Soil Science Society of America Journal 67:654-661 (2003)
© 2003 Soil Science Society of America
DIVISION S-9—SOIL MINERALOGY
Dissolution of Ripidolite (Mg, Fe-Chlorite) in Organic and Inorganic Acid Solutions
M. Hamer*,a,
R. C. Grahamb,
C. Amrheinb and
K. N. Bozhilovc
a Institut fuer Bodenkunde, Univ. Bonn, Nussallee 13, D-53115 Bonn, Germany
b Soil and Water Sciences program, Dep. of Environ. Sci., Univ. of California, Riverside, CA 92521-0424
c Central Facility for Advanced Microscopy and Microanalysis, Univ. of California, Riverside, CA 92521
* Corresponding author (hamer{at}boden.uni-bonn.de)
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ABSTRACT
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Although chlorites are an important source of Mg and micronutrients in soils, little is known about their weathering behavior in acidic environments. The effect of organic (acetic, oxalic, citric) and inorganic (hydrochloric, nitric, sulfuric) acids on the dissolution of ripidolite was studied at 25°C over an acid concentration range of 0.03 to 10 mM. The increasing ripidolite dissolution with increasing acid concentration can be described by the same empirical equation for all six acids used in this study. Because proton and ligand promoted dissolution mechanisms are additive, the influence of ligands can be calculated by subtracting the influence of protons from the total dissolution of ripidolite. We assume that the influence of Cl- in HCl systems can be neglected. Greater dissolution of ripidolite in the presence of the other acids at the same pH is attributed to the accompanying anion. At pH 3.5 and an anion concentration of 10-3.5 mol L-1, the relative effectiveness of the acids used in this experiment in promoting dissolution was nitric (enhancement factor: 0.97) 
hydrochloric (1.00) acetic (1.01) < sulfuric (1.19) < citric (2.70) < oxalic acid (3.27). The dissolution of ripidolite was nonstoichiometric with a preferential release of Si relative to Al, Fe, and, in some cases, Mg at low proton and ligand concentrations for all six acids. At higher acid concentrations the dissolution becomes almost stoichiometric in the presence of inorganic acids, whereas in the presence of oxalic acid Al, Fe, and Mg were released preferentially relative to Si. In the presence of citric acid, Fe is released preferentially relative to Si. The alteration of chlorites in soils and the amount of released elements into the soil solution therefore depends on the composition of acidifying agents in soils.
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INTRODUCTION
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ALTHOUGH CHLORITES ARE COMMON and widespread minerals, only a few studies on chlorite weathering under controlled laboratory conditions have been published (Brydon and Ross, 1966; Ross, 1967; Ross, 1975; Jones, 1981; Kittrick, 1982; Kodoma et al., 1983). Most weathering studies have primarily focused on the transformation of chlorites into secondary alterations products in soil environments (Bain, 1977; Rice et al., 1985; Graham et al., 1990; Cho and Mermut, 1992; Righi et al., 1993; Yoneda et al., 1995; Kitagawa and Itami, 1996; Carnicelli et al., 1997). Because the characterization of the chlorite species in soils is difficult and the chemical environment of soils is highly variable, little is known about the weathering behavior of chlorites. The objective of the present study was to characterize the dissolution behavior of a well defined chlorite (ripidolite) in various controlled chemical weathering environments.
Chlorites can be found in igneous as well as low- and medium-grade pelitic and mafic metamorphic rocks, usually derived by alteration of primary Mg- and Fe-bearing minerals such as pyroxenes, amphiboles, biotite, and other ferromagnesium minerals (Nesse, 2000). Furthermore they appear as a product of diagenesis in clay-bearing sediments and sedimentary rocks (Heim, 1990). Most commonly, chlorites occur as fine-grained scaly or foliated massive aggregates, characterized by their green color, micaceous habit and cleavage, and by the fact that the folia, in contrast to micas, are not elastic (Klein and Hurlbut, 1985).
Most chlorites are trioctahedral phyllosilicates with a 2:1:1 layer structure in which a 2:1 layer structure and an octahedral sheet alternate. The 2:1 layer (structurally similar to mica) is negatively charged and the octahedral sheet (interlayer hydroxide sheet) is positively charged. In most trioctahedral chlorites the principle substitutions are of Fe2+, Fe3+, and Al3+ for Mg2+ in both the talc and brucite-like layers, and Al3+ for Si4+ in the tetrahedral sites.
The breakdown and stability of soil minerals is quite complex and depends on the crystal structure and chemical composition of the mineral as well as the physical and chemical environment to which the mineral is exposed. Two of the most important chemical weathering reactions in soil environments are proton- and ligand-induced dissolution reactions (Furrer and Sticher, 1999). Depending on factors, such as climate, vegetation, and agricultural use, the amount and composition of organic and inorganic acids in soil solutions can vary over a wide range. Adsorption of H+-ions on mineral surfaces enhances the hydrolysis of X-O (X = Si, Al, Fe, Mg) framework bonds and thus the detachment of X into solution. Furrer and Stumm (1986) examined the dissolution of oxide minerals and found that the rate of dissolution is proportional to the concentration of adsorbed–exchanged protons or activated surface complexes. The dissolution of phyllosilicates may be much more complex. Furrer and Sticher (1999) suggested that ligands increase the dissolution of aluminosilicates by decreasing the activation energy for the rate-limiting step in hydrolysis by forming surface complexes with framework elements and that proton- and organic ligand-promoted mechanisms act simultaneously, but do not compete with each other and are thus additive.
Input of inorganic acids from the atmosphere because of anthropogenic acid deposition is an obvious source of soil acidification. The major sources of protons and ligands in natural soil acidification are organic acids derived by decomposition of organic substances and root exudates. Using inorganic and organic acids to study weathering reactions under laboratory conditions, chlorites are found to form soluble species, as indicated by the following reaction (equation calculated by using the chemical composition of the ripidolite used in this study):
 | [1] |
Low pH and the presence of complex-forming agents suppresses both readsorption of dissolved metals and the formation of insoluble Fe- and Al-oxides. Thus, under low pH conditions measurements of dissolution rates of framework elements can be used to characterize the influence of different chemical environments on the stoichiometry and overall dissolution rates of ripidolite.
In this study, three inorganic and three organic acids were used in batch weathering experiments to evaluate (i) the pH dependence of ripidolite weathering, (ii) the relative effectiveness of different ligands on promoting ripidolite dissolution, and (iii) the stoichiometry of ripidolite dissolution in different chemical environments.
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MATERIALS AND METHODS
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Preparation of Solids
The ripidolite sample (obtained from the Department of Environmental Sciences, University of California, Riverside, USA) was crushed mechanically with a disk grinder. The crushed material that passed through a 2.0-mm sieve was ground (Brinkmann Mortar Grinder, Brinkmann Instruments, Westbury, NY) and wet-sieved to obtain the 125- to 38-µm size fraction. To minimize the effect of high energy surface sites produced during grinding the 125- to 38-µm size fraction was washed (mineral:solution-ratio = 1:100) twice in 10 mM HCl and twice with double deionized water. The cleaned ripidolite was then dried at 60°C. X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and triple-point BET analyses were used to determine the crystal structure, surface morphology, and surface area of the starting material.
Experimental Solutions and Design
Stock solutions of the six acids (Table 1) were prepared from reagent grade chemicals and double deionized water. To prevent microbial breakdown of the organic acids, approximately 1 mL (2 drops) of chloroform per liter was added to all solutions. The dissolution experiment was conducted in acid-washed 50-mL polyethylene bottles at 25°C. The pretreated ripidolite (0.1 g) was placed into 25 mL of inorganic or organic solutions of different concentrations (0.03, 0.10, 0.30, 1.0, 3.0, and 10.0 mM). The reaction vessels were placed on a side-to-side shaker and agitated at 60 cycles per minute. Even if agitation causes abrasion of the particles, which could affect dissolution rates, all experiments were performed the same way thus comparison within different acids is possible. After 2 wk of reaction the bottles were centrifuged at 1395 g for 20 min. A 10-mL aliquot of the clear supernatant was removed and acidified with 2 mL concentrated HNO3 to prevent precipitation of Al and Fe during storage. Experimental solutions were analyzed for Si, Al, Fe, and Mg by means of inductively coupled plasma atomic emission spectroscopy (ICP-AES). The sum of these four elements was used as an index of dissolution rates. Although dissolution rates are influenced by sample preparation, agitation, equilibration time, and other experimental protocols, the rates within this experiment are comparable and thus provide information about the dissolution behavior of ripidolite in different environments.
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Table 1. Chemical formula, molecular weight, dissociation constant and calculated pH of inorganic and organic acids used in the experiment.
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RESULTS AND DISCUSSION
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According to powder XRD analysis, the mineral sample can be considered a nearly pure chlorite (Fig. 1)
. The cations which occupy the tetrahedral and octahedral sites influence d-spacing (Brindley and Brown 1980). Substitution of Al3+ for Si4+ in tetrahedral sites decreases basal spacing and can be described by the following equation: d(001) = 14.55 - 0.29x (Bailey, 1972), where d(001) is the spacing in Å and x the number of Al ions replacing Si in the formula (Si4-xAlx). Furthermore the content of heavy metals (e.g., Fe, Mn, Cr) in trioctahedral chlorites influences the b dimension of the chlorite unit cell. Brindley and Brown (1980) noted that the b is related to Fe2+ for Mg substitution (y) in octahedral positions by the following equation: b = 9.210 + 0.037y. By using the above relationships, one can calculate Al and Fe values from XRD data, yielding the following, a generalized chemical composition of the trioctahedral chlorite: (Mg2.80 Fe2+1.95Al1.25)(Si2.75Al1.25) O10(OH)8. To estimate the distribution of Fe and Mg between the 2:1 octahedral sheet and the interlayer hydroxide sheet, tabulated values of intensity ratios of odd-ordered diffraction peaks (Brindley and Brown 1980) can be used. On the basis of these XRD analysis, the chlorite used in this study can be described as a medium Fe-chlorite (ripidolite) with an almost symmetrical distribution of metals.
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Fig. 1. X-ray diffraction pattern of randomly oriented pretreated ripidolite from 2 to 65° 2 theta and enlargement of the 35 to 65° 2 theta area (125- to 38-µm fraction); spacing in Å.
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The surfaces of the pretreated particles were characterized by scanning electron microscopy (SEM). Figure 2
shows a micaceous ripidolite grain with a smooth surface and few fine particles adhering to the surface. Thus the treatment was mild enough to prevent etch pits or any other visible weathering traces on the surface, but it was not strong enough to remove all fine particles. Since ultrafine particles are associated with an initial rapid nonlinear dissolution rate (Holdren and Berner, 1979), the remaining particles may have little influence on the measured rates. The surface area of the pretreated material was 1.44 m2 g-1.
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Fig. 2. Scanning electron microscope image of a pretreated ripidolite grain (125- to 38-µm fraction; treated two times with 10 mM HCl and two times with distilled water).
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Dissolution Kinetics
The sums of dissolution rates of Si, Al, Fe, and Mg in the presence of 10 mM inorganic and organic acids are shown in Figure 3
. Although the sum of released elements for the different acids ranged from 0.10 mol m-2 s-1 x 10-10 (acetic acid) to 5.9 mol m-2 s-1 x 10-10 (oxalic acid), the ratios of released elements in each case approximates the ratios of elements in the mineral. The dissolution of ripidolite in HCl and HNO3 was similar whereas the dissolution in H2SO4 was more than twice as fast. This is because H2SO4 is a diprotic acid (Table 1).
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Fig. 3. Ripidolite dissolution rates (R), determined on the basis of the sum of released framework elements, in the presence of 10 mM inorganic and organic acid solutions.
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The dissolution rates of ripidolite in the presence of 10 mM organic acids increases in the order of acetic acid < citric acid < oxalic acid, which are mono-, tri-, and dicarboxylic acids, respectively (Table 1). Organic anions increase the weathering rate of silicates because of their ability to form complexes with framework metals which enhance both the solubility of the metals and the dissolution of minerals (Boyle and Voigt, 1973; Manley and Evans 1986; Amrhein and Suarez, 1988; Barman et al., 1992; Stillings et al., 1996; Zhang and Bloom, 1999).
Several studies have shown that the proton and ligand promoted dissolution of oxides (Furrer and Stumm, 1986) and aluminosilicates (Kalinowski and Schweda, 1996; Welch and Ullman, 1996; Malmström and Benwart, 1997) in acidic solutions is proportional to the hydrogen ion and ligand activity in solution. The dissolution kinetics can be described by the following exponential functions:
 | [2] |
where R is the mineral dissolution rate, k is a rate constant, a the hydrogen ion or ligand activity, and n an experimentally determined factor. The subscript H indicates proton promoted dissolution and the subscript L indicates ligand promoted dissolution.
Figure 4
shows the increasing dissolution rates of ripidolite–indicated by the sum of Si, Al, Fe, and Mg released into solution, in the presence of increasing HNO3 concentrations. The correlation coefficient of r2 = 0.998 reveals that the dissolution reaction can be described well by Eq. [2]. This empirical equation can be transformed into:
 | [3] |
where the solution pH can be plotted versus the log mineral dissolution rates. Thus we obtain the pH dependence of dissolution rates as a straight line with a slope equivalent to nH or nL of Eq. [2].
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Fig. 4. Ripidolite dissolution rates (R), determined on the basis of the sum of released framework elements, at increasing HNO3 concentrations (circles) and dissolution rates calculated by means of Eq. [2] (line).
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pH Dependence
By calculating the H+-ion activity of each acid in the concentration range used in this study, the pH dependence of ripidolite dissolution rates in inorganic and organic acids can be characterized as shown in Figure 5
. For all of the six acids used in our experiments the dissolution rates increase with decreasing pH. Compared with results from Kalinowski and Schweda (1996) who found a pH dependence of -0.14 for muscovite and -0.61 for biotite by using inorganic acids, the pH dependence of ripidolite (-0.45 in HCl), and therefore the weathering stability, can be considered less than muscovite but greater than biotite. These results also correspond with findings of Graham et al. (1989) who studied the weathering of iron-bearing minerals in soils and interpreted a higher weathering stability of Fe-containing chlorites compared with biotite.
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Fig. 5. Log dissolution rates (log R) of ripidolite vs. pH (data points) in inorganic and organic acid solutions and pH dependence calculated by means of Eq. [3] (straight lines).
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Proton- and Ligand-Promoted Dissolution
If the H+-ion activity alone influenced the dissolution of ripidolite, all acids should have the same pH dependence. Comparing the three inorganic acids, it can be seen that hydrochloric and nitric acid show approximately the same pH dependence, whereas the steeper slope of sulfuric acid indicates an additional effect besides the increasing H+-ion activity. The comparable pH dependence of hydrochloric and nitric acid, thus indicates a similar influence of Cl- and NO-3 on the dissolution, whereas HSO-4 and SO2-4 seem to increase the release of framework metals. This could be due to the different ability of these anions to form soluble complexes with the metals. According to Lindsay (1979), Cl- and NO-3 form weaker complexes with Al and Fe than SO2-4 does. Other authors have described higher weathering rates in the presence of SO2-4 compared with Cl- and NO-3 (Stumpf et al., 1984; Bloom and Erich, 1987).
The pH dependence of acetic acid is only slightly higher than hydrochloric and nitric acid, at least between pH 3.4 to 4.8. The monocarboxylic anion thus has only little effect on ripidolite dissolution rates. The di- and tricarboxylic anions of oxalic and citric acid, by contrast, substantially increase ripidolite dissolution. These acids are known to form complexes with Al and Fe (Tan, 1993; Strobel, 2001) in solution and to catalyze aluminosilicate dissolution by decreasing the activation energy for the rate-limiting step in hydrolysis at the mineral surface (Furrer and Sticher, 1999).
The only difference in the initial solution composition of the six acids at the same pH is the anion associated with the acid. Thus, differences in the dissolution behavior of ripidolite at the same pH are due to the accompanying anion. Because proton- and ligand-promoted mechanisms act simultaneously and are additive, it is possible to isolate the effect of ligands (RL) by calculating the difference between the total ripidolite dissolution rate (RT) and the dissolution rate of ripidolite promoted by the H+ activity (RH) at the same pH as follows:
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This calculation takes into account that surface protonation, induced by the adsorption of a ligand, is part of the ligand promoted effect (Stumm 1992). On the basis of experiments conducted with feldspars, neither chloride nor nitrate ions were expected to have significant effect on mineral dissolution (Welch and Ullman, 1996). The pH dependence of ripidolite in the presence of hydrochloric and nitric acid can thus be attributed solely to the H+-ion activity.
By using Eq. [4] and by assuming RT in HCl equal RH, one can caluculte the effect of proton- and ligand-promoted dissolution in the investigated acidic systems (Table 2). The total dissolution rates of ripidolite at pH 3.5 (RT(pH3.5)) range from 8.28 mol m-2 s-1 x 10-12 in nitric acid to 25.53 mol m-2 s-1 x 10-12 in citric acid. The ligand-promoted rates at pH 3.5 [RL(pH3.5)] can be obtained by subtracting RT of hydrochloric acid from the total dissolution rates of the other acids. However, to compare the effectiveness of different ligands it is necessary to calculate a rate, based not only on the same H+ activity but also on the same ligand activity [rate normalized to the same ligand activity: RL(pH3.5;pL3.5)]. This ligand-promoted dissolution rate yields an enhancement factor to describe the effect of ligands. Thus the relative ability of the ligands to increase the ripidolite dissolution rate is nitric (0.97) hydrochloric (1.00) acetic (1.01) < sulfuric (1.19) < citric (2.70) < oxalic acid (3.27). According to these values the dissolution of ripidolite by hydrochloric, nitric, and acetic acid appears to be related to a mainly proton-induced dissolution reaction, whereas the accompanied anion of sulfuric, citric, and oxalic acid increase the dissolution rate significantly by forming metal-ligand complexes with framework metals, in addition to the proton-promoted dissolution.
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Table 2. Rate constant, slope (pH dependence on dissolution rate; Eq. [2], [3], and [4]), calculated dissolution rate and enhancement factor for ripidolite in the presence of inorganic and organic acid solutions.
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Our data correspond with studies in the literature for other minerals. Bloom and Erich (1987), and Ridley et al. (1997) have shown that sulfate increases the dissolution of Al-bearing minerals, compared with other anions like chloride and nitrate. Several other studies have shown that organic acids enhance mineral dissolution of feldspars by a factor of 2 to 5 at pH 3 (Stillings et al., 1996), of hornblende by a factor of 1.4 to 4.9 at pH 4 (Zhang and Bloom, 1999), and of micas (Barman et al., 1992) by a combined action of acid attack and complexation.
Dissolution Stoichiometry
The dissolution stoichiometry of ripidolite at different pHs and with different acids can be assessed by comparing the ratios of released elements with the initial mineral composition. Figure 6 shows the Al/Si, Fe/Si, Mg/Si, and the Fe/Mg solution ratios in the presence of inorganic and organic acids between pH 1.79 to 4.80. Three of the ratios (Al/Si, Fe/Si, Mg/Fe) reveal a highly incongruent mineral dissolution at relatively high pHs (pH > 4) with a preferential release of Si for both inorganic and organic acids. With decreasing pH the dissolution becomes increasingly congruent. The Al/Si ratio reaches congruency at pH 3 in the presence of inorganic acids and shows slightly higher ratios in the presence of organic acids, indicating a preferential release of Al. The Fe/Si and Fe/Mg ratios in inorganic acid solutions reach congruency approximately at pH 2, whereas in organic acid solutions ratios indicating congruency are reached and partly surpassed at approximately pH 4. The Mg/Si ratios reveal an almost congruent dissolution in the presence of inorganic acids over the entire pH range and show only a few ratios lower than congruency in organic acid solutions between pH 4 and 5.
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Fig. 6. Element ratios in the presence of inorganic (IO) and organic acids (OA) vs. pH. Dotted line represents the element ratio of the initial mineral.
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The stoichiometry of dissolution can be examined in more detail by means of data presented in Table 3. Holdren and Speyer (1985) used a Relative Release Ratio to describe the stoichiometry of alkali feldspar dissolution at different pHs:
 | [5] |
where RRRAl/Si is the relative release of Al with respect to Si (ratio = 1: congruent dissolution; ratio < 1: preferential release of Si; ratio > 1: preferential release of Al). Table 3 shows a slightly preferential release of Al in the presence of 10 mM oxalic acid. All other acids show a highly incongruent dissolution of Al in 0.03 mM and an almost congruent dissolution in 10 mM solutions. This finding is comparable with the behavior of Fe. But besides oxalic acid, the ratios in sulfuric and citric acid at low pHs (10 mM = pH < 1,79 and < 2,41) also indicate a preferential Fe release. Except for acetic and citric acid, the dissolution of Mg seems to be congruent within the pH range investigated. Comparing the dissolution of Fe relative to Mg, only sulfuric, oxalic, and citric acid at low pHs induce a slightly preferential release of Fe.
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Table 3. Relative release ratios (RRR ) for Al, Fe, and Mg relative to Si and Fe relative to Mg for ripidolite in the presence of inorganic and organic acid solutions (0.03 and 10 mM).
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These results are consistent with Ross (1968)(1969) who showed that the dissolution of chlorites in very acid inorganic solutions (2 M HCl) was congruent, suggesting a dissolution process in which the octahedral and tetrahedral sheets were equally attacked. However, in acid solutions, containing complex-forming ligands, a preferential release of Fe and Mg can be seen (Table 3) indicating a preferential attack of octahedral sheets in the chlorite structure. The low release rates of Al and Fe at high pHs (pH > 4.22) and at low ligand concentrations (0.03 mM) indicated by RRR values < 1 may be caused by the precipitation of hydroxides or a readsorption of Al and Fe at or near the surface. These surface precipitates could affect dissolution rates calculated in Fig. 5. Furthermore an Al and Fe solution concentration close to the corresponding solubility product of Al and Fe hydroxides suggests the formation of hydroxides, even though XRD investigations after the experiment showed no evidence of new crystal phases. This is likely caused by the short time for mineral crystallization in this study relative to natural, field conditions of mineral formation in soils. Precipitates in this study are most likely poorly crystalline compounds and too small to be detected by XRD.
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CONCLUSIONS
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Chemical weathering of rock-forming minerals is of fundamental importance in controlling ground water geochemistry and the maintenance of soil fertility. Many laboratory experiments have been conducted to investigate the dissolution behavior of feldspars and 2:1 phyllosilicates, such as mica and biotite, in the presence of varying proton and ligand concentrations, but no such data are available for chlorites.
Our data show a linear relationship between ripidolite dissolution rates and pH within the investigated pH range in the presence of organic and inorganic acids. Comparable to the behavior of other aluminosilicates, complex forming ligands accelerate the dissolution rates of ripidolite.
Furthermore, the results show that the stoichiometry of chlorite dissolution and therefore the appearance of secondary weathering products depends on the solution chemistry in which the mineral dissolves. In neutral or slightly acidic soils with high contents of complex-forming acids, the octahedral sheets of chlorites are more vulnerable than the tetrahedral sheets. This may result in a swelling mixed-layer intermediate mineral or a vermiculite. However, chlorite weathering in soils with very low pHs or low contents of complex-forming acids results in a more congruent dissolution of the mineral. This latter conclusion supports the finding of Cho and Mermut (1992) that chlorites can be transformed directly to weathering products such as kaolinite, halloysite, Al- or Fe-oxides, amorphous materials, or soluble products.
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ACKNOWLEDGMENTS
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This work was performed with financial support from the German Research Foundation (Deutsche Forschungs Gemeinschaft, DFG), Bonn, Germany, and the Department of Environmental Sciences, University of California–Riverside, USA.
Received for publication March 11, 2002.
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