SOL-GEL SYNTHESIS OF FLUORIDE-SUBSTITUTED TALC

Fluoride-substituted talcs, Mg3Si4O10(Fr-x,OH1-x)(2)(fluorotalcs), whe re x is 0.45 or 0.71, were synthesized at 750 degrees C at confining p ressures between 150 and 200 MPa in 72 h from calcined amorphous xerog els, which were prepared from a fluoridated magnesium silicate sol. A soft, slippery product was obtained when the xerogel was fired with mo re than the stoichiometric amount of water necessary in the final stru cture. FTIR and XRD analyses showed decreases in the O-H stretching fr equency and in the position of the (060) peak to lower 2 theta values, respectively, with increase in x. The upper limit of the thermal stab ility of the synthetic fluorotalcs, estimated by thermal analysis (DTA and TG), was 1115 or 1075 degrees C when x was 0.45 or 0.71, respecti vely. The former represents an increase in thermal stability of simila r to 250 degrees C compared to synthetic talc without fluoride. The sl ightly lower limit obtained for the latter may indicate that too high fluoride substitution may destabilize the structure. Attempts to synth esize phases with x > 0.71 resulted in mixtures of a talc phase and Mg F2, indicating that fluoride does not fully substitute for hydroxide u nder the conditions used. Firing of xerogels above 800 degrees C under atmospheric pressure did not yield talc but formed forsterite and hum ite minerals including norbergite.