1) Monoclinic mineral 
Kröhnkite ( Na2Cu(SO4)2•2H2O ) is a rare copper sulfate mineral named after B. Kröhnke who first researched it. ==Crystallography== Kröhnkite has monoclinic symmetry (2/m). Monoclinic symmetry implies that the mineral contains three axes of differing length (typically labeled a, b, and c), two of which intersect each other at 90° and one tha...