Triangles - National Speleological Society
https://share.google/2MUYoc0m47Gq2OCs0
It would seem to be a rare habit of calcite. I have seen better images on t'web but of course can't find them now! Apparently it forms underwater, or at the surface. This may explain its friable nature?
Of course I am not guaranteeing anything and would like to know other ideas