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

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.