The Ravages of Time

Iron shale was once the most common meteoritic material to be seen around the area of Meteor Crater. But, after way more than a century of collecting not very much is left of the interesting by-product of the ancient meteorites. Typically, pieces of iron shale are flat plates of hard black magnetic rock. The metallic iron and nickel is nearly completely converted into oxide. Though most pieces if ground and examined very closely will have the tiniest specks of metal now and again. It is reminiscent of magnetite in color and luster but has really no cleavage and does not break at all the same way as magnetite. I would doubt that the iron shale would even classify as a mineral since it now lacks any internal crystalline structure. Still, it is the rusted remains of iron meteorites and therefore fascinating  Larger pieces may be layered but given enough time in the future these will also decompose further breaking apart on the layers to individual fragments of only a single major layer. In the photo below of two unusually large pieces many layers can be seen. Yet,  it is still just flat plate like structure. Any of the original crystalline structure of the meteorite is indistinguishable.

The early work at the crater yielded many of what were called Shaleball Meteorites. These large masses of shale preserved the shape of the original meteorite and when cut did still show the Widmanstätten pattern in the iron oxide. Sometimes these shaleball meteorites still had a metallic core but often they were completely masses of iron oxide. The layers of shale in these were curved somewhat also rather then being just flat sheets. The iron shale exfoliated off the outside of the surface meteorites and was spread by other processes, but for the shallballs which were buried the masses stayed together. In this photo of a large fragment of a shallball meteorite curvature can be seen and there is something of the original Widmanstätten pattern hinted at. But, is is not very clearly preserved.

On a rare occasion a piece of Canyon Diablo iron shale will show the Widmanstätten pattern more dramatically. The fragments of shale in the group below have features of the original crystal structure preserved. On some it is very hard to capture photographically.  In general it is triangular shapes in the layers that are the first tip-off. Then you will see that the whole area of the piece is kind of pyramid shaped.

Sometimes as in the enlargement of this one fragment there is a well defined Widmanstätten structure preserved. It would seem that even over the span of thousands of years the differences in the Taenite and Kamacite compositions allow for one to be preserved a geologic moment longer. Also, the corrosion that we see in etched slices which follows crystal boundaries seems to result after ages in cracks and the falling out of crystal shaped portions of iron shale. Please note the triangular pieces missing in some of my shots in this article.

Here is one more picture for you to consider. See what features of remnant crystal structure you can see.

Of all the impact craters on Earth only a handful have meteoritic material surviving around them. Iron shale is often one of the substances recovered at these few sites. Time and the environment are working relentlessly to breakdown the iron meteorites until nothing remains. After 50,000 or so years not too much remains at Meteor Crater, but we can still see in even the altered and heavily weathered material evidence of the meteorite’s cosmic structure.



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About the Author

James Tobin
The Meteorite Exchange, Inc. was born in 1996 with meteorite.com and Meteorite Times Magazine in 2002. Still enthusiastic about meteorites and all things related to them, we hunt, collect, cut and prepare specimens. We travel to gem shows and enjoy meteorites as much now as in the beginning. Please feel free to share any comments you have on this or any of our other sites.
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