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Oxidized tourmalines — a combined chemical, XRD and Mössbauer study

University of Mining and Metallurgy, Department of Mineralogy, Petrography and Geochemistry, al. Mickiewicza 30, 30–059 Kraków, Poland; e-mail: pieczka@uci.agh.edu.pl

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 _max. <Y-O> and _max. <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 Fe^2.5+ 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.

Key-words: tourmaline, X-ray powder diffraction, Mossbauer spectroscopy, Fe²⁺ thermal oxidation.

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