Incongruent and congruent dissolution of plagioclase feldspar: effect of feldspar composition and ligand complexation

Geoderma, 55 (1992) 55-78 Elsevier Science Publishers B.V., Amsterdam

55

Incongruent and congruent dissolution of plagioclase feldspar: effect of feldspar composition and ligand complexation William Shotyk a and H. Wayne Nesbitt b aGeological Institute, University of Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland hDepartment of Geology, University of Western Ontario, London, Ont., Canada N6A 5B7 ( Received November 28, 1991 ; accepted after revision April 2, 1992 )

ABSTRACT Shotyk, W. and Nesbitt, H.W., 1992. Incongruent and congruent dissolution of plagioclase feldspar: effect of feldspar composition and ligend complexation. Geoderma, 55:55-78. Secondary ion mass spectrometry (SIMS) was used to characterize the surfaces of plagioclase feldspar (two labradorite compositions and one anorthite) leached in water (pH 5.8), and in 10-4M HCI, HF, and oxalic acid, all at the same pH (4.0-4.1). Labradorite (An54 and An6o) leached at pH 4 for 72 days dissolved incongruently in HC1, oxalic acid, and HF, yielding altered layers strongly depleted in AI, Ca, and Sr and residually enriched in Si. The leached layers formed on the more calcic composition (An6o) were approximately 3 times thicker than those formed on the more sodic (An54) composition. The thinnest leached layers were produced by HCI (up to 1000 ~ on Ans4 and up to 3000 ~ on An6o), and the thickest leached layers by HF (up to 1700 ~ on An54 and up to 5000 ,~ on An6o). Because the H+concentration was the same in each solution, it was possible to clearly separate the relative importance of proton-promoted dissolution (in HCI) and ligand-promoted dissolution (in oxalic acid and in HF) for a given feldspar composition. To do this, the SIMS depth profiles obtained from An6o labradorite leached in oxalic acid and in HF were normalized to those produced in response to HCI. This comparison clearly shows that the ligands oxalate and fluoride significantly increased the formation of leached layers, compared to the effect of H ÷ alone. in contrast, anorthite (An~00) leached in the same solutions essentially dissolved congruently. In response to HC1 only very thin leached layers formed in which AI and Ca were only weakly depleted relative to Si. Chemical analyses of the output solutions showed that the molar ratio of Si to A1 in solution ( 1.1 to 1 ) is equal to the molar ratio of Si to AI in the fresh solid, confirming that the anorthite dissolved congruently. While the leached layers found on anorthite leached in oxalic acid and HF were thinner and less intensively depleted compared to the leached layers formed in response to HCI, the concentrations of Si and AI in the output solutions were significantly greater than those in the HCI solutions. While oxalate and fluoride again promoted the dissolution of the feldspar, Si and AI were still released to solution in the same molar proportion as they are found in the solid (congruent dissolution ). The early stages of feldspar dissolution, therefore, may be either incongruent or congruent, depending upon the chemical composition of the feldspar.

Correspondence to: W. Shotyk, Geological Institute, University of Bern, Baltzerstrasse 1, CH3012 Bern, Switzerland.

0016-7061 / 92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

56

W. SHOTYK AND H.W. NESB1TT

INTRODUCTION

The formation of leached layers on the surface of dissolving feldspars has been proposed by many investigators to explain the initial, non-stoichiometric stage of experimental feldspar dissolution in acidic solutions (Correns and von Engelhardt, 1938; Correns, 1962, 1963; Nixon, 1979; Gardner, 1983; Chou and Wollast, 1984, 1985; Holdren and Speyer, 1985, 1986; Wollast and Chou, 1985; Sj6berg, 1989; Hellmann et al., 1990). In acidic solutions (pH < 5 ), analyses of dissolved cations, A1, and Si indicate that feldspars initially dissolve incongruently: the cations and A1 are removed from the feldspar preferentially, apparently leaving behind a layer residually enriched in Si (Chou and Wollast, 1984, 1985; Holdren and Speyer, 1985, 1986). Recently, direct evidence for the formation of significant leached layers on naturally and experimentally dissolving feldspar surfaces has been obtained by investigators working independently, using a variety of surface analytical techniques (Schott and Petit, 1987; Hochella et al., 1988; Casey et al., 1988; Althaus and Tirtadinata, 1989; Casey et al., 1989a,b; Goossens et al., 1989; Muir et al., 1989; Petit et al., 1989; Hellmann et al., 1990; Muir et al., 1990). In HCI solutions at room temperature, the surface of dissolving labradorite feldspar is depleted in Na, Ca, and A1, relative to Si, over depths of hundreds of angstroms (Casey et al., 1988, 1989a,b; Muir et al., 1989, 1990; Casey and Bunker, 1990). Kinetic studies have shown that in acidic solutions the rate of dissolution of feldspars is promoted by hydrogen ions, and that the rate of dissolution increases as the hydrogen ion concentration increases (Wollast, 1967; Wollast and Chou, 1985; Helgeson, 1974; Helgeson et al., 1984; Chou and Wollast, 1985; Holdren and Speyer, 1985; Knauss and Wolery, 1986; Murphy and Helgeson, 1987; Blum and Lasaga, 1988; Brady and Walther, 1989; Schweda, 1990). Natural weathering solutions such as those found in soils, however, often contain complex-forming ligands which may also be involved in the dissolution process. The presence of simple, complex-formingligands such as oxalate has been shown to promote the rate of dissolution of aluminum oxides (Zutic and Stumm, 1982, 1984; Furrer and Stumm, 1983, 1986; Stumm et al., 1983, 1985 ). The effect of such ligands on the dissolution of aluminosilicates has been studied, but the results are ambiguous. There have been few, if any, studies of the surface chemistry of feldspars dissolving in solutions containing organic ligands and it is not known whether the ligands enhance or inhibit the formation of leached layers. In this study we have used secondary ion mass spectrometry (SIMS) to characterize the surfaces of three plagioclase feldspar compositions (two labradorite compositions, Ans4 and An6o, and anorthite, An~oo) leached in a variety of organic and inorganic acids, all at the same pH (4.0-4.1). With the same ligand concentration and the same pH in each acidic solution, our goal

DISSOLUTION OF PLAGIOCLASE FELDSPAR: COMPOSITION AND COMPLEXATION

57

was to clearly separate the effect of the ligands (oxalate and fluoride) from the effect of hydrogen ions (HCI) on the formation of leached layers on the surface of the dissolving feldspars. MATERIALS AND METHODS

Labradorite (An54) from Nain, Labrador, was kindly provided by Mr. John Forth of the Geology Department, UWO. Using an electron microprobe the composition of this labradorite was found to be Na0.41Ca0.saAll.545i2.490 8. A second labradorite composition called "Bytownite" (Sonora, Mexico) was purchased from Wards Natural Science Ltd., Mississauga, Canada. Despite its name, it is in fact labradorite in respect to its chemical composition (Na0.35Cao.6oAlt.575i2.4308). An anorthite was also purchased from Wards. The average composition of the anorthite (Hokkaido, Japan) is (Cal.ooAll.945i2.0508). The samples were prepared, leached, and analyzed as follows. Samples were cut into tablets approximately 1 c m × 1 c m × 0 . 5 cm thick and the surface of one side polished smooth using 0.5 pm diamond dust. We acknowledge the possibility of accelerated rates of dissolution because of the mechanical disruption of the surface (see the review by Aagaard and Helgeson, 1982). However, the primary objective of the present study is not to determine the absolute rates of feldspar dissolution, but rather to compare the relative importance of hydrogen ions and complex-forming ligands, and to observe any differences between different plagioclase feldspar compositions. By treating all of the samples in the same way, including the polishing step, such a comparison can be made. Furthermore, using SIMS we have compared the chemical composition of surface layers on both polished and naturally cleaved labradorite (Ans4) specimens leached in HC1; the chemical composition and thickness of the leached layers are not significantly different (Shotyk, unpublished ). The choice of reactor design also is important to the study (see Rimstidt and Dove, 1986, for a brief review of reactors designed for experimental studies of mineral dissolution ). A gravity-flow leaching apparatus similar to that described by Muir et al. (1989) was used for the experiments. The rate of flow of leaching solution was on the order of 250 to 400 ml day-l and this ensured that the concentrations of Ca, A1, and Si were kept well below equilibrium concentrations. Thus, the output solutions were maintained well below saturation with respect to any possible secondary phases.

Solution chemical analyses In order to clearly separate the effect of H + from the effect of the ligands, it was important to use the same ligand concentration and the same pH for each of the leaching solutions. Also, we wanted to leach the minerals at some pH

58

W. SHOTYK AND H.W. NESB1TT

value which is relevant to the soil solutions of the podzolic forest soils of central Ontario. The ligands selected also were meant to be environmentally relevant. Oxalic acid was selected on the basis of its simple structure and its relative importance among the aliphatic acids in soil solutions (Vedy and Bruckert, 1982 ). Fluoride was also selected as a ligand because A1 speciation studies have shown that the majority of inorganic monomeric A1 in the lakes of the area is in the form of fluoride complexes (LaZerte, 1984). Using acid concentrations of 10 -4 M, each of the acids HC1, oxalic acid, and HF give rise to a pH of approximately 4; this pH value is typical of the solution pH of podzolic soil solutions (Manley et al., 1987). Samples were leached for 72 days in 10-4M HC1 (pH 4.06_+ 0.03 ), 10-aM HF (pH 4.07 _+0.05 ), and 10-4M oxalic acid (pH 4.06 _+0.02 ). As a control, all mineral specimens were also leached in distilled water (pH 5.82 + 0.33). The pH values of the output solutions were not measurably different from those of the input solutions. Polished mineral specimens which were not leached were used as a second control. The output solutions were collected daily. Each day, 5 ml of eluant was collected for AI and Si determinations. These 5 ml aliquots were combined every 10 days to make a series of composite samples. Dissolved A1 was measured using ICP, and Si colorimetrically at 660 nm using a Technicon autoanalyzer with phosphomolybdate blue as a reagent (Technicon, 1973 ). To avoid any possible matrix interferences, Si standards were made up separately for each of HC1, HF, oxalic acid, and water. The detection limits for A1 and Si were each 10/tg 1-1. The pH of the output solutions was measured using a Radiometer PHM8e pH meter and Orion glass combination electrode in aliquots collected every second day.

Surface chemical analyses Secondary Ion Mass Spectrometry (SIMS) is the mass spectrometry of atomic species which are emitted when a solid surface is bombarded by an energetic primary ion beam (Vickerman, 1987 ). Secondary ions are ejected from the sample surface and analyzed in a mass spectrometer. By continually bombarding the surface of the sample with the primary ion beam (a process known as "sputtering" ), the atoms making up the material being studied are continually analyzed and a concentration depth profile is produced. The depth profile is simply an illustration of the chemical composition of a sample as a function of depth through the surface. Depth profiling of the feldspars was performed using a CAMECAims 3f secondary ion microscope in the specimen isolation (SI) mode (Metson et al., 1983; Metson et al., 1985 ). The principal advantage of the SI method for our studies is that no conductive surface coating is required to analyze the nonconducting aluminosilicates (Muir et al., 1989; Muir et al., 1990 ). A primary

DISSOLUTION OF PLAGIOCLASE FELDSPAR: COMPOSITION AND COMPLEXATION

59

ion beam of 160- (at approximately 15 kV [net] and 100 nA) was rastered over an area of 250/1m×250 pm on the sample surface. Positive secondary ions (23Na, 27A1, 28Si, 39K, 4°Ca, 888r) were detected from an area ( ~ 6 0 pm in diameter) well within the sputtered area. The SIMS analytical procedure used to depth profile aluminosilicate minerals is described in detail elsewhere (Muir, 1989; Muir et al., 1989). We depth profiled at either 2, 3, or 4 spots on each sample. The SIMS results presented here were obtained during several instrument sessions. The first labradorite composition studied (Ans4) was analyzed during two sessions (January and April, 1989 ). The second labradorite composition (An6o) and the anorthite were analyzed during the same instrument session (January 1989).

RESULTS

Surface chemical analyses Typical SIMS depth profile A typical SIMS depth profile for the An54 labradorite control (no leaching) is shown in Fig. l a. The most intense signals are 27A1and 288i (approximately 5 × 104 counts), and 4°Ca ( 10 4 counts), reflecting their abundance in the labradorite. The 39K and 88Sr signals are less intense (approximately 103 and 102 counts, respectively), reflecting their lower abundance in the mineral (trace elements). Shown on the abscissa is the sputtering time in seconds which corresponds to the depth of penetration by the primary ion beam. The correlation between depth of penetration by the primary ion beam with analysis time was investigated by Muir ( 1989 ) by determining the sputtering rate from freshly polished samples. The sputtering rate of this labradorite composition (An54) was found to be on the order of 2 A per second per 100 nA of primary beam current. Given that we used similar samples and analyzed them with the same SIMS and under the same analytical conditions, we assume that the sputtering rate during our analyses of labradorite also was 2 A per second. Thus, the sputtering time used here ( 1000 s) corresponds to a depth of penetration of the primary beam of approximately 2000 A. The SIMS depth profile of the An54 labradorite control shows that the intensities ofAl, Si, Ca, and Sr do not change significantly with time (Fig. la). In other words, these elements do not change in concentration from the surface of the mineral over the distance profiled (approximately 2000 A). The intensity of 39K is higher at the surface of the mineral compared to its intensity in the interior (Fig. la). This feature which is unique to Na and K has been reported earlier from both naturally and experimentally weathered feldspars (Nesbitt and Muir, 1988; Goossens et al., 1989; Muir et al., 1989). In these cases the Na and K depth profiles do not accurately reflect the true metal

60

W. SHOTYK AND H.W. NESB1TT

L

Labradorite An~Contro /

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400

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Sputter Time (s) AI

Si

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Fig. I. (a) SIMS depth profile oflabradorite (An54) control (no leaching). Notice that AI, Si, and Ca do not vary in intensity (concentration) with depth. The first ions detected (27A1 and 28Si ) increase in intensity immediately after the analysis begins because the beam is refocussed upon a fresh surface (even though the primary ion beam is focussed before the depth profile begins, as soon as the depth profiling starts the primary ion beam is refocussed to maximize the secondary ion yields). This generally increases the ion intensities by 2 to 5 times as the beam focus is improved. This explains why the first AI data point and the first Si data point are lower in intensity at the immediate surface of the control sample than they are in the interior. Despite the increase in absolute intensity of AI and Si from the first point of detection of each to the second, notice that the intensity of AI relative to Si does not change significantly. (b) SIMS depth profile of labradorite (An54) leached for 72 days in deionized water at pH 5.8. Notice that there is no significant difference between these depth profiles and those shown in (a) (no leaching). (c) SIMS depth profile of labradorite (Ans4) leached for 72 days in HC1 at pH 4. Notice that AI, Ca, and Sr are significantly depleted at the surface, relative to Si.

concentration profiles. Instead, both profiles (23Na and 39K) are affected by the charge-induced migration of Na + and K+in response to the negative primary beam (Streit et al., 1986). The SIMS depth profile of labradorite leached in water (Fig. lb) is not significantly different from that of the control (Fig. la). The SIMS depth profile of An54 labradorite leached in HC1 (Fig. 1c) shows that the intensities of AI, Ca, and Sr are significantly lower at the surface than in the interior of the sample. For example, the AI ion intensities at the surface of the mineral ( 103 counts per second) are one order of magnitude lower than the intensities at 1000 s ( 104 counts per second). Thus, compared to their concentrations in the interior of the mineral, AI, Ca, and Sr are depleted from the surface of the mineral (leached zone) toward the interior (unaltered zone ). These separations are emphasized when the intensities of these metals are viewed in rela-

DISSOLUTION OF PLAGIOCLASE FELDSPAR: COMPOSITION AND COMPLEXATION

61

tion to that of Si: the intensities of A1. Ca. and Sr compared to Si at the surface of the reacted mineral are significantly smaller than the corresponding ratios in the fresh interior. Compared to Si, the concentrations of A1, Ca, and Sr increase progressively from the surface of the reacted mineral to achieve constant concentrations after approximately 500 s of sputtering time, corresponding to a distance of approximately 1000/~ (Fig. 1c). In other words, a leached layer has formed on the surface of the feldspar which is strongly depleted in A1, Ca, and Sr, relative to Si, and is approximately 1000/~ thick.

Normalization calculations In order to emphasize the changes in element concentrations with depth through the samples, the SIMS depth profile data have been normalized to emphasize changes in ratios of the elements (Muir et al., 1989, 1990). These ratios emphasize any changes in A1 and Ca concentrations, relative to Si, with depth through the samples. The calculations are performed as follows: 1. the mobile constituents (A1, Ca, Sr) are normalized to a conservative element (in this case, Si). This contrasts the relative mobilities of the elements of interest. 2. the calculated ratios (A1/Si, Ca/Si, Sr/Si) in the leached layer of the leached specimen are then normalized to the unaltered zone of the leached mineral. This compares the element ratios in the weathered zone with those corresponding to fresh mineral. 3. the ratios are then normalized to the control specimens (no leaching). This takes into account any variations which may be due to instrumental effects.

Reproducibility of the SIMS depth profiles The excellent reproducibility of the SIMS depth profiles can be seen by comparing the A1/Si depth profiles recorded on different spots of the feldspar surface which were leached in different acidic solutions (Fig. 2 ). For example, four spots were depth profiled on the Ans4 labradorite leached in HC1 (Fig. 2a). Both the shapes and the thicknesses of the profiles measured at the different spots on the mineral surface are in excellent agreement. The two spots analyzed on the surface of Ans4 labradorite leached in oxalic acid are very similar (Fig. 2b). Finally, the four spots analyzed on the surface of Ans4 labradorite leached in HF are in excellent agreement (Fig. 2c ).

SIMS analyses of An6o labradorite Each of the SIMS depth profiles of An6o labradorite and the anorthite reacted in the different acidic solutions were taken during the same SIMS instrument session. Any differences in the depth profiles between samples, therefore, are unlikely to be the result of instrumental effects. Instead, we interpret the differences shown below to reflect real differences in mineral reac-

62

W. S H O T Y K

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AND H.W. NESBITT

LabradoriteAn~inHCI I

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LabradoriteAn~in oxalic acid

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LabradoriteAn~inHF 0

. . . . . . . . . .

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400

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Sputter Time (s) spot1 spot2 spot3 spot4

Fig. 2. (a) Percent change in A1/Si oflabradorite (Ans.) leached for 72 days in HCI. Four spots on the surface of the sample were depth profiled. The changes in A1/Si with depth at each of the different spots are very similar. (b) Percent change in AI/Si of labradorite (An s.) leached for 72 days in oxalic acid. Two spots on the surface of the sample were depth profiled. (c) Percent change in Al/Si.of labradorite (An54) leached for 72 days in HF. Four spots on the surface of the sample were depth profiled. Again, the changes in AI/Si with depth at each of the different spots are very similar. The oxalic acid and HF depth profiles were obtained in one SIMS session (January 1989), and the HCI profiles in another (April 1989). These profiles are used only to illustrate the excellent reproducibility of the depth profiles on the surface of a sample during a given SIMS session.

tivity due to differences in the compositions of the minerals and the leaching solutions. HCI The normalized A1/Si and Ca/Si ratios for An 60 labradorite leached in HC1 are shown in Fig. 3. AI and Ca are strongly depleted relative to Si from the beginning of the profile and remain depleted for approximately 1500 s of profiling. The Sr/Si depth profiles (not shown) resemble those of Al/Si and Ca/ Si. Thus, in response to HC1, a leached layer has formed on the surface of the An 60 labradorite which is depleted in A1 and Ca, relative to Si, on the order of 3000 ]~ thick. For comparison, the leached layer produced by HC1 on the An54 labradorite is approximately 1000 ~ thick (Fig. 2a). Oxalic acid Once again, A1 and Ca are strongly depleted relative to Si from the beginning of the profile and remain depleted for approximately 1500 s of profiling

DISSOLUTIONOFPLAGIOCLASEFELDSPAR:COMPOSITIONANDCOMPLEXATION

63

LabradoriteAnsoinHCI 2O 0 ....

~

'

+~,___

S.

-60

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_50 ~ " -100

lobo

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tsbo

~

2o0o

.....

o~O-20 -40

-80 -100

S 500

1000

1500

2000

SputterTime(s) • spot 1

o spot 2

Fig. 3. Percent change in AI/Si and Ca/Si of labradorite (An6o) leached in HCI. Both A1 and Ca are strongly depleted, relative to Si, for 1500 s of depth profiling. This corresponds to a depth of approximately 3000 A. Two separate SIMS depth profiles are shown, each taken on the same sample under identical SIMS operating conditions, but on two different spots on the mineral

surface. (Fig. 4). Thus, in response to oxalic acid, a leached layer has formed on the surface of the An6o labradorite which is depleted in A1 and Ca, relative to Si, on the order of 3000/k thick. Once again, this is significantly thicker than the leached layer produced by oxalic acid on the An54 labradorite (Fig. 2b). The three spots which were depth profiled show approximately the same depth of leaching, but clearly differ in respect to the extent of surface attack (Fig. 4). Spot 2 is the most extensively depleted depth profile and spot 1 the least. We attribute these differences in extent of leaching within a sample to zoning which is commonly observed within feldspar crystals (Barth, 1967). The less extensive dissolution probably corresponds to more sodic zones within the crystal, and the more extensively dissolved areas to more calcic zones. Inskeep et al. ( 1991 ) have observed similar variation in leached layer thicknesses on the surface of labradorite leached in acidic solutions. They reported leached layers up to 700 ~, in the sodic zones of the labradorite, compared to 1400 ~ leached layers in the calcic zones. HF

The normalized AI/Si and Ca/Si ratios for An6o labradorite leached HF are shown in Fig. 5. Once again, AI and Ca are strongly depleted relative to Si.

64

W. S H O T Y K

AND H.W. NESBITT

Labradorite An6oin oxalic acid 20 0 -20

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Fig. 4. Percent change in AI/Si and Ca/Si of labradorite (An6o) leached in oxalic acid. Once again, both A1 and Ca are strongly depleted, relative to Si, over a distance of approximately 3000/k. In this case the three spots on the surface are indicated different leaching intensities. Given the reproducibility of the SIMS depth profiles of the more sodic labradorite composition shown in Figure 2, the differences between these three depth profiles probably reflects chemical zonation in the mineral.

However, the A1/Si and Ca/Si ratios are depleted relative to their values in the fresh labradorite for approximately 2500 s, corresponding to a depth of alteration of approximately 5000/~. These leached layers are significantly thicker than those produced in response to the other acidic solutions (Figs. 3 and 4). Also, these leached layers are much thicker than those produced on the surface of An54 labradorite (Fig. 2c). Spot 1 is clearly more extensively leached than the other two spots analyzed on the surface (Fig. 5 ). Again, this difference is attributed to zoning within the feldspar crystal.

Separating the effects ofH +from the effects of the ligands Given that the pH was the same in all acidic solutions, the relative importance of hydrogen ion attack and the effects of the ligands can be compared by numerically separating the two processes. To do this, instead of normalizing the HF and oxalic acid depth profiles to the labradorite control, they have been normalized to the corresponding HC1 profiles thereby eliminating the effect of the hydrogen ions. This is done by selecting each metal/Si ratio for each of the depth profiles of labradorite dissolving in the complex-forming solutions and normalizing these to the corresponding ratios for the depth pro-

D I S S O L U T I O N O F P L AGI OCLASE FELDSPAR: C O M P O S I T I O N A N D C O M P L E X A T I O N

65

Labradorite An60in HF 20 I

d'.-" -60

~=/

-80 ~ i -100 , 0 500

, 1000

1500

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2O 0

~

--

-20

o5-4o ('~ -60 -80

~ f "

-I00

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1ooo

t~o

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Sputter Time (s) • spot 1 ° spot 2 • spot 3

Fig. 5. Percent change in Al/Si and Ca/Si of labradorite leached in HF. Notice that A1 and Ca both are strongly depleted, relative to Si, for 2500 s; this represents the formation of a leached layer approximately 5000 A thick.

files of labradorite leached in HCI. Specifically, the metal/Si ratios for individual depth profiles of An6o labradorite leached in the ligand-containing solutions were each normalized to the average metal/Si ratio for the two depth profiles ofAn6o labradorite leached in HC1. For example, in the case oflabradorite leached in oxalic acid, three depth profiles were obtained at three locations on the sample surface, and each was normalized to the average of the two HC1 depth profiles (Fig. 6a). After eliminating the effect o f p H by normalizing the oxalic acid data (three depth profiles) to HC1, A1 is still clearly depleted relative to Si over a distance of approximately 3000 ./~ (Fig. 6a). Similar depletions of Ca and Sr have been found, but are not shown here. Because all of the An6o depth profiles were obtained during the same SIMS session, the differences between the samples leached in HC1 and those leached in oxalic acid cannot be attibuted to instrumental effects. Thus, the more extensive depletion of A1 in oxalic acid relative to HC1 at the same pH is attributed to the oxalate ion (see Discussion ). The HF data (three depth profiles) normalized to HC1 shows a similar trend in Al/Si (Fig. 6b). The Ca/Si and Sr/Si profiles (not shown) are similar. Once again, after removing the effect of hydrogen ions, the A1 is clearly depleted relative to Si over a distance of approximately 5000/~. Again, because all of the An6o depth profiles were obtained during the same SIMS session, the differences between the samples leached in HC1 and those leached in HF

66

W. SHOTYK AND H.W. NESBITT

Labradorite An6o in oxalic acid normalized to HCl 20

o ............. o~ .2o

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Fig. 6 (a) Percent change in AI/Si oflabradorite leached in oxalic acid, normalized to labradorite leached in HCI. Because the pH of the HCI and oxalic acid solutions were the same, by normalizing the oxalic acid depth profiles to the average HC1 depth profile instead of to the control, the effect of H ÷ on leached layer formation can be eliminated. Thus, the extent of leaching shown here can be attributed to the oxalic acid. (b) Percent change in AI/Si of labradorite leached in HF, normalized to labradorite leached in HCI. Once again, by normalizing the depth profiles to the average HC1 depth profile instead of to the control, the effect of H÷on leached layer formation has been eliminated. The extent of leaching shown here can be attributed to the HF. The SIMS depth profiles of An6o labradorite leached in HC1, oxalic acid, and HF were all obtained during the same SIMS session. Thus, the observed differences between these solutions cannot be due to instrumental effects.

cannot be attibuted to instrumental effects. The depletion produced by HF compared to HCI is attributed to the presence of the fluoride ion. SIMS analyses of anorthite HCI

The ratio of A1/Si through the anorthite leached in HC1 is depleted relative to the composition of the fresh anorthite for approximately 800 s (Fig. 7a). Thus, much thinner leached layers (up to 1600 A) formed on the surface of anorthite leached in HC1 (Fig. 7a), compared to the An6o labradorite leached in HC1 (Fig. 3 ). Also, the intensity of A1 depletion, as measured by the percent change in A1/Si at the surface of the mineral relative to the interior, is significantly less in the case of anorthite (40 to 80%) compared to the An60 labradorite (95%).

DISSOLUTION OF PLAGIOCLASE FELDSPAR: COMPOSITION AND COMPLEXATION

20 [

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-lOO

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2°/ 01--~ ....... o~"

_20 ~ : , " /

::: .....

46o

660

a6o

lOOO

Anorthite in HF ~ -,% ~-~ l-ill r'~,j - r , ~,'1 : ~ . ~l : .....

05 < 4° -66f -80 100

, 0

,

400

,

600

•

800

C 1000

Sputter Time (s) • spot 1 • spot 2

• spot 3

Fig. 7 (a) Percent change in AI / Si of anorthite leached in HC1. Both A1 and Ca both are depleted relative to Si, but only for 800 s; this corresponds to a leached layer thickness of approximately 1600 ,~. Notice that the percentage change in A1/Si for the anorthite is much smaller than those for the labradorites shown in Figures 2 to 5. (b) Percent change in AI/Si ofanorthite leached in oxalic acid. Notice that the leached layer produced by oxalic acid is smaller than the slight leached layer produced in response to HC1, even though the concentrations of A1 and Si in the oxalic acid output solutions (shown in Table 1 ) are significantly greater than in HC1. (c) Percent change in A1/Si of anorthite leached in HF. Again, the leached layer produced by HF is smaller than the slight layer produced in response to HC1, even though the concentrations of AI and Si in the HF output solutions (shown in Table 1 ) are significantly greater than in HCI. All of the SIMS analyses of anorthite were obtained during the same SIMS session and the observed differences cannot be due to instrumental effects.

Oxalic acid The AI/Si ratios measured through the surface of anorthite leached in oxalic acid are depleted by only 20% compared to the fresh interior of the mineral (Fig. 7b). These depth profiles, in addition to those of Ca and Sr which are not shown, indicate that the anorthite effectively dissolves congruently in the oxalic acid solution, with only a very thin, weakly depleted leached layer left behind. HF The AI/Si profiles of anorthite leached in HF are similar to those of oxalic acid, and show that there is only a very small, weakly leached layer produced in response to the HF (Fig. 7c).

68

W. SHOTYK AND H.W, NESBITT

Solution chemical analyses An6o labradorite The results of the AI and Si determinations of the output solutions are shown in Table 1. Even though the concentrations of dissolved constitutents in the leaching solutions are low ( < 100/tg 1- ~) and the variations sometimes large, the measured concentrations of A1 and Si confirm that the output solutions are highly undersaturated with respect to possible secondary phases. Dissolved Si was below the limit of detection ( 10 ppb) in all of the An6o labradorite output solutions, but measurable concentrations of AI were found in the solutions containing complex-forming ligands (oxalic acid and HF). If the labradorite had been dissolving congruently, the molar ratio of Si to AI in the solutions would be similar to their molar proportion in the solid ( 1.5 to 1 ). In other words, Si would have been present in these solutions at concentrations exceeding those of AI. The lack of measurable concentrations of dissolved Si in these solutions is consistent with the SIMS depth profiles which show that the labradorite is dissolving incongruently, with AI being released preferentially over Si.

Anorthite In contrast to the labradorite, measurable concentrations of Si and significantly higher concentrations of AI are found in all the acidic solutions leaching anorthite. These data confirm that the anorthite is a more reactive feldspar composition than the labradorite. In the HCI solution, the molar ratio of TABLEI Average concentration o f dissolved constituents in output solutions (averaged over 72 days). The detection limits (d.l.) were approximately 10 #g 1- ~. Solution

concentration (/tg 1- ~) AI

concentration (/~mol I- J ) Si

Labradorite (Na0.35Cao.6oAl 1.57Si2.4308 ) Water < d.1. < d.1. HCI < d.l. < d.1. Oxalic 10_ 1
Molar ratio in solution (Si/AI)

AI

Si

< d.l. < d.l. 0.37 0.56


< d.l. 0.56 1.15 2.52


Anorthite (Cat ooAll 94Si2 osOs) Water HCI Oxalic HF

< d.l. 15___ 17 3 1 + 18 68 _+29

< d.l. 18+5 35+9 59 _+ 12

1.1 1.1 0.8

DISSOLUTION OF PLAGIOCLASE FELDSPAR: COMPOSITION AND COMPLEXATION

69

Si to A1 ( 1.1 to 1 ) is equal to the molar ratio of Si to A1 in the fresh solid. This finding confirms that the anorthite dissolves congruently in the HC1 solutions, with Si and A1 released in the same molar proportion as they are found in the fresh solid. Significantly higher concentrations of Si and AI were found in the oxalic acid and HF solutions, compared to anorthite in HC1. These higher concentrations reflect a more extensive dissolution of the solid phase. However, in both of these cases the molar ratio of Si to AI in the solution reflects the molar proportion of Si to A1 in the fresh solid. In oxalic acid and HF solutions, therefore, the anorthite dissolves to a greater extent than in HCI, but in all three cases the dissolution is effectively congruent, with no significant residual siliceous layer remaining behind. DISCUSSION

Incongruent dissolution of labradorite When plagioclase feldspars dissolve incongruently in acidic solutions, cations (Na and Ca) and A1 are released preferentially over Si (e.g. Chou and Wollast, 1984, 1985; Holdren and Speyer, 1985, 1986). The effect of this chemical separation is seen on the mineral surface where cation- and Al-depleted siliceous layers have been identified using a variety of surface analytical techniques (Schott and Petit, 1987; Hochella et al., 1988; Althaus and Tirtadinata, 1989; Goossens et al., 1989; Petit et al., 1989; Hellmann et al., 1990). Previous studies have shown that such leached layers are produced solely in response to the attack of the surface by hydrogen ions (Sj6berg, 1989). The mechanisms involved in the formation of such layers have been outlined by Casey and co-workers (Casey et al., 1988, 1989a, b; Casey and Bunker, 1990). The labradorite compositions examined here (An54 and An6o) both dissolve incongruently with A1 and the cations (Na, Ca, K) released preferentially over Si. The leached layers formed on the surface of the more calcic labradorite (An6o) are significantly thicker than those formed on the surface of the more sodic composition (An54). For example, in pH 4 HCI solutions reacted for 72 days, the leached layers formed on Ans4 and An6o are approximately 1000/~ and 3000/~, respectively. In response to HF the leached layers are approximately 1600/~ and 5000 A, respectively. This demonstrates a great sensitivity of leached layer thickness to plagioclase composition. This finding is consistent with the recent observation that plagioclase dissolution rates vary nonlinearly with mineral composition, and that the rates for the calcic compositions vary more with composition than the sodic compositions (Casey et al., 1991 ). The formation of such leached layers and the maximum thickness which any leached layer can achieve depends critically on the molar proportions of

70

w. SHOTYK AND H.W. NESBITT

Al and Si in the solid. In acidic solutions, AI-O bonds are attacked preferentially over Si-O bonds (Blum and Lasaga, 1988; Brady and Walther, 1989). Thus, the reactivities of plagioclase feldspars in acidic solutions increase with increasing A1 content. However, there must exist some m i n i m u m amount of Si in the feldspar framework necessary to maintain the integrity of such leached layers. The labradorite feldspar compositions described here contain sufficient Si that they are able to maintain siliceous residual layers during acid attack.

Congruent dissolution of anorthite The SIMS depth profiles of anorthite leached in HC1 reveal comparatively thin leached layers (approximately 1600/I ) which are depleted in Al and Ca, relative to Si (Fig. 7a). The molar concentrations of Al and Si in the output solutions are not significantly different from their molar proportions in the fresh solid, confirming that the anorthite effectively dissolves congruently, even in the HC1 solution. The leached layers observed on the anorthite leached in oxalic acid and in HF (Figs. 7b and c) are significantly less intensively depleted than those formed in response to HC1. This is interpreted as direct evidence that the anorthite is dissolving congruently in these solutions. Confirmation of the congruent dissolution of anorthite comes from the analyses of the output solutions which show A1 and Si in similar molar proportion to that of the fresh solid. The formation of a siliceous residual !ayer on the surface of a dissolving feldspar requires A1 and Ca to be removed from the minerals significantly faster than Si. A second requirement is sufficient residual Si to maintain the integrity of such a layer. If A1 and Si are removed from the surface at the same rate, the mineral dissolves congruently and no leached layer forms. The very thin leached layers detected using SIMS on the surface ofanorthite leached in HC1 (Fig. 7a) suggest that Si is being released at approximately the same rate as Al. Even though slight leached layers were detected, they are very thin compared to the corresponding leached layers produced on the surface of labradorite, a much less reactive feldspar composition, in response to the same acids (Figs. 2 to 5). Anorthite (Anloo) is made up of significantly more A1 and less Si than the labradorite (An6o). Because the anorthite contains much less Si compared to the labradorite, when AI-O bonds are broken in anorthite, proportionately more Si-O bonds are also broken. Not only is there less Si available in the anorthite to form a residual layer, any residual layer which does form on anorthite is comparatively more unstable than one forming on labradorite because there is proportionately less Si available to form it. As a result, the susceptibility of such a developing layer to subsequent dissolution is proportionately greater than a more siliceous one forming on the surface of

DISSOLUTION OF PLAGIOCLASE FELDSPAR: COMPOSITION AND COMPLEXATION

71

labradorite. As A1 is removed from the anorthite surface, the development of a siliceous surface layer is constrained by the amount of Si available to form it, and its rate of dissolution once it forms. It has been firmly established from field studies of soil weathering profiles (e.g. Goldich, 1938) and laboratory mineral dissolution experiments (e.g. Graham, 1950) that anorthite is a much more reactive feldspar composition than labradorite. The higher concentrations of AI and Si in the output solutions (Table 1 ) confirm the greater reactivity ofanorthite compared to labradorite. Despite the greater reactivity, only slight leached layers on the surface of anorthite leached in HC1 were detectable using the SIMS. This finding suggests that anorthite is incapable of sustaining the development of a significant leached layer. The absence of significant leached layers explains why the molar proportion of A1 and Si in the output solutions are comparable to their molar proportion in the fresh solid. In contrast to the response to HC1, even smaller leached layers were detected using the SIMS on anorthite dissolving in oxalic acid and in HF. However, it is clear from the A1 and Si concentrations of the output solutions that these two acids have significantly promoted the dissolution of anorthite, compared to HC1. On average the A1 concentrations in the oxalic acid solutions are approximately twice those of the HC1 solutions, and the A1 concentrations in the HF solution are approximately four times those of the HC1 (Table 1 ). In solutions containing these ligands, the attack of A1-O bonds is significantly greater, thus a proportionately greater number of Si-O bonds also are broken. As a result, there is no opportunity for a siliceous layer to form, and Si and A1 are released to the solution in the same molar proportion as they are found in the fresh solid (congruent dissolution).

Proton-promoted dissolution The SIMS depth profiles of labradorite leached in HC1 show that it dissolves incongruently, with A1 and Ca released preferentially, giving rise to an altered surface layer residually enriched in Si (Fig. 1 ). Because the chloride ion does not measurably affect the rate of labradorite dissolution in dilute solutions (Sjtiberg, 1989), the formation of this residual layer has resulted solely from H ÷ attack. The mechanisms by which this reaction is thought to takes place has been described elsewhere (Casey et al., 1988, 1989a,b, and others).

Ligand-promoted dissolution The leached layers produced on the surface of labradorite by oxalic acid and HF are much thicker than those produced by HC1, even though temperature, solution pH, and leaching times were the same for all experiments. To

72

W. SHOTYK AND H.W. NESBITT

emphasize these differences, the SIMS data for An6o labradorite leached in the ligand-containing solutions have been normalized to those of HC1 (Fig. 6 ). The differences which are observed after normalization to HC1 can only be attributed to the ligands because all other variables were held constant. The results show clearly that the complex-forming ligands have increased the thickness of the leached layer, relative to its thickness in HCI. The formation of a leached layer requires constituents to diffuse through (into and out of) the leached layer. Diffusion of species through the leached layer is driven by chemical potential gradients which, if appropriate standard states are chosen, are reflected in concentration (activity) gradients within the leached layer. Any process which affects the chemical potential of a species, necessarily affects the flux of that species through the leached layer (Fick's law of diffusion). Oxalate and fluoride form strong complexes with A1 in solution. Using the average AI concentration in the output solutions, speciation calculations using SOILCHEM (Sposito and Coves, 1988) indicate that the concentration of unbound A13+ is on the order of 10-11M in the oxalic acid solution, compared with l 0 - 8M in HC1. Complexation of A1 by oxalic acid in the bulk solution, therefore, is expected to have decreased the concentration of AI 3+, relative to its concentration in HC1, by about three orders of magnitude. Thus, as a consequence of A1 complexation in the bulk solution, the chemical potential of the A1 at the solid-solution interface has been signifcantly decreased, increasing the gradients of the metal species across the leached layer. In response, AI is expected to diffuse more rapidly through the leached layer to the bulk solution. Thisprocess would necessarily increase diffusion rates and most importantly, increase the rate of formation of the leached layer, as observed in our experiments (Fig. 6). Thus, as a result of the enhanced diffusion of AI, the leached layers formed in response to oxalic acid are significantly thicker than those formed solely from the attack by H + (i.e. in HC1 ). The effect of fluoride on leached layer formation is expected to be similar to that of oxalate. Using the average concentration of A1 in the output solutions, the calculated concentration of free Al 3+ in the HF solution also is on the order of l 0 - ~~M. Thus, HF also has increased the concentration gradient between the fresh feldspar and the bulk solution, thereby promoting the diffusive flux of Al out from the fresh mineral. The solutions containing oxalic acid and HF contain comparable concentrations of uncomplexed A13÷, and therefore similar concentration gradients between the fresh feldspar and the bulk solution must have existed. However, the thickness of the leached layer formed in response to HF is approximately 20% thicker than the layer produced in response to oxalic acid (compare Fig. 6a and b). The more extensive dissolution in HF compared with oxalic acid may be due to differences in their size. The small size of the fluoride anion relative to the oxalate anion may have allowed F - to diffuse from the bulk

DISSOLUTION OF PLAGIOCLASE FELDSPAR: COMPOSITION AND COMPLEXATION

73

solution through the leached layer and complex A1 at appropriate sites below the mineral surface. If this is true, then there are three ways in which HF solutions can attack the feldspar: (i) hydrogen ion attack; (2) metal complexation by F - in the bulk solution, and (iii) metal complexation by F - at the interface between the leached layer and the fresh feldspar. Although we consider these the most likely explanations for the increased thicknesses of the leached layers produced by oxalic acid and HF, additional mechanistic studies are needed.

Rates of plagioclasefeldspar dissolution in solutions containing organic ligands There are contradictory reports concerning the effect of simple organic ligands such as oxalate on the relative rates of plagioclase feldspar dissolution. Mast and Drever (1987) found that l m M oxalic acid had no effect on the overall rate of dissolution of oligoclase feldspar (Anl3). Their results now may be interpreted in respect to the development of residual siliceous layers on the surface of the mineral during acid dissolution. The complexation of A1 by oxalate in the bulk solution would have significantly decreased the concentration of AI 3+which, in turn, would have accelerated the diffusion of AI out from the fresh feldspar to the bulk solution. This process would have led to the formation of a residual siliceous layer on the surface of the dissolving oligoclase. Given the relative proportions of A1 and Si in the oligoclase, however, the siliceous layer would be relatively stable (as we have seen in the case of labradorite). The rate of dissolution of this siliceous layer is less than the rate of release ofAl (Casey et al., 1988, 1989a,b). Thus, the rate-limiting step in the dissolution of the oligoclase is the hydrolysis of the siliceous layer. The overall rate of oligclase dissolution, as measured by the release of Si, was therefore not influenced by the presence of oxalic acid (Mast and Drever, 1987). In contrast to the findings of Mast and Drever ( 1987 ), Amrhein and Suarez (1988) reported an accelerated rate of anorthite (An93) dissolution in the presence of similar concentrations of oxalic acid, relative to the rate of dissolution in the absence of oxalate. This accelerated rate of dissolution may now be interpreted as a reflection of the relative instability of this more aluminous feldspar composition. As the oxalate promoted the attack of A1-O sites, it destroyed a proportionately greater number of Si-O bonds, compared to its effect on the oligoclase. The rate at which the Si-O bonds were broken in this way would be comparable to the rate at which the A1-O bonds were broken, and the two elements would be released to solution in the same molar proportions ( ~ 1 : 1 ) as they are found in the solid (congruent dissolution). The overall rate of anorthite dissolution, as measured by the rate of release of Si,

74

W. SHOTYK AND H.W. NESB1TT

was cleared increased by the presence of oxalic acid (Amrhein and Suarez, 1988). The effect of complexing-forming organic ligands on the overall rate of plagioclase feldspar dissolution, as measured by the rate of release of Si, depends strongly upon the molar proportions of A1 and Si in the solid. Promotion of rates of feldspar dissolution by organic ligands is favoured by a relatively large ratio of AI to Si (e.g. 1 : 1 as in the case of the anorthite end member). Although we have not measured feldspar dissolution rates, based upon the leached layers on labradorite shown here, the overall rate of dissolution of more siliceous feldspar compositions (e.g. albite, oligoclase, andesine) is expected to be unaffected by organic ligands. While organic complexation will promote the release of A1, the rate of dissolution of the residual, siliceous surface layer expected to develope will control the overall rate of dissolution; this layer apparently is not directly affected by the organic acids. Additional experimental studies are needed to test this hypothesis. SUMMARY AND CONCLUSIONS

Labradorite (An54 and An6o) and anorthite have been leached for 72 days

in 10-4M solutions of HC1, oxalic acid, and HF all at pH 4, and in distilled water. The surfaces have been characterized using Secondary Ion Mass Spectrometry (SIMS) by depth profiling A1 and Ca, and the ratios A1/Si and Ca/ Si have been calculated as a function of depth through the feldspars. The concentrations of dissolved AI and Si have been measured in the output solutions. ( 1 ) The AI/Si and Ca/Si depth profiles show that substantial leached layers form on the surface of labradorite leached in HC1, confirming that they are dissolving incongruently. Using the thickness of the leached layers as an index of reactivity, the more calcic composition (An60) is significantly more reactive than the more sodic composition (An54): the leached layers produced in HC1 are approximately 3000 and 1000 A, respectively. These leached layers form solely in response to H ÷ ion attack. The two plagioclase compositions (An54 and An6o) have sufficient framework Si to be able to maintain a stable siliceous surface layer during the duration of the leaching experiments (72 days). (2) The leached layers formed on labradorite leached in oxalic acid indicate more extensively depletion compared to those formed in response to HC1 at the same pH. In this case the complexation of aqueous A1 in the bulk solution has promoted the diffusion of A1 through the leached layer and enhanced the formation of the leached layer, compared to HC1. ( 3 ) The leached layers formed on the surface of labradorite leached in HF on average also are more extensive (up to 5000 A) than those formed in response to HC1. Once again, complexation of aqueous AI in the bulk solution has promoted the diffusion of AI through the leached layer and enhanced its

DISSOLUTION OF PLAGIOCLASE FELDSPAR: COMPOSITION AND COMPLEXATION

75

formation. However, the leached layers produced in response to HF are significantly deeper than those produced in response to oxalic acid. The greater relative effectiveness of fluoride compared to oxalate may reflect the small size of the fluoride anion and its ability to diffuse through the leached layer and attack appropriate A1 and Ca sites in the fresh feldspar. (4) Comparatively thin leached layers form on the surface of anorthite leached in HC1, even though it is a significantly more reactive feldspar composition. The output solutions contain higher concentrations of Al and Si than the output solutions from the labradorite experiments, confirming that the anorthite has been more reactive. However, the concentrations of A1 and Si in these solutions are in the same molar proportions as they are found in the fresh solid, confirming that the anorthite dissolves congruently. It appears that this feldspar composition has insufficient Si to maintain the integrity of a substantial siliceous surface layer. Although oxalate and fluoride promote the dissolution of anorthite compared to its dissolution in HCI, the anorthite also dissolves congruently in these solutions. ACKNOWLEDGEMENTS All of the analytical work was performed by W.S. at the University of Western Ontario and supported by the Ontario Ministry of the Environment (with special thanks to Drs. Peter Dillon and Bruce LaZerte) and the Natural Sciences and Engineering Research Council of Canada. The staff at Surface Science Western (UWO) made the SIMS available to us and provided expert technical support. In addition, at UWO we wish to thank Dr. I.J. Muir for much help with the SIMS and for his comments on an earlier draft of this manuscript. Mr. J. Forth carefully polished the samples, and R.L. Barnett provided microprobe analyses. We are grateful to Mr. D.A. Tel of the Department of Land Resource Science, University of Guelph for skillful assistance with the aqueous Si analyses and Dr. G. Spiers of the same department for the ICP analyses of A1. Dr. Peter C. Lichtner of the Mineralogical Institute, University of Berne, kindly reviewed an earlier draft of the manuscript. Finally, thanks to Prof. G. Sposito of the Department of Soil Science, University of California, Berkeley, for the SOILCHEMcalculations. REFERENCES Aagaard, P. and Helgeson, H.C., 1982. Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions. I. Theoretical considerations. Am. J. Sci., 282: 237-285. AIthaus, E. and Tirtadinata, E., 1989. Dissolution of feldspar: the first step. In: D.L. Miles (Editor), Water-Rock Interaction. Balkema, Rotterdam, pp. 15-17. Amrhein, C. and Suarez, D.L., 1988. The use of a surface complexation model to describe the kinetics ofligand-promoted dissolution ofanorthite. Geochim. Cosmochim. Acta, 52- 27852793. Barlh, T.F.W., 1969. Feldspars. Wiley-Interscience, New York.

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Blum, A. and Lasaga, A., 1988. Role of surface speciation in the low-temperature dissolution of minerals. Nature, 331 : 431-433. Brady, P.V. and Walther, J.V., 1989. Controls on silicate dissolution rates in neutral and basic pH solutions at 25 °C. Geochim. Cosmochim. Acta, 53: 2823-2830. Casey, W.H. and Bunker, B. 1990 Leaching of mineral and glass surfaces during dissolution. In: M.F. Hochella Jr. and A.F. White (Editors), Mineral-Water Interface Geochemistry. Reviews in Mineralogy Vol. 23. Mineralogical Society of America, Washington, DC, pp. 397426. Casey, W.H., Westrich, H.R., and Arnold, G.W., 1988. Surface chemistry of labradorite feldspar reacted with aqueous solutions at pH=2,3, and 12. Geochim. Cosmochim. Acta, 52: 2795-2807. Casey, W.H., Westrich, H.R., Arnold, G.W. and Banfield, J.F., 1989a. The surface chemistry of dissolving labradorite feldspar. Geochim. Cosmochim. Acta, 53:821-832. Casey, W.H., Westrich, H.R., Massis, T., Banfield, J.F., and Arnold, G.W., 1989b. The surface of labradorite feldspar after acid hydrolysis. Chem. Geol. 78:205-218. Casey, W.H., Westrich, H.R. and Holdren, G.R., 1991. Dissolution rates of plagioclase at pH = 2 and 3. Am. Miner., 76:211-217. Chou, L. and Wollast, R., 1984. Study of the weathering of albite at room temperature and pressure with a fluidized bed reactor. Geochim. Cosmochim. Acta, 48:2205-2217. Chou, L. and Wollast, R. 1985 Steady-state kinetics and dissolution mechanism of albite. Am. J. Sci., 285: 963-993. Goldich, S.S., 1938. A study in rock weathering. J. Geol., 46:17-58. Goossens, D.A., Philippaerts, J.G., Gijbels, R., Pijpers, A.P., Van Tendeloo, S. and Althaus, E., 1989, A SIMS, XPS, SEM, TEM, and FTIR study of feldspar surfaces after reacting with acid solutions. In: D.L. Miles (Editors) Water-Rock Interaction. Balkema, Rotterdam, pp. 271-274. Correns, C.W., 1962. The experimental chemical weathering of silicates. Clay Miner. Bull., 26(4): 249-265. Correns, C.W. 1963. Experiments on the decomposition of silicates and discussion of chemical weathering, Clays Clay Miner., 10: 443-459. Correns, C.W. and von Engelhardt, W., 1938. Neue Untersuchungen tiber die Verwitterung des Kalifeldspates. Chem. Erde, 12: 1-22. Furrer, G. and Stumm, W., 1983. The role of surface coordination in the dissolution of o'-A1203 in dilute acids. Chimia, 37: 338-341. Furrer, G. and Stumm, W., 1986. the coordination chemistry of weathering: I. dissolution kinetics of a-A1203 and BeO. Geochim. Cosmochim. Acta, 50:1847-1860. Gardner, L.R., 1983. Mechanics and kinetics of incongruent feldspar dissolution. Geology, 11 : 418-421. Graham, E.R., 1950. The plagioclase feldspars as an index to soil weathering. Soil Sci. Soc. Am. Proc., 14: 300-302. Helgeson, H.C., 1974. Chemical interaction of feldspar and aqueous solutions. In: W.S. MacKenzie and J. Zussman (Editors), The Feldspars. Crane, Russak and Co., New York, pp. 184-217. Helgeson, H.C., Murphy, V.M. and Aagaard, P., 1984. Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions. II. Rate constants, effective surface area and the hydrolysis of feldspar. Geochim. Cosmochim. Acta, 48: 2405-2432. Hellmann, R., Eggleston, C.M., Hochella, M.F. Jr., and Crerar, D.A., 1990. The formation of leached layers on albite surfaces during dissolution under hydrothermal conditions. Geochim. Cosmochim. Acta, 54: 1267-1281. Hochella, M.F. Jr., Ponader, H.B., Turner, A.M., and Harris, D.W., 1988. The complexity of

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mineral dissolution as viewed by high resolution scanning Auger microscopy: Labradorite under hydrothermal conditions. Geochim. Cosmochim. Acta, 52: 385-394. Holdren, G.R. Jr. and Speyer, P.M., 1985. Reaction rate-surface area relationships during the early stages of weathering- I. Initial observations. Geochim. Cosmochim. Acta 49:675-68 I. Holdren, G.R., Jr. and Speyer, P.M., 1986. Stoichiometry of alkali feldspar dissolution at room temperature and various pH values. In S.M. Colman and D.P. Dethier (Editors), Rates of Chemical Weathering of Rocks and Minerals. Academic Press, New York, pp. 61-8 I. lnskeepk, W.P., Nater, E.A., Bloom, P.R., Vandervoort, D.S. and Erich, M.S., 1991. Characterization of laboratory weathered labradorite surfaces using X-ray photoelectron spectroscopy and transmission electron microscopy. Geochim. Cosmochim. Acta, 55: 787-800. Knauss, K.G. and Wolery, T.J., 1986. Dependence ofalbite dissolution kinetics on pH and time at 25 °C and 70°C. Geochim. Cosmochim. Acta., 50:2481-2497. LaZerte, B.D., 1984. Forms of aqueous aluminum in acidified catchments of central Ontario: a methodological analysis. Can. J. Fish. Aq. Sci., 4 l: 766-776. Manley, E.P,, Chesworth, W. and Evans, L.J., 1987. The solution chemistry of podzolic soils from the eastern Canadian Shield: a thermodynamic interpretation of the mineral phases controlling soluble Al s+ and H4SiO4. J. Soil Sci., 38:39-51. Mast, M.A. and Drever, J.I., 1987. The effect of oxalate on the dissolution rates of oligoclase and tremolite. Geochim. Cosmochim. Acta, 51: 2559-2568. Metson, J.B., Bancroft, G.M., Mclntyre, N.S. and Chauvin, W.J., 1983. Suppression of molecular ions in the secondary ion mass spectra of minerals. Surf. Interf. Anal., 5:181-185. Metson, J.B., Bancroft, G.M. and Nesbitt, H.W., 1985. Analysis of minerals using specimen isolated secondary ion mass spectrometry. Scan. Elect. Microsc., II: 595-603. Muir, l.J., 1989. Secondary ion mass spectrometry and its application to studies in geochemistry. Ph.D. thesis, University of Western Ontario, London. Muir, l.J., Bancroft, G.M. and Nesbitt, H.W., 1989. Characteristics of altered labradorite surfaces by SIMS and XPS. Geochim. Cosmochim. Acta, 53: 1235-1241. Muir, I.J., Bancroft, G.M., Shotyk, W. and Nesbitt, H.W., 1990. A SIMS and XPS study of dissolving plagioclase. Geochim. Cosmochim. Acta 54: 2247-2256. Murphy, W.M. and Helgeson, H.C., 1987. Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions. Ill. Activated complexes and the pH-dependence on the rates of feldspar, pyroxene, wollastonite, and olivine hydrolysis. Geochim. Cosmochim. Acta, 51: 3137-3153. Nesbitt, H.W. and Muir, I.J., 1988. SIMS depth profiles of weathered plagioclase, and processes affecting dissolved A1 and Si in some acidic soil solutions. Nature, 334: 336-338. Nixon, R.A., 1979. Differences in incongruent weathering of plagioclase and microline - Cation leaching versus precipitates. Geology, 7:221-224. Rimstidt, J.D. and Dove, P.M., 1986. Mineral/solution reaction rates in a mixed flow feactor: wollastonite hydrolysis. Geochim. Cosmochim. Acta, 50:2509-2516. Petit, J.-C., Dran, J.-C., Della Mea, G., and Paccagnella, A., 1989. Dissolution mechanisms of silicate minerals yielded by intercomparison with glasses and radiation damage studies. Chem. Geol., 78: 219-227. Schott, J. and Petit, J-C., 1987. New evidence for the mechanisms of dissolution of silicate minerals. In: W. Stumm (Editor), Aquatic Surface Chemistry. Wiley, New York, pp. 292315. Schweda, P., 1990. Kinetics and mechanisms of alkali feldspar dissolution at low temperature. Ph.D. thesis, Stockholm University, 99 pp. Sj6berg, L. 1989. Kinetics and non-stoichiometry oflabradorite dissolution. In: D.L. Miles (Editor) Water-Rock Interaction. Balkema, otterdam, pp. 639-642. Sposito, G. and Coves, J., 1988. SOILCHEM. Campus Software Office, Univ. of California, Berkeley, CA.

78

W. SHOTYK AND H.W. NESBITT

Streit, L.A., Hervig, R.L., and Williams, P., 1986. Quantitative SIMS microanalysis of trace elements in geological samples using in situ ion-implanted standards. In: A.D. Romig and W.F. Chambers (Editors), Microbeam Analysis. San Francisco Press, San Francisco, pp. 91-94. Stumm, W., Furrer, G. and Kunz, B., 1983. The role of surface coordination in precipitation and dissolution of mineral phases, Croat. Chem. Acta, 56:593-611. Stumm, W., Furrer, G., Wieland, E. and Zinder, B., 1985. The effect of complex-forming ligands on the dissolution of oxides and aluminosilicates. In: J.I. Drever (Editor), The Chemistry of Weathering. Reidel, New York, pp. 55-74. Technicon, 1973. Silicates in water and seawater. Technicon Auto Analyzer II, Industrial Method No. 186-72W. Technicon Industrial Systems, Tarrytown, New York. Vedy, J.C. and Bruckert, S., 1982. Soil solution: composition and pedogenic significance. In: M. Bonneau and B. Souchier (Editors), Constituents and Properties of Soils. Academic Press, New York, pp. 184-213. Vickerman, J.C., 1987. Secondary Ion Mass Spectrometry, Chem. Britain, October 1987, pp. 969-974. Wollast, R., 1967. Kinetics of the alteration of K-felspar ion buffered solutions at low temperature. Geochim. Cosmochim. Acta, 31: 635-648. Wollast, R. and Chou, L., 1985. Kinetic study of the dissolution of albite with a continuous flow-through fluidized bed reactor. In: J.I. Drever (Editor), The Chemistry of Weathering. Reidel, New York, pp. 75-96. Zutic, V. and Stumm, W., 1982. On the role of surface complexation in weathering reactions. Dissolution kinetics of hydrous alumina in the presence of organic ligands. In: H. Van Olphen and F. Veniale (Editors), International Clay Conference 1981. Elsevier, New York, pp. 613-621. Zutic, V. and Stumm, W., 1984. Effect of organic acids and fluoride on the dissolution kinetics of hydrous alumina. A model study using the rotating disc electrode, Geochim. Cosmochim. Acta, 48: 1493-1503.