An X-ray and optical investigation of the minerals cobaltite (CoAsS), glaucodot ((Co,Fe)AsS) , and arsenopyrite (FeAsS) from the Modum district, Norway, has disclosed that naturally occurring cobaltites have either ordered or disordered crystal structures. The presence of ordered cobaltite may be a useful indicator of metamorphism in ore deposits. It is predicted that a similarly disordered structure is possible in arsenopyrite. By means of an X-ray method for the determination of cobalt content of arsenopyrite, it is shown that when cobalt is present in excess of 9%, glaucodot may appear as a separate mineral phase. High cobaltian glaucodot could be considered a polymorphic form of cobaltite. The varietal names 'danaite' and 'alloclasite' are thought to be unnecessary and should be replaced by 'cobaltian arsenopyrite' and 'high-cobaltian glaucodot', respectively.