The crystal chemistry and thermal stability of sol-gel prepared fluoride-substituted talc

Stable, colloidal sols were prepared from the addition of methanolic (Mg(OCH₃)₂) to one equivalent of H₂O₂ in methanol. Fluoride was quantitatively incorporated by treatment of these sols with HF. Stable sols were obtained when sols, prepared from the acid catalyzed hydrolysis of tetramethylorthosilicate (TMOS), were combined with these fluoridated magnesium sols. Solvent removal gave xerogels, which were calcined, and treated with stoichiometric quantities of water at 750 °C and 1.6 kbar. The resulting products were high-purity, single-phase talcs with fluoride substitution as high as 75 mole percent; the fluoride substitution is higher than any previously reported. Powder XRD analysis showed a non-linear decrease in d(060) spacing with increasing fluoride substitution, which is attributed to a decrease in the b-dimension of talc when fluoride replaced hydroxide. FTIR spectroscopy showed a non-linear decrease in v_O-H in talc with increasing fluoride substitution, which was attributed to an increase in hydrogen bonding of the OH groups and an increase in the electronegativity of the octahedral layer in talc with increasing fluoride substitution. The thermal stability of the talcs was studied using DT and TG, powder XRD, FTIR, and fluoride ion-selective electrode analyses. Synthetic talc without fluoride decomposed at 860 °C, whereas talc with 68% fluoride substitution showed essentially no decomposition when heated to 1060 °C. When heated to 1200 °C, 68% fluoride substituted talc formed amorphous material, enstatite, protoenstatite, norbergite, and chondrodite. The upper stability temperature of talc, taken as the maximum rate of the first endothermic event in its DTA profile, was dependent on the extent of fluoride substitution. Talc with 100% fluoride substitution is predicted to be stable up to ～1100°C.