COMPOUNDS OF THE CRANDALLITE TYPE - THERMODYNAMIC PROPERTIES OF CA-PHOSPHATES, SR-PHOSPHATES, BA-PHOSPHATES, PB-PHOSPHATES, LA-PHOSPHATES, CE-PHOSPHATES TO GD-PHOSPHATES AND CA-ARSENATES, SR-ARSENATES, BA-ARSENATES, PB-ARSENATES, LA-ARSENATES, CE-ARSENATES TO GD-ARSENATES

Pure synthetic (arseno-)goyazite, (arseno-)gorceixite, (arseno-)plumbo gummite, La-, Ce- to Eu- (arseno-)crandallites, arsenocrandallite and Gd-crandallite, prepared from hydroxides, bayerite and phosphoric resp . arsenic acid at 200-degrees-C/15 bar, were used to determine their S tandard Free Energies of Formation DELTAG-degrees(f) resp. chemical po tentials mu-degrees at 333 K. This became possible through the determi nation of the equilibrium constants K for the dissolution equilibria i n aqueous solution after equilibration for at least one year. Because of the presence of gibbsite in the 333 K runs only the activities resp . concentrations of the metal cations, of H(aq)+ and of H3PO4 resp. H3 AsO4 had to be determined analytically. The value for crandallite, whi ch could not be synthesized, was derived from goyazite-crandallite mix ed crystals which turned out to exhibit ideal mixing. Substitution of phosphate by arsenate leads to a general increase of the thermodynamic stabilities; also pure arsenocrandallite becomes a stable phase at 47 3 K. Because the stability fields of crandallites are enlarged with fa lling temperature pure crandallite may become a stable phase in the we athering zone. The sequence of thermodynamic stabilities of the REE-(a rseno-)crandallites could be well discriminated and is characterized b y La > Nd > Ce > Sm > Pr > Eu > Gd > heavy REE for the phosphates and Ce > La is-approximately-equal-to Nd > Eu is-approximately-equal-to Pr > Sm > heavy REE for the arsenates. Presence of phases in an active s tate may alter the thermodynamic stabilities considerably.