THE TOURMALINE GROUP MINERALS: A CONSISTENT NOMENCLATURE

The International Mineralogical Association’s Commission on New Minerals, Nomenclature and Classification has accepted the recommendations by the Subcommittee on Tourmaline Nomenclature for tourmaline-group minerals. The nomenclature is based on chemical systematics for the generalized tourmaline structural formula: XY₃ Z₆(T₆O₁₈)(BO₃)₃V₃W where the common ions at each site are X = Na¹⁺, Ca²⁺, K¹⁺ and vacancy: Y = Fe²⁺, Mg²⁺, Al³⁺, Li¹⁺, Fe³⁺ and Cr³⁺; Z = Al³⁺, Fe³⁺, Mg²⁺ and Cr^3+; T = Si⁴⁺, Al³⁺ and B³⁺; B = B³⁺; V = OH^1- and O^2-; and W = OH^1-, F^1- and O^2- In terms of a classification scheme, most compositional variability occurs at the X, Y, Z, W and V sites. Tourmaline species are defined in accordance with the “dominant-valency” rule which is expressed as follows: “In a relevant site, the dominant ion of the dominant valence state is considered for nomenclature”. Additionally, the following considerations are made: (1) In tourmaline-group minerals dominated by either OH^1- or F^1- at the W site, the OH^1--dominant species is considered the reference root composition for that root name: e.g. dravite. (2) For a tourmaline composition that has most of the chemical characteristics of a root composition, but is dominated by other cations or anions at one or more sites, the mineral species is designated by the root name plus prefix modifiers, e.g. ”fluor-dravite”. (3) If there are multiple prefixes, they should be arranged in the order occurring in the structural formula, e.g. “potassium-fluor-dravite”. Tourmaline can be divided into several subgroups and subtypes. The principal subgroups are based on occupancy of the X-site which serves as a division into the alkali, calcic or the X-vacant subgroups. Because each of these subgroups involves cations (or vacancy) with a different charge, coupled substitutions are required to compositionally shift among the subgroups. Within each of these three principal subgroups, there are several subtypes related by heterovalent coupled substitutions. Within these subtypes, there are several tourmaline species related by homovalent substitutions. The 14 currently recognized tourmaline species are retained with some minor modifications, and additional probable species are proposed.