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Equations of State of Dense Hydrous Silicates

Crichton, Wilson Alexander; (2000) Equations of State of Dense Hydrous Silicates. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

Water has a profound effect on the seismic velocity, the rheology and melt properties of minerals. These affect the phase equilibria of minerals at depth and have implications in the origin of plumes and deep-focus earthquakes. In order to understand more fully the influence of hydrated minerals on mantle behaviour we must ascertain the divergence of the properties of hydrated minerals from those normally considered resident in the mantle. I present new data on the single-crystal equations of state of minerals in the MgO-FeO-SiO2-H2O system using a high-precision method of single-crystal x-ray diffraction. These include wadsleyite, OH-chondrodite, OH-clinohumite, phase A, phase B, superhydrous phase B and phase E. Single-crystals of nominally anhydrous wadsleyite and anhydrous phase B have also been studied, for comparison. The incorporation of water into the wadsleyite structure increases the compressibility by ~10% and the degree of anisotropy in compression increases to ~35%. Symmetry has no effect on compressibility, nor on the anisotropy of monoclinic and orthorhombic hydrous wadsleyites. The minerals along the Mg2SiO4-Mg(OH)2 join display near (decreasing) linearity in compression with respect to water content and increasing bulk modulus with density, even including the structurally dissimilar phase A. The effect of substituting fluorine on compressibility is presented, as are the predicted equations of state of OH-Mg humite and OH-Mg norbergite The anisotropy of the B-group minerals decreases with increasing water content because the stacking direction in these phases exhibits the dominant effects of hydration. The bulk moduli of the B-group decreases linearly with water content, if the effects of disorder in phase B are accounted for and also increases linearly with density. Phase E has the highest K' yet measured for a hydrous silicate. It is only slightly anisotropic in compression and a maximum of 70% of its layer structure is occupied.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Equations of State of Dense Hydrous Silicates
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
URI: https://discovery.ucl.ac.uk/id/eprint/10123954
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