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

Abstract

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.

Normalizing XRF-scanner data: A cautionary note on the interpretation of high-resolution records from organic-rich lakes

“…Although element-to-Al ratios are high in TOC-rich intervals, the absolute counts in Ti and Fe actually decrease, indicating a drop in the input of these elements. Weathering is primarily influenced by precipitation and temperature, and the rate of weathering for various minerals is generally the reverse of Bowen's reaction series, meaning that minerals rich in quartz and Al remain in the soil while more easily dissolved elements would be weathered from the soil and transported to the depositional basin (Brady, 1990;Fang et al, 2003;Selly, 1994). Consequently, the simultaneous increase of TOC and Ti-and Fe-to-Al ratios observed in LiYu Pond could be interpreted to be the result of periods of increased precipitation leading to increased chemical weathering in the catchment, enhanced flushing of soil organic matter into the lake, and possibly also to higher lake levels causing dysoxic conditions on the lake floor enhancing preservation of organic material (cf.…”

Magmatic architecture of dome-building eruptions at Volcán de Colima, Mexico

“…In explosive deposits, a fraction (approx. 10%) of the pyroclasts show some degree of oxidation and alteration with a devitrified groundmass and α-cristobalite partly replacing plagioclase, a commune occurrence in vapour phase alteration andesite rocks (Fang et al 2003). Geothermometry on two-pyroxene assemblages suggests storage temperatures of 960-1,020°C (Savov et al 2008).…”

Characteristics and Origins of Hot Springs in the Tatun Volcano Group in Northern Taiwan

“…In general, the volcanic rock is composed predominantly of twopyroxene and two-pyroxene hornblende andesites, possessing plagioclases as major phenocrysts, and the Wuchishan Formation is composed of course sandstone with abundant quartz grains (Chen 1963;Wu 1967). Previous studies have shown that the acidic thermal water could react with plagioclase to neutralize the water, reduce SO 4 2-ion concentrations and generate anhydite, montmorillonite, kaolinite, calcite and alunite as hydrothermally precipitated minerals (Chen and Yang 1984;Liu et al 1984;Wang 1991;Muir and Nesbitt 1992;Nogami and Yoshida 1995;Kempter and Rowe 2000;Varekamp et al 2000;Fang et al 2003;Marini et al 2003).…”

“…Meanwhile, laboratory experiments on andesitic chips from the TVG reacting with sulfuric acid of varying concentrations have been conducted (Fang et al 2003). The result showed that plagioclase will be decomposed, but not pyroxene and hornblende.…”