Hydroxyl defects in pure forsterite are usually ascribed to incorporation of protons fully compensating the electrostatic charge of cationic vacancies. However, partially compensated vacancies have been predicted from theoretical considerations. Here, we theoretically determine the structural, vibrational and infrared spectroscopic properties of partially protonated cationic vacancies in forsterite using a first-principles theoretical modeling approach. The results show that the partial protonation of Si vacancies strongly modifies their spectroscopic properties with respect to those of the fully protonated (4H)xSi defect, leading to a significant downshift of at least one of the OH-stretching absorption bands. Comparison with experimental observations shows that such defects are unlikely to significantly contribute to the speciation of OH groups in pure synthetic forsterite samples. A partial protonation of Mg vacancies has a weak effect on the spectroscopic properties of OH groups, compared with those of the fully protonated defect, making it difficult to assess their occurrence from spectroscopic observations only.

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