Crystal-chemical Features Research Articles

Kaersutitis in Azov Area Rocks (Ukrainian Shield): Petrological Aspect

The paper presents the results of new studies of kaersutites from lamprophyres of the Pokrovo-Kyriivo massif (PKM) of the Azov area (Ukrainian Shield). The comparison of mineralogical-petrographic and chemical features of kaersutites of lamprophyres with those of other dyke (microgabroid) and plutonic (pyroxenite) rocks of PKM is carried out and also, with this mineral from the camptonites of the Khlibodar quarry and the pyroxenites of the Chomutov massif. The paper presents the results of new studies of kaersutites from lamprophyres of the Pokrovo-Kyriivo massif (PKM) of the Azov area (Ukrainian Shield). The comparison of mineralogical-petrographic and chemical features of kaersutites of lamprophyres with those of other dyke (microgabroid) and plutonic (pyroxenite) rocks of PKM is carried out and also, with this mineral from the camptonites of the Khlibodar quarry and the pyroxenites of the Chomutov massif. The crystal chemical characteristics of the studied kaersutites were compared with those of other amphiboles in the Azov area. According to the chemical composition of kaersutites, the conditions of kaersutite formation in the mafic rocks of the Azov area region were estimated. Microprobe studies of the chemical composition of kaersutites from kaersutite lamprophyres were performed on JXA- 8520F (JEOL) X-ray microanalyzer equipped with five wave dispersion spectrometers (WDS) at the Institute of Natural Sciences of the Slovak Academy of Sciences (Bysterka Branch). The main characteristics of the chemical composition of kaersutite from kaersutite lamprophyres of PKM are low content of TiO2 (0.5-0.68 f.o.) and Al2O3 (up to 11.4%). Due to the low Al2O3 content, they are similar to kaersutites from afir gabroid and pyroxenite PKM. Kaersutites from other rocks of the Azov area are characterized by a higher content of Al2O3 (more than 12.3%, more often — 13%). The sample of kaersutites from PKM lamprophyres is not homogeneous, which indicates fluctuations in the conditions of rock formation. Most of the studied kaersutites of the Azov area are characterized by #Mg higher than 0.55. The exception is the measured ferrokaersutite from Hlibodarivka camptonite megacrysts (# Mg = 0.43). The vast majority of Azov area kaersutites have a high alkali content (more than 3%). According to their crystal chemical characteristics, kaersutites of the Azov area region differ significantly from other amphiboles of the district. The most highly baric are ferrokaersutites from Hlibodarivka camptonite megacrysts and kaersutite from porphyry amphibole-titanomagnetite microgabrroid PKM. The formation of kaersutite from the mafiс rocks of the Azov area corresponds to the conditions of high oxygen fugacity. Ferrokaersutite from Hlibodarivka camptonite was formed under conditions of medium oxygen fugacity. Keywords: kaersutite, lamprophyre, mafic dyke, pyroxenites, the Pokrovo-Kyriivo massif, camptonites of the Khlibodar quarry, Azov area.

Comparative study on structural and optical features of undoped and Eu3+ doped Pr6O11 synthesized via sol-gel and flame spray pyrolysis

Herein, it was aimed to differentiate features of undoped and europium (Eu3+) doped Pr6O11 particles fabricated via the sol-gel (SG) and flame spray pyrolysis (FSP) methods. The crystal phase structure, morphological, chemical, and optical features of synthesized Pr6O11 particles were characterized using an X-ray diffractometer (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), photoluminescence spectroscopy (PL), and UV–vis spectrophotometer. The results show that, compared with SG-based samples, Eu-doped Pr6O11 synthesized by FSP has excellent structural integrity, optical and electrical features, and morphology. This study outlines the development of a compatibility strategy between the production method and the final Pr6O11 powders, and that the optical, and electrical features may be controlled by selecting the proper dopant.

A highly hydrated variety of elpidite from the Khibiny alkaline complex, Kola Peninsula, Russia

AbstractAn unusual highly hydrated and Na-depleted variety of elpidite was identified in a hydrothermally altered peralkaline pegmatite at Mt. Yukspor in the Khibiny alkaline complex, Kola Peninsula, Russia. It differs from ‘ordinary’ elpidite, ideally Na2ZrSi6O15⋅3H2O, in its crystal chemical features, infrared spectrum and optical characteristics. The chemical composition (wt.%, electron microprobe, H2O by TGA) is: Na2O 5.45, K2O 0.67, CaO 0.05, SiO2 60.32, TiO2 1.34, ZrO2 18.43, Nb2O5 0.65, H2O 12.80, total 99.71. The empirical formula calculated on the basis of 6 Si and 15 O atoms is [(Na1.05K0.08Ca0.01)Σ1.14(H3O)0.74]Σ1.88(Zr0.89Ti0.10Nb0.03)Σ1.02Si6O15⋅3.47H2O; the H2O:H3O ratio was calculated from the charge balance requirement, taking into account the results of crystal structure refinement. The highly hydrated variety of elpidite is orthorhombic, Pma2, a = 14.5916(6), b = 7.3294(3), c = 7.1387(2) Å, V = 763.47(5) Å3 and Z = 2. The crystal structure was solved from single-crystal X-ray diffraction data, R1 = 3.43%. The structure is based upon an elpidite-type heteropolyhedral Zr–Si–O framework with Na+ and H3O+ cations and H2O molecules in the zeolitic channels. Hydronium cations substitute for water molecules in one of the extra-framework sites. This variety of elpidite could be considered as an intermediate product of natural ion-exchange reaction between ‘ordinary’ elpidite and a low-temperature hydrothermal fluid.

Photocatalytic activity enhancement for removal of dye molecules based on plasmonic Ag grafted TiO2 nanocubes under visible light driven

Introduction: Finding a novel photocatalyst for photocatalytic degradation operating in the wavelength range from UV to visible light has been considered a great potential for environmental remediation. Herein, TiO2 nanocubics (NCs) decorated Ag nanoparticles (NPs) with various concentrations were developed.
 Methods: The crystal structure, morphological and chemical characteristics of prepared photocatalysts were thoroughly analyzed by a series of main analyses (X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), and UVVis spectra).
 Results: The results revealed that a significantly promoting visible-light photocatalytic behavior of TiO2NCs@Ag photocatalyst was observed. The photocatalytic methyl orange (MO) degradation of the as-synthesized Ag anchored TiO2NCs photocatalyst (85% and 62% under UV light and visible light, respectively) exhibited outstanding photocatalytic efficacy compared with pristine TiO2 NCs. The achieved results could be assigned to the synergistic effects between TiO2NCs and Ag- NPs, leading to enhanced charge carrier separation and improved absorption ability in visible-light response.
 Conclusion: This work facilitates designing and developing high-efficiency heterostructure photocatalysts for practical works related to environmental deterioration.

Structural and Crystal-Chemical Features and Rb–Sr Age of Globular Glauconite in the Ust-Il’ya Formation (Lower Riphean, Anabar Uplift)

For the first time, mineralogical, structural, crystal-chemical and isotope-geochronological data were obtained for a glauconite sample from the Lower Riphean Ust-Il’ya Formation on the Magan River (western slope of the Anabar Massif, northern Siberia). It has been established that glauconite from the lenticular layer of friable glauconitite (glauconite grains ≥80%) contains two populations of mixed-layer crystals, in which mica layers (93%) alternate with either smectite-type (7%) or vermiculite-type layers (7%) with the short-range order factor R = 3. The ratio KAl = (VIAl / (VIFe3+ + VIAl) equal to 0.45 allows identifying the micaceous mineral as glauconite with the unit-cell parameter b = 9.065 A and K2O content of 8.29%. Comparison of new data with the previously obtained mineralogical and crystal-chemical characteristics of Ust-Il’ya glauconites from the Kotuikan River section (2.5 km above the Il’ya River mouth), located 65 km south of the Magan River section, showed both similarities and differences. Analysis of the crystal-chemical heterogeneity of Kotuikan glauconite samples from rocks of different lithological types revealed peculiarities in the distribution of individual mica varieties composing the glauconite globules. Isotopic dating of glauconite from the Magan River section was performed in combination with simulation of the distribution of octahedral cations and comparison of the results obtained with Mossbauer and infrared (IR) spectroscopy data. Such an approach combined with the mineralogical and crystal-chemical analyses contributes to correct interpretation of the stratigraphic significance of isotope data. The results obtained provide grounds for the conclusion that isotopic dates of glauconite from the Magan section (1474 ± 21 Ma) coincides within the error limit with the earlier Rb–Sr and K–Ar dating of the Ust-Il’ya Formation (1485 ± 13 and 1459 ± 20 Ma, respectively) based on the glauconite in the Kotuikan section (Zaitseva et al., 2016). The former value marks the stage of early diagenesis of sediments and is suitable for estimating the age of this formation.

Climate Signals in the Late Quaternary Bottom Sediments of Lake Baunt (Northern Transbaikalia)

Abstract —The bottom sediments of lakes with different mineralization located in the basins of inland areas are high-resolution archives of climate and environmental changes. We present results of study of the Late Pleistocene–Holocene sediments of freshwater Lake Baunt, one of the lakes of the Baunt (Tsipa) depression in the permafrost zone in northern Buryatia. The sediments were studied by XRD, IR spectroscopy, laser granulometry, elemental analysis, AMS dating, etc. Mineral analysis of the bottom sediments with an age of ~18 ka has revealed predominant phyllosilicates, quartz, and feldspars. Mathematical modeling of complex XRD spectra made it possible to identify chlorite, muscovite, illite, mixed-layer illite–smectite and chlorite–smectite, and kaolinite among the phyllosilicates. We have determined their structural and crystal-chemical features and quantitative proportions in the section (800 cm long core), which vary in accordance with the climate cycles and lake level fluctuations. The results obtained helped to reconstruct the evolution of the Lake Baunt basin controlled by the regional climate throughout the Late Pleistocene–Holocene. This paper continues a series of our publications concerned with the reconstruction of the Late Pleistocene–Holocene climate in East Siberia by comprehensive studies of the mineral composition of sedimentary sections of small lakes.

Synthesis and luminescent properties of (RE0.95Ln0.05)2O2S (RE = La, Y; Ln = Ho, Tm)

Solid solutions of oxysulfides (RE0.95Ln0.05)2O2S (RE = La, Y; Ln = Ho, Tm) were obtained by hydrogen reduction of the co-precipitated sulfates followed by sulfidation of the reaction products. The crystal chemical characteristics of the obtained compounds were refined by the Rietveld method. Morphological certification of particles in the dynamics of synthesis was performed. Most of the particles produced by chemical reactions have a cut that indicates the formation of a compound with a hexagonal syngony with angles of 60 and 120°. This indicates that the thermal effect of gaseous reagents H2, H2S on sulfates leads to heterogeneous reactions of thermal dissociation and the formation of new phases. Steady state luminescence properties displayed characteristic sharp bands corresponding to 4f-4f transitions. Luminescence decay curves of all studied samples showed monoexponential decay with microsecond and hundreds microsecond lifetimes depending on doping ions. Calculated color coordinates of Ho3+ and Tm3+-doped powders make them promising candidates to be used as phosphors.

Fedorite from Murun Alkaline Complex (Russia): Spectroscopy and Crystal Chemical Features

Fedorite is a rare phyllosilicate, having a crystal structure characterized by SiO4-tetrahedral double layers located between continuous layers formed by edge-sharing (Ca,Na)-octahedra, and containing interlayer K, Na atoms and H2O molecules. A mineralogical-petrographic and detailed crystal-chemical study of fedorite specimens from three districts of the Murun alkaline complex was performed. The sequence of the crystallization of minerals in association with fedorite was established. The studied fedorite samples differ in the content of interlayer potassium and water molecules. A comparative analysis based on polyhedral characteristics and deformation parameters was carried out. For the first time, EPR, optical absorption and emission spectra were obtained for fedorite. The raspberry-red coloration of the mineral specimens could be attributed to the presence of Mn4+ ions.

The nature of Zn-phyllosilicates in the nonsulfide Mina Grande and Cristal zinc deposits (Bongará District, Northern Peru): The TEM-HRTEM and AEM perspective

Abstract Zn-phyllosilicates are common minerals in nonsulfide Zn deposits and can give crucial information about the genesis of these oxidized mineralizations. They seldom represent the prevailing economic species but might have a significant impact on mineral processing. This study has been carried out on the Mina Grande and Cristal Zn-sulfide/nonsulfide deposits, which occur in the Bongará district (Amazonas region, northern Peru). The Cristal and Mina Grande orebodies are hosted by the sedimentary (prevailingly carbonate) successions of the Pucará Group (Condorsinga formation, Lower Jurassic), in an area affected by Neogene tectonics and characterized by Late Miocene and Pliocene-Early Pleistocene uplift phases (Andean and Quechua tectonic pulses). The Cristal deposit consists of both sulfide (sphalerite with minor pyrite and galena) and nonsulfide concentrations. The nonsulfides consists of smithsonite, hemimorphite, hydrozincite, chalcophanite, goethite, and greenockite, locally associated with Zn-bearing phyllosilicates. The Mina Grande deposit consists almost exclusively of Zn-oxidized minerals in limestone host rocks. The nonsulfides association consists of hydrozincite, hemimorphite, smithsonite, fraipontite, and Fe-(hydr)oxides, also containing a clayey fraction. The study deals with TEM-HRTEM and AEM investigations on clayey materials, to determine their crystal-chemical features and the origin of the complex Zn-clays-bearing parageneses. In both deposits, Zn-bearing illites (1Md and 2M polytypes) and I/S clay minerals (I3) are the main detected phases, with few compositions close to (Zn-bearing) muscovite. In the clayey fraction at Mina Grande, fraipontite, a Zn-bearing mica called K-deficient hendricksite, and (Zn-bearing) kaolinite also occur. Zn-illites and smectites (always containing Zn in variable amounts) characterize the mineral association at Cristal. The investigated compositional gap between di- and tri-octahedral Zn-phyllosilicates gives indications on the genetic relationships between them and advances on the knowledge of these species. The present work gives an insight into the Zn-bearing phyllosilicates systems by determining the amount/mode of metal incorporation in their lattices and understanding the relationships of natural occurring clay-rich complex associations, which can act as models for possible synthetic counterparts.

Crystal structure, conductivity, and phase stability of Cs3(H1.5PO4)2 under controlled humidity

Upon heating, a number of solid acids of structure type M3H(XO4)2 (M = NH4, K, Rb, Cs; X = S, Se) transform from a low conductivity monoclinic phase into a high conductivity (superprotonic) phase. Here, a new member of this material class, Cs3(H1.5PO4)2, is studied using a combination of single crystal X-ray diffraction, in-situ powder diffraction, thermogravimetric analysis, and A.C. impedance spectroscopy. The room temperature crystal structure of Cs3(H1.5PO4)2 was solved in the C2/m space group and found to be essentially isostructural to Cs3H(SeO4)2. On heating, no transition to a superprotonic phase was observed. Instead, Cs3(H1.5PO4)2 exsolves a small quantity of a CsH2PO4-like cubic phase at an onset temperature of ~192 °C prior to decomposition. The absence of a superprotonic phase transition is discussed in the context of other M3H(XO4)2 solid acids and the crystal-chemical features that define this class of materials.

Synthesis and Structure of New Rare Earth Cadmium Tellurite Halides

Single crystals of novel rare earth–cadmium tellurite halides, [Ln12(TeO3)12][Cd6X24] (Ln = La, X = Cl (1); Ln = La, X= Br (2), and Ln = Pr, X = Br (3)), were obtained from melts. All three compounds are isostructural to layered tellurite halides [Ln12(TeO3)12][Cd6Cl24] (Ln = Sm, Eu), described earlier, and crystallize in space group P4/nbm with unit cell parameters a ~ 16 A and c ~ 13 A. Their crystal structures have been refined to Robs = 0.052, 0.047, and 0.049, respectively. Bromides adopting this structure have been synthesized for the first time. Crystal-chemical features and prospects for development are discussed for the family of rare earth chalcogenite halides containing [Ln11Mx(ChO3)12] rare earth–chalcogenite slabs.

Dielectric properties of new oxide phases Ln0.65Sr1.35Co0.5Ti0.5O4 (Ln = La, Nd, Pr) with the K2NiF4 - type structure

New complex oxides of the composition Ln0.65Sr1.35Co0.5Ti0.5O4 (Ln = La, Nd, Pr) with the K2NiF4 – type structure were synthesized using the citrate-nitrate method. Their crystal chemical characteristics and dielectric properties are researched. Different values of frequency-independent dielectric constants are observed in a wide frequency range of 103–107 Hz in ceramics prepared at a temperature of 1300 °C. The highest permittivity ε is about 102 (at ambient temperature) in the frequency range from 1 kHz to 1 MHz for La0.65Sr1.35Co0.5Ti0.5O4 and Pr0.65Sr1.35Co0.5Ti0.5O4 samples. Co3+ and rare earth elements (La, Nd, Pr) co-doping Sr2TiO4 solid solution distorts the coordination polyhedra (Co,Ti)O6 and SrO9, which leads to an Re ε increase by several times. The surface microstructure of ceramics was investigated. The samples relative density correlates with the dielectric constant value. A sample of the Nd0.65Sr1.35Co0.5Ti0.5O4 composition has a lower relative density and, accordingly, a lower Re ε value. The results of dielectric properties researches show that the structure anisotropy and the samples morphology have a significant effect on the value of the dielectric constant.

Unusual silicate mineralization in fumarolic sublimates of the Tolbachik volcano, Kamchatka, Russia – Part 1: Neso-, cyclo-, ino- and phyllosilicates

Abstract. This is the initial paper in a pair of articles devoted to silicate minerals from fumaroles of the Tolbachik volcano (Kamchatka, Russia). These papers contain the first systematic data on silicate mineralization of fumarolic genesis. In this article nesosilicates (forsterite, andradite and titanite), cyclosilicate (a Cu,Zn-rich analogue of roedderite), inosilicates (enstatite, clinoenstatite, diopside, aegirine, aegirine-augite, esseneite, “Cu,Mg-pyroxene”, wollastonite, potassic-fluoro-magnesio-arfvedsonite, potassic-fluoro-richterite and litidionite) and phyllosilicates (fluorophlogopite, yanzhuminite, “fluoreastonite” and the Sn analogue of dalyite) are characterized with a focus on chemistry, crystal-chemical features and occurrence. Unusual As5+-rich varieties of forsterite, andradite, titanite, pyroxenes, amphiboles and mica are described. General data on silicate-bearing active fumaroles and the diversity and distribution of silicates in fumarole deposits are reported. Evidence for the fumarolic origin of silicate mineralization is discussed.

Crystal chemistry of fluorcarletonite, a new mineral from the Murun alkaline complex (Russia)

Abstract. This paper reports the first description of the crystal structure and crystal chemical features of fluorcarletonite, a new mineral from the Murun potassium alkaline complex (Russia), obtained by means of single-crystal and powder X-ray diffraction (XRD), electron microprobe analysis (EMPA), thermogravimetry (TG), differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR) spectroscopy. The crystal structure of fluorcarletonite, KNa4Ca4Si8O18(CO3)4(F,OH)⚫ H2O, a rare phyllosilicate mineral, contains infinite double-silicate layers composed of interconnected four- and eight-membered rings of SiO4 tetrahedra and connected through the interlayer K-, Na- and Ca-centered polyhedra and CO3 triangles. The X-ray diffraction analysis confirms the mineral to be tetragonal, P4∕mbm, a=13.219(1) Å, c=16.707(2) Å, V=2919.4(6) Å3 (powder XRD data), a=13.1808(5) Å, c=16.6980(8) Å, V=2901.0(3) Å3 (single-crystal XRD data, 100 K). The EMPA (average from 10 analyses) gave the following composition (wt %): SiO2 44.1(6), CaO 20.0(3), Na2O 11.1(3), K2O 4.5(2), F 1.3(5), TiO2 0.1(1) and Al2O3 0.03(3). The TG–DSC analysis confirmed the presence of H2O and CO2 (weight losses of 1.17 % and 14.9 %, respectively). The FTIR spectrum acquired in the range from 4000 to 400 cm−1 reveals the presence of H2O, CO3 and OH groups. The average formula of fluorcarletonite calculated from the results of EMPA and crystal structure refinement is K1.04Na3.89Ca3.87Ti0.01Si7.99Al0.01O18(CO3)3.86(F0.72OH0.28)⚫1.11H2O.

Arsenic and phosphorus in feldspar framework: sanidine–filatovite solid solution series from fumarolic exhalations of the Tolbachik volcano, Kamchatka, Russia

The influence of pentavalent arsenic and phosphorus on crystal chemical features of the feldspar framework was studied on the minerals belonging to the sanidine–filatovite solid-solution series from fumarolic exhalations of the Tolbachik volcano (Kamchatka, Russia). The reported feldspars demonstrate the first example of natural continuous solid solution between silicate and arsenate. The content of As2O5 varies from 0.0 to 45 wt% along with the highest phosphorus content found in natural feldspars: up to 5 wt% P2O5. The major substitution schemes in the studied feldspars are Al3+ + M5+ → 2Si4+ and M2+ + M5+ → Al3+ + Si4+, where M2+ = Zn and Cu and M5+ = As and P. The crystal structures of four samples with empirical formulae K0.95[(Si2.98Al0.93Fe0.08As0.02)Ʃ4.01O8] (space group C2/m), (K0.91Na0.06Ca0.03)Ʃ1.00[(Si2.81Al1.14As0.03P0.03Fe0.01)Ʃ4.02O8] (C2/m), K0.82[(Al1.92As1.11Si0.89Zn0.05Fe0.03Cu0.02P0.01)Ʃ4.03O8] (I2/c), K0.99[(Si1.87Al1.51As0.53P0.05Fe0.02Cu0.01)Ʃ3.99O8] (I2/c) were studied. The phase transition in the sanidine–filatovite solid-solution series is C2/m → I2/c. This is caused by the change of the Al:(Si + As) ratio from 1:3 in sanidine to 1:1 in filatovite along with significant ordering of Al, Si and As in the framework.

Zeolite-Containing Terra-Silicea as a Component of Composite Binders

The work is devoted to the study of the zeolite terra silicea (ZTS) for the subsequent development of composite binders based on its properties. Zeolite-containing rock - Terra silicea has high adsorption properties, due to the crystal-chemical features of the structure of its minerals and the predominance of capillary porosity in the rock. The rock-forming zeolite-containing Terra silicea mineral (clinoptilolite), due to the crystal chemical characteristics of its structure, is expected to act as a “battery” of the mortar part of the hydrating system, ensuring the solution intake in the later stages of hardening and, thereby, creating conditions for monolithic structure during the operational period.

Layered Perovskite-Like Oxides LnSr2CuTiO6.5 (Ln = La, Nd, or Pr): Synthesis, Structure, and Dielectric Properties

Layered perovskite-like oxides LnSr2CuTiO6.5 (Ln = La, Nd, Pr) were prepared, of them, the Pr and Nd compounds were for the first time. Their crystal-chemical characteristics and dielectric properties were studied. The dielectric permeability of the compounds was shown to increase considerably after thermobaric treatment (TBT) due to the effects of temperature and high pressure on the anisotropy of interatomic distances in coordination polyhedra (Ln, Sr)O9, (Cu, Ti)O6, and (Ln, Sr)O12. All the oxides studied have an activation conductivity type.

Кристаллохимические особенности глауконита Каринского месторождения (Южный Урал)

Кристаллохимические особенности глауконита Каринского месторождения (Южный Урал)

Crystal chemical features in the row of phosphates and vanadates with alkaline and transition metals

Crystal chemical features in the row of phosphates and vanadates with alkaline and transition metals

Fractionation of REE, U, and Th in natural ore-forming hydrothermal systems: Thermodynamic modeling

This contribution presents a thermodynamic model revealing the mechanisms responsible for separation of Heavy and Light Rare Earth (HREE and LREE) phosphates in natural hydrothermal systems at temperatures of 250–350 °C. Our calculations were performed for an isothermal column of rock containing 0.5 wt% of apatite (Ca phosphate), which served as an immobilizing agent for REE dissolved in the solution. REE were transported by 10 wt% NaCl acidic solution. The model accounted for formation of REE phosphate solid solutions through a regular mixing model. It demonstrates that hydrothermal flushing can efficiently separate REE forming xenotime (HREE phosphate solid solutions) at the beginning of the column, and re-transportation of monazite (LREE rich) to the end of the column. This separation is primarily due to the fact that at elevated temperatures stability LREE chloride complexes is significantly higher than that for HREE. The model also evaluates behavior of U and Th, which accompany REE in vast majority of natural locations. It was found that U strongly fractionates to xenotime, whereas Th fractionates to monazite. This phenomenon can be explained by the differences in crystal-chemical characteristics between monazite and xenotime, and the mobility of Th in aqueous solutions at elevated temperatures.