The formation, precipitation and structural characterisation of hydroxyaluminosilicates formed in the presence of fluoride and phosphate

Abstract

Hydroxyaluminosilicates (HAS) are important secondary mineral phases formed by the reaction of silicic acid (Si(OH) 4) with aluminium. Two discrete forms of HAS have been identified (HAS A and HAS B) and their structures and composition determined. Herein we have investigated the formation of HAS in the presence of equimolar Si(OH) 4 and fluoride (F −) or phosphate ( HPO 4 2 - ) . The latter resulted in the precipitation of aluminium hydroxyphosphate and inhibited the formation of HAS except where the concentration of Al was significantly in excess of HPO 4 2 - where HAS A was co-precipitated. There was no evidence of the formation of HAS which included phosphate in the structure. Fluoride did not prevent the formation of HAS, except, possibly, when it was present at four times the concentration of Al, and the inclusion of F in precipitated HAS was confirmed using electron microprobe and solid state NMR. Both HAS A and HAS B were found to incorporate F though evidence from NMR, in particular, suggested that F substituted for OH on Al but not on Si. In addition F was bound to octahedrally and not tetrahedrally co-ordinated Al and this preference appeared to inhibit or delay the dehydroxylation reaction which is involved in the transition between HAS A and HAS B. This is the first report of F-substituted HAS and further research will be required to determine if they are of environmental significance or, indeed, if these inorganic fluorinated polymers are of any value to materials science.