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Mineral trapping through the precipitation of carbonate minerals is a potential approach to reduce CO2 accumulation in the atmosphere. The temperature dependence of amorphous magnesium carbonate (AMC), a precursor of crystalline magnesium carbonate hydrates, was investigated using synchrotron X-ray scattering experiments with atomic pair distribution function (PDF) and X-ray absorption fine structure analysis. PDF analysis revealed that there were no substantial structural differences among the AMC samples synthesized at 20, 60, and 80 °C. In addition, the medium-range order of all three AMC samples was very similar to that of hydromagnesite. Stirring in aqueous solution at room temperature caused the AMC sample to hydrate immediately and form a three-dimensional hydrogen-bonding network. Consequently, it crystallized with the long-range structural order of nesquehonite. The Mg K-edge X-ray absorption near-edge structure spectrum of AMC prepared at 20 °C was very similar to that of nesquehonite, implying that the electronic structure and coordination geometry of Mg atoms in AMC synthesized at 20 °C are highly similar to those in nesquehonite. Therefore, the short-range order (coordination environment) around the Mg atoms was slightly modified with temperature, but the medium-range order of AMC remained unchanged between 20 and 80 °C.

We investigated the compressional behavior of i-AlCuFe quasicrystal using diamond anvil cell under quasi-hydrostatic conditions by in situ angle-dispersive X-ray powder diffraction measurements (in both compression and decompression) up to 76 GPa at ambient temperature using neon as pressure medium. These data were compared with those collected up to 104 GPa using KCl as pressure medium available in literature. In general, both sets of data indicate that individuald-spacing shows a continuous decrease with pressure with no drastic changes associated to structural phase transformations or amorphization. Thed/d0, whered0is thed-spacing at ambient pressure, showed a general isotropic compression behavior. The zero-pressure bulk modulus and its pressure derivative were calculated fitting the volume data to both the Murnaghan- and Birch-Murnaghan equation of state models. Results from this study extend our knowledge on the stability of icosahedrite at very high pressure and reinforce the evidence that natural quasicrystals formed during a shock event in asteroidal collisions and survived for eons in the history of the Solar System.

We investigated crystal structure and the local structure changes during the thermal decomposition of hydromagnesite by using in situ high-temperature XRD and ex situ high-temperature X-ray total scattering measurements. Hydromagnesite displayed anisotropic thermal expansion up to 220 °C. The a and c lattice parameters exhibited an increase trend with temperature, whereas the b lattice parameter and β angle did not show a regular trend with temperature. The relative expansion between 25 and 220 °C followed the c/c0 > a/a0 $$\gg$$ b/b0. At 260 °C, the a, b, and c lattice parameters significantly decreased. Above 280 °C, hydromagnesite underwent a structural collapse with dehydration and dehydroxylation reactions, but was never accompanied by nucleation and growth of crystal phases up to 425 °C. During the thermal decomposition from hydromagnesite to periclase, the Mg atoms maintained the octahedral coordination environments in the structure.

To reveal the decomposition mechanism with temperature under high pressure, crystal structure of a hydrogrossular, katoite Ca3Al2(O4D4)3 has been studied by high-pressure and high-temperature neutron diffraction technique. The crystal structure was investigated using Rietveld method under a constraint of the space group Ia $$\overline{3}$$ d. At pressure of approximately 8 GPa, the unit cell volume of katoite increased as a function of temperature. Although unusual expansion behavior was discerned in a temperature range between 200 and 400 °C, the unit cell was continuously expanded up to 850 °C. At 900 °C, katoite was decomposed and the residuals were dissolved completely in the dehydrated water. The results show that high-pressure strongly affects the increase of the katoite dehydration temperature from 300 to 900 °C. After cooling and decompression to ambient conditions, the peaks of katoite reappear together with those of corundum Al2O3 and portlandite Ca(OH)2. At 8 GPa, the volume of CaO8 dodecahedron and AlO6 octahedron expands with temperature up to 850 °C by about 8% and 13%, respectively. That of tetrahedral interstice, on the other hand, contracts by about 10%: the tetrahedral interstices are squeezed isotopically by the expansion of CaO8 dodecahedra and AlO6 octahedra. The neighboring D–D distance remains almost unchanged in this temperature range, while the O–D bond distance shrinks drastically just before decomposition. It is suggested that the shortening of O–D distance caused by the D–D repulsion destabilizes the O–D bond that results in the thermal decomposition of katoite.

Dypingite is one of the most common phases of crystalline magnesium carbonate hydrates, which are considered potential targets for CO2 mineral sequestration. Ex-situ high-temperature X-ray diffraction and ex-situ high-temperature synchrotron X-ray scattering experiments with pair distribution function analysis were used to explore the thermal transformation process of dypingite. Dypingite transformed to hydromagnesite until 100 °C, but its local structure remained unchanged. At the temperature range from 250 °C to 350 °C, where the amorphous phase was predominant, the second and third coordination spheres were continuously modified and crystallized to MgO at 400 °C. Although the local structure of dypingite was as that of hydromagnesite, dypingite exhibited a different thermal transformation pathway from hydromagnesite.

Single crystals of analcime were hydrothermally synthesized from a gel of analcime composition at 200 °C for 24 h. They were grown up to 100 μm in size with typical deltoidal icositetrahedron habit. The chemical composition determined by EPMA and TG analyses was Na0.84(Al0.89Si2.12)O6·1.04H2O. The single-crystal X-ray diffraction method was used to determine the symmetry and crystal structure of analcime. The analcime grown from a gel crystallized in cubic space group Ia3d with lattice parameter a = 13.713(3) A. In the cubic analcime, Si and Al cations were totally disordered over the framework T sites with site occupancy of Si:Al = 0.6871:0.3129(14). The single crystals of analcime with cubic symmetry were hydrothermally reheated at 200 °C in ultrapure water. After the hydrothermal treatment for 24 h, forbidden reflections for the cubic Ia3d symmetry were observed. The reflection conditions led to an orthorhombic space group Ibca with lattice parameters a = 13.727(2) A, b = 13.707(2) A, and c = 13.707(2) A. The unit-cell showed a slight distortion with (a + b)/2 > c, yielding a flattened cell along c. In the orthorhombic analcime, Al exhibited a site preference for T11 site, which indicates that the Si/Al ordering over the framework T sites lowers the symmetry from cubic Ia3d to orthorhombic Ibca. After the hydrothermal treatment for 48 h, reflections corresponding to orthorhombic space group Ibca were observed as well. The lattice parameters were a = 13.705(2) A, b = 13.717(2) A, and c = 13.706(2) A, retaining the flattened cell shape with (a + b)/2 > c. The Si and Al cations were further ordered among the framework T sites than the case of the hydrothermal treatment for 24 h. As a consequence, the Si/Al ordering was slightly but significantly accelerated with increasing the hydrothermal treatment time. During the hydrothermal reaction, however, chemical compositions were almost unchanged. The site occupancies of Na over the extra-framework sites remained unaffected with the heating time; thus, the hydrothermal heating influences the degree of ordering of Si and Al over the framework T sites rather than that of Na among the extra-framework sites.

Mirabilite (Na2SO4⋅10H2O) is one of the representative phase change materials. Temperature dependent structural behavior of mirabilite was investigated by using single-crystal X-ray diffraction (XRD) technique in the temperature range between 213 and 303 ​K. Thermal expansion of mirabilite was mainly ascribed to the expansion of two Na(H2O)6 octahedra constituting the ribbon structure along the c-axis. Three O atoms out of four apices of SO4 tetrahedron were split into two partially occupied O sites. The resulting disordered arrangement gave two orientations (major and minor) for the SO4 tetrahedron. The SO4 tetrahedron of the major orientation had the smaller volume than that of the minor orientation. The volume of the major SO4 tetrahedron showed a decreasing trend as increasing temperature. This shrinkage of the major SO4 tetrahedron would induce oxygen diffusion from the major SO4 tetrahedron to the minor SO4 tetrahedron. As a result, the O site occupancies belonging to the minor SO4 tetrahedron increased from 0.17 to 0.27. Crystal structure of mirabilite consists of infinite ribbons of edge-sharing Na(H2O)6 octahedra alternating with chains composed of SO4 tetrahedra and two H2O molecules to form sheets parallel to the b-c plane. Since the fundamental structure of mirabilite is essentially the same as that of thenardite (Na2SO4), phase change from mirabilite to thenardite immediately occurs after dehydration without formation of any intermediate phase.

Establishing a mechanistic understanding of dynamic fracture is critical for the design of efficient and safe structural materials. Transformation toughening describes the process by which a crystal structure change can reduce the crack-driving force. The understanding of the dynamic behavior of transformation-toughened materials is limited. To clarify the relationship between crystal structure transformations and toughening during a dynamic loading, we carried out a time-resolved x-ray diffraction study of yttria-stabilized tetragonal zirconia polycrystal under shock loading. The results show that the ambient tetragonal crystal structure is maintained on loading and phase transformation to the monoclinic phase occurs only after the onset of dynamic fracture, indicating that the transformation toughening is associated with dynamic fracture caused by wave interactions. This study not only revealed the reason for the high spall strength of zirconia ceramics but also demonstrated the great potential for time-resolved x-ray diffraction method to study fracture dynamics.

The combination of high-power laser and synchrotron X-ray pulses allows us to observe material responses under shock compression and release states at the crystal structure on a nanosecond time scale. A higher-power Nd:glass laser system for laser shock experiments was installed as a shock driving source at the NW14A beamline of PF-AR, KEK, Japan. It had a maximum pulse energy of 16 J, a pulse duration of 12 ns and a flat-top intensity profile on the target position. The shock-induced deformation dynamics of polycrystalline aluminium was investigated using synchrotron-based time-resolved X-ray diffraction (XRD) under laser-induced shock. The shock pressure reached up to about 17 GPa with a strain rate of at least 4.6 × 107 s–1 and remained there for nanoseconds. The plastic deformation caused by the shock-wave loading led to crystallite fragmentation. The preferred orientation of the polycrystalline aluminium remained essentially unchanged during the shock compression and release processes in this strain rate. The newly established time-resolved XRD experimental system can provide useful information for understanding the complex dynamic compression and release behaviors.

We report in-situ synchrotron X-ray diffraction study of icosahedral (i)-AlCuFe quasicrystal (QC) at high pressure up to 104 GPa under ambient temperature using the diamond anvil technique. With compression, there is no obvious change of the diffraction pattern within the measured pressure range. Five characteristic peaks from the i-AlCuFe QC maintain up to 104 GPa. The unit cell volume of the i-AlCuFe QC decreases monotonously until 72 GPa. The experimental P – V compression curve below 72 GPa was fitted to the Birch–Murnaghan equation of state with V 0 =2023(19) A 3 , K 0 =131(7) GPa, K ′=4.0 (fixed). The resulting EoS parameters are approximately consistent with those observed in previous study. A discontinuous volume change is observed between 72 and 75 GPa. These observed results indicate a phase transition from i-AlCuFe QC to an approximant crystal (AC) which has a similar local atomic arrangement as the QC. This study suggests that i-AlCuFe QC maintains its structure at least up to 72 GPa and then transforms to AC with small displacement of atoms.

Changes in composition during the transition from sediment to rock are usually attributed to long, complicated histories and atmospheric influences, while the contribution of benthic mat-building cyanobacteria is not typically considered. Here the goal is to understand the influence of cyanobacterial mats on mineral weathering in postdepositional settings of sandy, shallow subaquatic environments. Laboratory incubation experiments were done using ilmenite sands and ilmenite-enriched quartz sands colonized by cyanobacterial mats for five months at three temperatures: 25°C and 37°C, representative of postdepositional weathering regimes, and 70°C corresponding to early diagenesis. As a comparative control to represent abiotic processes, ilmenite sands and ilmenite-enriched quartz sands were also subjected to the same conditions without cyanobacterial colonization. The precipitation of minerals on cyanobacterial cells and extracellular polymeric substances (EPS) as well as the phase changes in natural ilmenites (FeTiO3) were documented to determine if cyanobacteria influence mineral reaction pathways. The precipitates, ilmenite grains, and permineralized cells were analyzed using complementary techniques of scanning electron microscopy (SEM), X-ray diffraction (XRD), and micro-Raman spectroscopy. The results of this study show that a variety of pure and mixed mineral phases precipitate under postdepositional conditions (T ≤ 70°C) in wet, sandy environments with or without cyanobacteria. Akaganeite, anatase, ankerite, lepidocrocite, gibbsite, kaolinite, and natrojarosite formed exclusively in the samples incubated with cyanobacteria. In the samples incubated with cyanobacteria, more mineral phases formed at 37°C, suggesting that cyanobacteria play a greater role in weathering than in early diagenesis. Sulfate phases that formed in the presence of cyanobacteria differed in chemical composition from the abiotic precipitates as Na, Al, Mg, and Si were incorporated into the structures of newly formed biotic phases. Understanding the possible fate of these precursor mineral phases will help redefine geochemical biosignatures that can be used for the detection of ancient microbial life in sedimentary rocks on Earth as well as for future missions exploring life on other planets.

A structural change in Fe2SiO4 spinel (ringwoodite) has been found by synchrotron powder diffraction study and the structure of a new high-pressure phase was determined by Monte-Carlo simulation method and Rietveld profile fitting of X-ray diffraction data up to 64 GPa at ambient temperature. A transition from the cubic spinel structure to a body centered orthorhombic phase ( I -Fe2SiO4) with space group Imma and Z = 4 was observed at approximately 34 GPa. The structure of I -Fe2SiO4 has two crystallographically independent FeO6 octahedra. Iron resides in two different sites of sixfold coordination: Fe1 and Fe2, which are arranged in layers parallel to (101) and (011) and are very similar to the layers of FeO6 octahedra in the spinel structure. Silicon is located in the sixfold coordination in I -Fe2SiO4. The transformation to the new high-pressure phase is reversible under decompression at ambient temperature. A martensitic transformation of each slab of the spinel structure with translation vector ![Formula][1] generates the I -Fe2SiO4 structure. Laser heating of I -Fe2SiO4 at 1500 K results in a decomposition of the material to rhombohedral FeO and SiO2 stishovite. Fe K β X-ray emission measurements at high pressure up to 65 GPa show that the transition from a high spin (HS) to an intermediate spin (IS) state begins at 17 GPa in the spinel phase. The IS electron spin state is gradually enhanced with pressure. The Fe2+ ion at the octahedral site changes the ion radius under compression at the low spin, which results in the changes of the lattice parameter and the deformation of the octahedra of the spinel structure. The compression curve of the lattice parameter of the spinel is discontinuous at ~20 GPa. The spin transition induces an isostructural change. [1]: /embed/mml-math-1.gif

The Jahn-Teller effect at Cu2+ in cuprospinel CuFe2O4 was investigated using high-pressure single-crystal synchrotron X-ray diffraction techniques at beamline BL10A at the Photon Factory, KEK, Japan. Six data sets were collected in the pressure range from ambient to 5.9 GPa at room temperature. Structural refinements based on the data were performed at 0.0, 1.8, 2.7, and 4.6 GPa. The unit-cell volume of cuprospinel decreases continuously from 590.8(6) to 579.5(8) A3 up to 3.8 GPa. Least-squares fitting to a third-order Birch-Murnaghan equation of state yields the zero-pressure volume V = 590.7(1) A3 and bulk modulus K = 188.1(4.4) GPa with K ′ fixed at 4.0. The structural formula determined by electron microprobe analysis and site occupancy refinement is represented as T (Fe3+0.90Cu2+0.10) M (Fe3+1.10Fe2+0.40Cu2+0.50)O4. Most of the Cu2+ are preferentially distributed onto the octahedrally coordinated ( M ) site of the spinel structure. With pressure, the arrangement of the oxygen atoms around the M cation approaches a regular octahedron. This leads to an increase in the electrostatic repulsion between the coordinating oxygen ions and the 3 d z2 orbital of M Cu2+. At 4.6 GPa, a cubic-tetragonal phase transition is indicated by a splitting of the a axis of the cubic structure into a smaller a axis and a longer c axis, with unit-cell parameters a = 5.882(1) A and c = 8.337(1) A. The tetragonal structure with space group I 41/ amd was refined to R 1 = 0.0332 and wR 2 = 0.0703 using 38 observed reflections. At the M site, the two M -O bonds parallel to the c -axis direction of the unit cell are stretched with respect to the four M -O bonds parallel to the a-b plane, which leads to an elongated octahedron along the c -axis. The cubic-to-tetragonal transition induced by the Jahn-Teller effect at Cu2+ is attributable to this distortion of the CuO6 octahedron and involves Cu 3 d z2 orbital, ab initio quantum chemical calculations support the observation. At the tetrahedrally coordinated ( T ) site, on the other hand, the tetrahedral O- T -O bond angle increases from 109.47° to 111.7(7)°, which generates a compressed tetrahedral geometry along the c -axis. As a result of the competing distortions between the elongated octahedron and the compressed tetrahedron, the a unit-cell parameter is shortened with respect to the c unit-cell parameter, giving a c / a ′ ratio ( ![Formula][1] ) slightly greater than unity as referred to cubic lattice ( c / a ′ = 1.002). The c / a ′ value increases to 1.007 with pressure, suggesting further distortions of the elongated octahedron and compressed tetrahedron. [1]: /embed/mml-math-1.gif

Composition dependence of the crystal structure between stibnite and antimonselite is investigated by using the single-crystal X-ray diffraction and the ab initio calculation methods to clarify the Se substitution effect on the crystal structure, especially focusing on the stereochemical behavior of Sb 5s 2 lone-pair electrons. The single-crystal X-ray diffraction measurements indicate no phase change throughout the solid solution range. The lattice parameters of a, b, c and unit cell volume are linearly increased as Se content increases. The lattice parameter variations normalized show an anisotropic expansion that the largest expansion is observed along the a-axis, followed by the c- and b-axes. The large Se atom exhibits a strong site preference for the X(1) and X(3) sites, while the small S atom prefers to occupy the X(2) site. The intra-ribbon Sb(1)–X and Sb(2)–X distances (X = S, Se) are continuously increased with the Se content. The three Sb(1)–X bond distances in the trigonal pyramids are changed within the similar range between 2.52 and 2.68 A, while the five Sb(2)–X distances in the tetragonal pyramids vary from 2.45 to 2.59 A, from 2.68 to 2.80 A, and from 2.86 to 3.00 A, respectively. With increasing Se content in the solid solution, the inter-ribbon distances where the Sb 5s 2 LPEs are located monotonously increase as well. However, the variations between the ribbons are considerably smaller than those of intra-ribbon distances. The polyhedral volumes of the Sb(1)X7 and Sb(2)X7 in which the Sb 5s 2 LPEs are accommodated constantly increase from 35.9 to 40.0 A3 and from 34.1 to 38.8 A3, respectively, and these eccentricity parameters decrease from 0.66 to 0.62 and from 0.57 to 0.55. As a result of ab initio calculation, the Sb 5s orbitals on the Sb(1) atoms remain almost unchanged throughout the solid solution. On the other hand, those on the Sb(2) atoms become smaller with the incorporation of Se. The result gives a more reasonable interpretation that the stereochemistry of Sb 5s 2 LPEs and the stereochemistry of the coordination polyhedra around the Sb atoms are affected by the Se substitution in the structure.

The fracture surfaces of analcimes reported by Sugano and Kyono (Phys Chem Miner, 2018) were re-investigated using field-emission scanning electron microscopy. The results of scanning electron microscopic observation show the fractures can be characterized by smoothly curved surfaces, called sub-conchoidal fracture, and neither the lamellar twin nor the domain walls of twin were found on the surfaces of the analcimes. In addition, the lamellar twin is generally formed by transformation from the high-temperature phase or from pseudomorphic replacement under strong alkaline conditions. Actually, the symmetry lowering of analcime reported by Sugano and Kyono (2018) occurs at 200 °C. The temperature is much lower than the transformation from the high-temperature phase. In the hydrothermal experiment (Sugano and Kyono 2018), moreover, the analcimes were synthesized under acidic condition and reheated in pure water. No twin domain is likely to be formed under the hydrothermal condition. These facts strongly deny the possibility that the twin domains cause the pseudo-symmetry of tetragonal analcime. Consequently, it can be concluded that the observed forbidden reflections for the cubic Ia3d symmetry are not due to the presence of twin domains, but due to the symmetry lowering of analcime from cubic Ia3d to orthorhombic Ibca.

The structural diversity of arsenic sulfide molecules in compositions between As4S6 and As4 was investigated using ab initio quantum chemical calculations. The As4S6 molecule consists of four trigonal pyramid coordinations of As atoms bonding to three S atoms. In the As4S5 composition, only one type of molecular configuration corresponds to an uzonite-type molecule. In the As4S4 composition, two molecular configurations exist with realgar-type and pararealgar-type molecules. Three molecular configurations are in the As4S3 composition. The first configuration comprises trigonal pyramidal As atom coordinations of two types: bonding to two S atoms and one As atom, and bonding to one S atom and two As atoms. The second is the molecular configuration of dimorphite. The third comprises trigonal pyramidal As atom coordinations of two types: bonding to three As atoms, and bonding to one As atom and two S atoms. The As4S2 composition allows molecular configurations of two types. One is comprised of trigonal pyramidal As atom configurations of one type bonding to two As atoms and one S atom. The other comprises trigonal pyramidal As atom coordinations of three types: bonding to two S atoms and one As atoms, bonding to one S atom and two As atoms, and bonding to three As atoms. The As4S molecule has trigonal pyramidal As atom coordinations of two types: bonding to one S atom and two As atoms, and bonding to three As atoms. The As4S composition permits only one molecular configuration, which suggests that the mineral duranusite comprises the As4S molecular geometry. In all, ten molecular configurations are predicted in the molecular hierarchy of the arsenic sulfide binary system. The simulated Raman spectral profiles are helpful in searching for undiscovered arsenic sulfide minerals.

The structural phase transition of the titanomagnetite (Fe3–xTixO4) solid solution under pressures up to 60 GPa has been clarified by single-crystal and powder diffraction studies using synchrotron radiation and a diamond-anvil cell. Present Rietveld structure refinements of the solid solution prove that the prefered cation distribution is based on the crystal field preference rather than the magnetic spin ordering in the solid solution. The Ti-rich phases in 0.734 ≤ x ≤1.0 undergo a phase transformation from the cubic spinel of Fd3m to the tetragonal spinel structure of I41/amd with c/a < 1.0. The transition is driven by a Jahn-Teller effect of IVFe2+ (3d6) on the tetrahedral site. The c/a < 1 ratio is induced by lifting of the degeneracy of the e orbitals by raising the dx2-y2 orbital below the energy of the dz2 orbital. The distortion characterized by c/a < 1 is more pronounced with increasing Ti content in the Fe3–xTixO4 solid solutions and with increasing pressure. An X-ray emission experiment of Fe2TiO4 at high pressures confirms the spin transition of FeKβ from high spin to intermediate spin (IS) state. The high spin (HS)-to-low spin (LS) transition starts at 14 GPa and the IS state gradually increases with compression.more » The VIFe2+ in the octahedral site is more prone for the HS-to-LS transition, compared with Fe2+ in the fourfold- or eightfold-coordinated site. The transition to the orthorhombic post-spinel structure with space group Cmcm has been confirmed in the whole compositional range of Fe3–xTixO4. The transition pressure decreases from 25 GPa (x = 0.0) to 15 GPa (x = 1.0) with increasing Ti content. There are two cation sites in the orthorhombic phase: M1 and M2 sites of eightfold and sixfold coordination, respectively. Fe2+ and Ti4+ are disordered on the M2 site. This structural change is accelerated at higher pressures due to the spin transition of Fe2+ in the octahedral site. This is because the ionic radius of VIFe2+ becomes 20% shortened by the spin transition. At 53 GPa, the structure transforms to another high-pressure polymorph with Pmma symmetry with the ordered structure of Ti and Fe atoms in the octahedral site. This structure change results from the order-disorder transition.« less

The crystal structure of chromite FeCr2O4 was investigated to 13.7 GPa and ambient temperature with single-crystal X-ray diffraction techniques. The unit-cell parameter decreases continuously from 8.3832 (5) to 8.2398 (11) A up to 11.8 GPa. A fit to the Birch–Murnaghan equation of state (EoS) based on the P–V data gives: K 0 = 209 (13) GPa, K′ = 4.0 (fixed), and V 0 = 588 (1) A3. The FeO4 tetrahedra and CrO6 octahedra are compressed isotropically with pressure with their Fe–O and Cr–O bond distances decreasing from 1.996 (6) to 1.949 (7) A and from 1.997 (3) to 1.969 (7) A, respectively. The tetrahedral site occupied by the Fe2+ cation is more compressible than the octahedral site occupied by the Cr3+ cation. The resulting EoS parameters for the tetrahedral and the octahedral sites are K 0 = 147 (9) GPa, K′ = 4.0 (fixed), V 0 = 4.07 (1) A3 and K 0 = 275 (24) GPa, K′ = 4.0 (fixed), V 0 = 10.42 (2) A3, respectively. A discontinuous volume change is observed between 11.8 and 12.6 GPa. This change indicates a phase transition from a cubic (space group Fd- $${\overline{3}}$$ m) to a tetragonal structure (space group I41 /amd). At the phase transition boundary, the two Cr–O bonds parallel to the c-axis shorten from 1.969 (7) to 1.922 (17) A and the other four Cr–O bonds parallel to the ab plane elongate from 1.969 (7) to 1.987 (9) A. This anisotropic deformation of the octahedra leads to tetragonal compression of the unit cell along the c-axis. The angular distortion in the octahedron decreases continuously up to 13.7 GPa, whereas the distortion in the tetrahedron rises dramatically after the phase transition. At the pressure of the phase transition, the tetrahedral bond angles along the c-axis direction of the unit cell begin decreasing from 109.5° to 106.6 (7)°, which generates a “stretched” tetrahedral geometry. It is proposed that the Jahn–Teller effect at the tetrahedrally coordinated Fe2+ cation becomes active with compression and gives rise to the tetrahedral angular distortion, which in turn induces the cubic-to-tetragonal transition. A qualitative molecular orbital model is proposed to explain the origin and nature of the Jahn–Teller effect observed in this structure and its role in the pressure-induced phase transition.

We report in situ Raman spectroscopic studies of ulvospinel in a diamond-anvil cell under hydrostatic conditions up to 57 GPa at room temperature. Two modes near 493 and 681 cm −1 are observed clearly at 1 GPa. In the cubic spinel structure, the lower frequency peak can be assigned to a mode of F 2 g symmetry and the higher frequency peak can be assigned to a mode with A 1 g symmetry. The remaining three modes could not be observed unambiguously in the measurements, although there are five Raman-active modes ( A 1 g + E g +3 F 2 g ) in the Fd 3 m space group of the spinel structure according to factor group analysis. The peak positions and shapes in the Raman spectra agree well with those measured under ambient conditions. With increasing pressure, the frequencies of the A 1 g and F 2 g modes increase continuously up to 9 GPa with pressure derivatives of 2.5 and 2.1 cm −1 /GPa, respectively. There is no obvious degradation of crystal symmetry or structural change observed within this pressure range. Upon increasing pressure to ~20 GPa, the F 2 g mode splits into B 1 g + E g modes, and then into B 1 g + B 2 g + B 3 g modes. The intensities of the Raman bands gradually decrease due to the tetragonal-orthorhombic phase transition. This mode completely disappears at a pressure of 29 GPa. The most striking characteristic of the Raman spectrum of ulvospinel is that compression leads to the extinction of the Raman-active mode derived from F 2 g symmetry. Only one peak resulting from the A 1 g mode can be observed continuously up to 57 GPa. The peak shift derived from the A 1 g mode and its full-width at half maximum (FWHM) values suggests another phase transition occurring around 30 GPa. The Raman spectrum of ulvospinel is in good agreement with the spectra of ZnCr 2 O 4 and ZnFe 2 O 4 spinels.

The crystal structure of a chromian pumpellyite K0.03Na0.07Ca7.82Mn2+0.01Mg1.95Fe3+0.05V3+0.01Cr3+3.77Al6.52Ti0.02Si11.82O42.06(OH)13.94 (Z = 1) from Sarany in the Russian Urals was studied using optical spectroscopy and single-crystal X-ray diffraction methods. Five discrete peaks were observed in an optical spectrum. Of these, three at 392(υ2), 574(υ1) and 726 nm were attributed to Cr3+; one at 456 nm to Fe3+; and a peak at 291 nm is related to M-O charge transfer. The crystal field parameters determined here (B, C and 10Dq) give 94% ionic character of Cr-O bonds and a mean VIM-O distance of 1.954 A. The crystal structure of chromian pumpellyite with a = 8.809(3), b = 5.9330(9), c = 19.137(1) A, β = 97.606(2)° at 90 K, and a = 8.807(6), b = 5.943(4), c = 19.18(1) A, β = 97.44(2)° at 293 K was refined using 1297 unique reflections measured at 90 K, converging at R = 2.3%. Site occupancies of the main constituent elements at the two independent octahedral X and Y sites are X = 0.49Mg + 0.308Cr + 0.202Al and Y = 0.742Al + 0.258Cr. Although Cr3+ ions occupy both the X and Y sites, the site preference of chromium for the X site is stronger than that for the Y site, as indicated by the distribution coefficient of chromium between the X and Y sites, KD = (Cr/Al)X/(Cr/Al)Y, of 4.39. The substitution of Cr3+ for Al in the Y site directly results in structural changes, whereas that in the X site is not effective for structural change.

Molecular crystals of an As4S4− x Se x (0 ≤ x ≤ 4) solid solution were grown at 400 °C using vacuum-sublimation and were characterized using single-crystal X-ray diffraction and electron microprobe analyses. The solid solution crystallizes in the monoclinic space group P 21/ n with lattice parameters of a = 9.33–9.57 A, b = 13.57–13.82 A, c = 6.60–6.74 A, β = 106.4–106.7°, V = 801–854 A3, and Z = 4. Substitution of Se for S in the As4X4 molecule causes nearly isotropic unit-cell expansion retaining identical molecular packing to the low-temperature form α-As4S4 (natural realgar). Crystal structure refinements show that Se is distributed with a strong preference for the X2 site. The As-X3 bond is slightly elongated in the solid-solution series. The bond length difference between As-X3 and the other As-X bonds increases with the Se content, thereby inducing atomic shift of the As4 bonds to X3 and X4 toward the side of X3. The atomic position of As4 returns to the place on the bisector between X3 and X4 at the As4Se4 end-member composition. Although As-As bonds shorten with Se content, the volume of the As4X4 molecule and the intercentroid distance between the nearest As4X4 molecules increase continuously, resulting in isotropic expansion of the unit cell in this solid-solution series. The strong site preference of Se for the X2 site suggests that S may be detached from that site more easily than from the remaining ones. On the contrary, very slight substitution of Se for S in the X3 site indicates that S in the X3 site is strongly bonded to two As atoms. Therefore, S is more stably incorporated into the X3 site than into the X2 site; these molecular features of the Se substitution in the As4X4 molecule are consistent with the proposed photoinduced transformation dynamics in realgar. Continuous illumination of the Se-containing realgar engenders alteration to pararealgar under identical transformation mechanisms to those of the realgar.

Guanacoite with high Mg content from the type locality (El Guanaco mine, Chile) was investigated to elucidate the crystal-chemical characteristics of this arsenate mineral. In this study, WDS X-ray intensity maps revealed that guanacoite single crystals from the investigated specimen show distinct and characteristic compositional zoning, which varies cyclically along [100], which is in the direction of the elongation of the prismatic crystals (Witzke et al. 2006); the boundaries separating different compositional regions lie perpendicular to the prism faces. The crystal structure of a selected fragment [ a = 5.459(2), b = 16.808(9), c = 6.917(3) A, β = 100.44(1)°, V = 624.1(5) A 3 , P 2 1 / c , and Z = 2], was solved using direct methods and refined to an R index of 3.09% for 1385 observed [ I o > 4σ( I o )] reflections measured at 103 K using Mo K α X-radiation. The chemical formula based on the refinement is (Cu 0.85 Mg 0.15 ) 2 Mg 2 (Mg 0.83 Cu 0.17 )(OH) 4 (H 2 O) 4 (AsO 4 ) 2 . The structure model obtained is in good agreement with the previous structure determination, although the occupancies of the three M sites (M = Mg or Cu) are distinctly different, i.e., the sites are considerably richer in Mg, especially M1 and M3. These structural results suggest that guanacoite’s chemical formula should be enlarged from the Cu-rich composition Cu 2 Mg 2 (Mg,Cu)(OH) 4 (H 2 O) 4 (AsO 4 ) 2 to (Cu,Mg) 2 Mg 2 (Mg,Cu)(OH) 4 (H 2 O) 4 (AsO 4 ) 2 to include the Mg-rich members (Mg up to 3.395 apfu). It appears that the chemical variability of guanacoite is mainly due to Mg-Cu substitution on the M1 and M2 sites.

This study estimates the influence of surface morphology of specimen on actual quantitative analysis results measured using the wavelength dispersive spectrometer (WDS) detector and the energy dispersive spectrometer (EDS) detector. The polishing plane of mineral specimens can yield reliable and reproducible results from the WDS measurement, but the measurement errors become larger with an increase of the surface roughness. On the other hand, the EDS semi-quantitative results greatly deviate from the ideal chemical compositions of minerals regardless of their surface roughness. Since rigorous quantitative analysis of a mineral is based on determining an intensity ratio between a characteristic X-ray peak in the mineral and the same peak in a standard measured under identical operating conditions, the measurement errors of chemical composition occur unless the X-ray intensities can be accurately measured under the identical conditions. Serious measurement errors may bring us to mischaracterized interpretations and erroneous descriptions.

Correlation between light-induced alteration of arsenic sulfide (mineral name: realgar, As 4 S 4 ) and irradiated light intensity was investigated using in situ single-crystal X-ray diffractometry. The exposure time required for the phase transformation increases gradually as the light intensity decreases. The X-ray diffraction intensities of (2 6 1) and (3 6 1) reflections decrease sharply, indicating that the As–As bonds in As 4 S 4 molecules are broken by insertion of an additional S atom. The As–As bond cleavage and As 4 S 4 molecule deformation are largely produced throughout the crystal structure when the unit cell volume reaches approximately 802 A 3 , followed by rearrangement of the As 4 S 4 pararealgar molecular packing. It continues until the unit the cell volume expands continuously to approximately 810 A 3 . The feature of the continuous increase of the a cell parameter was observed within the light intensity range. The value of the b cell parameter remained constant during light treatment. Nevertheless, the linear increase in the c sin β value was not confirmed as the light intensity decreased. Variation of the unit cell volume correlates with that of the c sin β value. Consequently, no continuous increase of unit cell volume was apparent in realgar exposed to light of 5 W/m 2 . Such a discontinuous increase in c sin β value is attributed mainly to the value of the c cell parameter, which varies widely with light exposure. The highest sensitivity for lighting is shown at the a and β angles in unit cell parameters. It increases or decreases linearly up to the loss of crystallinity.

The crystal structures of chromian epidotes from the Sambagawa metamorphic rocks, central Shikoku, Japan, were refined using single-crystal data to investigate the distribution of chromium among the three independent octahedral sites. On the basis of the electron probe microanalysis of the chromian epidotes on the polished surface of thin sections, three chromian epidote grains in a thin section were selected for X-ray intensity data collection, and their Cr2O3 values were as much as 1.79, 4.99, and 4.74 wt%, respectively, which led to values of 0.113, 0.315, and 0.299 Cr apfu, respectively. Nevertheless, their Fe2O3 contents were nearly constant (8.1-8.7 wt%, 0.49-0.52 Fe apfu) and their SrO contents were not significant (1.8-3.2 wt%, 0.08-0.15 Sr apfu). The selected chromian epidote grains were picked out using a binocular optical microscope from the thin sections, and were labeled as CrEp1, CrEp2, and CrEp3, respectively. The oxidation state of chromium was found to be trivalent by the optical spectrum. 57Fe Mossbauer doublets with isomer shift of 0.28-0.53 mm/sec and quadrupole splitting of 1.77-2.24 mm/sec indicate an exclusive distribution of Fe3+ at the M3 site. The crystal structure of CrEp1 [a = 8.8815(6), b = 5.5956(5), c = 10.1532(5) A, β = 115.159(2)], CrEp2 [a = 8.9165(4), b = 5.6226(2), c = 10.1728(5) A, β = 115.2365(9)], and CrEp3 [a = 8.895(2), b = 5.610(1), c = 10.146(2) A, β = 115.177(2)] were refined using 1145, 1853 and 1438 unique reflections, respectively, and the calculations converged at R factors of 5.8%, 6.6%, and 3.2%, respectively. The resulting structural formula were: [Ca]A1[Ca0.86Sr0.14]A2[Al]M1[Al]M2[Al0.43Cr3+0.09Fe3+0.48]M3Si3O12(OH) for CrEp1, [Ca]A1[Ca0.90Sr0.10]A2[Al0.86Cr3+0.14]M1[Al]M2[Al0.34Cr3+0.17Fe3+0.49]M3Si3O12(OH) for CrEp2, and [Ca]A1[Ca0.88Sr0.12]A2[Al0.90Cr3+0.10]M1[Al]M2[Al0.37Cr3+0.12Fe3+0.51]M3Si3O12(OH) for CrEp3. The selective distribution of Cr3+ at the M3 site in CrEp1, and the KD values (KD = (Cr3+/Al)M1/(Cr3+/Al)M3) of 0.35(3) for CrEp2 and 0.41(3) for CrEp3 show that the preference of Cr3+ in the octahedral sites is M3 > M1 » M2. The variations in the M1-Oi and M3-Oi distances indicate an equidimensional expansion of the M1O6 and M3O6 octahedra by the substitution of Cr3+(+ Fe3+) for Al3+.

Light-induced phase transformation from realgar to pararealgar has been studied by means of single-crystal X-ray diffraction and X-ray photoelectron spectroscopy. The photochemical degradation causes the increase of As4S4 intermolecular distances and the production of the As4S5 molecules. The results suggest the following cyclic process. 1st : an S atom in the As4S5 is released from one of the As-S-As linkages in As4S5 which becomes the As4S4 pararealgar molecule, 2nd : the free S atom is re-attached to another As4S4 realgar molecule and reproduces an As4S5 molecule, and 3rd : the As4S5 is again divided into an As4S4 pararealgar molecule and an S atom.

We report a crystal growth method of lead vanado-chlorapatite (vanadinite) Pb 5 (VO 4 ) 3 Cl single crystals using a CsCl flux. X-ray powder diffraction, scanning electron microscopy, electron microprobe analysis, and infrared spectroscopy characterized the crystals obtained in this study. We obtained only high-quality vanadinite single crystal phase from a two-step synthesis procedure. The as-grown single crystals with well-developed hexagonal prismatic shapes were as large as 6 mm. The crystals have an ideal chemical formula with highly homogeneous composition. Crystals grown from the CsCl flux contain no Cs impurities within the detection limit of EPMA. These results indicate that CsCl flux is a promising candidate for flux growth of single crystals in pyromorphite series.

Using flux method with cesium chloride solvent, Pb 5 (P x As 1− x O 4 ) 3 Cl single crystals were grown. Individual euhedral crystals consist of a hexagonal prism shape topped by a hexagonal pyramid, elongate [0 0 1] with a length-to-width ratio of approximately 1:5. These crystals were chemically homogeneous and exhibited no compositional zoning of P and As. Stopping the N 2 supply into the electric furnace during heating and growth formed pale yellow crystals together with colorless transparent crystals. These materials display identical X-ray powder patterns and infrared absorption spectra. Furthermore, no marked difference is apparent in the chemical compositions of colorless crystals and yellow ones. A relationship to color development is apparent in an O 1s binding energy in XPS measurements. The O 1s binding energy is higher for strongly colored crystals, and that for yellow crystals can shift from 530.65 to 530.73 eV depending on the degree of color development. The O 1s binding energies for naturally occurring green pyromorphite and yellow mimetite were greatly enhanced respectively to 531.44 and 531.11 eV. This color development process of materials reflects the environmental concentration of oxygen during its crystallization.

Light-induced degradation in realgar (arsenic sulfide) has been studied by means of four-circle single-crystal X-ray diffraction and X-ray photoelectron spectroscopy. Because of the alteration of realgar exposed to light, the a lattice parameter and c sinβ value increase linearly from 9.327 to 9.385 A and from 6.320 to 6.364 A, respectively. In contrast, the b lattice parameter remains substantially constant. Anisotropic variations of the lattice parameters engender a continuous increase of the unit-cell volume from 799.5 to 810.4 A3. Nevertheless, no correlation exists between the continuous increase of the unit-cell volume and the bond distance variations in As4S4 molecules because the As4S4 molecule in the unit cell expands very little during light exposure. The most pronounced change was in the distance between centroids in As4S4 cages. The spread of As4S4 intermolecular distances increases continuously from 5.642 to 5.665 A, which directly affects the unit-cell volume expansion of realgar. In addition, the O1s peak increases rapidly after light exposure. The result substantiates the following reaction proposed by Bindi et al. (2003): 5As4S4 + 3O2 → 4As4S5 + 2As2O3. That is, realgar is transformed into pararealgar if oxygen exists and produces the As4S5 molecule. The additional S atom contributes to anisotropic expansion for the a and c axes because the direction of the additional S atom points toward [414] in the unit cell. Furthermore, an S atom in the As4S5 molecule is released from one of the equivalent As-S-As linkages in As4S5 which becomes the As4S4 molecular of pararealgar. After the As4S5 molecule is divided into an S atom (radical) and the As4S4 (pararealgar type) molecule, the free S atom is re-attached to another As4S4 (realgar type) molecule, and reproduces an As4S5 molecule. The reproduced As4S5 molecule is again divided into an S atom (radical) and an As4S4 (pararealgar type) molecule. This cycle whereby realgar is indirectly transformed into pararealgar via the As4S5 molecule is promoted by light and repeated during light exposure.

The crystal structure of allanite from granitic pegmatite, the Daibosatsu Pass, Yamanashi, Japan, has been refined under the constraint of chemical composition determined by electron microprobe analysis of rare earth elements. Back-scattered-electron images and X-ray element maps of the allanites show that each of their crystal grains has chemically homogeneous distribution of major elements. A typical formula for the chemistry is: (Ca0.920☐0.080)Σ1.000(La0.238Ce0.443Pr0.048Nd0.100Sm0.019Th0.042Mn0.008☐0.102)Σ1.000(Al0.607Fe0.3173+Ti0.076)Σ1.000(Al1.000)(Fe0.5432+Fe0.3653+Mn0.055Mg0.037)Σ1.000(SiO4)(Si2O7)O(OH).The crystal structure of allanite, monoclinic, a 8.905 (1), b 5.7606 (5), c 10.123 (1) Å, β 114.78°(1), space group P21/m, Z = 2, has been refined to an unweighted R factor of 3.46% for 1459 observed reflections. Although the H atom position was not determined on the Difference-Fourier map, inspection of the bond valence sums demonstrates that the H atom is uniquely located at the O10 atom and involved in a hydrogen bond to O4. A systematic examination as to crystal chemistry of allanites suggests that the isolated SiO4 tetrahedron has the largest distortion of three kinds of the tetrahedron containing Si2O7 groups in the allanite structure. This observation is common to the epidote group minerals, while the larger distortion of A2 sites caused by occupancy by REE in allanites contrasts with the smaller one of A sites in other epidote group minerals. In the allanite groups the bond angles between the O10–H bond and hydrogen bond H···O4 are found to range from 170 to 180°.Compilation of the chemical compositions of the title allanite and the others from granitic rocks, Japan, which reveals Th-incorporation as the coupled substitution of 3Th4+ + ☐ (vacancy) ⇌ 4REE3+, provides an explanation for the observation that higher Th concentrations characterize allanites from the island arcs. The ternary Al2O3-Fe2O3-ΣREE diagram illustrates that allanites are grouped, according to their origins, into three classes suggestive of tectonic backgrounds for the crystallization localities; (1) intracontinental, (2) island arc and (3) continental margin.

The crystal structure of whewellite [Ca(C2O4)·H2O] and the dehydration mechanism of caoxite [Ca(C2O4)·3H2O] have been studied by means of differential thermal analysis, X-ray diffraction (powder and single-crystal) analysis and infrared analysis. The first and second analyses confirmed the direct transformation of caoxite into whewellite without an intermediate weddellite [Ca(C2O4)·2H2O] stage. Infrared spectra obtained from caoxite, weddellite and whewellite emphasize the similarity of the O–H-stretching band and O–C–O-stretching band in whewellite and caoxite and the unique bands of weddellite. The structure refinement at low temperature (123 K) reveals that all the hydrogen atoms of whewellite form hydrogen bonds and the two water molecules prop up the crystal structure by the hydrogen bonds that cause a strong anisotropy of the displacement parameter.Comparing the structural features of whewellite with those of weddellite and caoxite suggests that caoxite and whewellite have a sheet structure consisting of Ca2+ ions and oxalate ions although weddellite does not. It is additionally confirmed that the sheets of caoxite are corrugated by hydrogen bonds but whewellite has flat sheets. The corrugated sheets of caoxite would be flattened by dehydration so the direct transformation of caoxite into whewellite would not occur via weddellite. Essential for this transformation is the dehydration of interlayered water molecules in caoxite leading to the building of the crystal structure of whewellite on its intralayered water molecules. The difference in conformation of water molecules between those two crystal structures may explain the more common occurrence of whewellite than of caoxite in nature.