Ricerc@Sapienza

Tourmalines form the most important boron rock-forming minerals on Earth. They belong to the
cyclosilicates with a structure that may be regarded as a three-dimensional framework of octahedra ZO6
that encompass columns of structural “islands” made of XO9, YO6, BO3, and TO4 polyhedra. The overall
structure of tourmaline is a result of short-range and long-range constraints resulting, respectively on the
charge and size of ions. In this study, published data are reviewed and analyzed to achieve a synthesis
of relevant experimental results and to construct a crystal-chemical model for describing tourmalines
and their compositional miscibility over different length scales. Order-disorder substitution reactions
involving cations and anions are controlled by short-range structural constraints, whereas order-disorder
intracrystalline reaction involving only cations are controlled by long-range structural constraints. The
chemical affinity of a certain cation to a specific structural site of the tourmaline structure has been
established on the basis of structural data and crystal-chemical considerations. This has direct implications
for the tourmaline nomenclature, as well as on petrogenetic and provenance information. Some
assumptions behind the classification scheme of tourmaline have been reformulated, revealing major
agreement and significant improvements compared to earlier proposed scheme.