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Crystal structure of spessartine and andradite at elevated temperatures Rakai, Robert Joseph

Abstract

Single crystal x-ray intensity data have been collected with a manual diffractometer utilizing flat-cone geometry for a spessartine from Minas Gervais, Brazil at 25°, 350°, 575° and 850°C and for an andradite from Valmalen, Italy at 25°, 350°, 575° and 850°C. Weighted anisotropic least-squares refinements in space group Ia3d resulted in residual (R) factors ranging from 0.019 to 0.029. The refinements indicate that all of the positional parameters of the oxygen atom in andradite vary during thermal expansion. The x and y positional parameters of oxygen in spessartine likewise vary during thermal expansion while the z positional parameter remains essentially constant. Linear thermal expansion coefficients of the cell edges are 0.632(53) x 10⁻⁵°C⁻¹ and 0.764(23) x 10⁻⁵°C⁻¹ for spessartine and andradite, respectively. A high linear dependence is shown to exist for the cell edges of members of the silicate garnet solid solution as a function of temperature and composition collectively. Si-0 interatomic distances in both spessartine and andradite (uncorrected for thermal displacement) show a zero or slightly negative expansion while the Mn-0 and Al-0 interatomic distances of spessartine and the Ca-0 and Fe-0 interatomic distances of andradite show significant positive expansions as a function of increasing temperature. The rates of expansion of the Al-0, Mn(l)-0(4), and Mn(2)-0(4) interatomic distances in spessartine are 1.61(30) x 10⁻⁵Å°C⁻¹, 2.12(28) x 10⁻⁵Å°C⁻¹, and 3.28(31) x 10⁻⁵Å°C⁻¹, respectively. The rates of expansion of the Fe-0, Ca(l)-0(4) and Ca(2)-0(4) interatomic distances in andradite are 2.94(44) x 10⁻⁵Å°C⁻¹, 2.31(20) x 10⁻⁵Å°C⁻¹, and 3.28(13) x 10⁻⁵Å°C⁻¹, respectively. Relations between these values and the values reported for pyrope and grossularite (Meagher, 1975) as well as the concomitant polyhedral adjustments are considered in detail. In both spessartine and andradite, the shared octahedral edge increases at a greater rate than does the unshared octahedral edge and the X(2)-0(4) distance increases at a greater rate than does the X(l)-0(4) interatomic distance during thermal expansion. The spessartine garnet structure facilitates these variations mainly by rotation of the essentially rigid SiO₄ tetrahedron while the andradite garnet structure facilitates the same variations mainly by a distortion of the SiO₄ tetrahedron. The SiO₄ tetrahedron of andradite becomes more ideal as a function of increasing temperature.

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