Leaching rates of rock-forming components through acidic alteration

Journal of Volcanology and Geothermal Research65 (1995) 41-49

Leaching rates of rock-forming components through acidic alteration Kenji Nogami”,“, Minoru Yoshidab “Kusatsu-Shirane Volcano Observatory, Tokyo Institute of Technology, 641-36 Kusatsu, Agatsuma, Gunma, 377-17 Japan bDepartment of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, 152 Japan

Received 7 January 1994; revised version accepted 18 August 1994

Abstract A series of experiments on the interactions of a glassy basaltic andesite with acidic solution in a flowing system were carried out. Leaching indices of seven components (Na, K, Ca, Mg, Fe, Al and Si) were defined to compare the relative leaching behaviour of these components at all reaction stages. At 160°C and using 0.24 N HzS04 solution, Na, Ca and Al were leached easily, compared with K, Mg, Fe and Si. Although Si concentrated in the final residual rock, it was not always the hardest component to leach. The leaching behaviour of these seven components in a glassy rock agreed with that in crystalline rock. Results of three experiments at 80, 120 and 160°C using 0.12 N HCl solution showed that the rise of temperature accelerated the leaching rate of each component, whereas the leaching index for each component was not affected by temperature. This indicated that the leaching processes for these seven components were independent of temperature. On the other hand, in a series of the experiments at 160°C using 0.24.0.12 and 0.0024 N HCl solutions to examine the effect of acidity, the change in acidity of the reacting solution affected not only the reaction rate but also the reaction processes of the components. Na, Ca, Fe and Al became hard to leach as acidity diluted and Si became easy to leach and comparable to these components.

1. Introduction

In fumarolic and hydrothermal areas, rocks react with acid solutions. Rocks altered under acidic conditions become gradually enriched in SiOz as a result of the leaching of the other components, and finally change into SiOZ . nH,O, i.e., opaline silica (Minami et al., 1966; Ossaka, 1968). In order to simulate such processes, experiments were carried out by using thermal fluids and volcanic rocks whose chemical compositions, mineralogy and textures were known (e.g., Minatoetal., 1959; Iwasakietal., 1964; Ossaka, 1968). * Comesponding author. 0377-0273/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO377-0273(94)00039-l

Minato et al. ( 1959) soaked a block of andesite in hot strongly acid water over three months in the Tamagawa Hot Spring Area, Akita, Japan. They showed that the proportions of Si02 and HZ0 present in the residual rock increased, whereas those of other components, especially A1203, CaO and Na,O decreased markedly. They observed the formation of three zones in the residual rock, according to the alteration mineral assemblages formed and also pointed out that plagioclase was altered prior to pyroxenes in the intermediate zone. These facts correspond to the changes in the chemical composition of the residual rock. Many researchers have carried out experiments for rock and mineral alteration using acidic solutions, deionized water and electrolyte solutions in a batch

42

K. Nogami, M. Yoshida /Journal

of Volcanology and Geothermal Research 65 (1995) 41-49

and/or flowing system controlling pH values, temperature, flow rates and other reaction conditions (e.g., Kamiya, 1960; Chiba, 1962; Seyfried and Bischoff, 1979; Muir and Nesbitt, 1992; Walther and Woodland, 1993). Recent studies of rock-water interaction have been focused on the kinetics of mineral dissolution in electrolyte solutions (e.g., Brady and Walther, 1989; Dove and Crerar, 1990; Walther and Woodland, 1993) and observations of altered mineral surfaces using a variety of spectroscopic procedures (e.g., Hellmann et al., 1990; Muir and Nesbit, 1992). Sanemasaet al. ( 1972) investigated the dissolution reaction of olivine in inorganic acids at various temperatures. They estimated activation energy according to the Arrhenius plots and discussed effects of anions on activation energy. Dove and Crerar (1990) examined the kinetics of quartz dissolution in deionized water and electrolyte solutions. They also obtained data on activation energy and showed the effects of electrolytes on reaction rates. However, to investigate rock alteration processes under acidic conditions only by kinetics of the dissolution of individual phases is not the full story because volcanic rocks are aggregates of minerals and glass whose dissolution rates will be mutually affected by coexisting phases. In this work, experiments with acid solutions in a flowing system reproduce the leaching of rock-forming components and provide the basis for a method to investigate the leaching behaviour of these components. The experiments are also aimed to analyse the effects of temperature and acidity on leaching processes.

Table 1 Composition (wt.%) of45-74 pm fraction of sample used as material and LC- 1 lava from Izu-Oshima

SiO, TiOZ Al,03 FeOb CaO MgO MnO NazO K,O P,Os Total

starting

Starting material

LC-I”

54.64 I .26 14.43 13.20 9.08 4.15 0.22 2.2 1 0.48 0.10 99.77

54.15 I .24 14.56 14.09 9.02 4.07 0.20 2.05 0.47 0.10 99.95

Analyses by X-ray fluorescence. “Data from Fujii et al. ( 1988). “Total iron as FeO. ous magnetite and brownish glass exist in the groundmass. The sample was crushed in an agate mortar and sieved to obtain the 45-74 pm fraction. The separated sample was then washed with ethanol to eliminate very fine particles and was used as the starting material. Chemical compositions of LC-1 lava and the starting material used in the experiments are shown in Table 1. Segregation due to sieving is negligible.

2.2. Procedure Parts of an instrument for high-performance liquid chromatography were assembled into an apparatus for the experiments (Fig. 1) . The apparatus has a pump with a built-in damper whose maximum pressure is 7

2. Experimental 2.1. Material

The sample used is anonporphyritic basaltic andesite collected from a lava flow (LC- 1) of Miharayama, IzuOshima, which erupted in 1986. Small amounts of minute crystals of plagioclase and opaque mineral are scattered in its glassy groundmass. Fujii et al. ( 1988) and Nakano et al. ( 1988) reported that the microphenocrysts in the basaltic andesite comprise less than l2%. They concluded that the microphenocryst assemblage is plagioclase (An,,_,,) + clinopyroxene (augite and pigeonite) + orthopyroxene (bronzite and hypersthene) and that plagioclase, clinopyroxene, titanifer-

Fig. 1. Apparatus used for rock-acid solution interaction. 1 = acid solution; 2 = teflon tube; 3 = power switch; 4 = pump switch; 5 = display 6 = pump; 7 = damper; 8 = heater; 9 = Pyrex column; 10 = heat controller; 11 = valve; 12 = thermometer.

K. Nogami, M. Yoshido / Joumal of Volcanology and Geothermal Research 65 (1995) 4149 Table 2 Experimental

conditions

43

of EXP- 1 to EXP-6 EXP-I

Rock weight (g) Reaction temp. (“C) Pressure (kg/cm’) Reacting solution Total reaction time (min)

3.750 160 2-3 0.24 N H,SO, 5520

EXP-2

EXP-3

EXP-4

EXP-5

EXP-6

3.750 80 2-3 0.12 N HCI

3.750 120 2-3 0.12 N HCI

3.750 160 2-3 0.12 N HCI

1440

1440

1440

3.750 160 2-3 0.24 N HCI 1440

3.750 160 2-3 0.0024 N HCI 1440

kg/cm*. A Pyrex column of 10 mm inner diameter and 50 mm long was used for the reaction vessel. Samples of 3.750 g occupied about 80% of the capacity of the vessel. The column was connected to a Teflon tube via a glass filter supported with a Teflon stopper and heated with a 100 W mantle heater. The thermometer of the controller was set between the heater and the reaction vessel. HCl and H,S04 solutions were used as the acid media in the experiments and the acidity of solutions ranged from 0.0024 to 0.24 N. In preliminary experiments, differences in leaching behaviour of the components between HCl and H2S04 used as reaction fluids were not distinguishable in the same H30+ concentration. The flow rate of the acid solution was controlled with both an adjuster and a valve. Reaction temperature, run time and other experimental conditions are summarized in Table 2. Sample solutions were collected continuously in fractions. Based on the results of previous works (e.g., Iwasaki et al., 1976)) the decrease in the concentrations of dissolved components was expected to be rapid at the early stage of experiment and become slower. Therefore, the times taken for collecting solutions per fraction were extended as the reactions progressed. The first four fractions were collected at hourly intervals and the others a few hours apart.

2.3. Chemical analysis

Seven components - Na, K, Ca, Mg, Fe, Al and Si - in the solution were analysed by the following methods: Na and K with flame photometry; Ca, Mg and Si with ICP emission spectrometry; Fe and Al with colorimetry using 2,2’-bipyridine and 8-quinolinol, respectively.

3. Analysis of leached fluids

Data for EXP-1 performed at 160°C are given in Table 3. The flow rates ranged from 1.10 to 1.38 g/ min with 1.21 g/min as average. It was difficult to keep the flow rate more precisely constant because the packing condition of the sample changed as grains dissolved and secondary minerals, mainly opaline silica, were formed. The leached proportion P,(i) of component X, from the first to the ith fraction, is expressed by Eq. ( 1), where X is Na, K, Ca, Mg, Fe, Al and Si: i W,(k) P,(i) =k=IR x100 x

(1)

R, is the weight of component X present in the starting material and W,(k) is the amount of component X

leached in the solution of the kth fraction. P,(i) indicates the degree of leaching of component X from the original sample. The change of the leached proportion of each component with time is shown in Fig. 2. The seven components fall into two groups: one comprising Na, Ca and Al, showing the higher leached proportions; and the other, for K, Mg, Fe and Si, having less leaching. The leached proportions of Na, Ca and Al increase very sharply until about 1000 min but gradually afterwards, whereas the leached proportions of K, Mg, Fe and Si increase gradually throughout the experiment. The amounts of the components present in the residual rock obviously continued to decrease with time and the chemical and mineralogical compositions of the reacting solid material changed as the reaction proceeded. Accordingly, it is impossible to determine the relative facility with which the components leached at a particular reaction time from the leached proportions.

K. Nogami, M. Yoshida / Journal of Volcanology and Geothermal Research 65 (1995) 4149

44

=s,(i)

z,(i)

R,(i)

A

where S,(i) is the ratio of the amount of component X to the sum of seven components in the solution after the reaction to yield fraction i. R,(i) is a ratio of the amount of the component X to the sum of the amounts of seven components in the rock before the reaction to yield fraction i. Rx and W,(i) from Eq. ( 1) provide the amounts of the components present in the rock before the reaction to yield the ith fraction. The larger the leaching index, the more easily the component was leached. The variations of the leaching indices of the seven components against time are shown in Fig. 3. The indices of Na, Ca and Al are all above unity as these components were most easily leached. Those of K and Mg are lower than 1, although both gradually increase with time. The index of Fe gradually increases to be above 1 after about 4000 min. These variations mean that the

AI

0

Na

l A

cs

Fe Si UK

0 . v-

0

2000

4000

Mg

6000

Reaction time (min.)

Fig. 2. Change in leached proportions of Na, K, Ca, Mg, Fe, Al and Si vs. reaction time, resulting from leaching of these components from rock under acidic conditions in EXP- 1.

However, in order to compare the relative behaviour of the components at all reaction stages, we define a leaching index, Z,(i) , of component X in fraction i as:

Table 3 Concentration (in mg/kg) of major components in solution samples from EXP-I at 160°C using 0.24 N H,SO, Fraction No.:

I

2

Total reaction time (mm) Reaction time per fraction (min) Wt. of solution (g) Flow rate (g/min) Na K Ca Mg Fe Al Si

3

4

5

6

7

8

60 60 82.55 1.38

120 60 82.18 1.37

180 60 80.48 1.34

240 60 19.52 1.33

435 195 241 I .27

960 S25 663 I .26

1200 240 304 1.27

1440 240 309 I .29

69.7 3.95 256 11.8 147 389 183

51.6 2.16 171 5.36 113 283 125

50.0 2.08 161 5.32 105 263 126

45.3

40.9 I .82 127 5.37 91.6 209 139

18.7 0.97 57.8 3.60 56.2 92.3 130

8.58 0.70 28.5 3.31 16.4 44.4 124

5.80 0.52 19.7 3.20 30. I 21.5 113

1.96 151 5.33 98.2 241 129

Fraction No.

9

10

II

12

13

14

IS

16

Total reaction time ( min ) Reaction time per fraction (min) Wt. of solution (g) Flow rate (g/min)

1680 240 295 1.23

1800 120 151 1.26

2400 600 751 1.25

2520 120 147 1.23

2880 360 421

3000 120 146

4080 1080 1349

5520 1440 1588

5.22 0.56 17.2 3.65 29.6 22.8 118

4.16 0.52 15.0 3.93 27.8 18.2 117

2.44 0.41 9.80 3.41 20.0 10.5 88.1

2.18 0.50 8.81 3.62 19.7 8.62 84.6

Na K Ca Mg Fe Al Si

1.17

2.18 0.57 9.37 4.22 21.1 8.63 84.2

1.22 2.00 0.54 8.88 4.02 18.9 7.04 69.9

I .25

1.10

I .5 I 0.50 6.22 3.95 15.6 5.25 41.3

1.41 0.63 7.62 5.64 18.3 4.56 33.9

K. Nogami, M. Yoshida /Journal of Volcanology and Geothemal Research 65 (199s) 4149

A 0

P I¶ -0

2ow 4066 Reaction time (min.)

6666

Fig. 3. Leaching indices of Na, K, Ca, Mg, Fe, Al and Si in EXP-1 which indicate the relative ease of leachingfor the seven components, plotted against reaction time.

proportions of K, Mg and Fe present in the residual rock increased as the alteration progressed. The leaching index of Si becomes higher than 1 after about 1500 min, continues to increase until about 3000 min and then starts decreasing. This indicates that Si is not always the hardest element to be leached. Indeed, at the halfway stage of alteration, Si leached out more easily than K, Mg and Fe though it concentrated in the final residual rock.

45

K, Mg, Fe and Si were harder to leach than Na, Ca and Al. Although the rock used in the experiments is glassy and nonporphyritic, this tendency agrees with the alteration of crystalline phases shown by Minato et al. ( 1959). XRD patterns of the starting material and the residual rock of EXP-1 are shown in Fig. 4. Major minerals identified in the starting material are plagioclase and titaniferous magnetite. These minerals are also present in the residual rock, but the diffraction peaks of plagioclase are smaller. Broad peaks for SiOz. nH,O appear from 20 to 30” of 28. Sulfate minerals, such as anhydrite, gypsum and alunite, are not observed in the residual rock. These facts are in harmony with the results of the analysis of the solutions.

Residual rock

.i

30

B g

20

A A A0 a

E $10 :” I

0

,

20

I

40

60

28 (Cu Ku ) I degree Fig. 4. XRD patterns of starting and residual rocks used in EXP- 1. PI= plagioclase; Ti = titaniferous magnetite.

0 L 0

:

,P b4

PB

!

500

1000

Reaction time (min.)

Fig. 5. Effect of reaction temperature on leached proportions of Na, K, Ca, Mg, Fe, Al and Si. Change in leached proportions of seven components are plotted against reaction time in EXP-2, -3 and -4 using0.12NHCl. (a) 80°C. (b) 120°C. (c) 160°C.

46

K. Nogami, M. Yoshida/ Journal of Volcanology and Geothermal Research 65 (1995) 4149

0

2

4

6

0.6,

0.0

4 0

1

2

0.6,

0.0

Fig. 6. Leaching Cl = 160°C.

4 0

6

0

I

3-

I

04

3

0

1

4-

10

20

30

10

20

30

4

I 4 LeachLl proportion (%)

6

indices of Na, K, Ca, Mg, Fe, Al and Si plotted against leached proportions

4. Effect of reaction conditions on relative leaching facility 4.1. Reaction temperature In order to examine the effects of temperature on the leaching behaviour of all components, three experiments: EXP-2, -3 and -4 were carried out using 0.12 N HCl solution for 24 h at 80,120 and 160°C respectively (Table 2).

in EXP-2. -3 and -4. 0 =8O”C; l = 120°C;

The leached proportions of the seven components as a function of time at 80, 120 and 160°C, are shown in Fig. 5, b and c, respectively. These figures indicate that the higher is the reaction temperature, the larger are the leached proportions of all the components. In each experiments, the leached proportions of Na, Ca and Al show similar patterns and increase sharply with time, while the proportions of K, Mg, Fe and Si increase gradually.

K. Nogami, M. Yoshidn /Journal of Volcanology and Geothermal Research 65 (1995) 41-49

47

ture between 80 and 160°C. These results indicate that only the reaction rates change with reaction temperature, whereas the reaction processes do not essentially change. 4.2. Acidityof solution

(b):O.lZN A

z A

30. z r 8 g 20. P

A

::

:

Reaction time (min.)

Fig. 7. Effect of acidity of HCI solution on leached proportions of Na, K, Ca, Mg, Fe, Al and Si. Changes in leached proportions of seven components am plotted against reaction times in EXP-4, -5 and-6at 160°C. (a) 0.0024N. (b) 0.12N. (c) 0.24N.

In EXP-2, -3 and -4, the leached proportions of all the components increase as the reaction temperature rises. This indicates that the chemical composition of the reacting solid after the same reaction time varies with temperature. In order to compare the relative facility of the components leached at the same reaction stage, it is necessary to examine the leaching indices of the components against the leached proportions. Relations between the leaching indices for all components and their leached proportions in EXP-2, -3 and -4 are summarized in Fig. 6. Plots of the leaching indices versus the leached proportions for each component follow the same curve in all the experiments. Although the rise of temperature causes an increase of the leached proportions of the components, relative facility of the components to be leached is independent of tempera-

Hellmann et al. ( 1990) examined the pH dependence on dissolution rate and the leached layer thicknesses of albite. They showed that deepest leaching occurred at acid pH, lesser leaching at basic pH and little leaching at neutral pH. To investigate the effects of H30+ concentration in the acid pH region on the relative facility of the components to be leached, we carried out two experiments: EXP-5 and -6. Except for acidity, experimental conditions are the same as those given in EXP4 (Table 2), The leached proportions of all components as a function of time are shown in Fig. 7a, band c, using 0.0024, 0.12 and 0.24 NHCl solutions, respectively. The seven components in EXP-4 and -5 fall into two groups according to their leached proportions: one for Na, Ca and Al showing the higher proportions and the other, for K, Mg, Fe and Si, having the lower proportions. The components in EXP-6 are divided into four groups according to the leached proportions: one for Na, second for Ca and Al, third for K and Si and fourth for Mg and Fe. Plots of leaching indices of the seven components versus their leached proportions in EXP4, -5 and -6 are summarized in Fig. 8. In EXP-4 and -5, each component follows the same trend and the change of acidity from 0.24 to 0.12 N do not noticeably affect the facility of the components to be leached. In EXP-6 using 0.0024 N HCl solution, the plots of the indices of Na, Ca, Al and Fe follow trends quite different from those in EXP-4 and -5. The leaching indices of these components decrease, while the index of Si increases and becomes comparable to the indices for Na, Ca and Al. These results indicate that not only the reaction rates but also the reaction processes change in accordance with the change in the acidity of the reacting solution, from 0.12 to 0.0024 N. 5. Conclusions Experimental studies on the hydrothermal alteration of glassy basaltic andesite under conditions of high

48

K. Nogami, M. Yoshidn / Journal of Volcanology and Geothermal Research 65 (1995) 4149

2.8 ‘r

0

10

5

15

I

41

20

0

I

20

40

60

0.8 G

0.61

Ca

0.01

0.0

0

2

4

6

0

10

20

30

---_-I 0

20

LeAed

prbDportionl;%)

Fig. 8. Leaching indices of Na, K. Ca, Mg, Fe, AI and Si plotted against leached proportions in EXP-4. -5 and -6. 0 = 0.24 N; 0 = 0.12 N; A = 0.0024 N.

temperature and strong acidity indicate that Na, Ca and Al are leached out quickly compared with K, Mg and Fe. Si is mostly concentrated in the residual rock, but it is not always the hardest component to be leached out. Their leaching behaviours for glassy and nonporphyritic rock used in the present experiments coincide with those of crystalline rock previously reported. The reaction temperature markedly affects the leached proportion of the components, however, plots of the leaching indices against the leached proportions of the components follow the same trends regardless of

temperature. This indicates that an increase of the reaction temperature activates the leaching of the components, although the reaction mechanisms do not necessarily change. On the other hand, the acidity of the solutions strongly affects both the leached proportions and the leaching indices. The trends of the indices against the proportions of almost all the components depend upon the acidity. This indicates that not only the reaction rates but also the reaction processes are affected by changes in acidity of the reacting solutions.

K. Nogami, M. Yoshidn /Journal of Volcanology and Geothermal Research 65 (1995) 41-49

Acknowledgements

We would like to express our appreciation to Professor Masahiro Yamamoto of Okayama University for giving permission to use the ICP emission spectrometer. We also thank the members of Fujii Laboratory of University of Tokyo for analysing the rock sample. We are deeply indebted to Professor Emeritus Joyo Ossaka of Tokyo Institute of Technology for valuable discussions and instructive suggestions. Finally we thank the two reviewers for their constructive comments on the manuscript.

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lwasaki, I., Hirayama, M., Katsura, T., Ozawa, T., Ossaka, J., Kamada, M. and Matsumoto, H., 1964. Alteration of rock by volcanic gas in Japan. Bull. Volcanol., XXVII: 3-16. Iwasaki, I., Yoshiike, Y., Yoshida, S. and Ohmori, T., 1976. Variation in concentration of hot spring water by the reaction of rock with continuous flow of hot spring water of the Tamagawa hot springs. J. Balneol. Sot. Jpn., 27: l-27 (in Japanese withEnglish abstract). Kamiya, H., 1960. Leaching of andesite in acid media. Bull. Chem. Sot. Jpn., 33: 1731-1736. Minami, E., Ossaka, T. and Ossaka, J., 1966. The acid alteration and the formation of halotrichite and alunogen at the volcanoes and the hot springs in Japan. J. Balneol. Sot. Jpn., 17: 28-35 (in Japanese with English abstract). Minato, H., Nagashima, K. and Minami, E., 1959. Chemical and mineralogical change of rocks soaked in Yukawa at Tamagawa hot spring area. Gen. Stud. Tamagawa Hot Spring., 6: 3-7 (in Japanese). Muir, I.J. and Nesbitt, H.W., 1992. Controls on differential leaching of calcium and aluminum from labradorite in dilute electrolyte solutions. Geochim. Cosmochim. Acta, 56: 3979-3985. Nakano, S., Togashi, S. and Yamamoto, T., 1988. Bulk and mineral chemistry of products of the 1986 eruption of lzu-Oshima Volcano. Bull. Volcanol. Sot. Jpn., 33: S255-S264. Ossaka, J., 1968. Alteration of rocks in volcanoes and hot springs area. CHINETSU, 17: 65-79 (in Japanese with English abstract). Sanemasa, I., Yoshida, M. and Ozawa, T., 1972. The dissolution of olivine in aqueous solutions of inorganic acids. Bull. Chem. Sot. Jpn., 45: 1741-1746. Seyfried Jr., W.E. and Bischoff, J.L., 1979. Low temperature basalt alteration by seawater: an experimental study at 70°C and 150°C. Geochim. Cosmochim. Acta, 43: 1937-1947. Walther, J.V. and Woodland, A.B., 1993. Experimental determination and interpretation of the solubility of the assemblage microcline, muscovite, and quartz in supercritical H20. Geochim. Cosmochim. Acta, 57: 2431-2437.