Crystal-chemical Features Research Articles

Bond ionicities of high- Tc oxides

A semiempirical method for the calculation of the bond ionicity, which is applicable even to extremely anisotropic systems such as copper-oxide superconductors, is proposed as a generalization of the Phillips-Van Vechten-Levine scheme. The value of the ionicity calculated for oxides generally tends to increase as the crystal strain becomes more tensile. It is characteristic of cuprate superconductors that the values of the ionicity are high compared with other 3d transition-metal oxides. Also significant are the extremely high ionicities in the direction normal to the CuO 2 planes and the relatively high covalencies of the intraplanar bonds. These crystal-chemical characteristics may be intimately related to the remarkable insulator-to-metal transition and the associated high- T c superconductivity in the layered copper-oxide systems.

Synthesis, Structures, and Characterization of MMnZrF 7 ( M = Tl, Rb, NH4, K) Fluorides: An Example of 7-Coordination of Divalent Manganese

A series of phases with formula M IMnZrF 7(M I = Tl, Rb, NH 4, K) was synthesized and their crystal chemical characteristics were determined from X-ray powder diffraction data. The structures of both Tl and Rb representatives were refined from single crystal X-ray diffraction data. Both Tl and Rb fluorides are isostructural and crystallize in the monoclinic system with unit cell dimensions a = 6.450(1) Å, b = 8.250(1) Å, c = 6.459(2) Å, β = 118.0(2)° and a = 6.439(3) Å, b = 8.218(2) Å, c = 6.443(1) Å, β = 117.9(2)° respectively, P 2 1/ m (No. 11) space group, Z = 2. This structural type is built of chains of edge-shared alternated pentagonal bipyramids of Mn 2+ and Zr 4+ extending along the [101] direction, chains that are linked together by sharing corners to form (MnZrF 7) n- n layers perpendicularly to the b axis. Layers are connected through apical fluorine atoms of the pentagonal bipyramidal groups, into a three-dimensional network delimiting pseudohexagonal tunnels parallel to the [010] direction, within which the M I monovalent cations are located. Structural relationships with KIn 2F 7, β-KYb 2F 7 and β-U 3O 8 structures are discussed.

Crystal-chemistry of a complex Mn-bearing alkali amphibole ("tirodite") on the verge of exsolution

Electron- and ion-microprobe and Mossbauer analyses combined with X-ray structure refinement of a straw-yellow Mn-bearing alkali amphibole from Tirodi, Maharashtra, India, with no evidence of exsolution or inclusions, indicate a composition (Na 0.30 K 0.03 )(Na 0.87 Ca 0.39 Mn 0.57 Mg 0.17 )(Mg 3.84 Fe 3+ 0.72 Mn 0.38 Li 0.06 )(Si 7.88 Al 0.12 ) O 22 F 0.40 (OH) 1.60 , nearly half-way between (Ca-Na) and (Fe-Mg-Mn) amphiboles. The unit-cell dimensions are: a = 9.704 (7), b = 17.990 (12), c = 5.297 (3) A, β = 103.51 (5)°; space group C2/m and Z = 2. The single-crystal X-ray structure refinement (1363 reflections, R all = 0.039) was done to characterize the crystal-chemical features that might act as indicators of incipient exsolution phenomena due to immiscibility between (Ca-Na) and (Fe-Mg-Mn) amphibole compositions. An 57 Fe Mossbauer spectrum shows all iron to be in the trivalent state. All manganese is in the divalent state, and preferentially occurs at the split M4' site. The value of the β angle is related to the cation content at M4, and is consistent with the intermediate composition. C Mn 2+ is distributed almost equally over the three octahedral sites M1, M2, M3. A small amount of Li (detected by ion-microprobe) is ordered at the M3 site with concomitant substitution of Fe 3+ at the M2 site for charge balance

Asbecasite: crystal structure refinement and crystal chemistry

AbstractThe crystal structure of antimonian asbecasite in an ejectum of hypabyssal origin occurring at Tre Croci near Vetralla, Vico volcanic complex, Roman potassic province, Latium, Italy, has been refined to R = 0.042, and is compared to the original crystal structure determination carried out on the Sb-free asbecasite of hydrothermal metamorphic origin from the type-locality, Cherbandung in Binna valley, Monte Leone nappe, Switzerland. New electron microprobe analyses of samples from both localities demonstrate crystal-chemical features that permit distinction between asbecasites from the two occurrences, so far the only known localities for this mineral.

Crystal chemistry of Ba-rich trioctahedral micas-\M

Ten single-crystal X-ray structure refinements of igneous and metamorphic Ba-bearing micas were determined to define the exchange vectors required to compensate for the excess in positive layer charge caused by [XII] Ba 2+ for [XII] K + substitution. The determined exchange vector is BaAlK −1 Si −1 . The micas have a Ba 2+ content in the range 0.01-1.09 apfu (atoms per formula unit) with octahedra mostly populated by Mg 2+ (2.21 ≤ Mg 2+ ≤ 3.07 apfu) and Fe 2+ (0.13 ≤ Fe 2+ ≤ 1.53 apfu). All samples are 1M polytype and the structure were refined in space group C2/m (0.020 ≤ R ≤ 0.034). lncreasing the substitution of [IV] Al 3+ for [IV] Si 4+ distorts the tetrahedra by progressively increasing the dimensions of the basal edges. This increase in the lateral dimensions of the tetrahedral sheet allows for a greater rotation angle α (5.9 ≤ α ≤ 11.5). Octahedral sheet crystal chemical features (ordering pattern, size and distortion of both M(1) and M(2) sites) are similar to those reported for micas in the phlogopite-annite join. The most notable variation involves the length of M(1) unshared edges, which decreases in samples with the highest [XII] Ba 2+ content

Crystal-chemical features of phases with the A15 structure

Since Hume-Rothery factors are important in the stability of bounded and unbounded solid solutions and many ordered and intermediate phases, an attempt has been made to study the effect of the size factor and the electron density on the range of binary phases with the A15 structure. It has been found that a ±10% deviation of the atomic radii is predominant and the electron concentration is limited, its value depending on the electron configurations of the constituent components.

Trace and rare earth elements in cassiterite ? sources of components for the tin deposits of the Mole Granite, Australia

Trace and rare earth elements have been determined for cassiterite from deposits associated with the Mole Granite and hosted by granite, metasediments and metavolcanics. The ΣREE of cassiterite is controlled by the ΣREE of the the ore fluid and the rocks through which this fluid circulated. The REE distribution factor and ΣLREE/HREE value of cassiterite is strongly influenced by the associated mineral assemblage, the fluid chemistry and the crystal chemical characteristics of the host mineral. Cassiterite from deposits hosted by granite have trace and rare earth element characteristics similar to those determined for the Mole Granite. Cassiterite from deposits hosted by metasediments or acid volcanics have most trace and rare earth element characteristics similar to those of the enclosing rocks and some characteristics similar to the Mole Granite. The ore fluid had chemical components derived from the parental granite and components acquired by passage through the metamorphosed aureole.

Optical and m�ssbauer study of minerals of the eudialyte group

Twelve eudialyte specimens from the Khibini alkaline complex (Kola peninsula, USSR) have been studied by optical and Mossbauer spectroscopy methods. It has been found that in optically positive eudialytes Fe is mainly represented by Fe2+ ions in a rare planar four-fold coordination (Mossbauer hyperfine parameters — isomer shift IS=1.08 mm/s and quadrupole splitting QS=0.34 mm/s). It is these ions, not Mn3+ as it was assumed before, that are responsible for the crimson-red colour of eudialyte. In the optically negative red-brown and yellow-brown eudialytes (eucolites) Fe2+ ions are contained also in a tetragonal pyramid based on the above planar quadrangle (IS=1.33 mm/s and QS=2.24 mm/s). Major attention has been paid to an explanation of the colour and crystal-chemical features of optically positive in comparison with optically negative varieties of eudialytes.

ADSORPTION OF CYANAZINE ON PEAT AND MONTMORILLONITE CLAY SURFACES1

The adsorption of cyanazine by peat and montmorillonite is studied as a function of the crystal chemical characteristics of exchangeable cations and temperature. The adsorption kinetics of cyanazine on peat shows that the adsorption equilibrium is reached within 1 h. The values of the rate constants of the reaction seem to indicate that the adsorption rate of cyanazine by peat is mainly controlled by the diffusion process. The order of adsorption of cyanazine by peat is as follows: H+-peat > Cu2+-peat > Ca2+-peat > Co2+-peat > Mg2+-peat > K+-peat. The thermodynamic parameters seem to point to an adsorption mechanism by hydrogen bonds, although in the acidic and Cu2+ samples a protonation process and adsorption of the protonated species is also likely. The values of the adsorption rate constant of cyanazine by montmorillonite is ten times higher than that for peat. The adsorption of cyanazine by montmorillonite follows this order: Cu2+-mont > Co2+-mont > H+-mont; the adsorption on Cu2+-mont is five times greater than on the other samples. For the Cu2+ samples, the thermodynamic parameters seem to point to a mechanism of direct coordination cyanazine-Cu2+. The most probable mechanism for the adsorption of cyanazine by the Co2+ sample is by physical bonding. For the H+ sample there is a physical adsorption and an ionic one according to the following reaction: H+-mont + cyanazine → mont – H+-cyanazine.

Radionuclide behavior at the Oklo nuclear reactor, Gabon

The Oklo natural reactor, Gabon, remains the best natural analog for assessing the behavior of fission products, actinides, and actinide daughters in rocks. The rocks at Oklo are porous and permeable, as well as fractured and contain abundant water, yet many of the fission products and actinides have remained in place or close to their formation sites. The actinides Th, U, Np, Pu, and Am are similar in their crystal chemical characteristics, and were all retained in the host pitchblende. Elements incompatible in the pitchblende structure were lost by diffusion or other processes into the rocks surrounding the high-grade reactor ore. Alkali and alkaline earth elements Rb, Sr, Cs, and Ba were fixed in and very close to the reactor ores in clay minerals and in some secondary carbonates and sulfates. Local oxidizing conditions in the reactor zones caused some loss of Tc, Mo, Cd, adn Ru, but Tc, Mo, and Ru, were fixed in sulfides formed under close-by chemically reducing conditions. Local migration for Ag and Sn has been documented for some samples. Probable overall retention is indicated for Rh, Ru, Te, Nb, Zr, and Sb. Deficiencies in fissiogenic I and Kr, Xe have been noted, and data are not available for fissiogenic Br. Also, Oklo remains a logical site for in-depth study of present- and paleohydrology-stable isotopy, as well as distribution of fission products and actinides in UO 2 and surrounding minerals.

Systems on Gallium Polyphosphate Base and Classification of Diagrams

Abstract The interaction of gallium polyphosphate with phosphates of alkali and alkali-earth metals has been studied for the first time. Synthesis of Ga(PO3)3 was accomplished by solid phase reaction. IR-spectra of Ga(PO3)3 shows it to belong to polyphosphates. Diagrams have been studied by the differential-thermal method. The systems-Ga(PO3) 3-LiPO3 and Ga(PO3), -NaPO3 are eutactic. New compounds with composition of 1:1, melting with decomposition KGa(PO3)4 at 76O°C, RbGa(PO3)4 at 81O°C, CsGa(PO3)4 at 745°C have been revealed in systems Ga(PO3)3-KPO3, Ga(PO3)3-RbPO3, Ga(PO3)3-GsPO3. Analysis of IR-spectra of the isolated compounds made it possible to consider them as polyphosphates. The results of paper chromatography confirm polymer structure of anions of these compounds. The interaction of gallium polyphosphate with polyphospnates of alkali-earth metals shows the formation of eutactic type diagram. The data obtained as well as available in literature on diagrams of double systems from phosphates of univalent and three valent metals have been compared and analysed. The difference in properties of cations (charge, radius, polarization properties, covalency of bonds of metal-oxygen, crystal-chemical characteristics) are taken as a criterion of the interaction of components. This approach made it possible to compose the classification of systems where two groups are distinguished: the one where Al, Ga, Gn, T1 are tnree valent cation (systems with Fe (III) and Mn(II1) belong to them); the other-Sc, Y, La and Ln. The classification given permits to make prognosis of uninvestigated systems.

Orientation-Dependent "Metamorphic Grade" in Phyllosilicates Belonging to a Slaty Cleavage Fabric

The intimate coexistence of two microstructurally defined phyllosilicate populations that have apparently achieved two different degrees of diagenetic/metamorphic maturity underscores the importance of chemical processes in slaty cleavage development. Petrographic observations and analytical data on phyllites from Ocoee Gorge, Tennessee, indicate that most of the cleavage-parallel phyllosilicates concentrated in thin lamellae grew during low-grade metamorphism. On the other hand, the bedding-plane parallel phyllosilicates found between lamellae are detrital or diagenetically altered grains that were in place before slaty cleavage developed. Electron microprobe and XRD analyses have been used to characterize the mica, interstratified mica/chlorite, and chlorite found in the two microstructurally defined populations. Cleavage-parallel phyllosilicates are homogeneous and have crystal chemical characteristics indicative of low-grade metamorphism. Bedding-parallel phyllosilicates, on the other hand, are more heterogeneous and appear to be detrital or diagenetic. Specific differences in the two groups include greater K content of cleavage-parallel mica and the greater chemical variability of bedding-parallel mica. Cleavage-parallel interstratified mica/chlorite is limited to a maximum of about 50% chlorite, whereas there is no limit on chlorite in bedding-parallel interstratified grains. In addition, the proportion of mica in the total phyllosilicates is higher parallel to cleavage than parallel to bedding. The differences between the cleavage-parallel and bedding-parallel phyllosilicates suggest that new grain growth and recrystallization of pre-existing grains were more important than passive concentration and rotation in the development of this fabric.

Studies of Tie Electronic Structure By XPS f HTSC and Related Compounds

AbstractMeasurements by photoemission spectroscopy have shown that the chemical environments of alkaline earth cations in HTSC compounds are different from the environments of these ions in the prototype perovskite structures. A recently discovered HTSC compound (Pb2Sr2Ca0.5Y0.5Cu3O8) exhibits the exceptional feature of having a typical HTSC signature for Sr, while Ca shows a perovskite-like signature. The crystal-chemical features of the relevant compounds have been correlated with these observations. These correlations may be indicative of inter-layer electronic interactions.

Chemistry of the rock‐forming silicates: Multiple‐chain, sheet, and framework structures

The crystal chemistry of 16 groups of multiple‐chain, sheet, and framework silicates is reviewed. Crystal structure drawings are presented to illustrate crystal chemical features necessary to interpret chemical data for each mineral group. The 16 silicate groups considered in this review are the amphibole; nonclassical, ordered pyriboles; mica; pyrophyllite‐talc; chlorite; greenalite; minnesotaite; stilpnomelane; prehnite; silica polymorphs; feldspar; nepheline‐kalsilite; leucite‐analcite; sodalite group; cancrinite group; and scapolite. Electron microprobe analyses should be augmented by independent determinations of Fe2+/Fe3+ and H2O for many of the silicate groups discussed and by determinations of CO32−, SO42−, S2−, and Li in some of the others. However, microprobe data augmented as suggested will still be ambiguous for some of the silicate groups considered here because the structures are not completely determined or are variable, with disparate domains and/or structural modulations, e.g., pyriboles, greenalite, minnesotaite, and stilpnomelane. Nevertheless, the most rigorous way to interpret silicate mineral chemical data is based on the crystal structures involved.

Structural singularities of superconducting Y-Ba-Cu oxides

Singular features of the orthorhombic or tetragonal array assigned to the high-T c superconducting phase YBa 2 Cu 3 O 7−δ are pointed out: order-disorder of oxygen atoms and vacancies, and subsequent formation of anti-phase and out-of-step domains; nature and reversible character of the orthorhombic/ tetragonal phase transition; distortion trends of yttrium coordination polyhedron. Sample preparation conditions - sintering temperature, annealing atmosphere, cooling regime - are commented in terms of bulk crystal chemical characteristics. Calculated bond valence summations for the orthorhombic structure are presented.

Chemistry of the rock‐forming silicates: Ortho, ring, and single‐chain structures

The crystal chemistry of 21 ortho, ring, and single‐chain silicate structures is reviewed. Structure drawings are presented to portray the essential crystal chemical features necessary to correctly interpret chemical data for each mineral or mineral group. The groups considered are olivine, humite, zircon, sphene, garnet, vesuvianite, aluminosilicate, topaz, staurolite, chloritoid, epidote, melilite, beryl, cordierite, tourmaline, axinite, pyroxene, pyroxenoid, pectolite, sapphirine, and aenigmatite. Electron microprobe data for each group can be interpreted with various degrees of rigor, depending on whether H2O, Fe2+/Fe3+, and other elements not detected by the microprobe are present. Serious mineral chemists are encouraged to determine Fe2+/Fe3+ and H2O directly. Estimates of these values using microprobe data are usually imprecise and can lead to serious errors in interpretation of the chemical formula and thus any thermodynamic or petrologic inferences that are based on the correct formula.

Very-high-pressure metamorphism in the western Alps: implications for subduction of continental crust

The widespread occurrence of coesite and pure pyrope, the presence of several new mineral assemblages and new rock-forming minerals in common lithologies, but also crystal—chemical features such as the existence of silicate—phosphate solid solutions or the repeated occurrence among the new minerals of a new dense structure with face-sharing octahedra, are evidence that unusual pressures have been attained in part of the Dora Maira massif, western Alps, during Alpine regional metamorphism. Mineral assemblages suggest minimum pressures well in excess of 25 kbar (1 bar = 10 5 Pa) which, according to preliminary experimental data, may have reached 35 kbar in this at least 5x10 km 2 continental terrain clearly of supra-crustal origin. Obviously even continental material may be buried to depths of the order 100 km and then uplifted to the surface. Whereas the burial in a low temperature regime is readily explained by subduction of the continental lithosphere (of a peninsula?) 110 Ma ago or earlier, the uplift is much more problematic because it is petrologically constrained to proceed without significant temperature increase. The progressive migration 'in-plate’ of intra-continental thrusts once subduction was blocked, accounts for the stepwise decrease in age and grade of high-pressure metamorphism toward the external part of the chain; by the repeated underthrusting of cold material it might also have prevented a temperature increase in the most internal, early subducted zones.

スビネル型構造中の原子の熱運動

Since RM2X4 compounds having a spine! structure have been payed attentions not only for earth science but also for material science, crystal chemical features of spinel structures are reviewed. Atomic movement due to thermal vibration of atoms at high temperature and/or under high pressure comes to be one of the important subjects for the comprehension of structure dynamics with respect to transition, decomposition and melt. Analyses of anharmonic thermal vibrations of atoms of MgAl2O4 (a=8.0806Å, z=8, S. G.=Fd 3m) have been conducted by means of X-ray single-crystal diffraction study at high temperatures up to 1700°C besides the correction of thermal diffuse scattering. Structure refinements of the spinel in the series of the corrections of thermal effects, (1) elipsoidal harmonic vibrations of atoms, (2) thermal diffuse scattering, (3) anharmonic thermal vibrations of atoms, are presented as a function of temperature. Several theoretical approaches to anharmonic thermal vibrations are also reviewed and compared with each other.

Synthesis of ceramic pigments on the basis of augite

In order to extend the range of ceramic colors on the basis of available raw materials the authors investigated ceramic pigments made from diopside and augite which are related to pyroxenes. The crystal-chemical features of pyroxenes [i, 2] and their wide isomorphism create the prerequisites for the synthesis of ceramic pigments using raw materials in which the oxides of calcium, magnesium, and silicon are combined in significant amounts.

Crystal Structures and Electron Induced Damage in β-Alumina Related Compounds

During the transmission electron microscope observations of β''- and β''''-alumina with rhombohedral symmetry, electron damage due to the loss of the conduction planes takes place to form the defect blocks ; whereas the hexagonal counterparts, β- and β'''-alumina are relatively stable. Based on the structure images taken by a high-resolution 1 MeV electron microscope, new structure models of the defect blocks are proposed in place of the previous two models which were mutually incompatible. The results are discussed in terms of the differences in the crystal-chemical characteristics between the rhombohedral and the hexagonal groups of compounds.