CRYSTAL CHEMICAL AND STRUCTURAL ANALYSES OF SOME COMMON ROCK-FORMING MINERALS: SPINEL, KALSILITE, CLINOPYROXENE AND OLIVINE

Abstract

Natural and synthetic common-rock forming minerals were examined using single-crystal X-ray diffraction (SXRD) and electron microprobe (EMP) analyses. The influences of common defect features, such as inclusions in spinel and oxygen positional disorder and twinning in kalsilite, were reported on the respective structures. The case studies show that these defect features could lead to a misinterpretation of X-ray intensity data. The structural interpretations obtained from these XSRD analyses could be significantly different when physical properties of the crystals are considered.In the second part of my dissertation, comparative crystal chemical studies on mantle-derived minerals such as spinel, clinopyroxene, and olivine are reported. These studies were carried out to examine temperature, pressure, and compositional effects on the structures of these phases. In particular, packing arrangements of oxygen atoms were examined in detail to investigate how the packing affects element partitioning among upper-mantle minerals. At ambient conditions, oxygen packing is more distorted in the order of spinel < olivine < clinopyroxene. The packing of oxygen atoms in olivine might have a significant control on element substitutions at high pressure. Because elements whose radius is larger than that of Mg distort the packing of mantle olivine (Fo~89), olivine might limit the amount of those elements, such as Fe2+, entering the structure. In contrast, substitutions of smaller cations in C2/c clinopyroxenes increase packing distortion. For clinopyroxenes enclosed in peridotite and eclogite, higher equilibration pressures are associated with more distorted, less efficiently packed structures. Unlike many minerals reported in Thompson and Downs (2001), spinel becomes more packed with rising temperature when intracrystalline cation exchange reactions are possible. Despite wide chemical variations, spinel samples from one geological environment display a constant packing distortion, which might suggest that spinel is capable of achieving an optimal packing configuration at a given P and T.