Crystallochemical Structure of Tourmalines Inferred from Mössbauer Spectroscopy

In the Y octahedra of tourmalines theoretically may occur ions of Fe$\text{}^{2+}$ and Fe$\text{}^{3+}$. The authors discuss Mössbauer measurements and confirm the ordered model of ionic distribution at these sites: the presence of Fe$\text{}^{3+}$ ions in the Y octahedra and Al$\text{}^{3+}$ ions in the Z octahedra. The spectra of different tourmalines annealed at temperatures ranging from 873 K to 1153 K show that Fe$\text{}^{3+}$ ions can occur in both of these sites. The annealing has changed the symmetry of the neighbourhood of iron ions at the octahedral sites. It has been shown that the doublets with QS<2.00 mm/s should not been attributed to Fe$\text{}^{2+}$(Z). Also new results corroborating the existence of the iron ions in the mixed valence state in tourmalines have been presented.