The lanthanum hydroxide fluoride carbonate system: The preparation of synthetic bastnaesite

Abstract

Hydrothermal phase equilibria in the lanthanum + hydroxide + fluoride + carbonate system have been investigated along an isobaric and isothermal section of variable metal to fluoride ratio, x. Quantitative substitution of fluoride into LaOHCO3 proceeds with the formation of a continuous solid solution, La(OH)1-xFxCO3, for 0 0 [les] x [les] 1 and a two-phase region, LaFCO3 + LaF3, for 1 x 3 is an orthorhombic phase (a = 21.891(5), B = 12.639(3) and C = 10.047(2) A) which is not isostructural with LaFCO3. Hydrolysis of the La(OH)1-xFxCO3 phase to the corresponding UCl3-type La(OH)3-xFx compositions has been observed. Thermal decomposition reactions of the hydroxide fluoride carbonates are described, and a geochemical process for the formation of bastnaesite and tysonite is proposed.