Synthesis and crystal structure of Ga-rich, Fe-bearing tourmaline

Crystals of Ga-rich, Fe-bearing tourmaline (∼25 wt% Ga2O3, 11 wt% FeOtotal) were grown from hydrothermal solution at 100 MPa and 600°C. The crystals were grown on a seed as a newly formed, up to 200 μm thick layer and, in addition, as free, spontaneously nucleated elongated prisms, 10 to 200 μm in size. The crystal structure of the synthetic tourmaline was studied by Mossbauer spectroscopy and single-crystal X-ray diffraction (to an R -index of 0.032; a = 16.000 (1), c = 7.257(1) A). Gallium and iron cations are distributed over the Y and Z octahedral sites according to the crystal-chemical formula (Na0.95□0.05) (Ga1.34Fe2+ 0.62Al0.44Fe3+ 0.34Ni0.26) (Al3.28Ga1.92 Fe2+ 0.41Fe3+0.39) (Si6O18) (BO3)3 (OH2.29O0.71) (O), suggesting that the disordered distribution previously shown for Al3+ also holds for other trivalent cations in tourmalines.