  We report the characterisation of natural samples of the cubic pyrite mineral
MnS2 using very high resolution synchrotron X-ray diffraction techniques. At
low temperatures we find a new low temperature polymorph, which results from
coupling between magnetic and lattice degrees of freedom. Below the magnetic
ordering temperature T_N= 48 K, we detect a pseudo-tetragonal distortion with a
tiny c/a ratio of 1.0006. The structure can be refined in the space group Pbca.
The symmetry lowering reduces magnetic frustration in the fcc Mn2+ lattice and
is likely responsible for the previously reported lock-in of the magnetic
propagation vector. This behaviour is similar to the frustration driven
symmetry breaking reported in other three-dimensional Heisenberg magnets like
the chromate spinels
