Classification of the minerals of the tourmaline group

Abstract

A systematic classification of the tourmaline-group minerals, general formula X Y3Z6 (T6O18) (BO3)3 V3W, is proposed, based on chemical composition and ordering at the different crystallographic sites of the tourmaline structure. There are currently thirteen accepted tourmaline species. However, based on the actual chemical compositions of holotype material, several of these species were imprecisely or incorrectly defined. A proper definition of these species is proposed. A crystal-chemical feature that extends the number of possible end-members is the anion occupancy of the W-site (dominated by OH-, F- or O2-) and the V-site (dominated by OH- or, more rarely, O2-). Thus, based on the W-site alone, there can be hydroxy-, fluor-, or oxy-end-members. Furthermore, the presence of dominant O2- at the W-site commonly requires local cation-ordering at the Y- and Z-sites. The tourmaline-group minerals can be divided into three principal groups based on the dominant species at the X-site: alkali tourmalines (Na), calcic tourmalines (Ca) and X-site-vacant tourmalines ([]: vacancy). These groups are further divided, initially based on the W-site occupancy, then by the (actual or inferred) V-site occupancy, next by the (actual or inferred) Y-site occupancy and, finally, by the (actual or inferred) Z-site occupancy. A systematic classification procedure is developed that takes into account different levels of knowledge of the chemical composition and site occupancy of the tourmaline. Several examples are used to illustrate the application of this classification procedure.