Ulexite, Nacabsou(OH)R.Shro:Structure Refinement, Polyanion Configuration, Hydrogen Bonding,And Fiberoptics
American Mineralogist, Volume 63, pages 160-171, 1978 Ulexite, NaCaBsOu(OH)r.sHrO:structure refinement, polyanion configuration, hydrogen bonding,and fiberoptics SuBnnre GHosn, Cun'Nc WnN Department of Geological Sciences, Uniuersity of l(ashington S eatt le, Washington 98I 95 eNoJoeN R. Cmnx U. S. GeologicalSuruey,345 Middlefeld Road Menlo Park, California 94025 Abstract The crystalstructure of ulexite,NaCaBsOe(OH)6.5HrO, triclinic, PT. has been refined to a conventionalR:0.046 usingleast-squares methods, partially block-diagonal, for 49ll reflectionscollected on a single-crystaldiffractometer with graphite-monochromatizedMo radiation.AII l6 hydrogenatoms in theasymmetric unit werelocated, and all form hydrogen bonds.Refinedcellconstantsare:d=8.816(3),b=12.870(7),c=6.678(l)A,a:90.36(2)", B : 109.05(2)',7 = 104.98(4)",V : 688.4(4)A3,2 :2, density(calc) = 1.955g/cm3. The structurecontains isolated pentaborate-type polyanions composed of three tetrahedraand two triangles,plus chainsof Na octahedraand chainsof Ca polyhedra,all cross-linkedby polyanionbonds to thecations and by a networkof hydrogenbonds. Average bond distances (A) are:B-O, tetrahedrall.4T5,triangular 1.367; Na-O,2.421;Ca-O,2.484;hydrogen bonds, O-H 0.80,H . .O 2.08,O. .O 2.852.The octahedraland polyhedral chains are parallel to c, theelongation direction, and causethe fibrous habit of ulexitecrystals that is essentialto the opticalfiber bundles.The hydrogenbonds rangefrom strongwith a minimum O..'O distanceof 2.595(3)A to veryweak and possibly bifurcated with O. .'O distancesof 3.082(a) and3.194(3) A. The watermolecule environments are normal; each contacts at leastoneNa or Ca on its lone-pairside.
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