The problem of the existence of solid solutions between pyrrhotite and pentlandite, chalcopyrite, galena, and sphalerite was investigated by heat treatment. Pentlandite was put into solid solution in pyrrhotite above 425 degrees C. Upon slow cooling from 800 degrees C. it unmixed and oriented itself around pyrrhotite grains; under high magnifications was observed to contain oriented laths of a mineral tentatively called pyrrhotite. The formation of these laths may be caused by the unmixing of a solid solution of pyrrhotite in pentlandite or by the breakdown of pentlandite expelling pyrrhotite.Pyrrhotite and chalcopyrite form two solid solutions. Pyrrhotite will dissolve in chalcopyrite above 300 degrees C. The two minerals then react to form chalcopyrrhotite which makes a very fine intergrowth with chalcopyrite in an aureole about pyrrhotite masses in the samples treated. Above 600 degrees C., chalcopyrite will dissolve in pyrrhotite and, upon unmixing, forms oriented laths in the pyrrhotite.The eutectic intergrowth formed by pyrrhotite and galena has a composition of approximately 71 per cent galena and 29 per cent pyrrhotite by weight. The eutectic temperature lies between 765 degrees and 775 degrees C. The variation between these figures and those for the artificial PbS-FeS system may be caused by impurities.Crystallographic intergrowths between pyrrhotite and sphalerite were never produced, although such intergrowths have been described and observed in many ores and it is assumed that hydrothermal conditions lower the temperature of formation for such intergrowths considerably below that required in a dry melt.Small crystals of pyrrhotite, chalcopyrite, and galena were formed and the evidence indicates that they were produced by volatilization and subsequent deposition.