Ca Polyhedra Research Articles

A structural model for charoite

AbstractThe crystal structure of charoite was investigated mainly by using selected-area electron diffraction (SAED), X-ray diffraction (XRD) and high-resolution electron microscopy (HREM). SAED and XRD patterns indicate that the structure has a monoclinic cell: a = 32.296, b = 19.651, c = 7.16 Å, β = 96.3° and V = 4517 Å3. The space group inferred from systematic absences and HREM images is P21/m. A model of the charoite structure is proposed that is based on the features of related Ca-alkaline silicate structures and HREM images. The structure of charoite consists of three different silicon-oxygen radicals (polymerized SiO4 tetrahedra) which are located between Ca polyhedra. Two of these radicals form continuous tubular structures comprising pectolite-like tetrahedral chains. Calcium polyhedra are joined to form blocks, each of which consists of four columns sharing edges and apices. Potassium and H2O molecules are probably located inside the tubular silicate radicals. From these results, a general formula is derived: K6-7(Ca,Na)18[(Si6O17)(Si12O30)(Si18O45)](OH,F)2.nH2O with two formula units in the unit cell (Z = 2).

The compressibility and high pressure structure of diopside from first principles simulation

The structure of diopside (CaMgSi2O6) has been calculated at pressures between 0 and 25 GPa using the planewaves and pseudopotentials approach to density functional theory. After applying a pressure correction of 4.66 GPa to allow for the under-binding usually associated with the generalized gradient approximation, cell parameters are in good agreement with experiment. Fitting to the third-order Birch–Murnaghan equation of state yields values of 122 GPa and 4.7 for the bulk modulus and its pressure derivative. In addition to cell parameters, our calculations provide all atomic positional parameters to pressures considerably beyond those currently available from experiment. We have analyzed these data in terms of polyhedral rigidity and regularity and find that the most compressible Ca polyhedron becomes markedly less anisotropic above 10 GPa.

STRONTIOGINORITE: CRYSTAL-STRUCTURE ANALYSIS AND HYDROGEN BONDING

Strontioginorite, ideally SrCaB 14 O 20 (OH) 6 ·5H 2 O, from the Potash Corporation of Saskatchewan (New Brunswick Division) mine at Penobsquis, Kings County, New Brunswick, occurs in the Upper Halite member of the Windsor Group evaporites, and is associated with halite, sylvite, hilgardite, volkovskite and trembathite. Electron-microprobe analysis, supported by a single-crystal analysis of its structure, yield: CaO 7.47 (6.66–8.10), SrO 12.23 (11.30–13.69), (B 2 O 3 ) 61.22 and (H 2 O) 18.10, for a total of 99.02 wt.%. The empirical formula, based on 31 anions, is Ca 0.94 Sr 1.06 B 14 O 20 (OH) 6 ·5H 2 O. Strontioginorite is monoclinic, P 2 1 / a , with refined unit-cell parameters a 12.8171(4), b 14.4576(4), c 12.8008(4) A, β 101.327(1)°, V 2325.8(2) A 3 , Z = 4. The crystal structure refined to an R index of 0.024 for 6,784 unique, observed (>4σ F o ) reflections. The (010) sheets of borate polyhedra are weakly cross-linked by Sr and Ca polyhedra. The H 2 O and OH groups strengthen the cross-linkage with hydrogen bonding. The fundamental building block (FBB) within the structure, 8Δ6□:[ϕ] | | | −[ϕ] | | | −2Δ, is compared to that in the structure of other complex borates, such as nobleite and strontioborite.

Crystal structure of new synthetic calcium pentaborate Ca[B5O8(OH)] · H2O and its relation to pentaborates with similar boron-oxygen radicals

A new representative of pentaborates with the composition Ca[B5O8(OH)] · H2O was synthesized under hydrothermal conditions within the framework of the study of phase formation in the CaCl2-Na2CO3-B2O3 system. The crystal structure of the new pentaborate was established (a = 6.5303(9) A, b = 19.613(3) A, c = 6.5303(9) A, β = 119.207(2)°, V = 2513(2) A3, sp. gr. P21/c, Z = 4, d calcd = 2.74 g/cm3, automated Brucker SMART CCD diffractometer, 6871 reflections, λMo radiation, anisotropic refinement by least-squares, R hkl = 0.076). The structure of calcium pentaborate is built by isolated B-Ca-B stacks parallel to the (010) plane. The central fragments of these stacks consists of nine-vertex Ca polyhedra. The Ca layers are located between loose B-O networks composed of [B 2 t B 3 Δ O8(OH)]2− pentaborate groups. The arrangement of the polyhedra around large cations in pentaborates with groups of two B tetrahedra and three B triangles was analyzed in terms of crystal chemistry. It is established that the structures of these compounds consist of large isolated polyhedra, columns, layers, and three-dimensional frameworks.

Powder Rietveld refinement of armstrongite, CaZr[Si6O15]·3H2O

Crystal data and results of structure refinement using powder X-ray diffraction and Rietveld analysis for armstrongite, CaZr[Si6O15]·3H2O, are reported. Experimental data were collected using MoKα1 radiation over the range 2° - 51.98° 2θ. Armstrongite is monoclinic (space group C2, Z = 4), with a = 14.018(1) Å, b = 14.133(1) Å, c = 7.840(1) Å, β = 109.40(1)°. The crystal structure, reported by Kashaev and Sapozhnikov (1978) has been refined by Rietveld analysis, resulting in Rwp = 2.75. The structure is based on a heterogeneous octahedra-tetrahedra framework (ZrSi6O15) built by corrugated silicate sheets connected by ZrO6 octahedra. The seven-fold Ca polyhedron contains 2 H2O molecules. Two new sites for H2O molecules on the 2-fold axis in the voids of the zeolite like framework allowed to achieve the complete correspondence between chemical and structural data.

The pressure behavior of clinozoisite and zoisite; an X-ray diffraction study

Compressibility data of clinozoisite and zoisite were measured by single-crystal X-ray diffraction in a diamond-anvil cell up to a pressure of about 50 kbar. In both polymorphs, the unit cell parameters varied linearly with pressure but in an anisotropic pattern: l3a = 2.1(1) x 10-4, I3b= 2.8(1) x 10-4, 13,= 3.3(1) X 10-4 kbar-] for clinozoisite, and 13= 2.3(2) X 10-4, I3b= 2.9(1) x 10-4, 13,= 3.7(2) X 10-4 kbar-] for zoisite. The principal coefficients of the strain ellipsoid of clinozoisite are 13,= 2.0 x 10-4, 132 = 2.7 x 10-4, 133 = 3.3 X 10-4 kbar-]; 13]and 133were symmetrically oriented in the (010) plane with an angle of about 12° between 13]and the a axis, whereas 132 coincides with the b axis. Bulk moduli calculated as the reciprocal of cell-volume compressibility were 1300(20) kbar for the monoclinic and 1140(20) for the orthorhombic polymorph. Ko, determined by fitting the unit-cell parameters with a third-order Birch-Murnaghan equation of state, was 1270(45) kbar, with K' = 0.5(2) for clinozoisite and 1020(65) kbar with K' = 4.8(4) for zoisite. Structural refinements of clinozoisite performed at 0.5, 19.4, and 42 kbar, and also under ambient conditions, showed that the compression mechanism included both shrinking of the polyhedra (i.e., octahedra and Ca polyhedra) and tilting of the Si207 group, with reduction of the Si-O-Si angle. The different effect of these mechanisms explains the anisotropic compressional pattern in clinozoisite and the similar behavior observed in the two polymorphs. Comparison of high-pressure and high-temperature data for clinozoisite showed that a given increase in pressure produced structural effects very similar to those seen after a proportional decrease in temperature. The calculated volume-expansivity-to-compressibility ratio of 38 bar/K indicates that the cell volume of clinozoisite remains unchanged with geothermal gradients of about 10 °C/km. The crystallographic data support the results of experimental petrology in indicating that epidote is a good candidate for transporting H20 in down-going subduction slabs.

Molecular and crystal structure of 1?1 guest-host bis(hexafluoroacetylacetonato)calcium complex with 18-crown-6

We have carried out an X-ray structural study of the title complex (a CAD-4F diffractometer, λMoKα, graphite monochromator, ω/2Θ scan mode, θmax, direct methods, 1465 reflections, R=0.036). Crystals are monoclinic with a=17.971(13), b=13.475(3), c=13.379(8) A, β=106.67(5)°, Z=4CaO10C22H26F12, space group C2/c, d calc =1.537 g/cm3. The complex (C2 symmetry) has a molecular structure and belongs to the guest-host type. The Ca atom is located in the center of the 18-crown-6 cavity; bidentate hexafluoroacetylacetonate guest ligands occupy the trans-positions relative to the plane of a maxidentate macrocycle. The Ca−O distances in a ten-vertex Ca polyhedron are 2.474–2.666 A. The macrocycle conformation is characterized by six gauche C−C bonds and two gauche and ten trans C−O bonds. The dihedral COCC angle differs significantly (by 38.1°) from the angle of 60°, which is common to a gauche conformation. The six-membered cycle formed by oxygen atoms has a twist form with the annular O−O distances of 5.132–5.329 A. Structural features explaining an easy sublimation of the compound are discussed.

Structural modifications induced by dehydration in the zeolite gismondine

Gismondine from Montalto di Castro, Italy [Ca 3.91Al 7.77Si 8.22O 32·17.57 H 2O], a = 10.0199(4), b = 10.6373(5), c = 9.8316(5) A ̊ , β = 92.561(6)°, space group P2 1/ c, dehydrated in vacuum for 1 and 24 h and transformed into two new phases, here called gismondine (1 h) and gismondine (24 h), respectively. Gismondine (1 h) is characterized by 9.5% water loss and by a small decrease in the cell volume ( ΔV = 0.6%); cell parameters are a = 9.989(3), b = 10.616(3), c = 9.820(3) A ̊ , and β = 92.57(2)°. The framework is almost undistorted, but a rearrangement of water molecules causes a change in space group to P2 1, with formation of a more regular 6-coordinated Ca polyhedron. The final R w value (isotropic displacement factors) is 7.6%. Gismondine (24 h) is characterized by the orthorhombic space group P2 12 12 1 and a unit cell doubled with respect to the nondehydrated sample; cell parameters are a = 13.902(9), b = 8.892(4), and c = 13.952(5) A ̊ . More than 50% of water is lost, the framework is highly distorted, and the channels are strongly squashed. Residual water sites are fully occupied. Ca polyhedra are seven-fold coordinated and are linked by vertices to form infinite chains. The final R w value (isotropic displacement factors) is 7.6%.

ChemInform Abstract: Synthesis and Crystal Structure of the Mixed Double Orthopyrovanadate of Sodium and Calcium with the Composition Na3Ca2(VO4)(V2O7).

AbstractThe hitherto unknown title compound is prepared by heating a stoichiometric mixture of anhydrous Na2CO3, CaCO3 and NH4VO3 to 600 °C. The compound crystallizes in the space group C2/c with Z = 8. Its structure is composed of V2O74‐ and VO43‐ groups, which are linked by Ca polyhedra.

A new type of silicate double chain in agrellite, NaCa2Si4O10F

The most common type of silicate chain structure is the single silicate chain, formed by each silicate tetrahedron sharing two corners with two adjacent silicate tetrahedra as found in pyroxenes and pyroxenoids. Such single chains can polymerize further to form double chains as found in amphiboles. A different type of a silicate chain can be formed by silicate rings sharing tetrahedral corners. The only known example is the [Si40~1]oo chain in vlasovite, NaZrSi4Oll[1, 2], which is formed by four-membered silicate rings sharing corners. Agrellite provides the first example of a further polymerization of this chain into double chains, [Si4010]oo. Agrellite, a rare rock-forming silicate, occurs in regionally metamorphosed agpaitic alkalic rocks in Villedieu Township, Qu6bec, Canada [31. It is triclinic, space group Pi , with cell dimensions: a=7.759(2), b = 18.946(3), c=6.986(1)1k, ~=89.88(2), /~= 116.65(2), y=94.34(2) ~ and Z=4 . The crystal structure was determined by the symbolic-addition method and refined by the method of least squares to an R-factor of 0.054 based on 5343 x-ray reflections, measured on an automatic single-crystal diffractometer using MoK~ radiation. The crystal structure of agrellite consists of two crystallographically different [Si4Oto]oo double chains parallel to the c axis, two different Na polyhedra and four different Ca polyhedra. The silicate double chains are in fact hollow tubes, whose cross-sections are defined by chair-shaped six-membered rings (Fig. I). In addition, each silicate pipe contains two types of chair-shaped eight-membered rings. The sodium atoms connect the silicate pipes into layers, parallel to the ac plane; these layers alternate with the Ca polyhedral layers (also parallel to the ac plane), giving rise to a three-dimensional framework structure.