@jbauer, @egardner I checked out Mark Wilson’s blog that you mentioned. Technically…..he is incorrect calling them manganese dendrites. They are typically referred to the mineral pyrolusite – manganese oxide – in dendrite form. But! According to Mindat.org none of these dendrites are actually pyrolusite but any of a number of other manganese oxides https://www.mindat.org/min-26645.html So, still not manganese but manganese oxides. Sorry, I know enough about minerals to be dangerous. 🙂
We did receive some nice examples of these at Geier in with the donated collection from a deceased member. I suppose we need to give them to the mineral curator.