| Summary: | 博士 === 國立臺灣大學 === 地質科學研究所 === 91 === Hydrothermal experiments on an andesite sample reacted with H2SO4 solution have been carried out in teflon-lined autoclaves under the conditions of 0.05M~2.0M H2SO4 solution, 110oC~190oC, vapor pressures, and a renewal of the acid solution every 6 hours..
Experimental results show that the susceptibility of minerals of the andesite to the acid solution is decreased in the order of plagioclase, augite, hypersthene and opaque minerals (ilmenite and magnetite). Moreover, groundmass is more easy to be attacked then phenocrysts for the same mineral. As to the reaction of the minerals with the solution, it is generally initiated along the crystal weakness such as cleavages, microfractures and lattice defects, and then diffused sidewardly.
After reacting with the acid solution, chemical compositions of the minerals are changed as follows. The amount of the the compositions of (Na2O+CaO) and Al2O3 is decreased for plagioclase; and that of (FeO+MgO) and CaO is decreased for pyroxenes. On the other hand, the amount of FeO is decreased with TiO2 enriched for ilmenite and magnetite.
It is revealed that not only the cationic components of Na+ and Ca+2, for plagioclase, but also the framework Al+3 is readily dissolved by H+. For pyroxene, Ca+2 is more readily replaced than Mg+2 and Fe+2. In this study, the index of the mobility of elements of andesite is simply denoted by K value. The higher the K value is, the more immobile the element is. The order of mobility is a function of temperature and solution concentration, and is mutually related with the order of susceptibility of the minerals. Though the amount of REE of andesite is decreased, a positive anomaly of Eu of the REE pattern becomes more obvious as the experiment proceeded.
It is considered that dissolution, disintegration and rapid transformation of silica-rich residues to α-cristobalite are the three essential processes of the hydrothermal alteration of the andesite. The rapid transformation of altered residues to α-cristobalite preserves silica from further dissolution, and so α-cristobalite stays as the final essential residues during alteration. This is highly consistent with the field observations.
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