Fe-tourmaline synthesis under different T and f_O₂conditions

Abstract

Mössbauer spectrometry of natural samples of tourmaline shows the presence of Fe²⁺ and Fe³⁺ either in the Y (9b) or the Z (18c) sites, with intervalent iron delocalized between these sites. To understand the distribution of iron in natural samples, tourmaline was syn­thesized at temperatures from 400 to 700 °C, under a pressure of 100 MPa, by the trans­formation of an Fe²⁺-rich natural chlorite in the presence of Na_0.5K_0.5-feldspar, boric acid, and H₂O in stoichiometric proportions. The oxygen fugacity, f_O₂, of most experiments was buffered by solid assemblages including hematite (HM), nickel-nickel oxide (NNO), and quartz-fayalite-iron (QFI). It was possible to synthesize tourmaline with Fe²⁺ in the Z site and Fe²⁺ or Fe²⁺ + Fe³⁺ in the Y site. These site occupancies are similar to those observed in many natural samples.

For each f_O₂ buffer, Mossbauer spectrometry shows that Fe³⁺/Fe²⁺ in the Y site is cor­related positively with T and f_O₂ for T <550 °C. For higher temperatures, a negative cor­relation is observed. These relationships reflect structural changes involving the localiza­tion of Fe²⁺ in the Z site and an increasing amount of Al in the Y site. Fe³+/total Fe can be represented by general equations of the type [%Fe³⁺] = e^{(a log}f_O₂+^b). The curves corre­sponding to each temperature are roughly parallel and equidistant. Thus, Fe tourmaline is a potential geothermometer and an f_O₂ indicator.