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

Author(s)

Jan Filip, Ferdinando Bosi, Milan Novak, Henrik Skogby, Jiri Tucek, Jan Cuda, Manfred Wildner

Abstract

We present a detailed study of thermally-driven oxidn. and redn. 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 temps. of 700, 800 and 900 °C to oxidize or reduce the structural Fe. High-temp. changes were continuously monitored using 57Fe Moessbauer and IR 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 obsd., 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 oxidn./redn. of Fe at the Y and Z sites. Crit. samples of Cancrinite Hill tourmaline were investigated in detail by means of 57Fe Moessbauer and IR spectroscopy, single-crystal X-ray diffraction, electron microprobe and magnetometry. The optimized structural formulas are: X(Na0.93K0.02$$square$$0.05)Y(Ti0.13Al0.20Fe1.263+Fe1.102+Mn0.022+$$square$$0.30)Z(Al5.16Fe0.532+Mg0.31)BB3 T(Si5.88Al0.12)O27 V(OH)3 W(O0.12OH0.88) - untreated tourmaline (schorl);X(Na0.93K0.02$$square$$0.05)Y(Ti0.13Al0.52Fe1.143+Fe0.842+Mg0.05Mn0.022+$$square$$0.30)Z(Al4.85Fe0.902+Mg0.25)BB3 T(Si5.88Al0.12)O27 V(OH)3 W(OH) - tourmaline reduced in hydrogen (schorl);X(Na0.93K0.02$$square$$0.05)Y(Ti0.13Al0.40Fe2.003+Mg0.15Mn0.022+$$square$$0.30)Z(Al4.99Fe0.873+Mg0.14)BB3 T(Si5.90Al0.10)O27 V(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 intracryst. cation-exchange YR2++ZR3+$$leftrightarrows$$YR3++ZR2+ driven by elevated temp. 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 oxidn. of Fe within the tourmaline structure is possible. However, significant redn. of Fe cannot occur because an excess of H+ may not be incorporated within the tourmaline structure. Further redn. of Fe3+ will not occur until the breakdown of the tourmaline structure above its temp. of reductive decompn., where metallic Fe immediately appears as a sep. phase (a-Fe) together with cristobalite and Na-Al-Fe-borosilicate glass. The scarcity of Fe3+-rich tourmalines in nature and the sep. 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.

Organisation(s)

Department of Mineralogy and Crystallography

External organisation(s)

Palacký University Olomouc, Sapienza University of Rome, Masaryk University, Swedish Museum of Natural History

Journal

Geochimica et Cosmochimica Acta

Volume

86

Pages

239-256

No. of pages

18

ISSN

0016-7037

Publication date

2012

Peer reviewed

Yes

Austrian Fields of Science 2012

105113 Crystallography, 105116 Mineralogy

Portal url

https://ucrisportal.univie.ac.at/en/publications/acfc2e89-f46a-4a0d-98f8-eef1e7f094ba