Six-membered Rings Of Tetrahedra Research Articles

Crystal chemical models for the cancrinite-sodalite supergroup: the structure of a new 18-layer phase

Abstract The framework of the minerals belonging to the cancrinite-sodalite supergroup is very flexible and adapts itself to a wide range contents of extra-framework anions, giving rise to complex structures with different stacking sequences of layers that contain six-membered rings of tetrahedra. The knowledge of the crystal chemistry of these minerals was the key for the structure solution of a new phase showing a sequence with 18 layers. Geometrical and chemical constraints were imposed to the positions of the six-member rings so limiting the number of possible non-equivalent sequences of the 18 layers. The more probable sequences were single out on the basis of the chemical data and used as starting model vs. single crystal X-ray intensity data. The new 18-layer phase, ideal chemical formula (Na, K, Ca)72(Si54Al54O216)(SO4)16(Cl, H2O)8, Z = 1, space group P3, a = 12.904(2) Å, c = 47.802(4) Å, displays a stacking sequence ABCABACACABABACABC, Zdhanov symbol (8)211(2)112. The examined crystal was found in the sample MMUR (Museo di Mineralogia dell’Università di Roma “La Sapienza”) 24320, associated with sacrofanite, which is the 28-layer member of the supergroup. The sample was collected at Sacrofano, (Latium, Italy) in a volcanic ejected block, which originated from a pneumatolitic-pyroclastic explosion.

KIRCHHOFFITE, CsBSi2O6, A NEW MINERAL SPECIES FROM THE DARAI-PIOZ ALKALINE MASSIF, TAJIKISTAN: DESCRIPTION AND CRYSTAL STRUCTURE

Kirchhoffite, CsBSi 2 O 6 , is a new species of silicate mineral from the Darai-Pioz alkaline massif in the upper reaches of the Darai-Pioz River, in the area of the joint Turkestansky, Zeravshansky and Alaisky ridges, Tajikistan. It occurs as equant grains 10 to 80 μm in diameter, associated with quartz, pectolite, baratovite, polylithionite, aegirine, fluorite, leucosphenite, pyrochlore, neptunite and reedmergnerite. Kirchhoffite is colorless, transparent with a white streak, and a vitreous luster; it does not fluoresce under ultraviolet light. No cleavage or parting was observed. The Mohs hardness is 6 to 6½, and it is brittle with a conchoidal fracture. The measured and calculated densities are 3.62(2) and 3.639 g/cm 3 , respectively. Kirchhoffite is uniaxial (+) with indices of refraction ω = 1.592(2), ɛ = 1.600(2). The mineral is tetragonal, space group I 4 1 / acd , a 13.019(2), c 12.900(3) A, V 2186.3(1) A 3 , Z = 16, a : c = 1 : 0.9909. The six strongest lines in the X-ray powder diffraction pattern are [ d in A( I )( hkl )] are 3.26(100) (040), 3.48(82)(132), 2.770(67)(332, 233), 2.294(41)(044), 2.109(34)(352, 253), 5.32(32)(121). Chemical analysis by electron and ion microprobes gave SiO 2 40.47, B 2 O 3 11.27, K 2 O 0.11, Cs 2 O 48.16, Rb 2 O 0.09, for a sum of 100.10 wt.%. The resulting empirical formula on the basis of six atoms of oxygen is (Cs 1.02 K 0.01 ) ∑1.03 B 0.96 Si 2.02 O 6 , ideally CsBSi 2 O 6 . The crystal structure of kirchhoffite was refined to an R 1 index of 3.1% based on 487 observed reflections collected on a four-circle diffractometer with Mo K α X-radiation. In the structure, there are two tetrahedrally coordinated sites: Si is occupied by silicon, Si –O> = 1.610 A, B is occupied by boron, B –O> = 1.465 A, and Cs is [12]-coordinated, = 3.301 A. Tetrahedra form a [BSi 2 O 6 ] framework, the topology of which is identical to that of tetragonal pollucite, CsAlSi 2 O 6 . The [12]-coordinated Cs atoms occupy the channels along [111] formed by six-membered rings of tetrahedra. Kirchhoffite, CsBSi 2 O 6 , is a B analogue of the tetragonal modification of pollucite.

Marinellite, a new feldspathoid of the cancrinite-sodalite group

Marinellite, [(Na,K) 42 Ca 6 ](Si 36 Al 36 O144)(SO 4 ) 8 Cl 2 .6H 2 O, cell parameters a = 12.880(2) A, c = 31.761(6) A, is a new feldspathoid belonging to the cancrinite-sodalite group. The crystal structure of a twinned crystal was preliminary refined in space group P 31 c , but space group P 62 c could also be possible. It was found near Sacrofano, Latium, Italy, associated with giuseppettite, sanidine, nepheline, hauyne, biotite, and kalsilite. It is anhedral, transparent, colourless with vitreous lustre, white streak and Mohs9 hardness of 5.5. The mineral does not fluoresce, is brittle, has conchoidal fracture, and presents poor cleavage on {001}. D meas is 2.405(5) g/cm 3 , D calc is 2.40 g/cm 3 . Optically, marinellite is uniaxial positive, non-pleochroic, ω = 1.495(1), ϵ = 1.497(1). The strongest five reflections in the X-ray powder diffraction pattern are [d in A (I) ( hkl )]: 3.725 (100) (214), 3.513 (80) (215), 4.20 (42) (210), 3.089 (40) (217), 2.150 (40) (330). The electron microprobe analysis gives K 2 O 7.94, Na 2 O 14.95, CaO 5.14, Al 2 O 3 27.80, SiO 2 32.73, SO 3 9.84, Cl 0.87, (H 2 O 0.93), sum 100.20 wt %, less O = Cl 0.20, (total 100.00 wt %); H 2 O calculated by difference. The corresponding empirical formula, based on 72 (Si + Al), is (Na 31.86 K 11.13 Ca 6.06 )Σ=49.05(Si 35.98 Al 36.02 )Σ=72O 144.60 (SO 4 ) 8.12 C1 1.62 3.41H 2 O. The crystal structure of marinellite may be described as formed by the stacking along c of 12 layers containing six-membered rings of tetrahedra: the stacking sequence is ABCBCBACBCBC…, where A, B, and C represent the positions of the rings within the layers. Its structure consists of two liottite cages superimposed along [0, 0, z ] and of columns of cancrinite and sodalite cages along [1/3, 2/3, z ] and [2/3, 1/3, z ]. Sulphate groups, surrounded by sodium, calcium and potassium cations, occupy the liottite cages. Chlorine anions and sulphate groups occupy the sodalite cages, whereas H 2 O molecules are located within the cancrinite cages, bonded to Na cations. This structural model was refined in the space group P 31 c , conventional R = 0.098 for 2155 reflections. The structural relationships between marinellite and tounkite are discussed.

Crystal structure of diaquamagnesium (dihydrogenmonoboratemonophosphate), Mg[BPO4(OH)2](H2O)2, containing isolated sixmembered rings oftetrahedra

Article Crystal structure of diaquamagnesium (dihydrogenmonoboratemonophosphate), Mg[BPO4(OH)2](H2O)2, containing isolated sixmembered rings oftetrahedra was published on December 1, 2003 in the journal Zeitschrift für Kristallographie - New Crystal Structures (volume 218, issue JG).

Microsommite: crystal chemistry, phase transitions, Ising model and Monte Carlo simulations

Microsommite, ideal formula [Na4K2(SO4)] [Ca2Cl2][Si6Al6O24], is a rare feldspathoid that occurs in volcanic products of Vesuvius. It belongs to the cancrinite–davyne group of minerals, presenting an ABAB… stacking sequence of layers that contain six-membered rings of tetrahedra, with Si and Al cations regularly alternating in the tetrahedral sites. The structure was refined in space group P63 to R=0.053 by means of single-crystal X-ray diffraction data. The cell parameters are a=22.161 A=√3a dav, c=5.358 A=c dav; Z=3. The superstructure arises due to the long-range ordering of extra-framework ions within the channels of the structure. This ordering progressively decreases with rising temperature until it is completely lost and microsommite transforms into davyne. The order–disorder transformation has been monitored in several crystals by means of X-ray superstructure reflections and the critical parameters T c ≈ 750 °C and β ≈ 0.12 were obtained. The kinetics of the ordering process were followed at different temperatures and the activation energy was determined to be about 125 kJ mol−1. The continuous order–disorder phase transition in microsommite has been discussed on the basis of a two-dimensional Ising model in a triangular lattice with nn (nearest neighbours) and nnn (next-nearest neighbours) interactions. Such a model was simulated using a Monte Carlo technique. The theoretical model well matches the experimental data; two phase transitions were indicated by the simulated runs: at low temperature only one of the three sublattices begins to disorder, whereas the second transition involves all three sublattices.

Diffusion of silicon and gallium (as an analogue for aluminum) network-forming cations and their relationship to viscosity in albite melt

Self-diffusion of Si and tracer diffusion of Ga have been measured in an albitic melt, Na0.97Al0.99Si3.02O8, at 1 atmosphere between 1160 and 1558°C. Diffusion of these network-forming elements was studied for comparison with viscous transport and Nuclear Magnetic Resonance (NMR) relaxation parameters measured in albite melts and glasses. Gallium was used as an analogue for Al and estimated diffusion parameters for both Ga and Al are presumed to be equivalent. Diffusion can be described by Arrhenius lines for both Si and Ga (with 1-standard errors): +6.58 × 10−3Dsi = 1.37 × 10−4exp(−337.4 ± 52.6/RT)−1.35 × 10−4 +7.97 DAl ∼ DGa = 3.55 × 10−1 exp (−424.6 ± 42.5 /RT)−3.39 × 10−1, where diffusivities are in m2s−1 and activation energies are in kJ/mol. Comparison of Si and Ga diffusivities with Si/Al diffusivities in crystalline albite indicates relatively small differences between activation energies for diffusion of 3+ and 4+ network-forming cations in crystalline and in molten albite. Apparently, the energy barrier to diffusion of these network-forming cations is influenced significantly only by nearest neighbours, and is not strongly affected by changing the dominant structure from four-membered rings of (Si, Al)O4 tetrahedra in crystalline albite to three-, four-, and six-membered rings of tetrahedra in molten albite. Comparison of Na diffusion in albite crystals and glass demonstrates significant differences in activation energies for diffusion. The behaviour of oxygen diffusion appears to be intermediate between Na and network-forming cations, however, this observation is based upon limited oxygen diffusion data for rhyolitic, not albitic, melts. Activation energies for Ga diffusion in albite melt are similar to those measured in rhyolite melt at 1.0 GPa. The silicon activation energy for diffusion is three times higher in albite melt than in rhyolite melt at 1.0 GPa and does not correlate with the activation energy for the inverse of the NMR spin-lattice relaxation. The differences between Si activation energies in albite and rhyolite melts are suggested to be due to the presence of a mixed alkali-like effect in the latter melt. Calculated diffusivities based upon measured viscosities for albite melt and the Eyring equation agree almost within error with Ga diffusivities, but are typically 3–10 times greater than Si diffusivities. This correlation suggests that diffusion of Al in albite melt controls viscous transport. The activation energy for oxygen diffusion in crystalline albite and the estimated activation energy for oxygen diffusion in the melt are much lower than the activation energy for viscous transport and argue against a relationship between oxygen diffusion and viscous transport in this aluminosilicate melt. The more rapid diffusion of Ga and A1 compared to Si in albite melt is proposed to be due to the more frequent formation of five- or six-coordinated Ga-O and Al-O transition state complexes than similar Si-O complexes. The previously observed relationship between viscosity and chemical diffusivities during interdiffusion (Baker, 1992b) suggests that diffusion of Al also controls chemical diffusion in basaltic to rhyolitic melts.

The structure of reyerite, (Na,K)2Ca14Si22Al2O58(OH)8.6H2O

AbstractThe crystal structure of reyerite, (Na,K)2Ca14Si22Al2O58(OH)8.6H2O, Z = 1, was refined in the space group P, a = 9.765, c = 19.067Å, to R = 0.064 for 1540 reflections. The structure is composed of the following structural units: (a) tetrahedral sheets S1, with composition (Si8O20)8−, characterized by six-membered rings of tetrahedra; (b) tetrahedral sheets S2, characterized by six-membered rings of tetrahedra, with six tetrahedra pointing in one direction and two pointing in the other direction—the apical oxygens of these two tetrahedra connect two inversion-related S2 sheets to build double sheets, with composition (Si14Al2O38)14− and ordered distribution of aluminum cations; (c) sheets O of edge-sharing calcium octahedra. The various structural units are connected through corner sharing according to the schematic sequence ……; the corresponding composition is [Ca14Si22Al2O58(OH)8]2−. The charge balance is restored by alkali cations which are placed, together with water molecules, in the cavities of the structure at the level of the double tetrahedral sheet.