CaO8 Polyhedra Research Articles

Structural evolution of gypsum under high pressure: single-crystal X-ray experiments revisited

The structures of gypsum at pressures up to approximately 4 GPa are studied with density functional theory (DFT) and thoroughly compared with single-crystal X-ray diffraction experiments reported in the literature [Comodi et al. in (Am Miner 93:1530–1537, 2008)]. It is found that the exchange–correlation density functional revPBE (revised Perdew–Burke–Ernzerhof) in conjunction with a nonlocal van der Waals (vdW) correction is capable of modeling the lattice constants, axial compressibility, and bulk modulus with good accuracy, suggesting that the inclusion of the vdW functional is crucially important for understanding the structure of hydrous minerals. To gain further physical insights, the geometric parameters associated with the constituting components of gypsum (water molecules, SO4 tetrahedra, and CaO8 polyhedra) are analyzed and compared with the experimental values. DFT simulations show that, under pressure, the polyhedral layers remain as nearly planar sheets of interconnecting SO4 tetrahedra and CaO8 polyhedra without further crinkling. DFT analysis on the layer compressibility along the major crystal axis reveals that, in contrast to experimental reports, the hydrous interlayer is less compressible than the polyhedral layer. Squeezed by the lateral pressure, the water molecules in the hydrous interlayer become better affixed along the major axis, making the interlayer harder to compress along this axis.

Calciomurmanite, (Na, [])2 Ca(Ti, Mg, Nb)4 [Si2 O7 ]2 O2 (OH, O)2 (H2 O)4, a new mineral from the Lovozero and Khibiny alkaline complexes, Kola Peninsula, Russia

The new mineral calciomurmanite, (Na,□)2Ca(Ti,Mg,Nb)4[Si2O7]2O2(OH,O)2(H2O)4, a Na-Ca ordered analogue of murmanite, was found in three localities at Kola Peninsula, Russia: at Mt. Flora in the Lovozero alkaline complex (the holotype) and at Mts. Eveslogchorr (the cotype) and Koashva, both in the Khibiny alkaline complex. Calciomurmanite is a hydrothermal mineral formed as a result of late-stage, low-temperature alteration (hydration combined with natural cation exchange) of a high-temperature, anhydrous phosphate-bearing titanosilicate, most likely lomonosovite and/or betalomonosovite, in the peralkaline (hyperagpaitic) rocks. The holotype sample is associated with microcline, aegirine, lorenzenite and fluorapatite, whereas the cotype sample occurs with microcline, aegirine, lamprophyllite, tsepinite-Ca and tsepinite-K. The mineral occurs as lamellae up to 0.1 × 0.4 × 0.6 cm, sometimes combined in fan-shaped aggregates up to 3.5 cm. Calciomurmanite is pale brownish or purple; the streak is white. The lustre is nacreous on cleavage surface and greasy on broken surface across cleavage. The (001) cleavage is perfect, mica-like; the fracture is stepped. D meas = 2.70(3), D calc = 2.85 g cm−3. The mineral is optically biaxial (–), α = 1.680(4), β = 1.728(4), γ = 1.743(4), 2 V meas = 58(5)°. The IR spectrum is reported. The chemical composition (wt%, electron-microprobe data, H2O by the Alimarin method) is: Na2O 5.39, K2O 0.30, CaO 7.61, MgO 2.54, MnO 2.65, FeO 1.93, Al2O3 0.85, SiO2 30.27, TiO2 29.69, Nb2O5 6.14, P2O5 0.27, H2O 11.59, total 99.23. The empirical formula of the holotype sample, calculated on the basis of Si + Al = 4 apfu , is: Na1.34Ca1.04K0.05Mg0.49Mn0.29Fe0.21Nb0.36Ti2.85(Si3.87Al0.13)Σ4O16.40(OH)1.60(PO4)0.03(H2O)4.94. Calciomurmanite is triclinic, P -1, a = 5.3470(6), b = 7.0774(7), c = 12.146(1) A, α = 91.827(4), β = 107.527(4), γ = 90.155(4)°, V = 438.03(8) A3 and Z = 1. The strongest reflections of the X-ray powder pattern [ d ,A( I )( hkl )] are: 11.69(100)(001), 5.87(68)(011, 002), 4.25(89) (-1-11, -111), 3.825(44)(-1-12, 003, -112), 2.940(100)(-1-21, -121), 2.900(79)(004, 120). The crystal structure was solved by direct methods from single-crystal low-temperature (200 K) X-ray diffraction data and refined to R = 0.0656 for the holotype and 0.0663 for the cotype. The structure is based on a three-sheet HOH block: an octahedral ( O ) sheet containing alternating chains of NaO6 and TiO6 octahedra and two heteropolyhedral ( H ) sheets consisting of Si2O7 groups, TiO6 octahedra and CaO8 polyhedra. H2O molecules occupy two sites in the interlayer space.

Crystal structure of whiteite-(CaFeMg) from Crosscut Creek, Canada

The structural characterization of whiteite-(CaFeMg), a natural orthophosphate from Crosscut Creek, Yukon, Canada, with chemical composition (Ca0.86Na0.05)Σ = 0.91(Fe0.88Mn0.02)Σ = 0.90Mg2.17Al1.93(PO4)4(OH)2 · 8 H2O obtained by WDS-EMPA, was carried out by means of single crystal X-ray diffraction. The structure was solved in the P2/a monoclinic space group, with the following unit cell constants: a = 14.8700(15), b = 6.9785(5), c = 9.9268(10) Å, β = 110.110(1)°, V = 967.31(15) Å3. Structure refinement lead to the crystal chemical formula (Ca0.95Na0.11)Σ = 1.06Fe0.90Mg2.01Al2.05 (PO4)4(OH)1.99 · 8.17 H2O. Phosphorus atoms display tetrahedral (PO4) coordination, while magnesium, iron(II) and aluminum display regular octahedral coordination; calcium displays a complex CaO8 coordination. MgO2(H2O)4, FeO6, AlO4(OH)2, CaO8 polyhedra and PO4 groups are arranged such as to form a three-dimensional framework via edge- and vertex-sharing arrangements. Strong O—H … O hydrogen bonds contribute to stabilize the array: such interactions were also investigated by means of Libowitzky formula (Libowitzky, 1999) application, owing to the results of IR spectroscopy.

Crystal structure of dicalcium chromate hydrate

Direct methods and Rietveld analysis were applied to high-resolution synchrotron X-ray powder diffraction data to solve the crystal structure of dicalcium chromate hydrate (Ca2CrO5⋅3H2O). The compound crystallizes in monoclinic symmetry (space group Cm, Z=2), with a=8.23575(5) Å, b=7.90302(4) Å, c=5.20331(3) Å, and β=98.0137(3)°. The structure is built from double-layers of CrO4 tetrahedra and CaO8 polyhedra that run parallel to the (001) plane.