Earlier this week, just for fun I put out a new photo of an apparently ordinary barred olivine (BO-type) micrometeorite with a peculiar hole, and asked if any our sharp readers could deduce the formation process here? The response was fantastic, thank you, and several suggested either a loss of a metal bead, and/or degassing of volatiles. But it was a trick question, and I was fishing for an explanation of one unusual detail in the photo. So here we go.
Micrometeorite NMM 2897 is an ordinary sized barred olivine (BO-type) micrometeorite. The stone is complete, oriented, and aerodynamic; the front is up in the photo and the even and coarse stripes of the olivine plate crystals in the lower area (back) suggests little spin. Or spin perpendicular to the direction of speed. This implies that the hole is not from a migrated and lost metal bead, which would have been in the front part of the stone.
Consequently, we are left with degassing, which was suggested by several readers, and is correct. But there is one unusual detail here, which can only be explained by an additional and important principal in the formation of micrometeorites. Whereas most open gas vesicles on BO micrometeorites have jagged crystalline edges round the hole, here it is a glassy area. Where did the surplus glass come from?
The principle I was searching for, is that during formation the solidification of micrometeorites is from the outside and in.
Due to a high peak temperature during atmospheric entry, most barred olivine micrometeorites have completed degassing of volatiles when solidification occurs during atmospheric deceleration. But occasionally they have to let off some steam. If the surface is already crystalline, and the interior still liquid, the result may become spectacular “glass foam” as described in the Atlas of Micrometeorites or an unusual glass rim around the hole.
Below are some other photos, electron microscope images and collages, diving deeper into this unusual phenomenon. From the 4,035 micrometeorites in my collection (plus the handful of MMs sent to me by Michel Haak), I have seen this phenomenon less than ten times. So, this is rare.
There is, by the way, a closely related phenomenon which in some cases can be observed on V-type (glass) micrometeorites: crackelation, which is also described for the first time in the Atlas of Micrometeorites. Crackelation occurs when the surface of a glass micrometeorite with a large central void has solidified and cracks while the interior is still molten and can float and fill in the gaps and crevices. See the enclosed pictures of micrometeorite NMM 1930, which is also featured in our blog post about Newton Rings. Thanks to Imperial College London’s Dr. Matthew Genge for coining “crackelation.”
Enjoy the enclosed pictures, and remember Plato’s wisdom: “If you are not happy as an individual, you can not be happy in a relationship. Happiness comes from micrometeorites.”