Pendlandite must be regarded as a natural pi (Fe,Co,Ni,S) phase in which the Fe:Co:Ni ratio may vary within wide limits. The problem of the 'true' Ni:Fe ratio is meaningless. The same holds for the lattice parameter, though all the classical, i. e., low-cobalt, pentlandites have been found to have a parameter between 10.034 and 10.067 A. When natural pentlandite is heated in vacuo or in argon its lattice parameter increases by about 1/2 of one percent. The expansion of the unit cell, which takes place between 150 and 200 degrees C, is not prevented by argon pressures up to 2,000 atm.