Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Oleg S. Vereshchagin, Bernd Wunder, Ivan A. Baksheev, Franziska D.H. Wilke, Natalia S. Vlasenko, Olga V. Frank-Kamenetskaya

Ti⁴⁺ and Sn⁴⁺-bearing tourmalines - pressure control and comparison of synthetic and natural counterparts

Journal of Geosciences, volume 67 (2022), issue 2, 163 - 171

DOI: http://doi.org/10.3190/jgeosci.345

Me⁴⁺-bearing (Me⁴⁺ = Sn, Ti) dravite analogs were synthesized in the system MeO₂-MgO-Al₂O₃-B₂O₃-SiO₂-NaO-H₂O at 700 °C and 4 / 0.2 GPa in four hydrothermal experiments. Tourmalines form rosette-like aggregates and needle-like crystals that are chemically homogeneous. Tourmaline crystals obtained in high-pressure runs (4 GPa) are much smaller (up to 0.1 × 2 μm) and have lower Me⁴⁺ (0.27 wt. % SnO₂, 0.57 wt. % TiO₂) than those from the low-pressure (0.2 GPa) runs (up to 1 × 5 μm; 1.77 wt. % SnO₂, 2.25 wt. % TiO₂). Synthetic analogs of rutile, quartz and coesite were obtained in the system TiO₂-MgO-Al₂O₃-B₂O₃-SiO₂-NaO-H₂O, whereas synthetic analogs of cassiterite, tin-rich (up to ~19.55 wt. % SnO₂) Na-pyroxene, MgSn(BO₃)₂ (Mg-analogue of tusionite), quartz and coesite were synthesized in the system SnO₂-MgO-Al₂O₃-B₂O₃-SiO₂-NaO-H₂O. We suggest that at a high temperature (≤ 700 °C), the pressure negatively affects the Ti incorporation into the tourmaline structure. In contrast, at relatively low pressures, the Ti incorporation in tourmaline structures is governed by the Ti content in the mineral-forming medium. Low-pressure conditions are feasible for Sn incorporation in the tourmaline structure. The presence of Ti⁴⁺ and Sn⁴⁺ cations in structures of the synthesized tourmalines (probably at octahedrally coordinated sites), is also indicated by changes in the unit-cell parameters.