Iron redox reactions in the tourmaline structure: High-temperature treatment of Fe3+-rich schorl
| Authors | |
|---|---|
| Year of publication | 2012 |
| Type | Article in Periodical |
| Magazine / Source | Geochimica et Cosmochimica Acta |
| MU Faculty or unit | |
| Citation | |
| Doi | http://dx.doi.org/10.1016/j.gca.2012.02.031 |
| Field | Geochemistry |
| Keywords | crystal-structure; heat-treatment; ferric-iron; chemical-composition; site occupancies; dravite series; Mn-tourmaline; chemistry; Mössbauer; Fe |
| Description | 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. |
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