The crystal structure, infrared, Raman, excitation- and emission-mode photoluminescence spectra as well as thermal behavior of hydrogen- and REE-bearing natural arctite have been studied. The trigonal unit cell parameters are: a = 7.0773(1), c = 41.2197(6) Å, V = 1788.01(4) Å3; sp. gr. R -3 m. The crystal structure was solved and refined to the final R = 2.02
Strontium hexaferrite (SrM) powders were prepared by solid-state reaction using celestite and magnetite/hematite. Energy dispersive X-ray fluorescence spectrometry, X-ray diffractometry (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometry were used for analyses. The precursor ores were purified by hand-sorting and magnetic separation. Magnetic separation yielded 75 SrSO_4 via hand-sorting. XRD confirmed single-phase SrFe_12O_19 sample with hexagonal symmetry (space group P6_3/mmc ). EDS analyse confirmed that the percentage of elements is in agreement with the expected stoichiometry for the hexaferrite phase. Initial SrM sample exhibited magnetization at maximum field of 56 emu/g and coercive field of 632 Oe . Although remanence and coercivity were lower than expected for the initial SrM sample, due to wide particle size distribution observed in SEM images, by optimizing synthesis parameters, particularly annealing at 950^∘ C, the coercivity increased by nearly eightfold, reaching 4.96 kOe. The sample annealed at 950 ^∘ C also exhibited a maximum energy product of (BH)_max≈ 3.20 MGOe and an estimated magnetocrystalline anisotropy constant of K_1∼ 3.6× 10^6 erg/cm^3 . Microstructural evidence confirmed the predominance of particles in the monodomain regime, which justifies the increase in the coercive field. The results demonstrate that microstructural control is decisive for tuning the magnetic response of SrM, enabling the large-scale viability of this synthesis route for the production of high-coercivity hexaferrites, using a direct mixture of minerals instead of conventional industrial reagents.
The high-temperature volumetric and axial thermal expansion behaviors of ilmenite (FeTiO3) and geikielite (MgTiO3) were examined up to 773 K using in situ synchrotron X-ray diffraction. No phase transitions were observed within the experimental temperature range. The temperature–volume data were fitted using the Berman (1988)’s equation, yielding V0 = 316.50(3) Å3 and αV0 = 3.73(4) × 10–5 K−1 for FeTiO3, and V0 = 308.20(1) Å3 and αV0 = 2.95(9) × 10–5 K−1 for MgTiO3. In addition, axial thermal expansion analysis showed that FeTiO3 has a0 = 5.0865(1) Å, αa0 = 1.41(1) × 10–5 K−1, c0 = 14.0858(4) Å, and αc0 = 0.92(2) × 10–5 K−1, while MgTiO3 has a0 = 5.0585(1) Å, αa0 = 0.85(3) × 10–5 K−1, c0 = 13.9084(4) Å, and αc0 = 1.23(3) × 10–5 K−1. The results demonstrate that FeTiO3 exhibits more stable volumetric expansion with increasing temperature than MgTiO3. However, FeTiO3 and MgTiO3 display opposite trends in axial expansion: the a-axis expands more than the c-axis in FeTiO3 (αa0/αc0 = 1.53), whereas in MgTiO3, the c-axis expands more (αa0/αc0 = 0.69). These differences mainly reflect the distinct nature of the A-site cations: the 3d electrons of Fe2+ affect the rigidity and thermal response of Fe–O and Ti–O bonds, while Fe-related magnetic exchange interactions and associated magnetoelastic coupling may further constrain Fe–O bond expansion and reduce the thermal expansion of FeTiO3.
Grain boundaries are an important part of many polycrystalline mineral aggregates, possessing unique geometries and chemistries but they are often poorly understood due to the many complexities of studying such large geometrically diverse regions. In this work we shall examine the surfaces of the mineral forsterite in order to understand their controlling behaviour and mechanisms. We shall use a combination of density functional theory and both machine learning and classically trained forcefields. We find that for purposes of partitioning and surface energies most grain boundaries are similar to their constituent planar surfaces. Our main finding is that the relative energies of forsterite surfaces are highly sensitive to T and P and that their planar distributions are not static in real environments. Increasing T at low P can lead to growth of (010) surfaces while increasing T and P together can lead to the growth of (111) surfaces. In mantle conditions we predict a strong favourability of (111) surfaces that increases with depth but with large, sometimes non-monotonic shifts in the favourability of other surfaces. We also demonstrate that changing the surface distribution can lead to changes in crystal properties. The high temperature stabilisation of (010) is driven by a novel collective motion of Mg atoms along [100] channels which will also lead to a large, non-linear, anisotropic increase in grain boundary diffusion as (010) surfaces grow with temperature. We show that partitioning of water to forsterite surfaces is highly surface specific and driven by local chemistry enabling the possibility of hidden reservoirs in the Earth produced by changing surface plane distributions.
In-situ manufacturing and resource utilization on the lunar surface present a core challenge for establishing a sustainable lunar base, where the self-sufficiency of metal materials is particularly critical. Therefore, this study presented a novel method for extracting metallic iron from Fe3O4 via laser vacuum (1.0 × 10− 3 Pa) high-temperature (1930 K to 2360 K) decomposition, which could serve as a reference for in-situ metal extraction on the Moon. First, the feasibility of this approach was verified through thermodynamic calculations. Subsequently, the effects of laser power and irradiation time on the vacuum smelting process of Fe3O4 were systematically investigated. The results indicated that the content of metallic iron in the product reached its maximum value (31.71
Optical absorption spectroscopic investigation of a zircon from Afghanistan, unique in both size and color, show evidence that it represents a new type of purple color in this mineral. By microprobe and laser ablation-inductively coupled plasm (LA-ICP-MS) mass spectrometry analyses, the material is nearly pure ZrSiO4. The contents of Nb, P, F, Na and Mn are under the detection limits in all point analyses. The mean values of K, Ca, Ti, Sm, Gd, Pb and Ho contents are below 0.001 apfu. The dark-purple, nearly black color of the zircon is thermally unstable. At annealing from 400 to 900 °C it gradually vanishes due to decrease of intensities of the high-energy absorption edge, the broad absorption bands with maxima at 17,970 and 23,900 cm− 1 (E⊥c) and 18,620, 23,680 and 26,240 cm− 1 (E||c). Intensities of the weak narrow lines of absorption, which are very likely caused by electronic spin-forbidden ff-transitions of uranium and REE ions, remain unchanged. The origin of the zircon is unclear. By 206Pb/238U radiometric age 1909 million years, it is Proterozoic.
Cryptomelane (K-OMS-2) (K(Mn4+, Mn2+)8O16) is a manganese-oxide mineral that is abundant in soils and rock coatings globally and is gaining importance in materials science and industrial mineralogy. The crystal structure is composed of 2 × 2 octahedra that creates a tunnel space for K+. Low-weight percent (wt
Grain size controls diffusion, deformation, and the textural evolution of mantle rocks. Because grain growth is driven by a reduction in grain boundary energy, the relationship between grain boundary energy and grain size provides an important constraint on microstructural evolution. To better quantify this relationship, we analyzed olivine–olivine–pyroxene (ol–ol–px) dihedral angles in peridotite xenoliths from Ichinomegata (Japan) and San Carlos (USA), as well as in experimentally sintered olivine–pyroxene aggregates. In melt-free Ichinomegata peridotites, ol–ol–opx angles (≈100°) fit single normal distributions. In contrast, melt-bearing Ichinomegata peridotites yield lower-angle ol–ol–cpx triple junction populations (≈80°), which we interpret as relict ol–ol–melt triple junctions, although the distributions of dihedral angles might also reflect the combined effects of textural evolution and system conditions. The San Carlos peridotite and experimental samples yield uniform distributions with median dihedral angles of ≈100°. Moreover, the relationships between olivine grain size, secondary-phase grain size, and secondary-phase volume fraction follow the Zener equation in both natural and experimental peridotites across four orders of magnitude in olivine grain size. These findings suggest that microstructural processes, including Zener pinning, melt–rock interaction, and dynamic recrystallization, contribute to the grain-scale evolution of peridotites across both laboratory and geological scales.
The iron-bearing FexTiyOz minerals are important terrestrial materials for plate tectonics studies and paleo-magnetism in geophysics. The bulk modulus of the FexTiyOz minerals, such as Fe3O4 (magnetite), Fe2TiO4. (ulvöspinel), FeTiO3 (ilmenite), Fe2TiO5 (pseudobrookite) are related to the compressions of cation sites and vacant site. Cation site partly occupied by Ti atom shows a smaller Ko than that of only Fe atom. The compressibility increases with increasing Ti content in FexTiyOz. The vacant sites in the unit cell are much larger volumes than cation sites in these four structures. The bulk moduli of these samples are very similar to those of their vacant sites. The compressibility of the cation site is much larger than those of the vacant sites. The cation distribution in the solid solutions have been carried out by neutron diffraction and X-ray diffraction studies. The Fe/Ti distribution has been examined as a function of pressure. The cation sites and vacant sites show different compressibility with increasing pressure. The cation sites of these minerals at high-pressure conditions were investigated to understand their strong electronic correlations. The crystal internal potential change under high pressure is in an equilibrium state of the external pressure by virial theorem. The structure changes such as high-low electron spin transition, Jahn–Teller effect and d-p-π hybridization in the Fe–O bonds are elucidated by present high-pressure experiments. The d-p-π hybridization in the octahedral cation site was discussed by molecular orbital calculation and it brings the deformation of the octahedral cation site. The deformation triggers the structure changes in the high-pressure polymorphs.
The crystal structure of Na2Mg(SO4)2 has been redetermined from X-ray diffraction on a larger, higher-quality synthetic single crystal than that used for the original determination. This allowed the correction of important details of the crystal structure. The substitution of equal amounts of Mg and Na has been shown to occur at sites different from those previously believed to be involved. It explains the large, unusually anisotropic atomic displacement parameters (ADPs) and the splitting of many O sites as adjustments to the significantly different sizes of Mg and Na atoms that enter the same coordination environment. An orientation disorder at one S site has been identified and explained; likewise, splitting of a nearby Na site and its exceptionally large, anisotropic ADPs are due to movement within a large structural cavity within the [100] structural channel. The complex crystal structure contains [MgO6] octahedral coordinations, of which one has partial Mg/Na occupancy, with six [SO4] tetrahedral coordinations attached in a pinwheel arrangement. An exception is one Mg octahedron that shares an edge with an S tetrahedron. The Mg-S coordination units are assembled in three-layer (101) slabs, interconnected by additional Mg in octahedral coordination. Na atoms are located in voids within this framework, with coordination numbers ranging from 5 to 9.
Orthopyroxene is a major mineral in the Earth’s upper mantle. In this study, we conducted in-situ high pressure and temperature Raman spectroscopy measurements on single-crystal orthopyroxenes with varying (Fe, Al) concentrations using externally heated diamond anvil cells. The α-β transition was identified by systematic changes in characteristic Raman modes, and transition boundaries were determined in the range of 400–700 K up to 20 GPa. All samples exhibit the same structural transition pathway, while the transition pressure and Clapeyron slope vary systematically with composition. Our results demonstrate that increasing Fe content from 3 to 9 mol
Leucite (ideally K[AlSi₂O₆]) exhibits a complex sequence of structural transformations upon heating, the nature of which—particularly the existence of intermediate phases and the behaviour of the extra-framework potassium cation—has long been debated. In this study, highly accurate in-situ high-temperature powder X-ray diffraction (HT-PXRD) data were collected over the 303–1173 K range and analysed using the Rietveld method. The results support a continuous transition sequence from the low-temperature tetragonal I41/a phase to an intermediate tetragonal I41/acd modification, followed by the high-temperature cubic Ia -3 d phase. In the 873–898 K interval, the data are best described by the coexistence of two phases, namely I41/a + I41/acd at 873 K and I41/acd + Ia -3 d at 898 K. Given the long acquisition times and adequate thermal equilibration, the observed coexistence is best attributed to minor thermal gradients along the capillary rather than sluggish transition kinetics, providing a consistent explanation for previously reported lattice volume discontinuities. Furthermore, spontaneous strain analysis confirms that the ferroelastic transition is fully consistent, within experimental uncertainty, with a second-order Landau free energy expansion. While the intrinsic limitations of conventional PXRD preclude the refinement of Al/Si ordering, structural data indicate that the framework evolution is closely coupled to the extra-framework K cation. Bond-valence analysis indicates an apparent underbonding of K throughout the investigated range; however, this is consistent with pronounced dynamic disorder (rattling) within the framework cavities, prior to attaining a more regular 12-fold coordination in the cubic phase.
High-pressure spectroscopic measurements of ε-FeOOH were conducted up to 65 GPa at room temperature in diamond anvil cells. The pressure evolution of the Raman vibrational modes confirms that a hydrogen-bond-symmetrization-induced phase transition from P21nm to Pnnm occurs at 18 GPa. Infrared (IR) spectroscopic measurements suggest that the Pnnm phase has a disordered hydrogen state, and no spectroscopic evidence for fully centered hydrogen bonds is observed within the investigated pressure range. Above 45 GPa, Fe3+ in ε-FeOOH undergoes a high-spin to low-spin transition as indicated by a reduction of the unit cell volume, together with reductions in IR transmitted and Raman signals. These results demonstrate that ε‑FeOOH preserves a disordered hydrogen‑bond configuration up to at least 45 GPa, whereas δ-AlOOH transforms to a centered hydrogen-bond configuration at 18 GPa. This compositional contrast suggests that Fe‑bearing oxyhydroxides follow a distinct evolution of hydrogen bonding under compression, providing insight into hydrogen behavior in deep Earth materials.
Iron-rich phosphides are functional materials for the hydrogen evolution reaction and also analogues of Fe-Ni-P minerals found in meteorites and pyrometamorphic rocks on Earth. They are also potential components in solid metallic cores of the Earth, Mars, and other terrestrial planets. The thermal expansion coefficients (α) of three iron-rich phosphides (FeP, Fe2P, and Fe3P) were investigated by in-situ synchrotron X-ray powder diffraction from room temperature to 1073 K. By fitting the obtained volume-temperature (V-T) data using a function of α(T) = α0 + α1⋅T (Fei model), the parameters α0 and α1 for FeP’s volume are 2.60(22) ⋅10− 5 K− 1 and 3.57(36) ⋅10− 8 K− 2, for Fe2P’s volume are 3.26(37) ⋅10− 5 K− 1 and 3.70(64) ⋅10− 8 K− 2, and for Fe3P’s volume are 1.96(20) ⋅10− 5 K− 1 and 4.01(32) ⋅10− 8 K− 2, respectively. Axial thermal expansions are different in three iron phosphides. Combined with previous results, the density profiles of solid Fe3P and FeP in the Martian core conditions were calculated and compared with those of Fe3S, FeS and fcc Fe. Fe3P has a similar density to Fe3S under Martian core conditions, suggesting that phosphorus can exist in the Martian solid inner core if the core comprises iron sulfide.
Formation conditions, transformation mechanisms, and transformation product selection of magnesium carbonate hydrate phases at temperatures near the freezing point of water are poorly understood. In this study, carbonated Mg(OH)2 suspensions were aerated at T = 0 °C to obtain solutions from which either MgCO3·6H2O or lansfordite (MgCO3·5H2O) precipitated, with the selected phase depending on the aeration rate. Subsequently, magnesium carbonate hexahydrate was subject to transformation experiments yielding five different products. The product selection depended on the transformation condition. Among the products, a new phase was identified and characterized as a neutral magnesium carbonate hydrate with the chemical composition MgCO3·4H2O. Implications of the synthesis route and analytical results suggest that this tetrahydrate phase forms from MgCO3·6H2O by the release of two weakly bonded water molecules per formula unit, accompanied by a relaxation of the remaining MgCO₃·4H₂O layers without disrupting the edge-sharing octahedral pairs of the parental structure. Density functional theory calculations confirmed the stability of the proposed crystal structure through the examination of its phonon dynamics and subsequent vibrational analysis. Furthermore, the simulated X-ray powder diffraction pattern is in reasonable agreement with the experimental data when accounting for the high crystalline disorder inevitably introduced by the transformation process.
This research work reports synthesis, characterization and thermal expansion investigation of vanadium and molybdenum substituted langbeinite phosphates with the chemical formula KPbCr2P2.9V0.1O12, KPbCr2P2.8V0.2O12 and KPbCr2[P2.9Mo0.1]+0.1O12, KPbCr2[P2.8Mo0.2]+0.2O12. All the four-materials exhibit phase pure cubic structure with P213 space group. Rietveld refinement studies authenticate the single-phase formation with the least chi square values. The lattice parameters and lattice volume increased upon the substitution of bigger of VO4 and MoO4 for smaller PO4 group in the framework. The morphological and elemental mapping investigation reveal the crystalline nature of materials, shape of the particles and distribution of the elements. The peak shift and peak broadness in the vibrational modes of FTIR spectra affirms that the VO4 and MoO4 tetrahedral groups have effectively been substituted for the PO4 tetrahedra. The average coefficient of thermal expansion of KPbCr2P2.8V0.2O12 and KPbCr2[P2.8Mo0.2]+0.2O12 is found to be 1.44 × 10− 6 / K and 1.04 × 10− 6 / K.
Reichenbachite (Cu5(PO4)2(OH)4) is a rare copper-hydroxy-phosphate, the synthetic accessibility and phase evolution of which are still poorly understood. Here we report the first known reproducible hydrothermal synthesis to yield high purity reichenbachite, facilitated by alkaline conditions at 180 °C. Systematic time-controlled experiments clearly reveal a kinetic-thermodynamic bias: pseudomalachite forms rapidly as the metastable kinetic phase and reichenbachite emerges as the thermodynamic product after extended reaction times. Powder X-ray diffraction experiments combined with Rietveld refinements confirm phase selectivity as an outcome of reaction conditions. Scanning electron microscopy shows characteristic morphological features of each polymorph. Through a combination of order–disorder theory, classical nucleation theory, and Ostwald’s rule of stages we propose a mechanism of formation through polymorphic interconversion driven by synthesis duration and alkalinity. This study not only establishes a new reproducible synthesis for a previously elusive mineral but also provides insights into fundamental metastable-equilibrium phase transitions in copper hydroxy-phosphate systems, with implications for crystal growth theory and design of functional phosphate-materials.
The high-pressure behaviour of hydroboracite [ideally CaMg[B3O4(OH)(3)](2)center dot 3H(2)O, Sp. Gr. P2/c with a similar to 11.769, b similar to 6.684, c similar to 8.235 angstrom, and beta similar to 102.6 degrees at room conditions] has been studied by two in-situ single-crystal synchrotron X-ray diffraction experiments up to about 15 GPa, using He as pressure-transmitting medium. Between 14.51(5) and 14.72(5) GPa, hydroboracite undergoes a first-order phase transition to its high-pressure polymorph, hydroboracite-II (likely monoclinic with a similar to 11.29, b similar to 6.297, c similar to 7.48 angstrom, and beta similar to 106 degrees, space group unknown). The isothermal bulk modulus (K-V0 = beta(-1)(P0,T0), where beta(P0,T0) is the volume compressibility coefficient) of hydroboracite was found to be K-V0 = 41.4(6) GPa. The destructive nature of the phase transition prevented any structure resolution of hydroboracite-II or even the continuation of the experiments at pressures higher than 15.45(5) GPa. In the pressure range 0-14.45(5) GPa, the compressional anisotropy of hydroboracite, indicated by the ratio between the principal components of the unit-strain ellipsoid, is epsilon(1):epsilon(2):epsilon(3) = 2.2:1.3:1. The P-induced deformation mechanisms at the atomic scale in hydroboracite are here described.
Nitrogen-bearing K-cymrite (K, NH4)AlSi3O8·(N2,NH3,H2O), a clathrate phase that is stable in subduction zones, may play a crucial role in transporting potassium, water, and nitrogen to the depths of the mantle. Synchrotron X-ray diffraction in a diamond anvil cell with a methanol-ethanol medium was used to study its compressibility up to 10 GPa. N-bearing K-cymrite retains its hexagonal P6/mmm symmetry without signs of phase transitions across the pressure range. The third-order Birch-Murnaghan equation of state yielded the following values: V0 = 191.38(3) Å3, K0 = 47.1(2) GPa, and K0’ = 6.40(7). Unlike K-cymrite KAlSi3O8·H2O, which develops incommensurate modulation above 7–8 GPa, no satellite reflections appear in the diffraction patterns of N-bearing K-cymrite, even at 16 GPa. This suggests that N2 stabilizes the double tetrahedral layers against wave-like bending.
Olivine, a predominant mineral in the Earth’s upper mantle, plays a crucial role in geophysical and geochemical processes due to its abundance and elastic characteristics. This study investigates the elastic properties and structural variations of synthetic forsterite-fayalite solid solutions under high-pressure conditions through synchrotron single-crystal X-ray diffraction. A series of samples with varying fayalite contents were synthesized and characterized to evaluate the impact of Fe incorporation on unit-cell parameters, bulk modulus, and compression behavior. The results demonstrate that increasing fayalite content elongates the b-axis, increases polyhedral distortions, and enhances the bulk modulus, while maintaining the structural rigidity of [SiO₄] tetrahedra. By refining the fit parameters of the Birch-Murnaghan equation of state, this study establishes a composition-property relationship for elastic behavior. These results provide key constraints for modeling mantle dynamics and planetary evolution.
