Crystal Chemistry and Stability of Hydrated Rare-Earth Phosphates Formed at Room Temperature

Abstract

Abstract:In order to understand the crystal chemical properties of hydrous rare-earth (RE) phosphates, REPO 4,hyd , that form at ambient temperature, we have synthesized REPO 4,hyd through the interaction of aqueous RE elements (REEs) with aqueous P at room temperature at pH < 6, where the precipitation of RE hydroxides does not occur, and performed rigorous solid characterization. The second experiment was designed identically except for using hydroxyapatite (HAP) crystals as the P source at pH constrained by the dissolved P. Hydrated RE phosphate that precipitated at pH 3 after 3 days was classified into three groups: LREPO 4,hyd (La → Gd) containing each REE from La-Gd, MREPO 4,hyd (Tb → Ho), and HREPO 4,hyd (Er → Lu). The latter two groups included increasing fractions of an amorphous component with increasing ionic radius, which was associated with non-coordinated water. RE all PO 4,hyd that contains all lanthanides except Pm transformed to rhabdophane structure over 30 days of aging. In the experiments using HAP, light REEs were preferentially distributed into nano-crystals, which can potentially constrain initial RE distributions in aqueous phase. Consequently, the mineralogical properties of hydrous RE phosphates forming at ambient temperature depend on the aging, the pH of the solution, and the average ionic radii of REE, similarly to the well-crystalline RE phosphates.

A Spectroscopic and Computational Study of Cm3+ Incorporation in Lanthanide Phosphate Rhabdophane (LnPO4·0.67H2O) and Monazite (LnPO4)

“…Another possibility for the nonlinear compositional trend for the lattice parameter β could be small variations in the hydration state of the hydrated rhabdophane compounds; that is, water of hydration is slightly lower than the expected 0.67 for fully hydrated, monoclinic rhabdophane. According to literature, a first dehydration step for hydrated rhabdophane has been observed above ∼80 °C. , In this work, the precipitation of rhabdophane was performed at 90 °C for a total duration of one week, followed by drying of the solid phase at this temperature for 12 h. Thus, a small amount of dehydration could be expected, leading to a small deviation of the β angle. The dehydration reaction should be reversible, , but incomplete rehydration of the compounds cannot be excluded.…”

“…According to literature, a first dehydration step for hydrated rhabdophane has been observed above ∼80 °C. 34,53 In this work, the precipitation of rhabdophane was performed at 90 °C for a total duration of one week, followed by drying of the solid phase at this temperature for 12 h. Thus, a small amount of dehydration could be expected, leading to a small deviation of the β angle. The dehydration reaction should be reversible, 33,34 but incomplete rehydration of the compounds cannot be excluded.…”

Organic Ligand-Mediated Dissolution and Fractionation of Rare-Earth Elements (REEs) from Carbonate and Phosphate Minerals

“…The reference pattern of hydroxylbastnäsite-(Ce) is shown here because it is almost identical to that of hydroxylbastnäsite-(La) except for slight shifts, and the pattern of hydroxylbastnäsite-(La) is not available . Similarly, the reference patterns of rhabdophane-(La) and rhabdophane-(Nd) are almost identical to that of rhabdophane-(Ce) and are thus not shown here …”

Experimental study of apatite-fluid interaction and partitioning of rare earth elements at 150 and 250 °C

“…Particularly, little is known on apatite solubility in acidic and saline hydrothermal fluids at 100 to 300 °C, which are conditions that could be favorable for the formation of REE chloride complexes and REE moblization in aqueous fluids (Migdisov et al 2009(Migdisov et al , 2016Gysi and Williams-Jones 2013;Perry and Gysi 2020). Furthermore, experimental investigations show that rhabdophane [REE(PO4)•nH2O] is the stable REE phosphate and sink for REE at temperatures below 100 °C, whereas the anhydrous REE phosphate minerals monazite and xenotime control the solubility of REE at temperatures above 100 °C (Gysi et al 2015(Gysi et al , 2018Gausse et al 2016;Ochiai and Utsunomiya 2017;Arinicheva et al 2018;Van Hoozen et al 2020;Gysi and Harlov 2021). Several studies have also utilized experimentally derived thermodynamic models to calculate the Cl, OH, and F composition and solubility of apatite in aqueous fluids (Zhu and Sverjensky 1991;Mair et al 2017), in melts, and melt-fluids (Webster et al 2009(Webster et al , 2017Hermann 2015, 2017;McCubbin et al 2015;Riker et al 2018;Li and Costa 2020), which are supported by calorimetric experiments (Hovis and Harlov 2010;Hovis et al 2014).…”