Pyrrhotite  is a common ferrimagnetic mineral in terrestrial rocks and has been identified recently as the major remanence carrier in Martian rocks, as in SNC (Shergotty-Nakhla-Chassigny) type meteorites. This compound undergoes a low temperature magnetic transition around 32 K with a change in the natural magnetic remanence . This transition is going to be of growing importance in paleomagnetism and rock magnetism. To determine the structure change at 32K neutron diffraction was performed on two crystal samples at 50K and 20K (D10-ILL). Based on the magnetic torque measurements, performed on the same samples, a model of low-temperature triclinic structure was build. This model, explains previous Mössbauer experiments , the torque measurements, the magneto-crystalline anisotropy decreasing.