Kröhnkite ( Na₂Cu(SO₄)₂•2H₂O ) is a rare copper sulfate mineral named after B. Kröhnke who first researched it. 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 that intersects at an acute angle. Specifically, it belongs to the 2/m symmetry class meaning, the mineral has a 2-fold rotation axis about the b axis. It also has a unique motif of silicon tetrahedra chains and copper octahedra aligned along the c axis and linked together by sodium atoms. Kröhnkite exhibits the optical property birefringence; the difference in the two refractive indices of a mineral. Because this mineral is birefringent, it must be anisotropic. Anisotropic minerals cause the velocity of light to vary depending on the direction of travel through the mineral. Kröhnkite is biaxial negative, which reveals that the mineral has two optic axes.