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

Abstract

We present a detailed study of thermally-driven oxidation and reduction of Fe in the structure of schorl (as the most widespread tourmaline), Fe2+-bearing olenite and fluor-schorl. The principal Fe3+-rich tourmaline investigated in this study is a natural schorl sample from a unique occurrence in peralkaline rocks near Cancrinite Hill, east of Bancroft, southern Ontario. Tourmaline samples were thermally-treated in air and hydrogen at temperatures of 700, 800 and 900°C to oxidize or reduce the structural Fe. High-temperature changes were continuously monitored using 57Fe Mössbauer and infrared spectroscopy. Proportions of Fe2+ and Fe3+ vary as a function of the heat treatment. An increase in Fe3+ up to 100% after heating in air at 700°C was observed, whereas only small changes in the Fe3+/Fetot ratio after heating under hydrogen at 700°C was revealed. Partial deprotonation/protonation represents charge compensation for the oxidation/reduction of Fe at the Y and Z sites.Critical samples of Cancrinite Hill tourmaline were investigated in detail by means of 57Fe Mössbauer and infrared spectroscopy, single-crystal X-ray diffraction, electron microprobe and magnetometry. The optimized structural formulae are:X(Na0.93K0.02□0.05)Y(Ti0.13Al0.20Fe1.263+Fe1.102+Mn0.022+□0.30)Z(Al5.16Fe0.532+Mg0.31)BB3T(Si5.88Al0.12)O27V(OH)3W(O0.12OH0.88) – untreated tourmaline (schorl);X(Na0.93K0.02□0.05)Y(Ti0.13Al0.52Fe1.143+Fe0.842+Mg0.05Mn0.022+□0.30)Z(Al4.85Fe0.902+Mg0.25)BB3T(Si5.88Al0.12)O27V(OH)3W(OH) – tourmaline reduced in hydrogen (schorl);X(Na0.93K0.02□0.05)Y(Ti0.13Al0.40Fe2.003+Mg0.15Mn0.022+□0.30)Z(Al4.99Fe0.873+Mg0.14)BB3T(Si5.90Al0.10)O27V(O1.05OH1.95)W(O0.70OH0.30) – tourmaline oxidized in air (H+-rich “buergerite”).There is evident disorder of Al over the Y, Z and T sites as well as disorder of Fe2+ over the Y and Z sites, and ordering of Fe3+ at the Y site and Mg at the Z site. The fully oxidized tourmaline shows disorder of Fe3+ and Mg over the Y and Z sites. The occurrence of “extra” Fe2+ in the Z site of the reduced tourmaline relative to the untreated sample demonstrates the intracrystalline cation-exchange YR2++ZR3+⇆YR3++ZR2+ driven by elevated temperature. Increased disorder of Fe cations over the edge-shared YO6 and ZO6 octahedra enhances the antiferromagnetic exchange interactions in the tourmaline structure, which are stronger in reduced samples than in oxidized and untreated ones. This suggests that OH groups at the W site could mediate Y–Z inter-site exchange interactions.Complete oxidation of Fe within the tourmaline structure is possible. However, significant reduction of Fe cannot occur because an excess of H+ may not be incorporated within the tourmaline structure. Further reduction of Fe3+ will not occur until the breakdown of the tourmaline structure above its temperature of reductive decomposition, where metallic Fe immediately appears as a separate phase (α-Fe) together with cristobalite and Na–Al–Fe-borosilicate glass. The scarcity of Fe3+-rich tourmalines in nature and the separate existence of “buergerite” as the only tourmaline with almost all Fe as Fe3+ support its specific origin, such as from an HT-LP, high fO2 overprint.