An experimental dissolution study of feldspar chemistry under hydrothermal conditions

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dc.contributor.advisor Hoskin, P en
dc.contributor.advisor Swedlund, P en
dc.contributor.author Wong, Sin en
dc.date.accessioned 2012-04-02T20:44:34Z en
dc.date.issued 2012 en
dc.identifier.uri http://hdl.handle.net/2292/16429 en
dc.description Full text is available to authenticated members of The University of Auckland only. en
dc.description.abstract The aluminosilicate feldspars are by far the dominant silica-bearing class of mineral in the crust and yet little is known about feldspar behavior under geothermal conditions. Studying feldspar chemistry under geothermal conditions will contribute to a better understanding of the permeability of geothermal reservoirs which is directly influenced by mineral dissolution-precipitation reactions. In addition feldspar reactions are a potential source or sink of soluble silica that could influence the ever-present silica scaling issue associated with geothermal power generation. In this study feldspar chemistry was investigated as a function of pH and solution silica (SiO2(aq)) concentrations under hydrothermal conditions. This study focused on the sodium feldspar end member albite and the plagioclase intermediate member labradorite. All reactions were carried out in 2,000 ppm NaCl aqueous solutions at 200 oC at hydrostatic pressure in Teflon-lined autoclaves. Two variables were studied; pH and SiO2(aq). Reactions at an initial pH of 4, 7 and 9 were done with no added SiO2(aq) and reactions with initial SiO2(aq) of 0, 300, 550 and 800 ppm were done at a pH of 7. Optical microscopy, backscattered electron images, Electron Microprobe Analysis (EMPA), and Raman spectroscopy were used to analyze the unreacted and reacted surfaces. From optical microscopy iron oxyhydroxides and other dark-coloured impurities of unknown compositions were observed to be distributed at fractures, cleavage planes and topographical defects. This was considered to be a result of the preferential attack at the reactive sites of excess energy, contributing towards the spatial heterogeneity of the reacted samples. Changes in surface textural features in labradorite were observed to be more pronounced than ablite which was considered to be due to the higher susceptibility to weathering of calcic compositions in comparison to sodic compositions. The EMPA of unreacted albite and labradorite were consistent with the expected composition whereas weathered surfaces were highly variable due to surface heterogeneity and/or the surface roughness. The laser used for the Raman spectroscopy was chosen to provide a sensitive probe of changes in feldspar position on the feldspar ternary phase diagram but was not sensitive to the presence of weathering products e.g. the optically observed iron oxyhydroxides. Raman analysis showed no significant change in the underlying feldspar composition during weathering despite the large variability in the EMPA analyses. Due to the complex nature of the dissolution chemistry of feldspars and difficulties intrinsic to the analysis of weathered feldspars it was difficult to identify definitive reaction products or trends in reactivity from the EMPA or Raman spectroscopy. Numerical modeling was performed using Geochemists Workbench under conditions identical to the hydrothermal reactions. In the absence of SiO2(aq) diaspore (α-AlO(OH)) was the first phase to precipitate as albite dissolved at pH 4 and 7 followed by paragonite (NaAl2(OH)2(AlSi3O10) and then the feldspathoid analcime. At pH 9 only analcime was formed. In the presence of SiO2(aq) kaolinite was formed rather than diaspore followed by paragonite in addition to quartz. en
dc.publisher ResearchSpace@Auckland en
dc.relation.ispartof Masters Thesis - University of Auckland en
dc.rights Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated. Previously published items are made available in accordance with the copyright policy of the publisher. en
dc.rights Restricted Item. Available to authenticated members of The University of Auckland. en
dc.rights.uri https://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm en
dc.rights.uri http://creativecommons.org/licenses/by-nc-nd/3.0/nz/ en
dc.title An experimental dissolution study of feldspar chemistry under hydrothermal conditions en
dc.type Thesis en
thesis.degree.grantor The University of Auckland en
thesis.degree.level Masters en
dc.rights.holder Copyright: The author en
pubs.elements-id 318871 en
pubs.record-created-at-source-date 2012-03-13 en


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