The relation between the alum structures

Abstract

That the crystals which comprise the alums form an isomorphous series has generally been accepted almost as a self-evident fact. This is due to similarity of their chemical formulae and of their crystal classes, and the evidence was rendered almost complete when Cork showed that a series ranging from ammonium alum to thallium alum was based on the one space-group, Pa3. There were, indeed, some discrepancies in his measurements, but these could be ascribed to small parameter differences. The first indication of polymorphism among the alums was given by methyl ammonium alum, of which the structure was found to be different from that of potassium alum. §