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°. NaKZnP2O7 crystallizes in the space group P21/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)S2 – 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 P21 !
And this is how villamaninite looks like atomistically:
Metal-S6 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:
The Fascination of Crystals and Symmetry 