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The crystal structure of tlapallite has been determined using single-crystal X-ray diffraction and supported by electron probe micro-analysis, powder diffraction and Raman spectroscopy. Tlapallite is trigonal, space group P321, with a = 9.1219(17) Å, c = 11.9320(9) Å and V = 859.8(3) Å3, and was refined to R1 = 0.0296 for 786 reflections with I > 2σ(I). This study resulted from the discovery of well-crystallised tlapallite at the Wildcat prospect, Utah, USA. The chemical formula of tlapallite has been revised to (Ca,Pb)3CaCu6[Te4+3Te6+O12]2(Te4+O3)2(SO4)2·3H2O, or more simply (Ca,Pb)3CaCu6Te4+8Te6+2O30(SO4)2·3H2O, from H6(Ca,Pb)2(Cu,Zn)3(TeO3)4(TeO6)(SO4). The tlapallite structure consists of layers containing distorted Cu2+O6 octahedra, Te6+O6 octahedra and Te4+O4 disphenoids (which together form the new mixed-valence phyllotellurate anion [Te4+3Te6+O12]12−), Te4+O3 trigonal pyramids and CaO8 polyhedra. SO4 tetrahedra, Ca(H2O)3O6 polyhedra and H2O groups fill the space between the layers. Tlapallite is only the second naturally occurring compound containing tellurium in both the 4+ and 6+ oxidation states with a known crystal structure, the other being carlfriesite, CaTe4+2Te6+O8. Carlfriesite is the predominant secondary tellurium mineral at the Wildcat prospect. We also present an updated structure for carlfriesite, which has been refined to R1 = 0.0230 for 874 reflections with I > 2σ(I). This updated structural refinement improves upon the one reported previously by refining all atoms anisotropically and presenting models of bond valence and Te4+ secondary bonding.
We have employed mesoporous molecular sieves in polymer membranes in an effort to enhance the permselectivity. The principal advantage of these materials is that the polymer chains can penetrate the pores reducing the nonselective voids that are often observed with inorganic additives. In this study, we have prepared Matrimid® membranes with various loadings of the all silica molecular sieve DAM-1 (Dallas Amorphous Material) as well as DAM-1 functionalized with amines in the channel wall, to enhance the gas permeability characteristics of a high performance polymer. For all gases tested (N2, O2, CO2, CH4), the permeability increased in proportion to the wt % of the amine DAM-1 present in the membrane. The addition of the amine DAM-1 resulted in modest ideal O2/N2 permselectivity, while the ideal CO2/CH4 permselectivity values were >100, depending upon the moisture content of the feed. The ideal CO2/CH4 permselectivity values are among the highest for this type of composite membrane. Details of membrane fabrication as well as permeability and permselectivity results will be presented for a range of Matrimid®/molecular sieve composites.
We describe the factors affecting the electron transfer process between the different components of a self-assembled mixed monolayer. The system is comprised of mixed monolayers containing aminoalkanethiols (AMATs) and ferrocenylalkanethiols (FATs) of variable chain lengths. We study the effects of different ratio of the two mixed monolayer components on the permeability of the monolayer towards a Ru(NH3)6C13 redox probe. In order to study the electrical communication between the enzyme and the mediator molecules, the enzyme glucose oxidase (GOx) was attached to the AMAT sites to create a biosensor device. The relative efficiency of a biosensor of each chain-length combination of FAT and AMAT was examined. In light of this comparison, we consider the critical factors for efficient electron transfer between the ferrocene mediator and the GOx redox active site immobilized as part of the surface-confined system. We find that the biosensor response is greatest when the enzyme and the FATs are attached to the surface with different alkane chain lengths. We also find strong evidence for the existence of domains of FAT and AMAT in the mixed monolayer system.
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