More on Zinc Oxide

David Speck wrote with more on Zinc Oxide

I thought ZnO crystals were a well known demo specimen for SEM work. At least when I was at Cornell in 1975, they were a favorite item for demonstrating the power of a SEM. I remember going to the SEM lab on Olin Hall with a friend who had bought some cheap SEM time at 2:00 AM. The most memorable item we looked at was the ZnO smoke sample.

Zinc burning in air forms perfect tetrahedral pointy crystals in a broad range of sizes that resemble a caltrop from outer space. See:

http://embedded.eecs.berkeley.edu/caltrop/pic/caltrop.jpg

I thought I'd have no trouble finding an image of these crystals, but an extended Google search didn't bring up a single example. Perhaps they have been forgotten. It would be interesting to see them again.