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Met Name

Met Type

Thin Sections

  Iron, IAB-MG

TKW 12 kg. Fall not observed.


 

Steve writes:

While the stony-iron pallasite and mesosiderite meteorites have obvious silicate inclusions, abundant silicates are generally absent in the vast majority of iron-nickel meteorites. The exception is the type IAB iron group, which is the combination of the previous types IA and IB. The silicates in these meteorites can include such minerals as low- and high-calcium pyroxenes, graphite, olivine, various phosphates, plagioclase, troilite, and even traces of daubréelite and chromite; these silicates show a strong resemblance to those found in winonaites and other primitive chondrites, all of which show evidence of partial melting. In fact, where many iron meteorites groups are thought to have originated in the cores of planetesmials, the type IAB irons more likely formed from other processes that have been proposed to include impacts between molten or partly molten chondritic planetesimals during their formation process.

The silicated iron featured in this post is NWA 5549. Found in 2008 and classified by UCLA's Dr. John Wasson, it was later paired with NWA 6369, both having an anomalous iron texture described as a hexahedrite with octahedrite characteristics and rare lamellae of taenite and spheroidized plessite following the Widmanstätten direction – similar to Horse Creek, a hexahedrite with unique lamellae of nickel silicide that formed when silicon-rich metal broke down along the planes of its Widmanstätten structures. Silicon and other minor elements in this trio of meteorites have been proposed as the cause for a structure different from other irons that consist mainly of iron, nickel, and phosphorus, but the siderophile elements in Horse Creek are different than NWA 5549 and NWA 6369, the NWAs being considered kamacite irons.

The coarse Widmanstätten pattern seen in the polished surface of today’s end cut (Photo 1) exhibits an almost galvanized metal-like appearance similar to that seen in hexahedrites and some recrystallized irons; within the metal are obvious silicates, which mostly take the form of angular fragments, small grains, and dark rounded inclusions, many of the latter also including small metal blebs (Photo 2). The regmaglypts seen in its highly weathered exterior resulted from frictional heating as the meteorite plunged through Earth’s atmosphere and the hot gasses swirled more intensely in localized spots (Photo 3).

The texture of the iron in this meteorite made it ideal for an experiment I had been considering. I’d been applying reflective transformation imaging (RTI – an advanced form of polynomial texture mapping originally invented by HP Labs back in 2001) to iron meteorite photography. As Paul’s readers already know, different alloys will etch at different rates, as do different crystal faces of the same alloys, causing each to reflect light differently; these varied reflections give rise to the Widmanstäten patterns unique to metal meteorites. RTI post-processors interpolate images taken with multiple angles of illumination and corresponding software viewers let users dynamically change light angles to visually enhance subtle features.

Edwin Thompson had mentioned recalling an article back in the 90s that featured a photo of an iron meteorite taken with colored lights shining at different angles onto its etched surface, which more colorfully highlighted its Widmanstäten pattern. So I thought it might be interesting to combine multi-color illumination with RTI to further amplify this enhancement. The effect was surprisingly effective, but while Paul’s website doesn’t currently support viewing RTI outputs, I’ve captured a few screen images and made some corresponding animated GIFs to provide an idea of the kinds of results I’d obtained.

The animated GIF in Photo 4 alternates between a white light image with one of the many multi-colored light images captured at relatively low magnification when employing this technique. Of note is how various allows reflect the colored lights differently, giving rise to sometimes colorful contrast between adjacent crystals.

Photo 5’s animated GIF adds a few more colored images to highlight one of the advantages of this technique: some crystals are significantly more sensitive to angles of the colored lights than others. As an example, the crystal at the left edge of the image just below the top principally reflects blue light, while others more greatly vary in color through the sequence.

Photo 6’s animated GIF offers an example of how this technique can enhance features within crystals, as in the large crystal at the left of the image with small, presumably plessitic inclusions within it.

Finally, Photo 7’s animated GIF is a higher magnified view of a feature that shows a much more dramatic response to this multi-colored lighting approach (because there were more frames, I sped up the animation). I’m not smart enough to know what it might be, but two of my more educated friends suggest it could be scratches in a particularly soft localized alloy, complex shock artifacts, or exsolution features (knowing which would take additional analysis beyond what I’m capable of doing).

And yes, these photos do look a lot like achondrite thin sections, so I felt there was no need to add any xpol images of the silicates in this meteorite 😊.