Other
Scientific paper
Feb 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988gecoa..52..385h&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 52, Issue 2, pp.385-394
Other
4
Scientific paper
The dissolution of labradorite under hydrothermal conditions has been studied by exposing a packed core of fine-grained labradorite powder to deionized water at 300°C and 300 bars for 57 days using a single-pass flowthrough apparatus. The principal methods used for characterizing the labradorite dissolution were high lateral resolution scanning Auger microscopy (SAM), X-ray photoelectron spectroscopy (XPS), and measurement of the exit solution chemistry. Under these hydrothermal conditions, boehmite and halloysite precipitate throughout the core and the labradorite powder undergoes extensive dissolution. The reacted labradorite surfaces are incongruently modified to levels as deep as 20 Å (for Al and Si) to 30 Å (for Na and Ca), although the distribution of elements within this depth or deeper is not yet known. The modified surface chemistry not only changes systematically down the length of the core, but is also locally variable. Generally, and relative to the starting composition, the reacted surfaces are enriched in Al and depleted in Na, Ca, and Si, and the Ca surface concentration decreases with distance from the inlet. Locally, some portions of the reacted labradorite surfaces have had Ca (and presumably Na) almost entirely removed. The dissolution mechanism of labradorite under hydrothermal conditions as revealed by SAM is highly complex, and the variability in surface chemistry is probably controlled by the local density and distribution of microcracks and crystal defects, the starting morphology of each labradorite surface, and the local flow patterns of reacting solution over that surface. Our results are contrary to previous XPS studies of dissolving feldspars (at or near room temperature) which do not report dramatic changes in near-surface chemistries. Part of the difference between the XPS and Auger results may be due to the difference in the depth of analysis associated with the characteristic electrons utilized by the two techniques.
Harris David W.
Hochella Michael F. Jr.
Ponader H. B.
Turner Ari M.
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