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All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on NdPO4 by Materials Project

NdPO4 is Zircon-like structured and crystallizes in the hexagonal P6_222 space group. The structure is three-dimensional. Nd3+ is bonded to eight equivalent O2- atoms to form distorted NdO8 hexagonal bipyramids that share corners with four equivalent PO4 tetrahedra, edges with four equivalent NdO8 hexagonal bipyramids, and edges with two equivalent PO4 tetrahedra. There are four shorter (2.43 Å) and four longer (2.56 Å) Nd–O bond lengths. P5+ is bonded to four equivalent O2- atoms to form PO4 tetrahedra that share corners with four equivalent NdO8 hexagonal bipyramids and edges with two equivalent NdO8 hexagonal bipyramids. All P–O bond lengths are 1.55 Å. O2- is bonded in a 1-coordinate geometry to two equivalent Nd3+ and one P5+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on NdPO4 by Materials Project

NdPO4 crystallizes in the monoclinic P2_1/c space group. The structure is three-dimensional. Nd3+ is bonded in a 9-coordinate geometry to nine O2- atoms. There are a spread of Nd–O bond distances ranging from 2.45–2.80 Å. P5+ is bonded in a tetrahedral geometry to four O2- atoms. There is three shorter (1.55 Å) and one longer (1.56 Å) P–O bond length. There are four inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted single-bond geometry to two equivalent Nd3+ and one P5+ atom. In the second O2- site, O2- is bonded in a distorted single-bond geometry to two equivalent Nd3+ and one P5+ atom. In the third O2- site, O2- is bonded in a distorted single-bond geometry to two equivalent Nd3+ and one P5+ atom. In the fourth O2- site, O2- is bonded in a distorted single-bond geometry to three equivalent Nd3+ and one P5+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on NdPO4 by Materials Project

NdPO4 is Zircon structured and crystallizes in the tetragonal I4_1/amd space group. The structure is three-dimensional. Nd3+ is bonded in a 8-coordinate geometry to eight equivalent O2- atoms. There are four shorter (2.45 Å) and four longer (2.48 Å) Nd–O bond lengths. P5+ is bonded in a tetrahedral geometry to four equivalent O2- atoms. All P–O bond lengths are 1.56 Å. O2- is bonded in a 1-coordinate geometry to two equivalent Nd3+ and one P5+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2025

NdPO 4 solubility and aqueous Neodymium speciation in supercritical fluids: An experimental study at 500–700 °C and 1.7 kbar

A key aspect in the formation of rare earth elements (REE) deposits is the role of REE transport as aqueous REE complexes in supercritical hydrothermal solutions, where the nature of the aqueous complex is controlled by solution composition, temperature and pressure. Despite chloride being considered as one of the most abundant transporting ligands in magmatic-hydrothermal fluids, experimental investigations on the stability of aqueous REE chloride complexes are scarce above 300 °C. In this study, synthetic NdPO 4 crystals were reacted with non-saline and saline (0, 0.05 and 0.5 mNaCl), acidic (0.01 mHCl) aqueous solutions in a series of solubility experiments conducted at 500–700 °C and 1.7 kbar, where the solubilities were determined using a stable Nd isotope ( 145 Nd isotope spike) dilution technique. NdPO 4 solubility ranges between 28 ppm and 10,858 ppm, where solubility increases with both temperature and salinity. At 500 °C, log mNdPO 4 increases from –3.93 to –1.60 and there is a strong correlation between NdPO 4 solubility and NaCl concentrations (slope of 1.2 ± 0.3), indicating stabilization of the Nd chloride aqueous complexes with a stoichiometry corresponding to NdCl 2+ . At 600 °C, this correlation is weaker (slope of 0.4, log mNdPO 4 increases from –2.63 to –1.88) indicating the stabilization of both Nd chloride and hydroxyl species controlling solubility. At 700 °C, NdPO 4 solubility is largely independent of NaCl concentration indicating that solubility is controlled by Nd hydroxyl complexes, where stoichiometry suggests the neutral Nd(OH) 3 0 species is dominant. The solubility product (Ksp) of NdPO4 is derived from experimental data with the relation: log K sp = -41.81 – 0.057T – 20987/T, with T temperature in Kelvin. Comparison of the measured Nd phosphate solubility to thermodynamic predictions using the available Helgeson-Kirkham-Flowers equation of state parameters for aqueous Nd complexes indicate that predictions are up to three orders of magnitude lower compared to experimental observations. This discrepancy is most pronounced in saline solutions, suggesting that thermodynamic properties of the REE chloride species in supercritical fluids require revision. Numerical simulations of fluid-rock interaction between acidic, saline fluids and a Strange Lake felsic mineral assemblage demonstrates that NdPO 4 solubility predictions from models are four to six orders of magnitude lower than those calculated based on empirical fits from experiments, which suggests that acidic, saline fluids may play an important role in mobilizing large amounts of light REE from 450 to 700 °C.

58 GEOSCIENCES↗
DOE OSTI2026

The solubility of NdPO 4 and DyPO 4 and stability of Nd and Dy chloride and hydroxyl complexes as a function of pH and salinity to 450 °C

The mobility of rare earth elements (REE) in geological systems is often controlled by the stability of monazite and xenotime, which are important hosts for the light and heavy REE. These REE phosphates constitute important resources and provide information on ore formation conditions and timing due to their uses as geothermometers and geochronometers. While the thermodynamic properties for these minerals are well-established up to high temperature and pressure, the properties for the aqueous REE complexes are not well constrained with limited information up to 300 °C. In this study, the solubility of synthetic NdPO 4 and DyPO 4 endmembers were measured in hydrothermal sub- to supercritical NaCl-bearing aqueous solutions from 350 at saturated water vapor pressure to 450 °C at 700 bar. The speciation of Nd and Dy was determined in acidic to alkaline solutions (pH 25 °C values from 2 to 10), and indicates that the hydroxyl species REE(OH) 2 + and REE(OH) 3 0 predominate at low salinity (0.01 mol/kg NaCl) with some contribution of REE chloride species REECl 2+ and REECl 2 + in acidic fluids. The REE phosphate solubility measured in these experiments is up to two orders of magnitude higher than predicted using existing thermodynamic properties from literature for aqueous species extrapolated from lower temperature data to supercritical conditions. To address this discrepancy, the thermodynamic properties of the REE aqueous species were optimized using GEMSFITS to derive the formation constants for the REE chloride (β Cl ) and hydroxyl (β OH ) complexes in the studied temperature range. This study highlights the importance of the hydroxyl species REE(OH) 2 + and REE(OH) 3 0 over a wide range of pH and temperature. As a result, the revised thermodynamic properties for Dy and Nd chloride and hydroxyl species more accurately predict the solubility of NdPO 4 and DyPO 4 and provide insight into the speciation and mobility of the light and heavy REE in hydrothermal supercritical fluids in the crust.

58 GEOSCIENCES↗