Monazite and xenotime, the RE(P04) dimorphs, are the most ubiquitous rare earth (RE) minerals, yet accurate structure studies of the natural phases have not been reported. Here we report the results of high-precision structure studies of both the natural phases and the synthetic RE(P04) phases for all individual stable rare earth elements. Monazite is monoclinic, P2/n, and xenotime is isostructural with zircon (space group 14/amd). Both atomic arrangements are based on [001] chains of intervening phosphate tetrahedra and RE polyhedra, with a REOgpolyhedron in xenotime that accommodates the heavy lanthanides (Tb-Lu in the synthetic phases) and a RE09 polyhedron in monazite that preferentially incorporates the larger light rare earth elements (La-Gd). As the structure "transforms" from xenotime to monazite, the crystallographic properties are comparable along the [00I] chains, with structural adjustments to the different sizes of RE atoms occurring principally in (00I). There are distinct similarities between the structures that are evident when their atomic arrangements are projected down [001]. In that projection, the chains exist in (100) planes, with two planes per unit cell. In monazite the planes are offset by 2.2 A along [010], relative to those in xenotime, in order to accommodate the larger light RE atoms. The shift of the planes in monazite allows the RE atom in that phase to bond to an additional 02' atom to complete the RE09 polyhedron.

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Crystal chemistry of the monazite and xenotime structures

Inorganic lanthanide compounds with complex anions.

Synthesis of clay minerals

The enthalpies of formation for the compounds (RE3+)PO4, (where RE = Sc, Y, La–Nd, Sm–Lu) were determined by oxide-melt solution calorimetry. Calorimetric measurements were performed in a Calvet-type twin microcalorimeter in sodium molybdate (3Na2O · 4MoO3) and lead borate (2PbO · 2B2O3) solvents at 975 K. The experiments were carried out using both powdered single crystals grown by a flux technique and powders synthesized by precipitation. Formation enthalpies were derived from the drop-solution enthalpies for (RE)PO4, RE oxides, and P2O5. Enthalpies of formation for the (RE)PO4 compounds with respect to the oxides at 298 K become more negative with increasing RE3+ ionic radius; i.e., in going from ScPO4 (−209.8 ± 1.0 kJ/mol), to LuPO4 (−263.9 ± 1.9 kJ/mol), to LaPO4 (−321.4 ± 1.6 kJ/mol). From structural considerations, a similar trend is expected for the isostructural RE vanadates and arsenates, as well as for the tetravalent actinide orthosilicates.

Thermochemistry of rare-earth orthophosphates

Adsorption of Pb2+, Cd2+ and Sr2+ ions onto natural and acid-activated sepiolites

The redefinition of tengerite-(y), y2(co3)3.2-3h2o, and its crystal-structure

Kaolinite was hydrothermally synthesized from two kinds of silica-alumina gels to examine the effect of the structure of the starting material. Two kinds of gels were prepared by precipitation at different pH conditions (pH = 9.6 and 4.2) from solutions containing water glass and aluminum sulfate. Na ions in the gels were removed with a resin before the hydrothermal treatment, but a slight amount of sulfate ions was still present in the gels. The nuclear magnetic resonance spectra of the starting gels suggested that the gel prepared at pH 9.6 consists of networks with alternating SiO4- and A104-tetrahedra (partially A106-octahedra), whereas the gel prepared at pH 4.2 consists of a sheet structure related to allophane. After the hydrothermal treatment at 220"C for 9 days, kaolinite particles with spherical shape were obtained from the former gel, and platy kaolinite was crystallized from the latter one. The difference in morphology of synthetic kaolinite was attributable to the structures of the starting gels, and the pH values in the hydrothermal reactions were not very significant to the morphology.

Effects of the Structure of Silica-Alumina Gel on the Hydrothermal Synthesis of Kaolinite

Purification of Turkish sepiolite through hydrochloric acid treatment

Publisher Summary The rare earth minerals are defined as minerals containing rare earth elements as essential constituents. However, to judge this criterion, it is necessary to obtain structural as well as compositional information about the mineral. In addition, the determination of a chemical formula for a mineral also requires structural information in many cases, because the order or disorder of certain atoms in the crystal structure cannot be determined by chemical analysis alone. Therefore, minerals with significant contents of rare earths are treated as rare earth minerals in this chapter, even if the rare earth content appears “unessential” to the mineral. The rare earth minerals always contain a variety of rare earth elements. Crystal structures have been reported for about half of the known rare earth minerals. The structure analyses have revealed that many of them are solid solutions of the rare earth ions and other heterovalent ions that have ionic radii comparable to those of the rare earth ions; some of the rare earth minerals have independent crystallographic sites for rare earth atoms.

Chapter 108 Crystal structures of rare earth minerals

The formation process of kaolinite from the amorphous mixture of silica and alumina by hydrothermal reaction was studied by XRD, IR, DTA-TG, TEM, and high-resolution solid state 29 Si- and 27 Al-MAS/NMR. The amorphous mixture of the starting material prepared with commercially available silica-sol and alumina-sol was hydrothermally treated with distilled water in a Teflon pressure vessel at 220°C for a reaction time varying from 1 to 144 hours. The Q 4 state component of the amorphous starting material changed into an intermediate amorphous Q 3 state. The intermediate amorphous phase converted into crystalline kaolinite with spherical shape (Q 3 state). The yield and crystallinity of kaolinite increased with an increase in the reaction time

Ritsuro Miyawaki

Papers

The redefinition of tengerite-(y), y2(co3)3.2-3h2o, and its crystal-structure

Effects of the Structure of Silica-Alumina Gel on the Hydrothermal Synthesis of Kaolinite

Purification of Turkish sepiolite through hydrochloric acid treatment

Chapter 108 Crystal structures of rare earth minerals

Formation process of kaolinite from the amorphous mixture of silica and alumina