Hydroxide in pyroxene: Variations in the natural environment

A suite of 51 pyroxenes from a wide range of geological environments was examined with infrared spectroscopy. Small amounts of OH occur in nearly all the samples, suggesting that OH groups are a minor component of most pyroxenes. The OH concentrations vary (0.002-0.12 wt% OH) as a function of geological setting with the greatest amounts occurring in mantle-derived samples. The intensities of certain OH absorption bands are somewhat correlated with pyroxene composition, in particular with the presence of trivalent cations. This suggests that crystal chemical or compositional factors also control the OH incorporation. The OH incorporated in the pyroxene structure is easily distinguishable from OH owing to amphibole lamellae, which usually occur in diopsides and often in orthopyroxenes. OH in pyroxene is probably a function of the activity of hydrous components during crystallization but may also depend on postcrystallization changes in the geological environment