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Tenham The Meteorite That Teaches Us About Our Planet’s Mantle

I have to admit I have a soft spot for Australian Meteorites. That fondness stacked on top of my constant search for meteorites that are mineral type localities, and a separate love for meteorites that would qualify under the traditional definition of “antique”, makes Tenham a hat-trick type collection piece for me.

Tenham fell near Tenham Station, Queensland Australia in 1879. It was a nighttime fireball (somewhere around 2 to 3 A.M.) with several witnesses, but there is no exact date associated with the fall, just a that it was in the first quarter of the calendar year. With an impressive age of 144 years on earth, much of the writing on Tenham is itself vintage from the 30s through the 70s, full of terms mostly out of the vernacular today. The Aerolite shower produced a stone count of around 300, amounting to a total know weight of over 200 kilograms (as reported in Hellyer’s mass distribution study), 160 kilograms listed in the meteoritic bulletin. Many of recovered stones are described as complete or nearly so as recorded by Hellyer having used 250 Tenham meteorites as part of his study. Tenham was classified as a veined olivine-hypersthene chondrite, an L6 nowadays.

Tenham is a well-studied and referenced meteorite when it comes to shock related research. This is due to it sitting at the end of the shock scale, S6, (though a few studies put it at S5) along with the fact that the preponderance of the fall is curated in a few major institutions. So it is no surprise that the 4 new minerals found in the Tenham meteorite which get it a status as a major type locality collector piece are found within the shock veining. Shock veining is associated with high pressures and temperatures achieved as impact shock pressure passed through the parent body. In the case of Tenham that pressure was between 25 and 45 GPa, shock veining occurred due to shearing throughout the rock due to differential localized shock impedance characteristics (Langenhorst, 95).

The type locality minerals of the Tenham meteorite:

Akimotoite is a rarely seen member if the ilmenite group with a formula (Mg,Fe)SiO3. It was first discovered by N. Tomioka and K. Fujino in 1997, and published about in 1999 in American Mineralogist. It’s a polymorph of both pyroxene and bridgmanite (also with a type locality of Tenham). Since its formation environment precludes it from forming on the Earth’s surface its other representative examples are in meteorites like Zagami and Umbarger. Of note it is theorized that it is a significant constituent mineral of the Earth’s upper mantle 370-500 miles deep.

Bridgmanite is the (Mg,Fe)SiO3 magnesium end member of the Perovskitites group. It is theorized that this is the most prevalent mineral constituent on Earth’s mantle (making up 93% of the deep mantle), thus making it the most abundant mineral on Earth. Since it forms in the deep mantle (420 to 680 miles down) a terrestrial occurrence has yet to be found so meteorites and lab produced sources are all we have. Bridgmanite is a high pressure polymorph of enstatite. Though synthetic lab produced material was published about in the 1980s, following the rules of mineral naming however it wasn’t till it was found in natural form in Tenham in 2014 by Chi Ma and Oliver Tschauner, that it was formally allowed to be named.

Poirierite like bridgmanite was theorized to exist long before it was found, some 40 years ago by its namesake Jean-Paul Poirier. Poirier’s extensive research focused on the Earth’s mantle and core. This high pressure polymorph of olivine with a formula of Mg2SiO4 wasn’t describe from a natural occurrence till 2021.

The final type locality mineral in Tenham might be the most well-known among meteorite collectors, ringwoodite. This is the oldest first ever mineral found in Tenham with an official naming in 1969. Like Poirierite it too was named in honor of the scientist who theorized its existence, Alfred Ringwood, who happened to be a predominant Austrailian geologist and geochemist. It is a high pressure polymorph of the fosterite end member of olivine, but has a spinel structure. It is also a polymorph of Poirierite and Wadsleyite. What helps ringwoodite stand apart from most other minerals found in meteorites is, being a spinel, although it can be grey to colorless, it can also be quite colorful as bright blue to purple grains.

Tenham is a meteorite that has been critical to helping us better understand the composition of the depths of our planet. By allowing us access to naturally occurring mineral samples exposed to the pressures and temperatures similar to mantle of our own planet we would otherwise be unable to access. Tenham sat on my wish list for quite a long time. I avoided the pressures to get just a small bit and move along in my collecting. I wanted a nice crusted piece that showed off how well it has held up over nearly a century and a half, and I definitely wanted to have nice shock veining. With a little help from Roberto Vargas while he was at the Denver Show I was put on to the nice 26-gram piece that met all my requirements.

Till the next type locality- Mike

 


Photo image 5815 (left – before) and photo image 6150 (right – after polished). My 26.118-gram end cut, it came to me at 26.2 grams but being a perfectionist, I wanted the cut face immaculate, a perfectly polished face was well worth the 2 thousandth gram loss.

 


Crust still looking black and fresh after 144 years

 


Thin section I made out of a small secondary piece, it contained a nice thick shock vein displaying beautiful blue ringwoodite grains.

 


Robert’s collection piece, 18.16 grams and a Schwade number, a reminder for me to hold out and get a good one. Thanks for the assist in getting me steered in the right direction buddy!
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