抄録
In order to predict the effect of acid precipitation on building stone, artificial chemical weathering of polished plates of Kitagi Granite, Okayama, southwestern Japan, was conducted. An improved Soxhlet extraction apparatus was used with distilled water and HNO3, H2SO4 and HCI solutions of pH 3 at 55°C for different periods of time up to 824 days. The granite was composed mainly of quartz, plagioclase (Ab86An12Or2), alkali feldspar (0r92Ab8) and biotite. The leached sample solutions were analyzed for major and minor elements using ICP, ICP-MS and ionchromatography. Morphological and chemical changes of each mineral surface were studied by SEM, EPMA and microscopic techniques. Alteration products were collected from the surface of each mineral on the polished plates by hand picking and examined using an XRD.
Molar ratios of each element in the leached solutions to those in the fresh granite varied depending on the experimental period. The ratios of elements such as Mn, Fe, Mg, Sc and Zn, which reflect the dissolution of biotite, were relatively high. Similarly, the ratios of Ca, Na, Gd, Sm, Nd, La and Sr, which reflect the dissolution of plagioclase, were relatively high. It seems, therefore, that plagioclase and biotite are easily weathered. It was also estimated from the ratios that the reactivity order of the acid solutions for plagioclase is H2SO4, HCI and HNO3, and for biotite is HNO3, H2SO4, and HCl.
It is evident that biotite changed into mica-clay minerals and/or interstratified minerals of mica and vermiculite by exposure to the various acid solutions used in the experiment. Smectite appeared to be formed from plagioclase during the artificial chemical weathering. However, altered products from quartz and alkali feldspar, were not detected.
