Abstract
Thermal oxidation of Fe2+ in tourmalines of the schorl-dravite series induced by gradual annealing of powdered samples in air above 600°C was studied using X-ray diffraction and Mossbauer spectroscopy. The a unit-cell parameter distinctly decreases, while the c parameter even slightly increases during the oxidation as a result of thermal modification of the X, Y, Z, B and T polyhedra sizes, particularly those primarily occupied by Fe2+ or traces of Mn2+. The Y octahedron shrinks proportionally to the initial content of Fe2+(Fe2++Mn2+), while the Z octahedron even shows a small expansion. The Δmax. <Y-O> and Δmax. <Z-O> values estimated for 3 Fe2+ (Fe2++Mn2+) per formula unit (pfu) are -(0.072–0.070) Å and +0.009 Å, respectively. They indicate that in the tourmaline structure the primary Fe2+ and traces of Mn2+ occupied only the Y sites. In Mossbauer spectra of oxidized tourmalines the Fe2+ doublets with QS > 2 mm/s decreased, while the doublets with QS < 2 mm/s slightly increased during oxidation at elevated temperatures. Simultaneously, the Fe2.5+ mixed valence doublet and the Fe3+ doublets progressively increased with advancing oxidation. These changes are explained in term of progressive generation of Fe3+ in place of primary Fe2+ within the triad of the Y sites. The process results in progressive formation of the Me2+Fe2+Fe3+ Y-triads, followed by the Fe2+Fe3+Fe3+ triads, sometimes with an electron delocalized among adjacent Fe2+ and Fe3+. Finally, triads with the Y sites occupied only by Fe3+ instead of the primary Fe2+ are generated and tourmaline completely transformes into the oxidized form. Annealing of tourmalines after complete oxidation increases Fe3+—Al disordering among the Y and Z sites up to the structure's decomposition at around 900°C. The Fe2+ doublets with QS < 2 mm/s commonly assigned to Fe2+ at the Z sites result rather from the presence of Fe2+ at the Y sites and the temperature- or oxygen-fugacity-induced changes in the W site occupancy by hydroxyls and oxygens.
