Plagioclase feldspars are abundant primary minerals in nature and represent an important source of Ca and Na in noncarbonate systems. Two phase intergrowths of calcic and sodic rich phases (exsolution lamellae) are common features in plagioclase feldspars, yet few studies have evaluated their role in the rates, stoichiometry and mechanisms of plagioclase dissolution. The objectives of the proposed collaborative study are to (i) determine the effects of exsolution lamellae on the rates and stoichiometry of plagioclase feldspars, (ii) determine the effect of preferentially weathered lamellar phases on the interpretation of data collected using surfaced sensitive spectroscopies such as x-ray photoelectron spectroscopy (XPS) and (iii) examine the potential role of exsolved phases in preferential release of Ca from plagioclase samples collected from well characterized watershed laboratories. Collaboration among the principal investigators will combine expertise and facilities necessary to accomplish these objectives. Dissolution experiments will be conducted at pH 4 using batch pH stat, flow through cell, and column reaction vessels using plagioclase samples over a range of an contents with and without exsolution lamellae. In addition, plagioclase samples from watershed laboratories will be used in characterization and dissolution studies to evaluate the potential role of exsolution lamellae in natural systems. The surfaces of reacted minerals will be characterized using scanning electron microscopy (SEM-EDX), x-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM-EDX, HRTEM-ED). Results from this study will be used to model the rate and stoichiometry of plagioclase dissolution as a function of An content in minerals with and without exsolved phases. Hypotheses concerning the mechanisms of plagioclase dissolution and the preferential release of Ca relative to Si and Na during dissolution will be tested for both laboratory and watershed weathering environments.
