Browsing by Author "Cooper, Brian J."

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    The effects of heating and dehydration on the crystal structure of hemimorphite up to 600{u00B0}C
    Cooper, Brian J. (Virginia Polytechnic Institute and State University, 1978)
    The crystal structure of hemimorphite, Zn₄Si₂O₇(OH)₂H₂O, has been refined from X-ray intensity data collected at 300°, 600°, and 22°C (after heating to 600°C). The space group throughout the experiment was Imm2, Z=2, and the cell dimensions at each temperature, in the above-mentioned order, are: a=8.337(5), b= 10.724(6), c=5.116(4)Ȧ; a=8.268(5), b=10.784(8), c=5.113(3)Ȧ; and a=8.206(4), b=10.815(6), c=5.089(2)Ȧ. Anisotropic least-squares refinements resulted in unweighted R-factors of 0.046, 0.074, and 0.061, respectively. The structure can be described as a framework consisting of interconnected corrugated sheets composed of 3-membered rings of one SiO₄ and two Zn(OH)O₃ tetrahedra. Heating to 600°C, causing dehydration, along with subsequent cooling to 22°C resulted in the contraction of a and c, expansion of b, and a decrease in the unit cell volume. The unit cell volume decrease is directly proportional to the volume decrease of the cavity initially containing the water molecule. These changes are rationalized in terms of atomic positional shifts, especially those of the hydroxyls which are no longer hydrogen-bonded to a water molecule. The hydroxyls move towards the position occupied by the water molecule prior to dehydration. The effects of dehydration on the crystal structure of hemimorphite were to found to be similar to those found in some zeolites. Although hemimorphite may not qualify for a molecular sieve due to its small aperture size, it may be useful as an isotopic sieve.
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    The investigation of the optical properties of polytypic minerals
    Cooper, Brian J. (Virginia Polytechnic Institute and State University, 1988)
    A new approach to the investigation of the optical properties of polytypic minerals that combines spindle stage techniques, X-ray diffraction methods, electron microprobe analysis, and dielectric tensor calculations has been developed and applied to zinc sulfides and chloritoids. For the first time, X—ray diffraction studies of natural anisotropic zinc sulfides have documented the simultaneous occurrence of twinning and stacking disorder along more than one of the four symmetry equivalent <111> directions of sphalerite. Precession photographs of optically anisotropic zinc sulfides are characterized by twin—equivalent diffraction maxima and diffuse diffraction streaking along lattice rows with (h-k) ≠ 3n (equivalent hexagonal indices) in one or two <111> directions. A system of linear equations has been used to calculate the approximate volume fractions of each twin domain and the sphalerite host domain. Dielectric tensor calculations have been performed to illustrate that mixtures of cubic and hexagonal zinc sulfide may be optically biaxial if the intergrowth occurs along more than one of the symmetry equivalent <111> directions of sphalerite (cubic). The dependence of the optical properties upon the chemical variation and polytypic intergrowth in the Hg-Fe chloritoids has been investigated. The effects of the variation in chemical composition of specific polytypic compositions were analyzed first. The refractive indices of 10 approximately pure 2M₂ Hg-Fe chloritoids show strong correlations (R² ≥ 0.094) to the proportion of Hg cations in the H(1B) site, Hg/(Hg + Fe + Hn) - HGN. Correlations between the optical orientation angles and HGN were weaker (R² ≤ 0.87). The optical orientation is very sensitive to small variations in the polytypic composition, especially orientation angles that have fixed values in 2M₂ chloritoid. The parameter showing the most sensitivity is ∠ X ⋀ b, which is 0° in an ideal 2M₂ chloritoid, but increases to about 6° for a chloritoid containing 10% by volume 1Tc polytype. The sum of ∠ Y ⋀ c* and ∠ Z ⋀ c* for an ideal 2M₂ chloritoid has a value of 90°, whereas a chloritoid with 10% 1Tc has a sum of 92°. Although not as sensitive as ∠ X ⋀ b, this parameter can be determined with only a spindle stage or universal stage. The observed dependence of the optical properties on polytypic intergrowth and chemical variation has been modeled using dielectric tensor calculations based on the properties of a 1Tc layer and assuming that the 2M₂ polytype is derived by twinning the 1Tc polytype about [010] with an (001) composition plane.