CRYSTALLOCHEMICAL FORMULAS AND DEFINITION OF SPECIES OF TOURMALINE-GROUP MINERALS

Abstract

The complex chemical composition of tourmalines, in combination with various mechanisms of isomorphic substitutions, creates difficulties in recalculating the chemical data to the crystallochemical formulas. In each case, the calculated formula is no more than a result of an author’s interpretation and hence any formula is a far or close approach to the reality. Different modes of representing the composition of tourmaline-group minerals are discussed in this paper. Special attention is drawn to uncertainties, controversies, and mistakes in writing the crystallochemical formulas and to the problems that emerge in attempts to define species of tourmaline-group minerals from the calculated formulas.