THE OH-F SUBSTITUTION IN TI-RICH POTASSIUM RICHTERITE - RIETVELD STRUCTURE REFINEMENT AND FTIR AND MICRO-RAMAN SPECTROSCOPIC STUDIES OF SYNTHETIC AMPHIBOLES IN THE SYSTEM K2O-NA2O-CAO-MGO-SIO2-TIO2-H2O-HF

Amphiboles were synthesized along the join K(NaCa)Mg5(Si7.4Ti0.6)O22(OH,F)2 from (OH) = 2 to F = 2 apfu (atoms per formula unit). Rietveld structure refinement shows that Ti4+ is completely ordered at the T2 site. For the OH end-member starting composition, a single-phase amphibole is obtained with 0.6 apfu of Ti. Along the OH-F join, amphibole +/- priderite +/- titanite +/- rutile is produced, and the Ti content of the amphibole decreases to 0.16 apfu in end-member fluor-richterite. The infrared spectra in the principal OH-stretching region show a main band for the Ti-rich OH end-member centered at 3728 cm-1. This band is assigned to a MgMgMg-OH --> K configuration. When F is present, a new band appears at 3711 cm-1; the position of this band remains constant while its intensity increases with increasing F content, suggesting that the OH-F substitution is not random. The Raman spectra in the region of 1200-30 cm-1 show bands assigned to vibrations of Ti4+O4 groups that decrease in intensity with decreasing Ti4+ in the amphibole. The decrease in solubility of Ti in potassium richterite for increasing F content at O3 is compatible with the crystal-chemical model for [4]Ti4+ half arrow right over half arrow left Si substitution in richterite proposed by Oberti et al. (1992).