Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Peter Bačík, Lenka Skřápková

Position of Ti⁴⁺ in the tourmaline structure based on bond-valence topological modelling

Journal of Geosciences, volume 71 (2026), issue 1, 3 - 19

DOI: http://doi.org/10.3190/jgeosci.0052.25

Tourmalines commonly contain minor amounts of Ti. However, Ti-rich compositions are relatively rare, with only two approved tourmaline species - dutrowite and magnesio-dutrowite, showing significant Ti⁴⁺ occupancy at the Y site. According to the currently valid tourmaline nomenclature, Ti⁴⁺ is assigned exclusively to the Y site. To evaluate the structural mechanisms capable of introducing Ti⁴⁺ into the tourmaline structure, a series of modelled mechanisms was examined to identify the most plausible and locally stable arrangements involving Ti. For this purpose, short-range bond-valence requirements, distortion indices characterizing local polyhedral geometry, and long-range constraints expressed through the <Y-O> and <Z-O> ratios were applied to assess the likelihood of individual components containing Ti⁴⁺ at various crystallographic sites and within different local environments. Most tested structural mechanisms exhibited severe anion underbonding, overbonding, or produced unrealistic distortion indices. Nevertheless, the calculations indicate that Ti⁴⁺ can, in principle, be accommodated at the Y, Z, and even T sites, and prove that its higher valence and larger cation size are efficiently redistributed within the structure. The total amount of Ti⁴⁺ appears to be structurally limited to approximately one apfu, as higher Ti⁴⁺ contents (>1 apfu) lead to substantial structural deformation resulting in an unstable arrangement.