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Carbonate minerals are major contributors to carbon sequestration in geological deposits; however, their nature and behavior remain unclear. Amorphous magnesium carbonate (AMC) is formed as a precursor to crystalline magnesium carbonates and as a product of thermal decomposition of nesquehonite (NSQ). In this study, the AMCs formed during the crystallization and decomposition of NSQ were investigated using X-ray diffraction (XRD) and atomic pair distribution function (PDF) methods. An AMC with a hydromagnesite-like structure (AMC-I) was formed immediately after mixing MgCl2 and Na2CO3 solutions. After 5 min of stirring, no change was observed in the XRD pattern; however, the PDF pattern changed. This suggests that the medium-range ordered structure of AMC-I transformed into an intermediate structure (AMC-II) between AMC-I and NSQ. After 10 min of stirring, the AMC-II crystallized into NSQ. In the case of Rb2CO3, the AMC-II structure was formed immediately after the mixing of solutions and was stable for three days. AMC-II in the Rb2CO3 solution appeared to be in equilibrium with energetic local minima, indicating the existence of polyamorphism in AMC. When Cs2CO3 solution was used, the first precipitate had an AMC-I structure. By stirring for 5 min, the AMC-I was transformed to AMC-II, and after 10 min of stirring, a few quantities crystallized into NSQ. After three days, NSQ dissolved and transformed back into AMC-I. Thus, it is inferred that the crystallization of NSQ is significantly influenced by alkali cations in aqueous solutions. The AMC formed during the thermal decomposition also possesses the AMC-I structure. [ABSTRACT FROM AUTHOR]

In this study, we use laser-driven shock compression coupled with in situ X-ray diffraction to interrogate the phase transition dynamics of shock-compressed zircon (ZrSiO4) for the first time. A phase transition from zircon to the high-pressure reidite phase was observed during the nanosecond timescale of a laser-driven shock. At high laser power, diffraction peaks of zircon and reidite appeared superimposed on two broad features. This diffuse background was ascribed to liquid scattering from a partial melt. At the highest laser power, the diffuse scattering dominated, with minimal evidence for crystal diffraction. On release, the melt recrystallized into a combination of zircon and reidite. Decomposition of zircon to SiO2 and ZrO2 was not observed. This study revealed that on laser-shock timescales, the zircon–reidite phase transition readily occurs. However, the decomposition of zircon into ZrO2 and SiO2 is kinetically inhibited. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 ≫ 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. [ABSTRACT FROM AUTHOR]

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 individual d-spacing shows a continuous decrease with pressure with no drastic changes associated to structural phase transformations or amorphization. The d/d0, where d0 is the d-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. [ABSTRACT FROM AUTHOR]

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 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. [ABSTRACT FROM AUTHOR]

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) Å. 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) Å, b = 13.707(2) Å, and c = 13.707(2) Å. 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) Å, b = 13.717(2) Å, and c = 13.706(2) Å, 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. [ABSTRACT FROM AUTHOR]

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 5 s 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 Å, while the five Sb(2)-X distances in the tetragonal pyramids vary from 2.45 to 2.59 Å, from 2.68 to 2.80 Å, and from 2.86 to 3.00 Å, respectively. With increasing Se content in the solid solution, the inter-ribbon distances where the Sb 5 s 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)X and Sb(2)X in which the Sb 5 s LPEs are accommodated constantly increase from 35.9 to 40.0 Å and from 34.1 to 38.8 Å, 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 5 s 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 5 s LPEs and the stereochemistry of the coordination polyhedra around the Sb atoms are affected by the Se substitution in the structure. [ABSTRACT FROM AUTHOR]

The structural diversity of arsenic sulfide molecules in compositions between AsS and As was investigated using ab initio quantum chemical calculations. The AsS molecule consists of four trigonal pyramid coordinations of As atoms bonding to three S atoms. In the AsS composition, only one type of molecular configuration corresponds to an uzonite-type molecule. In the AsS composition, two molecular configurations exist with realgar-type and pararealgar-type molecules. Three molecular configurations are in the AsS 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 AsS 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 AsS 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 AsS composition permits only one molecular configuration, which suggests that the mineral duranusite comprises the AsS 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. [ABSTRACT FROM AUTHOR]

The crystal structure of chromite FeCrO 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) Å up to 11.8 GPa. A fit to the Birch-Murnaghan equation of state (EoS) based on the P-V data gives: K = 209 (13) GPa, K′ = 4.0 (fixed), and V = 588 (1) Å. The FeO tetrahedra and CrO octahedra are compressed isotropically with pressure with their Fe-O and Cr-O bond distances decreasing from 1.996 (6) to 1.949 (7) Å and from 1.997 (3) to 1.969 (7) Å, respectively. The tetrahedral site occupied by the Fe cation is more compressible than the octahedral site occupied by the Cr cation. The resulting EoS parameters for the tetrahedral and the octahedral sites are K = 147 (9) GPa, K′ = 4.0 (fixed), V = 4.07 (1) Å and K = 275 (24) GPa, K′ = 4.0 (fixed), V = 10.42 (2) Å, 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 I4 /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) Å and the other four Cr-O bonds parallel to the ab plane elongate from 1.969 (7) to 1.987 (9) Å. 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 Fe 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]