MPOD submissions, but Photo 10 directly compares three variations. The upper half shows the edges of the respective crust whose surfaces are seen directly below them (from left to right, the pieces weigh 0.64 gram, 0.70 gram, and 0.42 gram; for size reference, the black cube in the bottom center image is 1 cm, while the maximum dimensions of the left and right fragments are both 10 mm). Each piece obviously exhibits slightly different characteristics (I especially like the rightmost one – it reminds me a little of some of the pictures of Mars). Surprisingly, none demonstrated any examples of fluorescence on their interiors.

And for those wanting a little history, the meteorite fell around 4:00 pm on February 18, 1948, showering fragments over a large area of Norton County, Kansas and Furnas County, Nebraska. The event was heralded by a brilliant fireball, and several witnesses reported loud explosions followed by a roaring sound described as being like that of a jet engine. In the smoke train preceding the fireball, puffs denoted places the meteorite broke apart as it streaked across the afternoon sky. The blast from its impact was large enough to shake buildings and crack a few windows.

Almost seven weeks passed before the first stone was discovered. As reported in The Norton Daily Telegraph:

Mr. Tansill on April 6 found the first fragment to be located. It lay in a clover field on land he owns about a mile and a quarter north of his home, where he was working with a tractor and suddenly noticed the fragment sparkling "like a million diamonds".

In the months that followed, thousands of stones would be recovered from a large strewn field on the Kansas/Nebraska border. The largest piece of the Norton County meteorite was found on July 3, 1948, when two ranchers discovered in a wheat field a nearly circular hole six feet in diameter and depth. Digging down a mere inch, they struck something hard; more digging exposed a huge stone about three feet across, buried with its "nose" pointing downwards. A recovery team led by Dr. Lincoln LaPaz (who was the Director of the University of New Mexico’s Institute of Meteoritics at the time) covered the stone in a thick coat of plaster-of-Paris, lifted it out of the hole with a crane, and transported it to Albuquerque for further analysis. It’s remained in UNM's Meteorite Museum since the Museum opened in 1974, and at approximately 2000 pounds, it’s the largest single piece of an achondrite meteorite in the world (while many iron meteorites weigh more, large stone meteorites don’t usually survive their fall through the Earth's atmosphere).

Norton County is a rare Ca-poor aubrite fragmental breccia. Observed falls are already uncommon, and of those, only about one percent are of this kind. Aubrites are enstatite achondrites principally made up of magnesium and calcium pyroxenes. Most varieties also contain iron, but on Norton County’s parent body (which spectral studies suggest is the near-Earth Apollo asteroid 3103 Eger), almost all of the iron turned into metal and separated into a metallic core, making its enstatite Fe-poor (its parent body was also quite lacking in oxygen, leading to early speculation that Norton County might have originated on oxygen-poor Mercury).

Because enstatite achondrites are reduced, they contain several exotic minerals that are uncommon or not found on Earth, including: caswellsilverite, discovered first in the Norton County meteorite in 1982; daubréelite, normally found in iron-nickel meteorites but also seen in Norton County and the Martian ALH84001 meteorite; perryite, a nickel silicide unique to meteorites; oldhamite, a calcium magnesium sulfide only rarely seen terrestrially; and niningerite, a galena group mineral also unique to meteorites and named after the famous meteoriticist Harvey Nininger. Not being a geologist, I naively wonder whether one of these more exotic minerals is responsible for Norton County’s sparse fluorescence, though I understand it might also be due to depletion of iron in some of the enstatite crystals (iron quenches fluorescence in terrestrial minerals).