Note on a connexion between the molecular volume and chemical composition of some crystallo-graphically similar minerals

The molecular volumes of the members of an isomorphous group of minerals in general show an approach to equality. In other words, minerals having an analogous chemical composition show not only closely related crystalline forms, but have also approximately equal molecular volumes. Many years ago J. D. Dana endeavoured to show that a simple relationship as regards molecular volume also subsisted in the case of various minerals which were closely related crystallographically, although unlike chemically. He concluded that in these cases the similarity in form depended solely on the relationship in molecular volume, and had nothing whatever to do with the chemical composition.