Most pyroxenes contain trace amounts of OH- that are lost when heated at 700 °C (1 atm) under either air or H2. When Si-deficient Fe3+-rich pyroxenes (ferrian diopside and esseneite) are heated in H2, 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. Mossbauer and optical spectroscopy indicate that these samples have substantial amounts of Fe3+ in the tetrahedral position. After the uptake of H2, optical spectra show that the amount of Fe2+ in the M(2) site has increased, corresponding to charge compensation according to the reaction Fe3+ + O2- + 1/2H2 = Fe2+ + 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 H2. Hydrothermal experiments (600 to 800 °C and 1-to 2-kbar H2O pressure) do not increase the amount of OH- but do redistribute some of the absorption intensity among the OH- bands. Experiments performed in D2O 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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