Powder data and accurate cell dimensions of both polymorphous forms of 3Cu(OH) 2 · Cu(NO 3 ) 2 were determined with the de Wolff focusing monochromator camera. The orthorhombic mineral gerhardtite has a = 5.592, b = 6.075, c = 13.81 2 Å, and the synthetic monoclinic modification a = 5.605, b = 6.087, c = 6.929 Å, β = 94°29′. The two structures must be closely related. All the methods which should yield specifically orthorhombic or monoclinic crystals according to the literature, always gave monoclinic ones, whereas some other methods gave mixtures of both forms. The orthorhombic modification seems to be stable at low temperatures (below about 140°C) according to density and reaction data, while at higher temperatures (between about 140°C and 340°C) the monoclinic form is more stable. Even at low temperatures, however, most synthetic methods favour the formation of the monoclinic form as a metastable intermediate. The transformation monoclinic–orthorhombic is strongly inhibited. Thus natural gerhardtite must have formed very slowly. We succeeded in preparing samples with as much as 80% of the orthorhombic form. Powder diagrams of natural gerhardtite (from Kalabi, Jadotville, Katanga) showed some very weak monoclinic lines too. Layers of both modifications can probably occur in the same “crystal”.