Hydrothermal Alteration of Andesite in Acid Solutions: Experimental Study in 0.05 M H 2 SO 4 Solution at 110 C

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1 Journal of the Chinese Chemical Society, 2003, 50, Hydrothermal Alteration of Andesite in Acid Solutions: Experimental Study in 0.05 M H 2 SO 4 Solution at 110 C Jiann-Neng Fang a * ( ), Huann-Jih Lo b ( ), Sheng-Rong Song b ( ), San-Hsiung Chung c ( ), Yaw-Lin Chen b ( ), I-Chieh Lin b ( ), Bing-Sheng Yu d ( ), Huei-Fen Chen b ( ), Li-Jun Li b ( ) and Chia-Mei Liu b ( ) a Department of Earth Science, National Taiwan Museum, Taipei 100, Taiwan, R.O.C. b Department of Geosciences, National Taiwan University, Taipei 106, Taiwan, R.O.C. c Central Geological Survey, MOEA, Taipei 235, Taiwan, R.O.C. d Department of Resources Engineering, Dahan Technology University, Hualien 971, Taiwan, R.O.C. Hydrothermal experiments on an andesite have been carried out under the condition of 110 C, autogeneous pressure, 0.05 M H 2 SO 4 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 SiO 2, but a decrease of Al 2 O 3, CaO and Na 2 O as the experiments proceeded. The chemical change of the treated andesite, it indicates that the relative amounts of SiO 2,TiO 2, MgO, FeO, MnO and K 2 O increase, while those of Al 2 O 3, CaO, Na 2 O and P 2 O 5 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. Keywords: Andesite; Acid hydrothermal alteration; Elemental mobility. INTRODUCTION In fumarolic and hydrothermal areas, volcanic rocks generally react with acid solutions, and suffer from different extents of reaction. Such reacted volcanic rocks may gradually be enriched in SiO 2, and eventually change mainly to opaline silica (SiO 2.nH 2 O), 1 low-cristobalite (SiO 2 ) and other forms of SiO Simulation of such natural processes have been attempted by many workers In general, both batch and flow systems have been designed for the study of the stated natural processes. In a batch system, solid and acid solution are usually reacted in a closed vessel for a certain period, and then the solution is analyzed. The difficulty encountered in this method is the problem of precipitation which can generally occur when a reacting solution reaches a saturated concentration for a compound or mineral. This will decrease the solution concentrations of the precipitated components. Precipitation on the dissolving rock or mineral chips will decrease dissolution rate and hence gradually decrease the solution concentrations also. So, a batch system can only be used to investigate short-term dissolution behavior before precipitation. On the other hand, in a flow system, the solution is continuously flowed through a reacting vessel, and the solution is collected from the flowout fraction at scheduled time intervals for analyses. In this method, the problem of precipitation also can not be avoided, yet loss of powdered sample through flowout will decrease the amount of the original sample so as to decrease the solution concentrations gradually. 11 A revised batch system is designed for the present experiments. In this revised system, it takes repeated renewing of the acid solution within a certain time interval. The longest time, in which there is no precipitated material stuck on the residual rock chips during the experiments, is taken as a suitable period for renewing the acid solution. So, the residual rock chips can be directly analyzed instead of the conventional solution analysis. Practically, the longer the rock chips are reacted with the acid solution, the more the precipitated materials tend to stick on the residual rock chips. This will

2 240 J. Chin. Chem. Soc., Vol. 50, No. 2, 2003 Fang et al. make it difficult to clean off the precipitated materials on the residual rock chips through ultrasonic treatment. On the other hand, though it may avoid the sticking problem, it makes the experiments become impractical, if the period of renewal is too short. This study emphasizes the reaction of the minerals in andesite on the acid solution so as to ascertain the mobility of chemical elements and the chemical changes of volcanic rocks in hydrothermal areas. EXPERIMENTAL SECTION An andesite from Tatunshan volcanic rocks in northern Taiwan was collected for the solid starting material in this study. The rock samples were crushed and screened to obtain rock chips with diameters ranging from 0.34 mm to 0.40 mm. The rock chips were cleaned in deionized water with the aid of an ultrasonic vibrator for 15 minutes. After cleaning and then drying at 110 C for 24 hours, they were stocked for experiments. H 2 SO 4 solution with the concentration of 0.05 M was used as the liquid medium. Experiments were carried out in teflon-lined autoclaves, with an internal volume of about 300 ml. for each, under the conditions of 110 C and autogenous pressure. Each experiment was started with an amount of about 7.5 g rock chips and 150 ml H 2 SO 4 solution. Sample charged autoclaves were taken out of the oven every 6 hours to renew the acid solution to avoid precipitated compounds or minerals sticking on the surface of the rock chips. The solid sample was collected in each scheduled reaction time for analyses. The renewal of solution can be repeated untill reaching a cumulative reaction time of about 360 hours when the rock chips are too small (< 100 mesh) to separate from precipitates by sieving. It should be noted that precipitation can generally be observed after two hours as the experiments proceed, and six hours are found to be the longest reaction time to avoid sticking of precipitates on the rock chips so as to make the separation of precipitates from residual rock chips free from difficulty. The residual rock chips in each reaction time were separated from precipitates, usually as micrometer-sized minerals, through a 100-mesh sieve. Chemical compositions of the rock samples and plagioclase were analyzed by using X-ray fluorescence (XRF) and microprobe techniques, respectively. Mineral constituents and processes of alteration of the rock samples were investigated by using microscopy. EXPERIMENTAL RESULTS Microscopic observations show that the fresh andesite is composed of coarse-grained plagioclase (30%), augite (5%), hypersthene (5%) and opaque minerals (3%), and about 57% glass, and fine-grained plagioclase, pyroxenes and apatite. It is found that the coarse-grained plagioclase and pyroxenes in treated samples show micropits and microfractures (Fig. 1A, B). In general, fine-grained crystals readily suffered from alteration, as compared with coarse-grained ones (Fig. 2A), and the susceptibility of the minerals to the acid solution is decreased in the order of plagioclase, augite, hypersthene and opaque minerals (magnetite and ilmenite) with plagioclase more easier to react than others (Fig. 2B). Chemical compositions of the coarse-grained plagioclase, which is the most reactive mineral in the experiments, and of the fresh and hydrothermally treated andesite are Fig. 1. Micrographs showing micropits (micp) and microfractures (micf) in coarse-grained plagioclase and hypersthene [Plates I to V (except Plate IIIB) being taken from the sample treated for 312 hours].

3 Hydrothermal Alteration of Andesite J. Chin. Chem. Soc., Vol. 50, No. 2, Table 1. Chemical Compositions of Plagioclase of the Fresh and Hydrothermally Treated Andesite Oxide (wt%) SiO Duration(hrs) 2 TiO 2 Al 2 O 3 MgO FeO MnO CaO Na 2 O K 2 O Total 0(fresh) : denoted to not determined. Reported analyses for the treated samples being of altered portions. shown in Table 1. As shown in Table 1, plagioclase shows an increase of SiO 2, but a decrease of Al 2 O 3, CaO and Na 2 Oas the experiments proceeded. Chemical compositions of the fresh and hydrothermally treated andesite are listed in Table 2 and illustrated in Fig. 3. Obviously, the relative amounts of SiO 2,TiO 2, MgO, Fig. 2. Micrograph showing coarse-grained hypersthene (hyp) being more resistant than finegrained minerals (fm) in the rim of a rock chip. Micrograph illustrating plagioclase (completely) being more easily reacted than hypersthene (hyp) and augite (aug). FeO, MnO and K 2 O increase, while those of Al 2 O 3, CaO, Na 2 O and P 2 O 5 decrease with an increase in duration of the experiments. DISCUSSION AND CONCLUSIONS As previously stated, the andesite changes composition after the acid treatment. Some oxides such as SiO 2,K 2 O, MgO, FeO, MnO and TiO 2 increase, but others such as Na 2 O, Al 2 O 3, CaO, and P 2 O 5 decrease in weight percent. Certainly, the chemical elements show a different mobility. In this paper, the index of the mobility of chemical elements is simply expressed by K = (wt.% oxide i) treated /(wt.% oxide i) fresh, where i is denoted to a given oxide. Fig. 4 indicates the relationships between K values and experimental durations of different chemical elements. Obviously, those elements with K values greater than one increase, whereas those with K values less than one decrease during the experiments. Thus, the relative mobility of the elements listed in decreasing order is P, Ca, Al, Na, Si, K, Mg, Fe, Mn and Ti in terms of the K value. This agrees well with the results from analyses of solutions in a conventional batch system of many workers However, the order may change somewhat for a different concentration of an acid solution. 13 The compositional change of the andesite seems to relate to susceptibility of its constituent minerals to the acid solution. As mentioned previously, the susceptibility of the minerals to an acid solution decreases in the order of plagioclase [(Ca,Na)(Al,Si)AlSi 2 O 8 ], augite [Ca(Mg,Fe,Al)(Al,Si) 2 O 6 ], hypersthene [(Mg,Fe)SiO 3 ] and opaque minerals [ilmenite (FeTiO 3 ) and magnetite (Fe 3 O 4 )] with plagioclase more reactive than others. So, it is no wonder that Ca, Al and Na are more mobile than others because these elements are readily released into solution from the plagioclase, while Mg, Fe etc. still stay in the less reactive pyroxenes and opaque minerals. Basically, reaction is easier on the surface than in the interior of a rock chip because of the direct exposure of the former to solution. Nevertheless, alteration simultaneously

4 242 J. Chin. Chem. Soc., Vol. 50, No. 2, 2003 Fang et al. Table 2. Chemical Compositions of the Fresh and Hydrothermally Treated Andesite Oxides Duration(hrs) SiO 2 TiO 2 Al 2 O 3 FeO MnO MgO CaO Na 2 O K 2 O P 2 O 5 Total 0(fresh) Fig. 3. Compositional changes of the andesite during the experiments.

5 Hydrothermal Alteration of Andesite J. Chin. Chem. Soc., Vol. 50, No. 2, Fig. 4. Relationships between K values and durations of different elements. appears both on the rim and in the interior, and yet the latter occasionally indicates a high degree of alteration for an experiment (Fig. 5A). Reaction generally occurs along microfractures and vesicles (Fig. 5B) in the interior. These pathways were developed during the cooling of the andesite. So, a high degree of alteration in the interior may be due to a larger effective reaction area of the microfractures and vesicles than that of the surface of a rock chip. As to the reaction of the minerals with the solution, it is generally initiated along cleavages and/or microfractures (Fig. 6A, B), and then is diffused sideways. Lattice defects may also be the focus for the initial reaction, as has been noted by many authors. 14,15 Microscopic observations also indicate that small crystals are more susceptible to reaction than large ones for the same mineral (Fig. 7A). EDS determinations show that the residues of the strongly reacted plagioclase (Fig. 7B) are silica-rich materials. It is also found that some plagioclase crystals appear to be completely dissolved for longer experiments (Fig. 7A). So, it is likely that Na, Ca and Al are readily released, and then Si is lost later in the plagioclase. Structurally, Na and Ca are the main cationic constituents, while Al and Si constitute the Fig. 5. Micrograph showing a high degree of alteration of plagioclase (plag) in the interior of a rock chip. Micrograph displaying microfractures (micf) and vesicles (ves) in the interior of a fresh rock chip. Fig. 6. Micrographs showing the reaction of augite and plagioclase being initiated along cleavages (clev) and microfractures (micf) after an experiment of 312 hours.

6 244 J. Chin. Chem. Soc., Vol. 50, No. 2, 2003 Fang et al. structural framework of the plagioclase. Obviously, not only the cationic components of Na and Ca but also the framework Al is readily released, while the framework Si is less easily attacked. The behavior of chemical reactions of less reactive pyroxenes is not clearly known because they are only partly reacted in the experiments. If the chemical reaction is similar to that of the plagioclase, the cationic constituents of Ca, Mg and Fe of the pyroxenes will react first, while the framework Si will be released into the solution later. These arguments seem to support essentially the assumption proposed by many authors that the chemical reaction between minerals and acid solution is a two-stage process. The first stage is an ion exchange reaction, and the second one is an attack of the structural framework. It seems that plagioclase does not follow strictly the above assumption because its framework Al reacts with the acid solution as readily as its cationic constituents of Na and Ca, though the framework Si is surely to be attacked later. ACKNOWLEDGEMENT This study was partly supported by a grant from the National Science Council of the R.O.C. (NSC M ). Received June 4, REFERENCES Fig. 7. Micrograph showing fine-grained plagioclase (plag) being more susceptible to reaction than coarse-grained one (plag) ones and the fine-grained plagioclase (plag) being completely dissolved. Micrograph showing residues (res) of a strongly reacted plagioclase (plag). 1. Minami, E.; Ossaka, T.; Ossaka, J. J. Balneol. Soc. Jpn. 1966, 17, Wang, Y. Proc. Geol. Soc. China 1973, 16, Chen, P. Y. Acta Geological Taiwanica Science Reports of the National Taiwan University 1961, 9, Ellis, A. J.; Mahon, W. A. J. Geoc. Cosm. Acta 1964, 28, Reyes, A. G. J. Volcanol. Geotherm. Res. 1990, 43, Minato, H.; Nagashima, K.; Minami, E. Gen. Stud. Tamagawa Hot Spring 1959, 6,3. 7. Ossaka, J. Chinetsu 1968, 17, Seyfried, W. E. Jr.; Bischoff, J. L. Geoc. Cosm. Acta 1979, 43, Muir, I. J.; Nesbitt, H. W. Geoc. Cosm. Acta 1992, 56, Walther, J. B.; Woodland, V. Geoc. Cosm. Acta 1993, 57, Nogami, K.; Yoshida, M. J. Volcanol. Geotherm. Res. 1995, 65(1-2), Kamiya, H. Bull. Chem. Soc. Jpn. 1960, 33, Chiba, S. Sci. Rep., Fac. Educ., Fukushima Univ. 1962, Berner, R. A.; Holdren, G. R. Geochim. Acta 1979, 43, Fung, P. C.; Sanipelli, G. G. Geoc. Cosm. Acta 1982, 46, Garrels, R. M.; Howard, P. Clays Clay Miner. 1959, 6, Marshall, C. E. Econ. Geol. 1962, 57, Laguche, M. Geoc. Cosm. Acta 1976, 40, Busenberg, E.; Clemency, C. V. Geoc. Cosm. Acta 1976, 40, Fung, P. C.; Bird, G. W.; Mclntyre, N. S.; Sanipelli, G. G.; Lopata, V. J. Nucl. Tschnol. 1980, 51, 188.