TOURMALINE CHEMISTRY AND THE FLEXIBLE B-SITE

Tourmaline is a complex borosilicate mineral group with over 30 species currently approved by the IMA. Due to its complex chemistry and difficulties with full chemical analysis, assumptions are commonly made during formulae normalizations, often with little corroborative support. These include Fe²⁺/Fe³⁺ ratios and the amounts of the light elements Li, B and H. These assumptions can lead to misidentification of tourmaline species and the resulting interpretation of the geochemical environment of formation. Past research has shown the stereochemistry of the B-site to be sensitive to variations in chemistry, responding to the occupancy of the Y- and Z-sites in particular. We are further investigating this relationship to better understand why the (BO₃) triangle responds to the chemistry of the octahedral sites, and what elements and sites have the strongest effect. This will allow crystallographic data to support assumptions made during the tourmaline normalizations. This information can also be used to validate, identify, and differentiate similar species among the tourmaline group.