NWA 11805, Polymict Diogenite, 5.5g full slice

Description

NWA 11805, Polymict Diogenite, 5.5g full slice with one crusted edge.

NWA 11805 is a minimally weathered Polymict Diogenite that was originally submitted for classification and brought to market by my good friend, and fellow IMCA member, Chris Colvin. Chris and I have been friends since he first started collecting, and we have shared many bits of knowledge together over lunch at our favorite Chinese restaurant. When he told me he was working on his first classification I was excited to see what would come of it, but little did I know that it would be such a fascinating and rare petrology of an HED.

Chris had bought a ~110g lot of HED specimens from another merchant we are both familiar with, who had himself acquired it from a Moroccan merchant soon before this. Tony Irving performed the classification, revealing an interesting brecciated combination of both angular mineral debris, lithic diogenite clasts and microgabbroic eucrite. The bulk of the mineral debris composition (>90%) is diogenitic, with the rest being minor eucritic exsolved pigeonite, calcic plagioclase, silica polymorphs and titanium-rich chromite and troilite.

Eucrites and diogenites are both igneous rocks, and as you may know, they both originate from the crust of Vesta; with diogenites thought to originate deeper within the crust. It is not unexpected therefor that NWA 11805 would be something of an amalgam between the two petrologies. Examining the cut surfaces closely you can easily make out the distinction between the breccia components, particularly of note are a smattering of lovely green orthopyroxene clasts.

When Chris decided to bring NWA 11805 to market I wanted to acquire a specimen, and ultimately I ended up buying the main mass with the intention of cutting slices. This was an awkwardly shaped specimen with something of a dome shape, one side having near complete coverage by a brown weathered fusion crust. As a result, cutting the mass in order to produce specimens with some good surface area was somewhat tricky. I needed to angle the specimen in the vice grip just right so that the cut went along the meat of the curved specimen. Even with the ultra-low kerf of my blade this resulted in an expensive 9g of cutting and polishing loss out of a 50.3g mass, but the specimens came out pretty good. Each specimen is a full slice of the main mass with some crust on at least one surface. All of them have enough surface area to produce an appreciable specimen and they have been polished to 2500 grit in order to best reveal those features. If you have an optical loupe you, and can get your hands on a specimen, you are in luck because the interior is even more stunning under the loupe.

When Polymict Diogenites are on the market they tend to be priced at the 40$/g range, making this specimen a relatively affordable example of the classification. This is a relatively rare classification with only 34 specimens classified, and 33 of them having very low total known weights. Only four Polymict Diogenites have been found outside of the hot dessert region. I have always said that the greatest gift that Northwest Africa gave us collectors is a plentitude of rare petrological types to enjoy. This classification is a fine example of that…

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