Single-crystal X-ray Structure Refinements Research Articles

From Cs[C2N3] to Cs3[C6N9] – a thermal and structural investigation

Abstract Cesium dicyanamide Cs[C2N3] (≡ Cs[N(CN)2] or Cs[dca]) was obtained by a metathesis reaction in form of transparent colorless platelets. The results of single-crystal X-ray structure measurements and refinements (C2/c, Z = 8) with the monoclinic cell parameters a = 932.31(8), b = 1274.67(9), c = 824.94(7) pm, β = 110.803(3)° at −70 °C and a = 939.59(7), b = 1281.58(8), c = 827.57(6) pm, β = 110.610(3)° at 20 °C corroborate earlier results for this compound. The Raman and IR spectra of Cs[C2N3] are presented for the first time and the result compares well with those of NaCs2[C2N3]3. The heat-driven cyclotrimerization process of Cs[C2N3] was studied by thermal analyses (DSC) and temperature-dependent X-ray powder diffraction methods. At 370 °C, its trimerization product Cs3[C6N9] is formed, crystallizing in the orthorhombic space group Pbam with the cell parameters a = 3043.0(3), b = 1052.4(1) and c = 415.21(4) pm for Z = 4. The IR spectrum of this cesium tricyanomelaminate (Cs3[C6N9] or Cs3[TCM]) is presented, but a well-resolved Raman spectrum could not be acquired owing to fluorescence phenomena. An overview about the cyclotrimerization reactions of all pseudo-binary alkali-metal dicyanamides (A[C2N3]) to their corresponding tricyanomelaminates (A 3[C6N9]) with A = Li–Cs gives a basis for a discussion of the different thermal and structural characteristics.

Recommended X-ray single-crystal structure refinement and Rietveld refinement procedure for tremolite

A detailed description of the structure of the amphibole-supergroup minerals is very challenging owing to their complex chemical composition that renders the process of cation partition extremely difficult, particularly because of the occurrence of multivalent elements. Since amphiboles naturally occur under a fibrous morphology and have largely been used to produce asbestos, there is a growing demand for detailed and accurate structural data in order to study the relationships between structure, composition and toxicity. The present study proposes a recommended refinement procedure for both X-ray single-crystal structure refinement (SREF) and Rietveld analysis for tremolite, selected as a test case. The corresponding structural results are compared to estimate the `degree of confidence' of the Rietveld refinement with regard to SREF. In particular, it is shown that the interpretation of the electron density of the tremolite structure by SREF is model dependent. By assuming that the site-scattering values from SREF should be as close as possible to those from electron microprobe analysis, as a crucial constraint for the correct description of the final crystal-chemical model, it is found that it is best satisfied by using partially ionized scattering curves (SCs) for O and Si, and neutral SCs (neutral oxygen curves or NOCs) for other atoms. This combination leads to the best fit to the diffraction data. Moreover, it is found that Rietveld refinement using NOCs produces the best structural results, in excellent agreement with SREF. It is worth noting that, due to the complexity of the diffraction pattern and the fairly large number of freely refinable parameters, refinements with different combinations of SCs produce results almost indistinguishable from a statistical point of view, albeit showing significant differences from a structural point of view.

Multi-methodical study of the Ti, Fe2+ and Fe3+ distribution in chevkinite-subgroup minerals: X-ray diffraction, neutron diffraction, 57Fe Mössbauer spectroscopy and electron-microprobe analyses

Three non-metamict chevkinite-subgroup minerals, space group P21/a, from Cape Ashizuri, Japan, (No. 1), Tangir Valley, Diamar District, Pakistan (No. 2) and Haramosh Mts., Skardu district, Pakistan, (No. 3) were studied by crystal chemical techniques. Powder X-ray diffraction and transmission electron microscopic observations confirmed well crystalline samples. Electron-microprobe analyses indicated the general composition [(REE, Ca)4Fe2+(Fe2+, Fe3+, Ti)2Ti2(Si2O7)2O8] known for chevkinite-(Ce). Site scattering values determined by single-crystal X-ray structure refinements suggested assignment of subordinate Nb to the octahedral M3 and M4 sites, minor Th to M1 for the Ashizuri sample and minor Mg to M1 for both samples from Pakistan. Neutron time-of-flight powder diffraction studies were applied to determine the Ti/Fe distribution among octahedral sites for all samples and Mossbauer spectroscopy served for the Fe valence assignment at the four octahedral sites. Combined results gave the simplified formulas with Ce as dominant REE: No. 1 A(REE,Ca)4M1(Fe2+,Ti)M2(Ti,Fe3+)2M3,M4(Ti,Fe2+,Nb)2(Si2O7)2O8, No. 2 A(REE,Ca)4M1(Fe2+,Fe3+,Mg)M2(Ti,Fe3+)2M3,M4(Ti,Fe2+)2(Si2O7)2O8 and No. 3 A(REE,Ca)4M1(Fe3+,Fe2+,Mg,Ti)M2(Ti,Fe3+)2M3,M4(Ti,Fe2+)2(Si2O7)2O8. The dominant iron valence at M1 of the Haramosh sample (No. 3) is ferric whereas for samples Nos. 1 and 2 iron is ferrous. The structure of chevkinite-subgroup minerals consists of octahedral layers (M2, M3 and M4) alternating with an intermediate layer containing disilicate groups, REE polyhedra and strongly distorted M1 octahedra. Increased O atomic displacement parameters within the intermediate layers indicate disorder and/or strain to fit the extension of the embracing octahedral layers. A statistical analysis of chevkinite-subgroup structures (space groups P21/a, C2/m and P21/m) indicates a positive correlation between bond length and M1 bond angle variance. A negative correlation is found between bond length and the disilicate bending angle Si–O–Si, varying between ca. 160° and 175°. Outliers of this trend are delhuyarite-(Ce) Ce4Mg(Fe3+2W)□(Si2O7)2O6(OH)2, mainly due to octahedral W6+ and vacancies in the octahedral layers associated with OH groups, and synthetic Nd end-members with strong bonds between Nd and the intersecting O of the disilicate group. The observed trends reflect the main mechanisms in the intermediate layer to adopt to the size of the embracing rigid octahedral layers formed by M2, M3 and M4 octahedra.

Synthesis and Crystal Structure of Three Ga-rich Lithium Gallides, LiGa6, Li11Ga24, and LiGa2.

Three new binary phases have been synthesized in the Ga-rich part of the Li-Ga system: LiGa6, Li11Ga24, and LiGa2. Their crystal structures and the respective phase formation conditions have been investigated with X-ray single crystal structure refinements, Rietveld refinements of X-ray powder diffraction data, and thermal analyses. They complete the Ga-rich part of the Li-Ga phase diagram together with the reported phases Li6-xGa14 with 2 ≤ x ≤ 3 and LiGa3.42. The compositions of two of the new gallides, LiGa6 and LiGa2, had been predicted in previous thermoanalytical studies, but their crystal structures remained unknown. All three new binary main group compounds adopt new structure types. LiGa6 crystallizes with the trigonal space group R3̅c (No. 167, a = 6.1851(8) Å, c = 23.467(4) Å), Li11Ga24 crystallizes with the hexagonal space group P63mc (No. 186, a = 13.7700(19) Å, c = 23.250(5) Å), and LiGa2 crystallizes with the orthorhombic space group Cmce (No. 64, a = 8.51953(4) Å, b = 14.44163(7) Å, c = 15.29226(7) Å). All phases form air- and moisture-sensitive crystals of bright metallic luster. They can be synthesized starting from the pure elements and taking into account their incongruent melting behavior by adequate tempering sequences derived from differential scanning calorimetry (DSC) studies of the system. Lithium gallides do not form electron-precise Zintl phases. The electronic structures of these polar intermetallic phases combine ionic, covalent, and metallic bonding contributions and have been analyzed by density functional theory (DFT) calculations in the cases of LiGa6 and LiGa2. Measurements of the specific electronic resistivities have also been performed and prove the metallic behavior.

Rare-earth crystal chemistry of thalénite-(Y) from different environments

ABSTRACTThalénite-(Y), ideally Y3Si3O10F, is a heavy-rare-earth-rich silicate phase occurring in granite pegmatites that may help to illustrate rare-earth element (REE) chemistry and behaviour in natural systems. The crystal structure and mineral chemistry of thalénite-(Y) were analysed by electron microprobe analysis, X-ray diffraction and micro-Raman spectroscopy from a new locality in the peralkaline granite of the Golden Horn batholith, Okanogan County, Washington State, USA, in comparison with new analyses from the White Cloud pegmatite in the Pikes Peak batholith, Colorado, USA. The Golden Horn thalénite-(Y) occurs as late-stage sub-millimetre euhedral bladed transparent crystals in small miarolitic cavities in an arfvedsonite-bearing biotite granite. It exhibits growth zoning with distinct heavy-rare-earth element (HREE) vs. light-rare-earth element (LREE) enriched zones. The White Cloud thalénite-(Y) occurs in two distinct anhedral and botryoidal crystal habits of mostly homogenous composition. In addition, minor secondary thalénite-(Y) is recognized by its distinct Yb-rich composition (up to 0.8 atoms per formula unit (apfu) Yb). Single-crystal X-ray diffraction analysis and structure refinement reveals Y-site ordering with preferential HREE occupation of Y2 vs. Y1 and Y3 REE sites. Chondrite normalization shows continuous enrichment of HREE in White Cloud thalénite-(Y), in contrast to Golden Horn thalénite-(Y) with a slight depletion of the heaviest REE (Tm, Yb and Lu). The results suggest a hydrothermal origin of the Golden Horn miarolitic thalénite-(Y), compared to a combination of both primary magmatic followed by hydrothermal processes responsible for the multiple generations over a range of spatial scales in White Cloud thalénite-(Y).

Microporous crystal structure of labuntsovite-Fe and high-pressure behavior up to 23 GPa

Labuntsovite-Fe, an Fe-dominant member of the labuntsovite subgroup, was first discovered in the Khibiny alkaline massif on Mt Kukisvumchorr [Khomyakov et al. (2001). Zap. Vseross. Mineral. Oba, 130, 36–45]. However, no data are published about the crystal structure of this mineral. Labuntsovite-Fe from a peralkaline pegmatite located on Mt Nyorkpakhk, in the Khibiny alkaline complex, Kola Peninsula, Russia, has been investigated by means of electron microprobe analyses, single-crystal X-ray structure refinement, and IR and Raman spectroscopies. Monoclinic unit-cell parameters of labuntsovite-Fe are: a = 14.2584 (4), b = 13.7541 (6), c = 7.7770 (2) Å, β = 116.893 (3)°; V = 1360.22 (9) Å3; space group C2/m. The structure was refined to final R 1 = 0.0467, wR 2 = 0.0715 for 3202 reflections [I > 3σ(I)]. The refined crystal chemical formula is (Z = 2): Na2K2Ba0.7[(Fe0.5Ti0.1Mg0.05)(H2O)1.3]{[Ti2(Ti1.9Nb0.1)(O,OH)4][Si4O12]2}·4H2O. The high-pressure in situ single-crystal X-ray diffraction study of the labuntsovite-Fe has been carried out in a diamond anvil cell. The labuntsovite-type structure is stable up to 23 GPa and phase transitions are not observed. Calculations using the BM3 equation of state resulted in the bulk modulus K = 72 (2) GPa, K′0 = 3.7 (2) and V 0 = 1363 (2) Å3. Compressing of the heteropolyhedral zeolite-like framework leads to the deformation of main structural units. Octahedral rods show the gradual increase of distortion and the wave-like character of rods becomes more distinct. Rod deformations result in the distortion of the silicon–oxygen ring which is not equal in different directions. Structural channels are characterized by a different ellipticity–pressure relationship: the cross-section of the largest channel I and channel II demonstrates the stability of the geometrical characteristics which practically do not depend on pressure: ∊channel I ≃ 0.85 (4) (cross-section is rather regular) and ∊channel II ≃ 0.52 (2) within the whole pressure range. However, channel III is characterized by the increasing of ellipticity with pressure (∊ = 0.40 → 0.10).

The role of Fe content on the Fe-Mg exchange reaction in augite

The study of the intracrystalline Fe-Mg exchange between M 1 and M 2 crystallographic sites in clinopyroxene on samples a variety of geological settings has provided a framework to understand the thermal history of pyroxene-bearing rocks. The Fe-Mg exchange reaction has successfully been exploited as a geothermometric tool in the study of orthopyroxene and pigeonite-bearing rocks, but relatively few data are available for clinopyroxene. A strong correlation between total iron content and the slope of the Fe 2+ -Mg equilibrium distribution coefficient ( k D ) as a function of temperature has been found for orthopyroxene and pigeonite, and we investigate this relationship in augite. We carried out new equilibrium annealing experiments at 800, 900, and 1000 °C followed by single-crystal X-ray diffraction and structural refinement to obtain a new geothermometric calibration for augite from a 120 m thick lava flow from Ontario, Canada [Theo’s Flow, En 49 Fs 9 Wo 42 hereafter also referred as Fs 9 where Fs = 100·ΣFe/(ΣFe+Mg+Ca) with ΣFe = Fe 2+ +Fe 3+ +Mn]. This new calibration enabled us to evaluate the compositional effects (mainly Fe content) by comparison with the data previously obtained on augite from MIL 03346 martian sample (En 36 Fs 24 Wo 40 hereafter referred to as Fs 24 ). The extremely good agreement observed between the data obtained on Theo’s Flow and Miller Range (MIL 03346) augite demonstrate that for the range of compositions between Fs 9 and Fs 24 , total iron content has a negligible or null influence on equilibrium behavior. Furthermore, linear regression of data from Theo’s Flow and MIL 03346 gave a single calibration equation: ln k D = − 4040 ( ± 180 ) / T ( K ) + 1 . 12 ( ± 0 . 17 ) ( R 2 = 0 . 988 ) . This new calibration describes the equilibrium behavior of augite and can be reliably used to determine the closure temperature ( T c ) of augite with composition ranging between Fs 9 and Fs 24 .

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.

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).

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.

Nanoscale building blocks in a novel lithium arsenotungsten bronze: Synthesis and characterization

We report on a novel compound Li3AsW7O25 obtained by solid-state reaction and characterized by diffraction and spectroscopic methods. The bronze-type compound crystallizes in the orthorhombic space group Pbca with a=724.38(3)pm, b=1008.15(4)pm, c=4906.16(17)pm and Z=8. The structure is built up by chains of WO6 octahedra interconnected by AsO4 tetrahedra and WO6 octahedra forming a polyhedral arrangement as seen in intergrowth tungsten bronzes. The X-ray single crystal structure refinement allows solving the complex arsenotungstate framework. The powder neutron diffraction data analysis locates the lithium atoms. Thermal analysis showed that Li3AsW7O25 is stable up to its melting at 1135(3)K followed by a decomposition at 1182(5)K. The Kubelka–Munk treatment of the UV–vis spectrum revealed a wide band gap in the range of 2.84–3.40eV depending on the presumed electron transition type.

Optical absorption spectroscopy study of the causes for color variations in natural Fe-bearing gahnite: Insights from iron valency and site distribution data

Four gahnite single crystals with variable colors from pale blue to green have been studied by a multi-analytical approach with the aim to evaluate existing assignments of optical absorption bands. Combined information from electron microprobe analyses, Mossbauer spectroscopy, IR-spectroscopy, single-crystal X-ray structure refinements, and optical absorption spectroscopy confirms the conclusions of earlier studies that the absorption bands recorded in the visible spectral region up to ~540 nm (above ~18 500 cm −1 ) are related to electronic d-d transitions in tetrahedrally coordinated Fe 2+ . It also demonstrates that a set of absorption bands between ~550–625 nm (~16 000–18 200 cm −1 ) are caused by spin-allowed and spin-forbidden d-d transitions in tetrahedrally coordinated Co 2+ . Two absorption bands at higher wavelengths (~680 and ~800 nm, i.e., ~14 700 and ~12 500 cm −1 ) are assigned to electronic transitions in exchange coupled VI Fe 3+ - IV Fe 2+ pairs and a band at ~950 nm (~10 500 cm −1 ) is assigned to a spin-allowed electronic transition in VI Fe 2+ . Low-Fe gahnite crystals owe their blue color to traces of cobalt at concentration levels in the order of 200 ppm and less, while the green color of gahnite crystals with higher Fe-contents is due to a combination of electronic ligand-metal transitions causing strong UV-absorption and electronic transitions in exchange coupled Fe 2+ -Fe 3+ cation pairs that absorb in the red region of the visible spectrum. A detailed characterization of samples that includes cation site occupancy and iron valency data is demonstrated to be crucial for interpreting optical absorption spectra. Also electronic transitions in trace element chromophores below the detection limit of electron microprobe may participate to light absorption. All this information contribute to the comprehension of the causes of crystal color of minerals, gemstones, and ceramic pigments.

Temperature dependence of Uiso constraints in riding hydrogen treatments

"Anisotropic parametrisation of the thermal displacements of hydrogen atoms in single-crystal X-ray structure refinement is not possible with independent atom model (IAM) scattering factors. This is due to the weak scattering contribution of hydrogen atoms. Only when aspherical scattering factors are used can carefully measured Bragg data provide such information. For conventional structure determinations parameters of ""riding"" hydrogen atoms are frequently constrained to values of their ""parent"" heavy atom. Usually values of 1.2 and 1.5 times X-U_eq are assigned to H-U_iso in these cases. Such constraints yield reasonable structural models for room-temperature data. However, todays small molecule X-Ray diffraction experiments are usually carried out at significantly lower temperatures. To further study the temperature dependence of ADPs we have evaluated several data sets of N-Acetyl-L-4-Hydroxyproline Monohydrate at temperatures ranging from 9 K to 250 K. Methods compared were HAR [1], Invariom refinement [2], time-of-flight Neutron diffraction and the TLS+ONIOM approach [3]. In the TLS+ONIOM approach non-hydrogen ADPs from Invariom refinement provided ADPs for the TLS-fit. Hydrogen atoms in all methods were grouped and analyzed according to their Invariom name. We reach a good agreement of the temperature dependence of H-U_iso/X-U_eq. At very low temperatures the ratio H-U_iso/X-U_eq can be as high as 4, e.g. for Hydrogen attached to a sp3 carbon atom with three non-Hydrogen atom neighbors. Since all methods consistently show that the H-U_iso/X-U_eq ratio is temperature dependent, this effect should be taken into account in conventional structure determinations."

Growth, structural and magnetic characterization of Al-substituted barium hexaferrite single crystals

Large single crystals of aluminum-substituted M-type barium hexaferrite BaFe12−xAlxO19 were obtained from carbonate flux. The Al content in the crystals can be controlled via the Al content of the flux up to x=1.1 according to single crystal X-ray structure refinements. Al shows a distinct preference to substitute Fe on crystallographic sites with high coordination numbers by oxygen atoms, whereas no significant amounts of Al can be found on a tetrahedrally coordinated site. An increasing amount of the aluminum dopant results in a monotonous reduction of the Curie temperature from 440 to 415°C and the saturation magnetization at room temperature from 68 to 57emu/g for single crystal and from 61 to 53emu/g for powder samples.

Ion substitutions and structural adjustment in Cr-bearing tourmalines

The geological settings of Cr-bearing tourmaline occurrences are reviewed and the crystal chemistry of Cr-bearing tourmalines is analysed on the basis of new X-ray single-crystal structure refinements ( R 0.019–0.025) of three crystals from the Ural deposits (Cr 2 O 3 content ~6 wt.%) and one from Umba Valley, Tanzania (~1 wt.% Cr 2 O 3 ) and published data (13 structures) on tourmalines from different deposits with Cr content up to 36 oxide wt.%. The boundary between the ordered and the disordered distribution of Cr between Y and Z octahedral sites is determined near 1.03 ± 0.27 Cr apfu and relationships between Cr content and structural parameters (unit-cell parameters, octahedral bond-lengths, charge of Y and Z sites and polyhedral deformation) are established. An increasing Cr content in tourmaline causes an increase of both lattice parameters, of and bond-lengths and a regular variation of the charge of the Y and Z octahedra. In general, the structure becomes less deformed by reducing the distortion of the octahedra and of the tetrahedral ring.

In situ dehydration behavior of veszelyite (Cu,Zn)2Zn(PO4)(OH)3{middle dot}2H2O: A single-crystal X-ray study

The rare mixed copper-zinc phosphate mineral veszelyite (Cu,Zn)2Zn(PO4)(OH)3·2H2O space group P21/c, a = 7.5096(2), b = 10.2281(2), c = 9.8258(2) A, β = 103.3040(10)°, V = 734.45(3) A3 was investigated by in situ temperature-dependent single-crystal X-ray structure refinements. The atomic arrangement of veszelyite consists of an alternation of octahedral and tetrahedral sheets. The Jahn-Teller distorted CuO6 octahedra form sheets with eight-membered rings. The tetrahedral sheet composed of PO4 and ZnO3(OH) tetrahedra shows strong topological similarities to that of cavansite, gismondine, and kipushite.Diffraction data of a sample from Zdravo Vrelo, near Kresevo (Bosnia and Herzegovina) have been measured in steps of 25 up to 225 °C. Hydrogen positions and the hydrogen-bond system were determined experimentally from the structure refinements of data collected up to 125 °C. At 200 °C, the hydrogen-bonding scheme was inferred from bond-valence calculations and donor-acceptor distances. The hydrogen-bond system connects the tetrahedral sheet to the octahedral sheet and also braces the Cu sheet.At 150 °C, the H2O molecule at H2O2 was released and the Cu coordination (Cu1 and Cu2) decreased from originally six- to fivefold. Cu1 has a square planar coordination by four OH groups and an elongate distance to O3, whereas Cu2 has the Jahn-Teller characteristic elongate bond to H2O1. The unit-cell volume decreased 7% from originally 734.45(3) to 686.4(4) A3 leading to a formula with 1 H2O pfu. The new phase observed above 150 °C is characterized by an increase of the c axis and a shortening of the b axis. The bending of T-O-T angles causes an increasing elliptical shape of the eight-membered rings in the tetrahedral and octahedral sheets. Moreover a rearrangement of the hydrogen-bond system was observed.At 225 °C, the structure degrades to an X-ray amorphous residual due to release of the last H2O molecule at H2O1. The stronger Jahn-Teller distortion of Cu1 relative to Cu2 suggests that Cu1 is fully occupied by Cu, whereas Cu2 bears significant Zn. H2O1 is the fifth ligand of Cu2. Zn at Cu2 is not favorable to adopt planar fourfold coordination. Thus, if the last water molecule is expelled the structure is destabilized.This study contributes to understanding the dehydration mechanism and thermal stability of supergene minerals characterized by Jahn-Teller distorted octahedra with mixed Cu, Zn occupancy.

Crystal chemistry and hydrogen bonding of rustumite Ca10(Si2O7)2(SiO4)(OH)2Cl2 with variable OH, Cl, F

Three samples of the skarn mineral rustumite Ca10(Si2O7)2(SiO4)(OH)2Cl2, space group C 2/ c , a ≈7.6, b ≈ 18.5, c ≈ 15.5 A, β ≈ 104°, with variable OH, Cl, F content were investigated by electron microprobe, single-crystal X-ray structure refinements, and Raman spectroscopy. “Rust1_LCl” is a low chlorine rustumite Ca10(Si2O7)2(SiO4)(OH1.88F0.12)(Cl1.28,OH0.72) from skarns associated with the Rize batholith near Ikizedere, Turkey. “Rust2_F” is a F-bearing rustumite Ca10(Si2O7)2(SiO4)(OH1.13F0.87) (Cl1 96OH0.04) from xenoliths in ignimbrites of the Upper Chegem Caldera, Northern Caucasus, Russia. “Rust3\_LCl\_F” represents a low-Cl, F-bearing rustumite Ca10(Si2O7)2[(SiO4)0.87(H4O4)0.13](OH1.01F0.99) (Cl1.00 OH1.00) from altered merwinite skarns of the Birkhin massif, Baikal Lake area, Eastern Siberia, Russia. Rustumite from Birkhin massif is characterized by a significant hydrogarnet-like or fluorine substitution at the apices of the orthosilicate group, leading to specific atomic displacements. The crystal structures including hydrogen positions have been refined from single-crystal X-ray data to R1 = 0.0205 (Rust1\_LCl), R1 = 0.0295 (Rust2\_F), and R1 = 0.0243 (Rust3\_LCl\_F), respectively. Depletion in Cl and replacement by OH is associated with smaller unit-cell dimensions. The substitution of OH by F leads to shorter hydrogen bonds O-H⋯F instead of O-H⋯OH. Raman spectra for all samples have been measured and confirm slight strengthening of the hydrogen bonds with uptake of F. This study discusses the complex crystal chemistry of the skarn mineral rustumite and may provide a wider understanding of the chemical reactions related to contact metamorphism of limestones.

Cu- and Mn-bearing tourmalines from Brazil and Mozambique: crystal structures, chemistry and correlations.

Cu- and Mn-bearing tourmalines from Brazil and Mozambique were characterised chemically (EMPA and LA-ICP-MS) and by X-ray single-crystal structure refinement. All these samples are rich in Al, Li and F (fluor-elbaite) and contain significant amounts of CuO (up to ~1.8 wt%) and MnO (up to ~3.5 wt%). Structurally investigated samples show a pronounced positive correlation between the <Y-O> distances and the (Li + Mn2+ + Cu + Fe2+) content (apfu) at this site with R 2 = 0.90. An excellent negative correlation exists between the <Y-O> distances and the Al2O3 content (R 2 = 0.94). The samples at each locality generally show a strong negative correlation between the X-site vacancies and the (MnO + FeO) content. The Mn content in these tourmalines depends on the availability of Mn, on the formation temperature, as well as on stereochemical constraints. Because of a very weak correlation between MnO and CuO we believe that the Cu content in tourmaline is essentially dependent on the availability of Cu and on stereochemical constraints.

Crystal structure and phase stability of the Φ phase in the Al–Mg–Zn system

We report on the phase equilibria, the homogeneity range, the crystal and electronic structure of the Φ phase in the Al–Mg–Zn system. The homogeneity range is similar at 360 °C and 330 °C and has a wedge-like shape. Al and Zn vary about 13 at.% and Mg maximal 2.5 at.%. The crystal structure has been characterized by X-ray single crystal structure refinement at two compositions Al23.2Mg54.6Zn22.2 ((Al1.856Mg0.368Zn1.776)Mg4, Z = 19, oP152, Pbcm, a = 8.9374(7) Å, b = 16.812(2) Å, c = 19.586(4) Å) and Al17.1Mg53.4Zn29.5 (Al1.368Mg0.272Zn2.360)Mg4, a = 8.8822(3) Å, b = 16.7741(7) Å, c = 19.4789(8) Å). The structure contains four different types of layers stacked along the c axis which can be classified as pentagon–boat, boat, pentagon–triangle and square–triangle tilings. These tilings are decorated by Al and Zn centred icosahedra. The valence band structure can be described by a nearly free electron model except the 3d Zn contributions at lower energy. The large solubility for Al and Zn is caused by substitutional disorder at the icosahedral sites. Total energy calculations indicate, that the homogeneity range at higher temperatures splits up at 0 K into two ordered structures with compositions Al16Mg50Zn34 and Al10Mg50Zn40.