Hydroxyl groups in nonmetamict chevkinite-(Ce):a crystal chemical discussion

The minerals of chevkinite group were commonly considered to be anhydrous minerals. The infrared absorption spectrum of natural nonmetamict chevkinite-(Ce) from the aegirine-alkali granite, Mianning, Sichuan Province, China, exhibited two broad peaks in the 3600-2800 cm-1 region owing to the OH stretching. The corresponding H2O content required for the charge balance in formula was 1.27%. The O-H ··· O bond lengths maight cover from 0.2658 to 0.2794 nm by the correlated OH stretching energies. An electrostatic charge balance for chevkinite-(Ce) based on the assigned site-population from chemical data was calculated without the hydrogen contribution. The resulting empirical bond-valence sum on O6, O8, O2, O3, O5, and O4 ranged from 1.73 to 1.95 vu. The partial substitution of O by OH may occur in four atom sites: O6, O2, O4, and O5. The small differences in the bond-valence sums between the supposed donors and acceptors may mean a mixed donor/acceptor role of the involved oxygen atoms. The IR spectral features between 3394 and 3035 cm-1 consisted of various hydrous species at different structural sites and orientations. The OH groups in the chevkinite-(Ce) appeared to be involved in local charge imbalance in the structure and to be present when the mineral crystallized hydrothermally.