In Situ High-Pressure Structural Changes of Diopside-Anorthite (CaMgSi2O6-CaAl2Si 2O8) Glasses: A Multi-spectroscopic Approach

In this thesis the structure of haplobasaltic glasses is investigated using a multi-spectroscopic approach. At ambient conditions, X-ray absorption near edge structure spectroscopy is used to determine the local environments around each element within these glasses which range in composition from diopside (CaMgSi2O6) to anorthite (CaAl2Si2O8). These results show that silicon and aluminum are tetrahedrally coordinated throughout all compositions. The mean Si-O bond length is estimated to range from 1.58 to 1.66 Ë A. The Si L2,3-edge results show low lying d orbitals which appear to respond to changes in the tetrahedral bonding environment. The direct observation of d orbitals in Si - O bonding is an unexpected result. The two alkaline earth elements, Mg and Ca, display separate behaviors. Magnesium appears to be dominantly in five-fold coordination in the diopside glass and does not appear to change coordination number until the Di60An40 composition, after which four-fold coordination is thought to be important. Calcium appears to show a linear shift in coordination from seven- to eight-fold between anorthite and diopside glass, respectively. At high pressure, Brillouin, X-ray Raman and Raman spectroscopies have been used to characterize the local to intermediate range order within haplobasaltic glasses. In situ high pressure O K -edge X-ray Raman results suggest [5]Si forms in diopside glass as low as 3 GPa. Raman spectra on diopside glass are interpreted to show a closure of the inter-tetrahedral angle of 2-3â Ś/GPa between 0 and 4 GPa, and 1â Ś/GPa thereafter. In situ Brillouin spectra of anorthite glass display no change upon compression until 5 GPa, after which the Brillouin frequency shift becomes positive. This behavior is comparable to that of magnesium metasilicate glasses and natural basaltic glasses. This may be due to changes in the tetrahedral ring structure populations or the formation of highly coordinated network formers. High pressure Raman spectroscopy shows that the bridging oxygen (BO) vibrations of intermediate haplobasaltic glass compositions display comparable evolution with pressure. The vibrations of BOs associated with more polymerized parts of the silicate network appear to diminish between 7 and 9 GPa, whereas BO vibrations associated with more depolymerized parts of the silicate network are observed up to 14 GPa. I suggest that the loss of these vibrations corresponds to the inability of the silicate network to densify by kinking of connected tetrahedra and/or by wrinkling of ring structures. These densification mechanisms may be found in a wide range of silicate melt compositions, including those of natural magmas.