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Crystal structure and stability of Ni-rich synthetic tourmaline. Distribution of divalent transition-metal cations over octahedral positions

Abstract The crystal structure of synthetic Ni-rich tourmaline with a nickel content of 18.96 wt.% NiO (a = 15.890(2), c = 7.1815(8) Å) has been refined to R = 3.1% using single-crystal X-ray diffraction data. It has been shown that Ni is distributed not only over the Y, but also over the Z sites according to the ideal formula Na(Ni2+2 Al)(Al5Ni2+)(Si6O18)(BO3)3(OH)4. Based on bond valences and charge balance analysis it has been shown that this composition corresponds to the stable disordered member of the solid solution series NaAl3Al6(Si6O18)(BO3)3(O)3(OH) - NaNi2+3 Al6(Si6O18)(BO3)3(OH)4. Taking into consideration available structural data on Me2+ (Ni, Cu, Co, Fe, Mn)-rich tourmalines, cation-size mismatch and bond-valence calculations we assume that there are no structural constraints precluding occurrence of these cations in both octahedral sites. Divalent transitional metal-rich tourmalines have not been found in Nature probably due to the insufficient concentration of these elements in B-rich geological systems.