|MadSci Network: Earth Sciences|
I have a question about the identity of a rock. Description: A deposit of globular, mushroom shaped fulgerites(?) on the shore of Lake Michigan. Each specimen has a hard black coating (0.5 mm) on the outside, but can easily be broken open by hitting each with a heavy object or applying other mechanical stress. Sand on the inside appears to be fused together with a hard / spongy consistency. The rock is soft enough that you can push a needle into it. A cross section reveals ringlike structures of blackened sand. Microscopically, the sand grains appear to have fibers surrounding each grain. Macroscopically they fook fuzzy. Bits of the rock produce bubbles in 1 N HCl, but do not fall apart after prolonged exposure to acid, and take on a more spongy consistency. What the heck are these things? Some kind of fungal colony? Maybe a strange variant of a fulgurite? Please let me know what you think when you get a chance.
Re: What makes a rock spongy?
Try the links in the MadSci Library for more information on Earth Sciences.