Effect of heat treatment on tourmaline from Brazil

Abstract

The crystal-chemical behaviour of tourmaline from Aracuai, Minas Gerais state, Brazil, when subjected to heating in air atmosphere has been studied by several techniques, including EMPA, UV - Vis, TGA, and Mossbauer spectroscopy. The tourmaline samples are typically intermediate members of the elbaite-schorl series. The origin of colour and of its change after treatment has been discussed in terms of local disorder, presence of metal transition elements, oxidation of ferrous iron at the octahedral site, and simultaneous trap of the excess electron. These findings may be used to enhance the colour in tourmaline crystals or generate wanted colour changes

Iron redox reactions in the tourmaline structure: High-temperature treatment of Fe3+-rich schorl

“…1c and 2). The possibility of full oxidation of Fe in tourmaline structure under oxidizing conditions of air is consistent with previous studies (Fuchs et al, 2002;Pieczka and Kraczka, 2004;Castañ eda et al, 2006;Bačík et al, 2011). Importantly, a low-temperature 57 Fe Mö ssbauer spectrum (25 K; Fig.…”

“…being in line with previous studies of thermal treatment of tourmaline in oxidizing conditions (see Pieczka and Kraczka, 2004;Castañ eda et al, 2006;Bačík et al, 2011 and references therein). As a principal conclusion of presented high-temperature experiments, the possibility to fully oxidize Fe within the studied samples (i.e., Fe 3+ -rich schorl, Fe 2+ -bearing olenite and fluor-schorl) but not to reduce Fe to significantly higher extent that was the original value is due to the inability of the structure to incorporate an excess of H + .…”

“…In fact, both site-scattering refinement and bond-length considerations are consistent with the B site being fully occupied by B and no or negligible amounts of B at the T site. All Mn was assumed to be divalent, as crystal-structure studies have shown that this is the most common valence state of Mn in tourmaline (Bosi et al, 2005a), even at elevated temperatures under oxidizing conditions (Castañ eda et al, 2006). The OH content of the untreated tourmaline could then be calculated by charge balance with the assumption that T+Y+Z = 15.00 apfu.…”

“…Their main results include: (1) thermal oxidation of Fe 2+ to Fe 3+ in tourmaline starts at a temperature of about 600°C (Bogdanova et al, 1981;Povondra and Laštovičková, 1989;Afonina et al, 1993;Fuchs et al, 2002;Pieczka and Kraczka, 2004;Castañ eda et al, 2006;Bačík et al, 2011), leading to a change of color to brown or reddish brown (Castañ eda et al, 2006;Ertl and Rossman, 2007;Bačík et al, 2011); (2) at elevated temperature, Al and Fe 3+ become disordered over the Y and Z sites (Afonina et al, 1993;Pieczka and Kraczka, 2004;Castañ eda et al, 2006;McKeown, 2008); (3) oxidized tourmaline loses H and F (Pieczka and Kraczka, 2004;Castañ eda et al, 2006;McKeown, 2008;Bačík et al, 2011); (4) oxidation of Fe 2+ in schorl changes the dimensions of the Y site and partly also of the Z site, and moreover, the unit-cell parameters approach those characteristic of "buergerite" (Bogdanova et al, 1981;Povondra and Laštovičková, 1989;Pieczka and Kraczka, 2004;Bačík et al, 2011); (5) above a temperature of about 900°C the tourmaline decomposes (Povondra and Laštovičková, 1989;Afonina et al, 1993;Pieczka and Kraczka, 2004;Castañ eda et al, 2006;McKeown, 2008;Bačík et al, 2011). The published experiments were almost exclusively performed under oxidizing conditions (i.e., in air); however, different structural aspects might appear under reducing conditions.…”

Anisotropy of hydrogen diffusion in tourmaline

“…At the temperatures and annealing durations reported in Table 2, the samples do not show any optical or IR evidence for destabilization. The shape of infrared OH and OD bands, as well as fundamental absorption of BO 3 groups at 1370 cm À1 , remain unchanged-even after 257 h annealing at 1073 This is in agreement with thermal analyses experiments that show the release of OH and H 2 O groups in tourmaline only occurs above 1123 K (Castañ eda et al, 2006).…”