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Title:The elastic properties of fine-grained polycrystalline and amorphous samples by Brillouin scattering
Author(s):Zhang, Guimiao
Advisor(s):Bass, Jay D.
Department / Program:Geology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):elastic properties
Brillouin Scattering
Grain Boundaries
water content
Grain Size
Abstract:Near-perfect single crystals are often used for elasticity measurements when available, though in nature, fine-grained polycrystalline and amorphous minerals compose a large part of it. Therefore, the relationship between the properties of single-crystal and polycrystalline and amorphous samples is of fundamental importance in applying lab measurement results to real Earth. In this study we have investigated the sound velocities and elastic properties of fine-grained polycrystalline and amorphous materials using Brillouin scattering. Our initial experiments were performed on silica minerals, among the most abundant minerals in the crust, and MgO, one of the major components of Earth’s lower mantle. Four silica minerals (wood opal, hyalite, opal-CT and agate) and three MgO samples have been measured. The grain size of the silica minerals is around 1 to 7 micron in average, whereas that of the MgO is around 1 to 10 microns. Samples were ground and polished into plate-like shapes (~30 microns thick) for 90° geometry symmetric Brillouin scattering at room conditions, and for 50° geometry Brillouin scattering at high-pressure conditions. Our initial results at room conditions indicate that the velocities of agate, hyalite and wood opal are nearly 9%, 10% and 20% lower than the velocities obtained from single-crystal quartz measurements using Voigt-Reuss-Hill averaging respectively. Several features, such as the grain size, the inclusions and the water content, may contribute to the velocities drops. However, Brillouin spectra from MgO samples up to 19GPa show broad peaks that are roughly symmetric in shape. Peak shifts and velocities of the fine-grained MgO are close to the velocities obtained from single-crystal MgO measurements. The sharpness of the peaks increases with the decreasing grain size. Some of our observations may be a result of grain boundary effects.
Issue Date:2013-05-24
Rights Information:Copyright 2013 Guimiao Zhang
Date Available in IDEALS:2013-05-24
Date Deposited:2013-05

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