Identification of a low-energy OH-valence vibration in zoisite

By using the H–D isotope effect on the vibrational spectrum of orthorhombic zoisite, we confirm the assignment of a band at 2160 cm⁻¹ to a low-energy OH-stretching vibration. These OH groups occur in strong hydrogen bridges with a bonding energy of approximately 40kJ/mole, along with the “normal” OH groups in the O(10)–H ⋯ O(4) bridge. Furthermore, four OH librational modes could be identified in the region below 1100 cm⁻¹. The samples studied by IR spectroscopy were synthesized at 14 kbar and 700°C in the systems CaO−Al₂O₃−SiO₂−H₂O and −D₂O, respectively.