STABILITY RELATIONS OF ALUMINUM HYDROXY-FLUORIDE HYDRATE, A RALSTONITE-LIKE MINERAL, IN THE SYSTEM ALF3-AL2 O3-H2O-HF

Abstract

Phase relations in the system AlF3- Al2O3-H2O-HF have been investigated between 400° and 700°C using sealed gold tubes and conventional hydrothermal techniques. Stable crystalline assemblages synthesized include (1) Al2F6y(OH)y•nH2O (pyrochlore structure) + corundum (or diaspore), (2) Al2F6-y (OH)y•nH2O, (3) AlF3-x (OH)x (α-AlF3 structure) + Al2F6-y (OH)y•nH2O, 4) AlF2(OH) (β-AlF3 structure) + Al2F6-y (OH)y•nH2O, (5) AlF2(OH) + corundum, (6) AlF2(OH) + AlF3-x(OH)x, (7) AlF3-x(OH)x + corundum, and (8) AlF3-x(OH)x. Compositions of solids were determined using XRD methods after calibration with results of electron-microprobe analyses. The maximum thermal stability of Al2 F6-y(OH)y•n H2O is 575°C in assemblages with AlF2(OH) or corundum, and 435°C in the presence of AlF3-x(OH)x. AlF2(OH) replaces Al2F6-y(OH)y•nH2O in assemblages synthesized above these temperatures and, in turn, decomposes to AlF3-x(OH)x above 625°C. Al2F6-y(OH)y•nH2O hydrolyzes with decreasing temperature in the F-deficient portion of the system and with increasing temperature in the F-rich portion of the system, with the result that the range of Al2F6-y (OH)y•nH2O compositions on the join AlF3-Al(OH)3 contracts from almost 15 mol.% Al(OH)3 at 400°C to a single composition, 58 mol.% Al(OH)3, at 575°C.