When Meteorites Get Old
One of the things often associated with large iron meteorites that have been around awhile is iron shale. A name coined in connection with material found at Barringer Meteorite Crater. It has a layered structure so there is a similarity to shale. Though from my experience the layers in real shale are usually much thinner than layers in iron shale.
Iron shale is a product created by the slow weathering of the exterior of a meteorite. It requires a considerable amount of time to form. In the case of the specimen shown here from Barringer Meteorite Crater (Meteor Crater) many thousands of years were involved in the formation. Iron shale is hard and brittle. It is often responsive to a magnet and some pieces will very strongly hold a magnet. It is possible to find small particles of unchanged metal in iron shale. I have seen it occasionally in specimens ground smooth from Meteor Crater. But, it often requires a close examination under a microscope to see the small spots and lines of metal.
Meteorites just do not do very well in the moist atmosphere of Earth. They will quickly decompose or terrestrialize into rocks that become harder and harder to recognize as cosmic in origin. In the case of iron meteorites they will often split and peel like the layers of an onion as the iron shale develops. Many weathered meteorites were found at Meteor Crater in such a condition. They were named Shale Ball Meteorites by Barringer. Occasionally when these weathered meteorites were cut the Widmanstä tten pattern was still discernable. In some cases a remaining metallic center was found. Iron shale was found commonly around the crater in the early days. It is found at Monturaqui crater which is very old, it is found at the site of the Hoba iron and many other old iron meteorites. Sometimes iron shale is the only remaining meteorite evidence that there is at old impact sites.
Are chunks of iron shale meteorites. I think they can be if they are whole intact shale balls. That is my opinion of course. But, when they are chunks that have exfoliated off of a large weathering meteorite; then I think they have become a weathering product only. To me they can no longer be called a meteorite themselves. But, they are meteoritic.
The mass of iron shale shown here is 975 grams in weight.