The infrared OH-stretching region of synthetic richterites in the system Na20-K20-CaO-MgO-Si02-H20-HF
Robert, Jean-Louis; Della Ventura, Giancarlo; Thauvin, Jean-Louis
European Journal of Mineralogy Volume 1 Number 2 (1989), p. 203 - 212
published: May 3, 1989
manuscript accepted: Jan 11, 1989
manuscript received: Aug 22, 1988
ArtNo. ESP147050102002, Price: 29.00 €
Abstract The results of an infrared investigation are presented for the OH-stretching region in synthetic (OH,F) substituted Na- and K-richterites. All the synthesized amphiboles exhibit a "tremolite-type" band at 3,670 cm-1. This band suggests a partially vacant A-site ; it even persists in samples that were synthesized in concentrated N a d and KC1 solutions, rather than pure water. The departure from ideal composition is interpreted as due to structural constraints, at least under the present experimental conditions. For the OH end-member Na-richterite, the principal OH-stretching band is a single sharp band, centred at 3,730 cm-1. In the OH end-member K-richterite, this high-intensity band is split into two components, of almost equal intensity, at 3,735 and 3,730 cm-1. This splitting is interpreted in terms of positional disorder of the K ion on the A-site: A(m) and A(2/m) positions are occupied, which perturbs the repulsive H+-K+ interaction. Along the (0H,F) solid solution, a new band appears at 3,714 cm- 1 whose intensity regularly increases as a function of XF. 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-wavenumber component of the OH-stretching doublet (at 3,735 cm-1) disappears first. The strong anisotropy of cell dimension variations with increasing XF, in both the Na- and K-series (i.e. a strong decrease of the a parameter compared to b, c and ß), also reflects the weakening of repulsive interactions between the hydroxyl proton H+ and the Na+ or K+ ion in the A-site, as F" replaces OH".