Pete J. Dunn

Flexibility of the zeolite RHO framework : effect of dehydration on the crystal structure of the beryllophosphate mineral, pahasapaite

Pahasapaite, a hydrated beryllophosphate mineral (Ca 5.5 Li 3.6 K 1.2 Na 0.2 □ 13.5 )Li 8 Be 24 P 24 O 96 ·38 H 2 O, has a distorted cubic, zeolite RHO-type structure with a = 13.781 A. On dehydration, the structure distorts further to give a cell with a = 13.100(3) A. As with the aluminosilicate RHO framework, Li + is associated with the 6-ring sites and Ca 2+ is sited in the double 8-ring sites, leading to severe distortion of the double 8-rings. The double 8-rings consist of elliptically distorted single 8-rings orthogonal to each other and the degree of their distortion can be described by the Δ/a parameter, where Δ is one-half the difference between the major and minor axes of these elliptical 8-rings and a is the cell edge. This parameter, Δ/a, varies from 0 to 0.178 for the aluminosilicate system, but is 0.201 for the dehydrated pahasapaite mineral. This is clearly the greatest distortion yet observed for this framework. This combination of new framework chemistries (Be/P/O vs. Si/Al/O) and the increased distortion is being examined by theory and experiment with respect to the effect on guest molecules.

Crystal-structure determination of pinalite

Abstract The crystal structure of pinalite, Pb3WO5Cl2, is orthorhombic Amam, with a = 11.073(2), b = 13.067(3), c = 5.617(1) Å, V = 812.81(3) Å3, and Z = 4. It has been refined to an R index of 0.049 on the basis of 593 unique, observed reflections. There are two crystallographically distinct Pb cations with different coordinations: Pb1 has eightfold-coordination with ligands of 4 Cl atoms, 4 O atoms, and one stereoactive lone-pair, whereas Pb2 has ninefold-coordination with ligands of 4 Cl atoms and 5 O atoms. The W cation has fivefold-coordination with 5 O ligands. The pinalite structure is layered parallel to (100) with Pb-O-Cl layers interleaved with Pb-W-O layers. Pinalite belongs to the bismuth oxychloride group of structures. The Pb-O-Cl layer of pinalite duplicates the basic structure of thorikosite and the Pb-W-O layer of pinalite is able to conform to this fundamental cell through differences in the two Pb coordinations and a unique W coordination. Similarly the structures of nadorite and kettnerite are also shown to replicate the basic bismuth oxychloride structure.

Sverigeite, a new tin beryllium silicate mineral from Långban, Värmland, Sweden

Abstract Sverigeite, NaMgMnBe2SnSi3O12(OH), is orthorhombic, space group Ibmm or Ibm2 with a = 6.818(6), b = 13.273(8), and c = 10.815(8) A. The strongest reflections in the X-ray powder diffraction pattern are: 2.884(202); 2.826(042); 4.35(121); 5.77(101); 2.644(222); and 6.63 A (020). Chemical analyses yielded SiO2 33.5, FeO 0.3, ZnO 1.2, MgO 8.1, MnO 11.8, Na2O 6.1, BeO 9.7, SnO2 28.5, H2O 1.2, sum = 100.4 weight percent, in good agreement with the proposed formula with Z = 4. Sverigeite is yellow in color; the hardness (Mohs) is approximately 6 1/2; the density is 3.60 (meas.), 3.61 (calc)g/cm3; cleavage is perfect on {010}. Optically, sverigeite is biaxial positive, 2V = 67°, with indices of refraction α = 1.678(4), β = 1.684(4) and γ = 1.699(4); dispersion is strong r > v; pleochroism is moderate: X = yellow, Y ≃ Z = pale yellow; absorption X > Y ≃ Z; orientation: Z = b. Sverigeite is found associated with mimetite, jacobsite, manganoan calcite, and amphibole. The only known sample was found on the ...

Fredrikssonite, a new member of the pinakiolite group, from Långban, Sweden

Abstract Fredrikssonite is a new member of the pinakiolite group with ideal formula Mg2Mn3+(BO3)O2. It is orthorhombic, space group Pbam or Pba2 with a = 9.18(1), b = 12.555(6), c = 2.954(2) A, Z = 4. The strongest reflections in the X-ray powder diffraction pattern are (d, I, hkl): 5.1680120; 2.590100240; 2.48690201; 2.20130250; 2.01350321, 430; 1.51340521, 171. Microprobe analysis yielded Al2O3 1.9, Fe2O3 5.4, Mn2O3 35.5, MgO 40.3, B2O3 17.9 (estimated by ion microprobe), sum = 101.0%. The chemical formula, calculated on the basis of total cations = 3, yields: Mg1.93(Mn3+ 0.87Fe3+ 0.13Al0.07(BO3)0.99O2.05. Fredrikssonite is reddish brown, slightly transparent, but nearly opaque in one direction. The hardness (Mohs) is approximately 6; the density is 3.84(5) (meas.), 3.80(calc.) g/cm3. The luster is vitreous. Cleavage was observed only on small fragments; its orientation is unknown. Optically, fredrikssonite is biaxial, positive, α = 1.82(2), β = < 1.86, γ = ≈ 1.99; pleochroism is strong with X = golden ...

Crystal structure of naturally occurring mercury(II) amidonitrate

A naturally-occurring mercuroammonium compound from F’itkin County, Colorado, is shown to be the natural analog of synthetic HgNH,N03. The crystals are isometric, P4132 orP

A contribution to the crystal chemistry of ellestadite and the silicate sulfate apatites

32, with a = 10.254( 1) A and twelve formula weights per cell. Using 437 symmetry-independent reflections, the crystal structure was partially determined and refined to a residual of 0.090. The positions of the Hg atoms and the N and 0 atoms of the nitrate group were determined, but the amide ion could not be located, probably due to positional disorder. The structure contains mercury atoms arranged in equilateral triangles 3.421(l) 8, on a side. These triangles are linked through shared vertices into helical chains wound around the fourfold screw axes. Similar triangular units occur in other inorganic Hg(II) compounds. The distortion of the nitrate ion from trigonal planar symmetry is also discussed.