The infrared OH-stretching region of synthetic richterites in the system Na (sub 2) O-K (sub 2) O-CaO-MgO-SiO (sub 2) -H (sub 2) O-HF

All the synthesized amphiboles show a 'tremolite-type' band at 3670 cm (super -1) which suggests a partially vacant A-site. The departure from ideal compositions is interpreted as due to structural constraints. For the OH end-member Na-richterite the principal OH-stretching band is a single one centred at 3730 cm (super -1) but for K-richterite the band is split into two at 3735 and 3730 cm (super -1) . This is interpreted in terms of positional disorder of the K ion on the A-site. Along the (OH,F) solid solution, a new band appears at 3714 cm (super -1) whose intensity regularly increases as a function of X (sub F) . No band shift is observed. This modification of the IR spectra is interpreted as due to an ordered OH-F substitution. In K-richterites, OH groups which are in stronger interaction with K ions in the A(m) position are selectively replaced by F; the high-wave number component of the OH-stretching doublet disappears first. The strong anisotropy of cell dimension variations with increasing X (sub F) , in both the Na- and K-series, also reflects the weakening of repulsive interactions beween the hydroxyl proton H (super +) and the Na (super +) or K (super +) ion in the A-site, as F (super -) replaces OH (super -) .