OH- in pyroxene: An experimental study of incorporation mechanisms and stability

Abstract

Most pyroxenes contain trace amounts of OH^- that are lost when heated at 700 °C (1atm) under either air or H_2. When Si-deficient Fe^(3+) -rich pyroxenes (ferrian diopside and esseneite) are heated in H_2, an up to 70-fold increase of the intensity of the infrared OH absorption bands occurs, which is well correlated with Si deficiency in the tetrahedral site. Mössbauer and optical spectroscopy indicate that these samples have substantial amounts of Fe^(3+) in the tetrahedral position. After the uptake of H_2, optical spectra show that the amount of Fe^(2+) in the M(2) site has increased, corresponding to charge compensation according to the reaction Fe^(3+)O^(2-) + 1/2H_2 = Fe^(2+) + OH^-. Heating in air at 600-700 °C decreases or removes the OH^- bands. The OH^- bands from air-heated samples are restored by subsequent heating in H_2. Hydrothermal experiments (600 to 800 °C and Ito 2-kbar H_2O pressure) do not increase the amount of OH^- but do redistribute some of the absorption intensity among the OH^- bands. Experiments performed in D_2O show that OD^- readily replaces OH^-. Therefore the diffusion of H^+ through the pyroxene crystal cannot be a rate-limiting process, and hence a more fundamental change than just a mechanical introduction is required to incorporate OH^- in the pyroxene structure. The thermal stability of the OH^- in pyroxene is comparable to OH^- in Fe-rich amphiboles and suggests that pyroxenes can provide information about the activity of hydrous components prevalent during their crystallization.

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