Magnesium sulphate (orthorhombic) takes up ferrous sulphate (monoclinic) to the extent of 19%, forming isomorphous orthorhombic crystals; ferrous sulphate, on the other hand, takes up magnesium sulphate to the extent of 54% to form monoclinic crystals.
Clarke (1889-1893) supposes them to be substitution derivatives of normal aluminium orthosilicate A14(S104)3, in which part of the aluminium is replaced by alkalis, magnesium, iron and the univalent groups (MgF), (A1F2),(AlO), (MgOH); an excess of silica is explained by the isomorphous replacement of H 4 SiO 4 by the acid H4S130s.
Zinc sulphate, like magnesium sulphate, unites with the sulphates of the potassium metals and of ammonium into crystalline double salts, ZnS04 R2S04-+-6H20, isomorphous with one another and the magnesium salts.
All these are strikingly alike in appearance and general characters, differing essentially only in chemical composition, and it would seem better to reserve the name cerargyrite for the whole group, using the names chlorargyrite (AgC1), embolite (Ag(Cl, Bl)), bromargyrite (AgBr) and iodembolite (Ag(C1, Br, I)) for the different isomorphous members of the group. They are cubic in crystallization, with the cube and the octahedron as prominent forms, but crystals are small and usually indistinct; there is no cleavage.