Single-crystal Structure Refinement Research Articles

Oxo-mangani-leakeite from the Hoskins mine, New South Wales, Australia: occurrence and mineral description

AbstractOxo-mangano-leakeite, a newly approved end-member of the amphibole supergroup (IMA-CNMNC 20150-35), has been found in a rock containing manganese silicate and oxide at the Hoskins Mine, a Mn deposit 3 km west of Grenfell, New South Wales. The end-member formula of oxo-mangani-leakeite is ANaBNa2C(Mn3+4Li)TSi8 O22WO2, which would require SiO2 53.15, Mn2O3 34.91, Li2O 1.66, Na2O 10.28, total 100.00 wt.%. The empirical formula derived for the sample of this work from electron and ion microprobe analysis using constraints resulting from single-crystal structure refinement is A(Na0.65K0.36)∑ = 1.01B(Na1.94Ca0.06)∑ = 2.00C(Mg1.60Zn0.01 Li0.58)∑ = 5.01T(Si7.98Al0.02)∑ = 8.00O22W(O1.34OH0.66)∑ = 2.00. Oxo-mangano-leakeite is biaxial (–), with α = 1.681, β = 1.712, γ = 1.738, all ± 0.002, and 2V (meas.) = 81.0(4)°, 2V (calc.) = 83.5°. The unit-cell dimensions are a = 9.875(5), b = 17.873(9), c = 5.295(2) Å, β = 104.74(3)°, V = 903.8 (7) Å3; the space group is C2/m, with Z = 2. The strongest ten reflections in the powder X-ray pattern [d values (in Å), I, (hkl)] are: 8.423, 100, (110); 3.377, 46, (131); 4.461, 40, (040); 4.451, 40, (021); 3.134, 37, (310); 2.694, 37, (151); 2.282, 27, (); 2.734, 25, (31); 2.575, 24, (061); 2.331, 24, [() ()]. The holotype material is deposited in the Canadian Museum of Nature, Ottawa, under the catalogue number CMNMC 86895.

Towards a revisitation of vesuvianite-group nomenclature: the crystal structure of Ti-rich vesuvianite from Alchuri, Shigar Valley, Pakistan.

Vesuvianite containing 5.85 wt% TiO2 from an Alpine-cleft-type assemblage outcropped near Alchuri, Shigar Valley, Northern Areas, Pakistan, has been investigated by means of electron microprobe analyses, gas-chromatographic analysis of H2O, X-ray powder diffraction, single-crystal X-ray structure refinement, 27Al NMR, 57Fe Mössbauer spectroscopy, IR spectroscopy and optical measurements. Tetragonal unit-cell parameters are: a = 15.5326 (2), c = 11.8040 (2) Å, space group P4/nnc. The structure was refined to final R1 = 0.031, wR2 = 0.057 for 11247 I > 2σ(I). A general crystal-chemical formula of studied sample can be written as follows (Z = 2): [8-9](Ca17.1Na0.9) [8]Ca1.0[5](Fe2+0.44Fe3+0.34Mg0.22) [6](Al3.59Mg0.41) [6](Al4.03Ti2.20Fe3+1.37Fe2+0.40) (Si18O68) [(OH)5.84O2.83F1.33]. The octahedral site Y2 is Al-dominant and does not contain transition elements. Another octahedral site Y3 is also Al-dominant and contains Fe2+, Fe3+ and Ti. The site Y1 is split into Y1a and Y1b predominantly occupied by Fe2+ and Fe3+, respectively. The role of the Y1 site in the diversity of vesuvianite-group minerals is discussed.

The U-Nb-Al ternary system: experimental and simulated investigations of the phase equilibria and study of the crystal structure and electronic properties of the intermediate phases

The phase relations in the ternary U-Nb-Al system were established for the whole concentration range for 900 K and 1200 K. They were derived from quenched samples annealed at 900 K for three months and at 1200 K for two months by using x-ray powder diffraction, scanning electron microscopy and energy dispersive spectroscopic analyses. The formation of the two ternary phases, UNb2Al20 and U6Nb4Al43 was confirmed. Both phases form by peritectic reaction, at 1200(5) K and at 1262(5) K for UNb2Al20 and U6Nb4Al43 respectively. Single-crystal structure refinements confirmed that UNb2Al20 adopts the CeCr2Al20 type (cubic, Fd3¯m) and that U6Nb4Al43 crystallizes with the Ho6Mo4Al43 type (hexagonal, P63/mcm). The isothermal sections are characterized by (i) the extended homogeneity ranges due to mutual exchange between U and Nb and between Nb and Al in the bcc-phases (γU and Nb), at 1200 K, and (ii) a substantial ternary extension with limits of about 6(1) to 10(1) at.% U at 900 K and 1200 K respectively, of the σ-phase (Nb2Al, tetragonal, P42/mnm). The modeling of these isothermal sections was carried out by the Calphad method. The liquidus projection and the invariant reactions along with their corresponding temperatures were calculated using our optimized database. The electronic properties of both intermediate compounds were measured by means of dc-susceptibility, specific heat and resistivity measurements in the temperature range 2–300 K, revealing for UNb2Al20 enhanced Pauli paramagnetism and complex magnetic features for U6Nb4Al43 at low temperature. For the latter compound a composite behavior involving long-and short-range orderings, resulting from intra and inter U dimeric chains along the c-axis is suspected below 12 K.

Symmetry reduction due to gallium substitution in the garnet Li6.43(2)Ga0.52(3)La2.67(4)Zr2O12.

Single-crystal structure refinements on lithium lanthanum zirconate (LLZO; Li7La3Zr2O12) substituted with gallium were successfully carried out in the cubic symmetry space group I [Formula: see text]3d. Gallium was found on two lithium sites as well as on the lanthanum position. Due to the structural distortion of the resulting Li6.43(2)Ga0.52(3)La2.67(4)Zr2O12 (Ga-LLZO) single crystals, a reduction of the LLZO cubic garnet symmetry from Ia[Formula: see text] d to I [Formula: see text]3d was necessary, which could hardly be analysed from X-ray powder diffraction data.

Structural anomalies in tobelite-2M2 explained by high resolution and analytical electron microscopy

AbstractA transmission electron microscopy (TEM) investigation was undertaken in order to elucidate the nature of the structural disorder in a tobelite specimen from the sedimentary rocks of the Armorican sandstones (western France). This structural disorder may be the origin of residual electron-density maxima in difference-Fourier maps reported previously in single-crystal XRD studies of tobelite. The TEM investigation of tobelite confirmed that it is the 2M2 polytype in subfamily-B, but the ordered sequence is interrupted by numerous stacking faults parallel to (001), for which the stacking vectors belong to both subfamilies-A and -B of mica polytypes, with a prevalence of the latter. Chemical heterogeneity depending on the Si/Al ratio and Na and Mg concentration was observed at the nanoscale among different mica lamellae in a single crystal. The observed variations are consistent with a change in mica chemistry leading to interlayer vacancies which may cause shortening of the interlayer separation, as revealed by the single-crystal structure refinements.

High-temperature solution growth and vapour transport equilibration of (1−x)K1−yNaYNbO3–xLiNbO3 lead-free piezo-/ferroelectric single crystals

In order to develop lead-free piezo-/ferroelectric materials, single crystals of 0.98K0.8Na0.2NbO3–0.02LiNbO3 (KNN–LN) have been grown in the perovskite structure using the high-temperature solution growth method. Dielectric measurements reveal structural phase transitions at TC (cubic to tetragonal)=411°C and TO–T (tetragonal to orthorhombic)=189°C, respectively, and an additional phase transition at a lower temperature of −78°C which corresponds to the transition from the orthorhombic to a rhombohedral phase. Single crystal structural refinements based on X-ray diffraction data indicate that there are no oxygen octahedral tilts present at room temperature and at −103°C, which suggests that the crystal is K-rich. Composition analysis by energy dispersive spectroscopy and laser ablation – inductively coupled plasma – mass spectrometry confirms the K-rich composition. A vapour transport equilibration technique is successfully developed to optimise the composition of the as-grown crystals by enriching the Li-content and it is demonstrated to be a viable approach to increase the Li-concentration of KNN–LN and other piezo-/ferroelectric crystals.

Special Collection: Advances in Ultrahigh-Pressure Metamorphism: Tetrahedral boron in natural and synthetic HP/UHP tourmaline: Evidence from Raman spectroscopy, EMPA, and single-crystal XRD

Olenitic tourmaline with high amounts of tetrahedral B (up to 2.53 [4] B pfu) has been synthesized in a piston-cylinder press at 4.0 GPa, 700 °C, and a run duration of 9 days. Crystals are large enough (up to 30 × 150 μm) to allow for reliable and spatially resolved quantification of B by electron microprobe analysis (EMPA), single-crystal X-ray diffraction, and polarized single-crystal Raman spectroscopy. Tourmalines with radial acicular habit are zoned in [4] B-concentration [core: 2.53(25) [4] B pfu; rim: 1.43(15) [4] B pfu], whereas columnar crystals are chemically homogeneous [1.18(15) [4] B pfu]. An amount of 1.4(1) [4] B pfu was found in the columnar tourmaline by single-crystal structure refinement (SREF) ( R = 1.94%). The EMPA identify [T] Si −1 [V,W] O −1 [T] B 1 [V,W] (OH) 1 as the main and [X] □ −1 [T] Si −1 [X] Na 1 [T] B 1 as minor exchange vectors for [4] B-incorporation, which is supported by the SREF. Due to the restricted and well-defined variations in chemistry, Raman bands in the OH-stretching region (3000–3800 cm −1 ) are unambiguously assigned to a specific cation arrangement. We found the sum of the relative integrated intensity ( I rel ) of two low-frequency bands at 3284–3301 cm −1 (ν1) and 3367–3390 cm −1 (ν2) to positively correlate with the [4] B concentrations: [4] B [pfu] = 0.03(1) × [ I rel (ν1) + I rel (ν2)]. Hence, those bands correspond to configurations with mixed Si/B occupancy at the T site. Our semi-quantitative correlation also holds for well-characterized natural [4] B-bearing tourmaline from the Koralpe, Austria. This work shows the potential for Raman spectroscopy as a non-destructive method for the chemical classification of (precious) natural tourmaline, and as a tool to rapidly characterize chemical zonation of tourmalines in thin section.

Synthetic Potassic-Ferro-Richterite: 1. Composition, Crystal Structure Refinement, and HTBehavior ByIn OperandoSingle-Crystal X-Ray Diffraction

Abstract The high-temperature behavior of synthetic potassic-ferro-richterite was addressed to obtain data relevant to understanding petrogenetic processes as well as to testing complementarity and mutual calibration of single-crystal X-ray diffraction (XRD) analysis plus structure refinement (SREF) with single-crystal FTIR spectroscopy. This experimental approach aims at: (1) better quantifying the onset of deprotonation, its development and the amount, if any, of relict OH at the end of the process; (2) verifying whether or not the process is homogeneous within the crystal; and (3) evaluating local changes in cation environments close to the OH dipole. In this first part of a series of two papers, we report on the crystal-chemical characterization of potassic-ferro-richterite and on a single-crystal XRD study at high T . Detailed analysis of the available data allowed us to obtain a full characterization of the bulk and crystal chemistry of the studied crystal, hence improving the unit formula suggested by Redhammer & Roth (2002). In operando H T measurements up to 1073 K showed quite anomalous behavior with respect to pargasite/kaersutites, specifically a much lower T for the onset of the deprotonation process (around 500 K), and strongly anomalous behavior of the β angle, which shows inverse slopes for protonated and deprotonated phases. “Oxo-potassic-ferro-richterite” is formed upon deprotonation and remains stable at least up to 1073 K under the conditions of this study. Structure refinements from data collected at different temperatures allowed us to detect changes in the crystal-structure geometry and in turn to decipher the way in which amphiboles with such a peculiar composition respond to increasing T and deprotonation. The thermal expansivity coefficients α (×10 −5 K −1 ) are: potassic-ferro-richterite: α a = 1.30(6), α b = 0.93(6), α c = 0.12(3), α β = −0.49(5), α asinβ = 1.34(8), α V = 2.59(2); “oxo-potassic-ferro-richterite”: α a = 1.71 (3), α b = 0.97(1), α c = 0.193(8), α β = 0.22(1), α asinβ = 1.59(4), α V = 2.74(2).

In-situ crystal structure determination of seifertite SiO2at 129 GPa: Studying a minor phase near Earth’s core–mantle boundary

Seifertite SiO2 likely exists as a minor phase near the core–mantle boundary. By simulating the pressure and temperature conditions near the core–mantle boundary, seifertite was synthesized as a minor phase in a coarse-grained, polycrystalline sample coexisting with the (Mg,Fe)SiO3 post-perovskite (pPv) phase at 129 GPa and 2500 K. Here we report the first in situ single-crystal structure determination and refinement of seifertite at high pressure and after a temperature quench from laser heating. We improved the data coverage of a minor phase from a diamond-anvil cell (DAC) by merging single-crystal data of seifertite from six selected grains that had different orientations. Observed systematic absences of reflections from the six individual grains allowed only one space group: Pbcn. The refined results of seifertite are in good agreement with the predictions from previous first-principles calculations at high pressure. This approach provides a method for structure determination of a minor phase in a mineral assemblage synthesized under P-T conditions representative of the deep Earth.

FLUORNATROPYROCHLORE, A NEW PYROCHLORE SUPERGROUP MINERAL FROM THE BOZIGUOER RARE EARTH ELEMENT DEPOSIT, BAICHENG COUNTY, AKESU, XINJIANG, CHINA

Fluornatropyrochlore is a new pyrochlore supergroup mineral species found in the intrusive alkali granite of the Boziguoer rare earth element deposit in Baicheng County, Akesu, Xinjiang Autonomous Region, China. This new mineral is found to be closely associated with microcline, albite, aegirine, a sodic amphibole, biotite, zircon, rutile, thorite, fluorite, fluocerite-(Ce), columbite-(Fe), xenotime-(Y), astrophyllite, chevkinite-(Ce), and fergusonite-(Y). Fluornatropyrochlore occurs mostly as anhedral crystals, rarely as subhedral grains, with a grain size of 0.02 to 0.25 mm. The color is brownish yellow to reddish orange. Other physical properties are: brittle, translucent to transparent, non-fluorescent, adamantine luster, light yellow streak, with Mohs hardness 4–4½, and calculated density 5.275 g cm−3. Fluornatropyrochlore is isotropic, n = 2.10(5) (589.9 nm). It has a cubic crystal structure, with unit-cell parameters determined using single-crystal X-ray diffraction as: a 10.5053(10) A, V 1159.4(2) A3, Z = 8, space group Fd 3 m . The strongest ten X-ray diffraction (XRD) peaks in the powder XRD pattern are [ d in A( I )( hkl )]: 6.0741(3)(111), 3.0416(100)(222), 2.6280(38)(004), 1.8568(34)(044), 1.5820(15)(226), 1.5148(4)(444), 1.3137(2)(008), 1.2045(3)(266), 1.1726(2)(048), and 1.0712(1)(448). Chemical analyses by a combination of electron microprobe, Fourier transform infrared spectroscopy (FTIR), and single crystal-structure refinement give its chemical composition as: Na2O 6.80, K2O 0.01, CaO 2.01, MgO 0.01, FeO 0.05, SrO 0.03, PbO 16.17, Ce2O3 4.29, La2O3 1.65, Nd2O3 0.41, Y2O3 0.42, SiO2 0.03, TiO2 1.36, UO2 5.81, Ta2O5 3.00, Nb2O5 53.42, F 3.19, Cl 0.02, ThO2, Sb2O5 0.01, ZrO2 0.01, MnO 0.04, SnO2 0.34, H2O not detected, with a sum of 98.21 wt.%. The empirical formula derived from its averaged composition is (Na1.03Pb0.34Ca0.17U0.10Th0.01Ce0.12La0.05Y0.02Nd0.01)Σ1.85(Nb1.88Ti0.08Ta0.06Sn0.01)Σ2.03O6.21F0.79 based on 7 anions per formula unit. The crystal structure was refined to an R index of 0.053 using Mo Ka X-ray intensity data. Site-scattering refinement analysis shows that the new mineral is characterized by F dominance at the Y site, Na dominance at the A site, and Nb dominance at the B site, based on the overall electroneutrality requirement of the structural formula.

Single crystal structure determination and refinement of AgZrCu4 and Ag-containing Cu10Zr7 by precession electron diffraction and tomography techniques

The Cu–Zr–Ag ternary system belongs to the Cu-based bulk amorphisable alloy systems. The solidification of the amorphisable alloy is still not well-understood. The identification and characterization of the crystal structure of the solidified phases are essential for describing the ternary system. A typical Cu-rich solidified phase in the Cu–Zr–Ag ternary system is the so-called m-phase. The structure of this phase was not known before. Another typical phase is the Ag-containing Cu10Zr7 phase. We determined and refined the crystal structures of single crystals of these two phases by transmission electron microscopyusing precession electron diffraction technique and diffraction tomography (performed by ADT3D software). The determination and refinement procedure was achieved in SIR2008, SIR2011 and SHELXL software packages. In conclusion, precession electron diffraction technique combined with diffraction tomography makes it possible to determine the crystal structure of materials with sub-micrometer sized crystals such as alloys when single crystal X-ray diffraction based structure determination fails.

Influence of the X-site composition on tourmaline’s crystal structure: investigation of synthetic K-dravite, dravite, oxy-uvite, and magnesio-foitite using SREF and Raman spectroscopy

The crystal structures of synthetic K-dravite [XKYMg 3 Z Al 6 T Si6O18(BO3) 3 V (OH) 3 W (OH)], dravite [XNaYMg 3 Z Al 6 T Si6O18(BO3) 3 V (OH) 3 W (OH)], oxy-uvite [XCaYMg 3 Z Al 6 T Si6O18(BO3) 3 V (OH) 3 W O], and magnesio-foitite [X☐Y(Mg2Al)ZAl 6 T Si6O18(BO3) 3 V (OH) 3 W (OH)] are investigated by polarized Raman spectroscopy, single-crystal structure refinement (SREF), and powder X-ray diffraction. The use of compositionally simple tourmalines characterized by electron microprobe analysis facilitates the determination of site occupancy in the SREF and band assignment in the Raman spectra. The synthesized K-dravite, oxy-uvite, and magnesio-foitite have significant Mg–Al disorder between their octahedral sites indicated by their respective average 〈Y–O〉 and 〈Z–O〉 bond lengths. The Y- and Z-site compositions of oxy-uvite (YMg1.52Al1.48(10) and ZAl4.90Mg1.10(15)) and magnesio-foitite (YAl1.62Mg1.38(18) and ZAl4.92Mg1.08(24)) are refined from the electron densities at each site. The Mg–Al ratio of the Y and Z sites is also determined from the relative integrated peak intensities of the Raman bands in the O–H stretching vibrational range (3250–3850 cm−1), producing values in good agreement with the SREF data. The unit cell volume of tourmaline increases from magnesio-foitite (1558.4(3) A3) to dravite (1569.5(4)–1571.7(3) A3) to oxy-uvite (1572.4(2) A3) to K-dravite (1588.1(2) A3), mainly due to lengthening of the crystallographic c-axis. The increase in the size of the X-site coordination polyhedron from dravite (Na) to K-dravite (K) is accommodated locally in the crystal structure, resulting in the shortening of the neighboring O1–H1 bond. In oxy-uvite, Ca2+ is locally associated with a deprotonated W (O1) site, whereas vacant X sites are neighbored by protonated W (O1) sites. Increasing the size of the X-site-occupying ion does not detectably affect bonding between the other sites; however, the higher charge of Ca and the deprotonated W (O1) site in oxy-uvite are correlated to changes in the lattice vibration Raman spectrum (100–1200 cm−1), particularly for bands assigned to the T 6O18 ring. The Raman spectrum of magnesio-foitite shows significant deviations from those of K-dravite, dravite, and oxy-uvite in both the lattice and O–H stretching vibrational ranges (100–1200 and 3250–3850 cm−1, respectively). The vacant X site is correlated with long- and short-range changes in the crystal structure, i.e., deformation of the T 6O18 ring and lengthening of the O1–H1 and O3–H3 bonds. However, X-site vacancies in K-dravite, dravite, and oxy-uvite result only in the lengthening of the neighboring O1–H1 bond and do not result in identifiable changes in the lattice-bonding environment.

Synthesis and magnetic properties of centennialite: a new S = ½ Kagomé antiferromagnet and comparison with herbertsmithite and kapellasite

Minerals of the atacamite group such as herbertsmithite and kapellasite have recently attracted enormous attention as the S = ½ Kagome antiferromagnets for achieving the quantum spin liquid (QSL) state with diverse technological applications. Herein we report on the synthesis of the newly discovered mineral centennialite by using an unconventional “solid-state” reaction method at 463 K. Synthetic centennialite, Ca1.06Cu2.94Cl2.01(OH)5.99·0.73H2O, has been characterized by scanning electron microscopy, electron microprobe analyses, Fourier-transform infrared spectroscopy, thermogravimetric and differential scanning calorimetric analyses, single-crystal X-ray diffraction structure refinements, and magnetic susceptibility measurements. The crystal structure of centennialite is characterized by a perfect (threefold symmetry) Kagome layer with <5 % substitution between Ca and Cu and therefore differs from those of herbertsmithite and kapellasite, in which 15–25 % mixing between similar Zn and Cu atoms dramatically affects the QSL state. Centennialite remains antiferromagnetic down to ~7 K with a moderate spin frustration (i.e., a Weiss temperature θ = −56 K and a spin frustration parameter f = 8), but exhibits a canted antiferromagnetic ordering with a ferromagnetic component at lower temperatures.

Crystal Growth through Field-Assisted Electrochemical Redox and Ion-Exchange Reactions: A Case Study of K4.2Na3.8Si46 Clathrate-I

Single crystals of the ternary clathrate-I compound K4.2Na3.8Si46 were grown by a unique ion-exchange process employing spark plasma sintering (SPS). Single crystal structure refinements of clathrate-I K4.2Na3.8Si46 at 100 and 300 K indicate that the 6d crystallographic site contains both Na and K. Extended Huckel tight binding calculations suggest that K4.2Na3.8Si46 is metallic and that the Fermi level lies within bands having Si–Si antibonding character. The selective synthesis of clathrate-I and -II compositions was possible by changing the reaction temperature. Our results indicate that SPS can be employed in electrochemical redox and ion-exchange reactions simultaneously, thus allowing for the rapid synthesis of single crystals of multinary inorganic clathrate phases that cannot be accessible by traditional crystal growth techniques.

Standard-state thermodynamic properties of annite, KFe3[(OH)2AlSi3O10], based on new calorimetric measurements

The heat capacity ( C p) of synthetic annite was measured using a Physical Properties Measurement System (PPMS) for temperatures ranging from 2 to 300 K, and by differential scanning calorimetry (DSC) for temperatures between 282 and 363 K. The sample was synthesized in a hydrothermal apparatus at 2 kbar and 800°C, with a log f O2 of −20. It was characterized using electron microprobe analysis, Mossbauer spectroscopy, XRD, SEM, and single-crystal structural refinement methods and contained the well-established minimum possible Fe3+ content of 10% ( i.e. , Fe3+/Fetot = 0.1), representing the closest possible synthetic composition to ‘ideal’ annite. A distinct kink in the slope of C p plotted against T at around 49 K indicated a heat-capacity anomaly, which is interpreted to have been caused by a magnetic ordering phase transition. In order to compute the magnetic contribution ( C mag) to the overall C p, the C p data between 40 and 65 K were therefore excluded from the measured C p data set and the remaining data fitted to a combination of Debye, Einstein, and Schottky functions. These functions then served as a model for estimating the vibrational part of the heat capacity ( C vib) within the temperature range of the magnetic phase transition. Integrating the C p data with respect to temperature yields a calorimetric entropy of 411.4±2.9 J/mol·K at 298 K for ‘pure’ annite (excluding any configurational terms). The magnetic entropy amounts to only 1.5 J/mol·K, which is much less than predicted theoretically. The DSC data, combined with C p values from published literature, are represented by the following polynomial, which can be used to compute the C p of annite at temperatures above 298 K: ![Formula][1] Applying the calorimetrically determined entropy of annite (and adding a configurational entropy of 11.53 J/mol·K, representing Al-avoidance) to published phase-equilibrium data on the reaction annite + quartz = sanidine + fayalite + H2O resulted in a standard enthalpy of formation for annite of Δ H of=−5132.5±2.0 kJ/mol. The published experimental data on the reaction annite = sanidine + magnetite + H2 (hydrogen-sensor experiments and conventional oxygen-buffer brackets) were analysed using the calorimetrically derived entropy of annite. With the exception of magnetite–wustite buffered experiments, all experimental constraints could be satisfied using the annite standard-state properties S o =422.9±2.9 J/mol·K and Δ H of, values ranging between 5128 and −5135 kJ/mol. A plot is presented showing contours for the Fe3+/Fetot ratio of annite within the assemblage annite–sanidine–magnetite (+H2) as function of log f O2 and temperature. [1]: /embed/mml-math-1.gif

Single-crystal synthesis and structure refinement of La2Li0.5Al0.5O4 with K2NiF4-type structure

Single crystals of lanthanum lithium aluminum oxide, La2Li0.5Al0.5O4, were synthesized for the first time by a flux method at 773 K. The La2Li0.5Al0.5O4 single crystal is colorless, has an average size of 60 μm with a square platelet shape, and crystallizes in the tetragonal K2NiF4-type structure, space group I4/mmm with a = 3.7742(9) Å, c = 12.753(4) Å, V = 181.66(10) Å3, and Z = 2. The structure was determined using single-crystal X-ray diffraction data and refined to the conventional values R = 1.25% and wR = 1.28% for 524 observed reflections. The octahedral site was randomly occupied by Li and Al atoms. The corresponding polycrystalline sample was also prepared by a conventional solid-state reaction method at 1123 K.

Structural and photoluminescence properties of doped and REE-endmember mixed-framework rare-earth sorosilicates

Photoluminescence (PL) properties of lanthanide-doped mixed-framework barium yttrium sorosilicates Ln:BaY2(Si3O10) (Ln=Pr3+, Sm3+, or Er3+) and Ln:BaY4(Si2O7)(Si3O10) (Ln=Pr3+, Sm3+, Er3+, or Ho3+), and selected REE-endmembers BaREE2(Si3O10) (REE=Y, Dy) and BaREE4(Si2O7)(Si3O10) (REE=Y, Er, Ho) are presented. These silicates were synthesized by a high-temperature, molybdate-based flux-growth method. Results from single-crystal structure refinements are given and briefly discussed in the spectroscopic context. Sm3+, present in both sorosilicate structure types, causes transitions in the visible yellow to orange spectral range at 600 and 570nm, according to the 4G5/2→6HJ transitions. Pr3+ is responsible for dominant PL bands around 500nm due to the 3P0→3H4 transition, showing a blue–green emitting color. Er3+ exhibits green–yellow emission with strongest bands between 540 and 570nm, which can be associated with the 4S3/2→4I15/2 transition. Ho3+ shows a dominant characteristic PL emission in the red spectral region, which is assigned to the 5F5→5I8 transition. The photoluminescence of emission of Dy3+ centers, with the main emission at 575nm in the visible yellow region, is assigned to the 4F9/2→6H13/2 transition. Strong differences in the intensity of PL emissions of the REE-endmembers and Ln-doped silicates are caused by self-quenching effects due to high concentration of Ln3+ cations in the crystal structure. Unexpected PL emission spectra of the host sorosilicates BaY2(Si3O10) and BaY4(Si2O7)(Si3O10) are caused by trace impurity contents of other Ln3+ cations.

Crystal structures of deuterated sodium molybdate dihydrate and sodium tungstate dihydrate from time-of-flight neutron powder diffraction.

Time-of-flight neutron powder diffraction data have been measured from ∼90 mol% deuterated isotopologues of Na2MoO4·2H2O and Na2WO4·2H2O at 295 K to a resolution of sin (θ)/λ = 0.77 Å(-1). The use of neutrons has allowed refinement of structural parameters with a precision that varies by a factor of two from the heaviest to the lightest atoms; this contrasts with the X-ray based refinements where precision may be > 20× poorer for O atoms in the presence of atoms such as Mo and W. The accuracy and precision of inter-atomic distances and angles are in excellent agreement with recent X-ray single-crystal structure refinements whilst also completing our view of the hydrogen-bond geometry to the same degree of statistical certainty. The two structures are isotypic, space-group Pbca, with all atoms occupying general positions, being comprised of edge- and corner-sharing NaO5 and NaO6 polyhedra that form layers parallel with (010) inter-leaved with planes of XO4 (X = Mo, W) tetra-hedra that are linked by chains of water mol-ecules along [100] and [001]. The complete structure is identical with the previously described molybdate [Capitelli et al. (2006 ▸). Asian J. Chem. 18, 2856-2860] but shows that the purported three-centred inter-action involving one of the water mol-ecules in the tungstate [Farrugia (2007 ▸). Acta Cryst. E63, i142] is in fact an ordinary two-centred 'linear' hydrogen bond.

Manganese incorporation in synthetic hercynite

AbstractManganese incorporation in synthetic hercynite, and partitioning between hercynite and silicate melt synthesized at 1.0 GPa, 1250°C, and at an fO2 buffered by Fe–FeO, has been studied by X-ray absorption spectroscopy and single-crystal X-ray structure refinement. Spectra indicate the presence of both Mn2+ and Mn3+ (and possibly also Mn4+) in synthetic hercynite and partitioning of Mn2+ into the melt phase, and Mn3+ into hercynite, respectively, under run conditions. X-ray refinement is consistent with partial disorder of Fe and Al across tetrahedral and octahedral sites. A higher than expected degree of Fe-Al disorder in the Mn-bearing hercynite can be explained by preferential incorporation of Mn2+ onto the tetrahedral site, and indicates that Fe-Al disorder in pure, stoichiometric hercynite cannot necessarily be used to determine closure temperatures in natural spinel. However, partitioning of Mn2+ and Mn3+ between melt and hercynite suggests that Mn incorporation in hercynite could be used as a measure of fO2 conditions in magmas during spinel crystallization.

Gatedalite, Zr(Mn2+2Mn3+4)SiO12, a new mineral species of the braunite group from Långban, Sweden

AbstractGatedalite, Zr(Mn22+Mn43+)SiO12, is a new mineral of the braunite group. It is found in hausmannite-impregnated skarn together with jacobsite, Mn-bearing calcite, tephroite, Mn-bearing phlogopite, långbanite, pinakiolite and oxyplumboroméite at the Långban Mn-Fe oxide deposit, Värmland, central Sweden. The mineral occurs as very rare, small (≤60 μm), grey, submetallic, irregularly rounded anhedral grains. Gatedalite has a calculated density of 4.783 g/cm3. It is opaque and weakly anisotropic with reflectivity in air varying between 17.1 and 20.8% in the visible spectral range. Gatedalite is tetragonal, space groupI41/acd, with the unit-cell parametersa= 9.4668(6) Å,c= 18.8701(14) Å,V= 1691.1(2) Å3andZ= 8. The crystal structure was refined to anR1 index of 5.09% using 1339 unique reflections collected with MoKαX-ray radiation. The five strongest powder X-ray diffraction lines [din Å, (I), (hkl)] are: 2.730(100)(224), 2.367(12)(040), 1.6735(12)(440), 1.6707(29)(048) and 1.4267(16)(264). Electron microprobe analyses in combination with single-crystal structure refinement resulted in the empirical formula: (Zr0.494+Mn0.402+Mg0.07Ca0.02Zn0.01Ce0.013+)Σ1.00(Mn4.443+Fe0.593+Mn0.572+Mg0.41Al0.01)Σ6.02Si0.99O12. Gatedalite is a member of the braunite group (general formula AB6SiO12). It is related to braunite (Mn2+Mn63+SiO12) through the net cation exchange (Zr4++ Mn2+) → 2Mn3+, which results from the substitutions Zr4+→ Mn2+at the 8-fold coordinated site (Ain the general formula) coupled with a 2Mn2+→ 2Mn3+substitution at the 6-fold coordinated sites (Bin the general formula).