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Weathering microenvironments on feldspar surfaces: implications for understanding fluid-mineral reactions in soils

M. R. Lee^1,*, D. J. Brown¹, M. E. Hodson², M. MacKenzie^3, and C. L. Smith^1,

¹ Department of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow G12 8QQ, UK
² Department of Soil Science, School of Human and Environmental Sciences, The University of Reading, Whiteknights, Reading RG6 6DW, UK
³ Department of Physics and Astronomy, Kelvin Building, University of Glasgow, Glasgow G12 8QQ, UK

^* E-mail: m.lee{at}ges.gla.ac.uk

The mechanisms by which coatings develop on weathered grain surfaces, and their potential impact on rates of fluid-mineral interaction, have been investigated by examining feldspars from a 1.1 ky old soil in the Glen Feshie chronosequence, Scottish highlands. Using the focused ion beam technique, electron-transparent foils for characterization by transmission electron microscopy were cut from selected parts of grain surfaces. Some parts were bare whereas others had accumulations, a few micrometres thick, of weathering products, often mixed with mineral and microbial debris. Feldspar exposed at bare grain surfaces is crystalline throughout and so there is no evidence for the presence of the amorphous `leached layers' that typically form in acid-dissolution experiments and have been described from some natural weathering contexts. The weathering products comprise sub-µm thick crystallites of an Fe-K aluminosilicate, probably smectite, that have grown within an amorphous and probably organic-rich matrix. There is also evidence for crystallization of clays having been mediated by fungal hyphae. Coatings formed within Glen Feshie soils after 1.1 ky are insufficiently continuous or impermeable to slow rates of fluid-feldspar reactions, but provide valuable insights into the complex weathering microenvironments on debris and microbe-covered mineral surfaces.

KEYWORDS: alkali feldspar, TEM, smectite, weathering