Hydrothermal Alteration of Andesite in Acid Solutions: Experimental Study in 0.05 M H2SO4 Solution at 110 °C

• Published in: Journal of the Chinese Chemical Society (Taipei) 2003-04, Vol.50 (2), p.239-244
• Main Authors: Fang, Jiann-Neng, Lo, Huann-Jih, Song, Sheng-Rong, Chung, San-Hsiung, Chen, Yaw-Lin, Lin, I-Chieh, Yu, Bing-Sheng, Chen, Huei-Fen, Li, Li-Jun, Liu, Chia-Mei
• Format: Article
• Language: English
• Subjects: Acid hydrothermal alteration; Andesite; Elemental mobility
• ISSN: 0009-4536; 2192-6549
• DOI: 10.1002/jccs.200300035

Hydrothermal experiments on an andesite have been carried out under the condition of 110 °C, autogeneous pressure, 0.05 M H2SO4 solution and renewal of acid solution every 6 hours. The experimental results indicate that the plagioclase and pyroxenes in the treated samples show micropits and microfractures. Small crystals readily suffered from alteration, as compared with large ones, and the susceptibility of the minerals to the acid solution is decreased in the order of plagioclase, augite, hypersthene and opaque minerals. Plagioclase, which is the most reactive mineral in the experiments, shows an increase of SiO2, but a decrease of Al2O3, CaO and Na2O as the experiments proceeded. The chemical change of the treated andesite, it indicates that the relative amounts of SiO2, TiO2, MgO, ΣFeO, MnO and K2O increase, while those of Al2O3, CaO, Na2O and P2O5 decrease with an increase of the experimental duration. The relative mobility of chemical elements listed in decreasing order is P, Ca, Al, Na, Si, K, Mg, Fe, Mn and Ti in terms of the K value. It is no wonder that Ca, Al and Na are more mobile than others because these elements are readily released into solution from the more reactive plagioclase, while Mg, Fe etc. still stay in the less reactive pyroxenes and opaque minerals.