C , ▁but ▁ 6 H ▁or ▁ 6 M ▁depending ▁on ▁the ▁symmetry . ▁The ▁mon oc lin ic ▁forms ▁are ▁stable ▁at ▁temper atures ▁below ▁ 2 5 4   ° C , ▁whereas ▁the ▁hex ag onal ▁forms ▁are ▁stable ▁above ▁that ▁temperature . ▁The ▁exception ▁is ▁for ▁those ▁with ▁high ▁iron ▁content , ▁close ▁to ▁the ▁tro il ite ▁composition ▁( 4 7 ▁to ▁ 5 0 % ▁atomic ▁percent ▁iron ) ▁which ▁exhib it ▁hex ag onal ▁symmetry . ▁ ▁Mag net ic ▁properties ▁The ▁ideal ▁Fe S ▁lattice , ▁such ▁as ▁that ▁of ▁tro il ite , ▁is ▁non - mag net ic . ▁Mag net ic ▁properties ▁vary ▁with ▁Fe ▁content . ▁More ▁Fe - rich , ▁hex ag onal ▁p yr r hot ites ▁are ▁ant ifer romagnet ic . ▁However , ▁the ▁Fe - def ic ient , ▁mon oc lin ic ▁ ▁Fe 7 S 8 ▁is ▁f err im agnet ic . ▁The ▁fer romagnet ism ▁which ▁is ▁widely ▁observed ▁in ▁p yr r hot ite ▁is ▁therefore ▁attributed ▁to ▁the ▁presence ▁of ▁relatively ▁large ▁concentr ations ▁of ▁iron ▁vac an cies ▁( up ▁to ▁ 2 0 %) ▁in ▁the ▁cry stal ▁structure . ▁V ac an cies ▁lower ▁the ▁cry stal ▁symmetry . ▁Therefore , ▁mon oc lin ic ▁forms ▁of ▁p yr r hot ite ▁are ▁in ▁general ▁more ▁defect - rich ▁than ▁the ▁more ▁symmet rical ▁hex ag onal ▁forms , ▁and ▁thus ▁are ▁more ▁magnetic . ▁Mon