In the last post we presented a kind of an empirical proof that a crystal belonging to the monoclinic crystal system can have an angle *β* of exactly 90°. NaKZnP_{2}O_{7} crystallizes in the space group *P*2_{1}/*n, *with the following cell parameters:* a* = 12.585(5) Å, *b* = 7.277(5) Å, *c* = 7.428(5) Å, *β* = 90.00(5)°. Monoclinic symmetry – but orthorhombic metric.

Our blog reader Michael Fischer now found an even more impressive example: The mineral villamaninite (Cu, Ni, Co, Fe)S_{2} – a pyrite-derivative – is *pseudo-cubic* with cell parameters *a* = 5.709 (2), *b* = 5.707 (2), *c* = 5.708 (2) Å, *α* = 90.00, *β *= 90.01(1), *γ* = 90.00°. However, the symmetry is only monoclinic with the space group *P*2_{1} !

And this is how villamaninite looks like atomistically:

Metal-S_{6} octahedra, all 6-fold corner-connected to each other but not edge-connected.

The CIF and the reference to the research article can be found here:

http://rruff.geo.arizona.edu/AMS/minerals/Villamaninite