Artificial weathering of hornblende particles was conducted in buffered aqueous solutions at room temperature (25 °C) over a pH range of 3.4-4.6. Both ultrasonically cleaned starting materials and samples weathered for various periods of time (one to six months) were examined by scanning electron microscopy (SEM). Dissolution was incongruent with preferential removal of Ca, Al, Mg and Fe relative to Si. Surface weathering was discontinous. The weathered areas were uneven with preferential pitting along cleavage planes. High resolution transmission electron microscopy (TEM) showed altered hornblende with a layered structure. Smectite-like materials were removed from the weathered surfaces by sonication. It is unclear whether the smectite-like phase was a weathering product produced during the experiment or an alteration product of a mica impurity.
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5 ACKNOWLEDGEMENTS This work was supported in part by a University of Minnesota Graduate School Grant-in-Aid, #426-0952-4909-02.W e thank Sara Gruenhagen for her help in x-ray diffraction analysis, Gilbert Ahlstrand for his help with SEM, and John Humenansky for his help with STEM.