An experimental study of dissolution--reprecipitation in fluorapatite: fluid infiltration and the formation of monazite

In a series of timed experiments, monazite inclusions are induced to form in the Durango fluorapatite using 1 and 2 N HC1 and [H.sub.2]S[O.sub.4] solutions at temperatures of 300, 600, and 900 [degrees]C and pressures of 500 and 1,000 MPa. The monazite inclusions form only in reacted areas, i.e. depleted in (Y+REE)+Si+Na+S+Cl. In the HC1 experiments, the reaction front between the reacted and unreacted regions is sharp, whereas in the [H.sub.2]S[O.sub.4] experiments it ranges from sharp to diffuse. In the 1 N HC1 experiments, Ostwald ripening of the monazite inclusions took place both as a function of increased reaction time as well as increased temperature and pressure. Monazite growth was more sluggish in the [H.sub.2]S[O.sub.4] experiments. Transmission electron microscopic (TEM) investigation of foils cut across the reaction boundary in a fluorapatite from the 1 N HCI experiment (600[degrees]C and 500 MPa) indicate that the reacted region along the reaction front is characterized by numerous, sub-parallel, 10-20 nm diameter nano-channels. TEM investigation of foils cut from a reacted region in a fluorapatite from the 1 N [H.sub.2]S[O.sub.4] experiment at 900[degrees]C and 1,000 MPa indicates a pervasive nano-porosity, with the monazite inclusions being in direct contact with the surrounding fluorapatite. For either set of experiments, reacted areas in the fluorapatite are interpreted as replacement reactions, which proceed via a moving interface or reaction front associated with what is essentially a simultaneous dissolution-reprecipitation process. The formation of a micro- and nano-porosity in the metasomatised regions of the fluorapatite allows fluids to permeate the reacted areas. This permits rapid mass transfer in the form of fluid-aided diffusion of cations to and from the growing monazite inclusions. Nano-channels and nano-pores also serve as sites for nucleation and the subsequent growth of the monazite inclusions.