The composition and crystallinity of the near-surface regions of weathered alkali feldspars

Lee, M.R. , Hodson, M.E., Brown, D.J., MacKenzie, M. and Smith, C.L. (2008) The composition and crystallinity of the near-surface regions of weathered alkali feldspars. Geochimica et Cosmochimica Acta, 72(20), pp. 4962-4975. (doi: 10.1016/j.gca.2008.08.001)

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Our ability to identify thin non-stoichiometric and amorphous layers beneath mineral surfaces has been tested by undertaking X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) work on alkali feldspars from pH 1 dissolution experiments. The outcomes of this work were used to help interpret XPS and TEM results from alkali feldspars weathered for <10,000 years in soils overlying the Shap Granite (north-west England). The chemistry of effluent solutions indicates that silica-rich layers a few nanometers in thickness formed during the pH 1 experiments. These layers can be successfully identified by XPS and have lower Al/Si, Na/Si, K/Si and Ca/Si values than the outermost not, vert, similar9 nm of unweathered controls. Development of Al–Si non-stoichiometry is coupled with loss of crystal structure to produce amorphous layers that are identifiable by TEM where >not, vert, similar2.5 nm thick, whereas the crystallinity of albite is retained despite leaching of Na to depths of tens to hundreds on nanometers. Integration of XPS data over the outermost 6–9 nm of naturally weathered Shap feldspars shows that they have stoichiometric Al/Si and K/Si ratios, which is consistent with findings of previous TEM work on the same material that they lack amorphous layers. There is some XPS evidence for loss of K from the outermost couple of nanometers of Shap orthoclase, and the possibility of leaching of Na from albite to greater depths cannot be excluded using the XPS or TEM results. This study demonstrates that the leached layer model, as formulated from laboratory experiments, is inapplicable to the weathering of alkali feldspars within acidic soils, which is an essentially stoichiometric reaction.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Brown, Dr David and MacKenzie, Dr Maureen and Lee, Professor Martin
Authors: Lee, M.R., Hodson, M.E., Brown, D.J., MacKenzie, M., and Smith, C.L.
Subjects:G Geography. Anthropology. Recreation > G Geography (General)
College/School:College of Science and Engineering > School of Geographical and Earth Sciences
College of Science and Engineering > School of Physics and Astronomy
Journal Name:Geochimica et Cosmochimica Acta
ISSN (Online):1872-9533
Published Online:14 August 2008

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