- Email: [email protected]
Nontronite after acid or alkali attack
Chemical Geology, 32 (1981) 95--102
95
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
N O N T R O N I T E A F T E R ACID OR ALKALI ATTACK L. HELLER-KALLAI and I. ROZENSON
Department of Geology, The Hebrew University, Jerusalem (Israel) (Received June 30, 1980; revised and accepted February 2, 1981)
ABSTRACT Heller-Kallai, L. and Rozenson, I., 1981. Nontronite after acid or alkali attack. Chem. Geol., 32: 95--102. X-ray diffraction patterns, infra-red and MSssbauer spectra of samples of nontronite reacted with 0.1 N HCI at 70--90°C for various lengths of time were indistinguishable from those of the starting material. After appropriate corrections for exchange of interlayer cations, constant Al/Mg and Al/Fe ratios of the solid were maintained. It was concluded that dissolution may be incongruent in a narrow reaction rim, but that the structure of the bulk material is not affected by acid attack. In contrast, minor changes were observed in the infra-red and MSssbauer spectra of samples reacted with 0.3 N NaOH solution. These were attributed to partial deprotonation of the entire structure. Scanning electron micrographs showed that alkali attack changed the morphology of the clay appreciably while acid attack did not.
INTRODUCTION
The question whether clay minerals dissolve congruently or incongruently on acid attack has been debated for many years. Osthaus' (1954, 1955) conclusions that octahedral cations of montmorillonite and nontronite are extracted in preference to tetrahedral ones on drastic acid attack has found wide acceptance, though other investigators reported that the rates of dissolution of tetrahedral and octahedral cations from these minerals are equal (e.g., Mathers et al., 1954). Similar controversies pertain to the results of acid attack of chlorites (Brindley and Youell, 1951; Ross, 1969). Churchman and Jackson (1976) presented evidence to show that A1 dissolved o u t of montmorillonite was partially adsorbed on to the clay interlayers. The presence of a metastable Si-rich secondary phase, "depleted montmorillonite", was also inferred and attributed to preferential leaching of Mg and A1. Many of the previous studies of acid attack were designed to establish the reaction kinetics and emphasis was placed on the chemical aspects of the reactions and on determination of the thermodynamic constants. Churchman and Jackson (1976) showed that at least in the pH range they investigated (1.5--4.2) it is invalid to treat acid dissolution as a single-step process. Kinetic studies of alkali attack are impracticable, since precipitation of 0009-2541/81/0000---0000/$02.50 © 1981 Elsevier Scientific Publishing Company
96 hydroxides occurs at the high pH-values of the experimental solutions. Dudas and Harward (1971) studied the effect of a combined NaOH or KOH and sodium dithionite--citrate bicarbonate extraction of clay minerals, but in such combined treatments it is difficult to assess the role of alkalinity alone. Russell (1979) showed that interaction of alkali-metal hydroxides with dioctahedral smectites leads to deprotonation of OH groups. In the present study, attention was focussed on possible changes in the structure of clay minerals after acid or alkali attack, without consideration of the reaction kinetics. Nontronite was chosen for detailed investigation. The relatively drastic conditions of attack adopted render the results of the acid attack directly comparable with those reported by Osthaus.
EXPERIMENTAL Materials
Nontronite from the State of Washington (Source Clay Mineral) was used for the experiments. The structural formula deduced after correction for the presence of quartz and iron oxides is:
(Ca0.34 Mg0.14 ) (SiT.~s A10.Ts) (A10.ss Fe2.TS Mg0.33 Ti0.0s )O20(OH)4 The a m o u n t of Fe203 in the sample is 1.2% of the total, or 5.5% of the iron present (Rozenson, 1975). Some experiments were also carried out with nontronite from Garfield, Washington (American Petroleum Institute). Procedure
In each experiment 40 mg of sample were mixed with 20 ml of solution. Series 1: Nontronite was heated with 0.1 N HC1 in test-tubes immersed in an oil bath at 88 + 2°C for periods of time ranging up to 1 hr. Series 2: Nontronite was heated with 0.1 N HC1 in sealed ampoules maintained at 70 + 2 ° C for periods of time ranging up to 91 hr. Series 3: Similar to series 2, but the temperature was maintained at 90 -+ 2 ° C and heating times ranged up to 140 hr. Series 4: Nontronite was heated with 0.3 N NaOH in test-tubes immersed in an oil bath at 88 + 2°C for periods of time ranging up to 24 hr. After various periods of heating the samples were centrifuged and washed with distilled water. The supernatant solutions of all the samples of series 1 were analysed for A1, Mg and Fe by atomic absorption spectroscopy and those of series 2 and 3 for A1 and Fe. In the course of the experiments of series 1 and 4 some evaporation occurred and the solutions became more concentrated with time. Since no attempt was made to establish reaction kinetics or any thermodynamic constants, this does not affect the validity of the results. Series 5: Samples were covered with 15 mmol/g 0.5 N NaOH solution
97
and evaporated to dryness, following the procedure outlined by Russell (1979). Infra-red (IR) spectra were recorded of all the samples. Selected specimens were subjected to X-ray diffraction (XRD), MSssbauer spectroscopy and scanning electron microscopy (SEM). The MSssbauer spectra were recorded and resolved into two Fe 3+ doublets, as previously described (Rozenson and Heller-Kallai, 1976). RESULTS AND INTERPRETATION
Chemical analyses The fraction of Fe, Mg and A1 which remained in the solid state after various reaction times was deduced from duplicate analyses of the supematant solutions of the samples of series 1. The results are shown in Table I. To obviate possible changes introduced in pretreatment of the nontronite, this was used in its natural state, without cation exchange or removal of iron oxides. To avoid the necessity of introducing arbitrary assumptions, only the ratios of elements remaining in the solid state were considered. The original sample contained exchangeable Mg and adsorbed Fe. It is evident that the A1/Mg ratio increased gradually in the course of the first few hours, indicating that the exchange of Mg ions was not instantaneous, but required several hours for completion. The high value observed after 24-hr. treatment is probably due to experimental error. The reproducibility of the analysis was lower than for the less reacted samples. Moreover, since the amount of Mg in the solid phase is low, the error in the A1/Mg ratio is large. The fact that the A1/Fe ratio did not vary appreciably with time and, in particular, did not increase with time, indicates that under the prevalent experimental conditions TABLE I
Dissolution of nontronite on acid attack (series 1 ) Time (hr)
0 0.5 1.5 2 6 12 18 24
% e l e m e n t in s o l u t i o n
% e l e m e n t in solid phase .1
AI
A1
-0.49 0.71 0.71 1.27 1.47 1.63 1.91
Fe
-1.47 2.28 2.44 4.00 4.92 5.92 7.02
Mg
-0.28 0.35 0.37 0.51 0.55 0.58 0.67
4.02 3.53 3.31 3.31 2.75 2.55 2.39 2.11
, I (% in original solid) -- (% in s o l u t i o n ) . ,2 I r r e p r o d u c i b l e results.
Fe
15.13 13.66 12.85 12.69 11.13 10.21 9.21 8.11
R a t i o o f e l e m e n t s in r e m a i n i n g solid
Mg
1.08 0.80 0.73 0.71 0.57 0.53 0.50 0.41
Al/Mg
A1/Fe
3.72 4.41 4.50 4.63 4.78 4.77 4.78 5.14 *2
0.27 0.26 0.26 0.26 0.25 0.25 0.26 0.26 *=
98 dissolution of iron oxides and of the mineral itself occurred simultaneously. Chemical analyses of A1 and Fe in the supernatant solutions of selected samples of series 2 and 3 were carried out to monitor the extent of the reaction. The A1/Fe ratio in the solid phase remained approximately constant for all the samples of series 2. Samples of series 3 showed a similar A1/Fe ratio in the initial stages of the reaction, but after 15 hr. or more the results were erratic. After 50-hr. reaction time in sealed ampoules at 90°C - 65% of the A1 ions and after 140 hr. over 70% had passed into solution. The results obtained with series 4 were irreproducible. At the prevalent high alkalinity polymerisation, precipitation and/or adsorption of some of the products is expected to occur.
X-ray diffraction, infra-red and MSssbauer spectroscopy All the acid-treated samples but one gave rise to XRD patterns, IR and MSssbauer spectra which are indistinguishable from those of the original sample. The single exception is the sample which was heated for 40 hr. at 90°C in a sealed ampoule. This drastic treatment led to the formation of a new unidentified phase with a featureless absorption in the OH stretching region and a shoulder extending from 940--910 cm -~. This sample will be excluded from the following discussion. All the alkali-treated samples of series 4 remained fully expandable with glycol. Unlike the acid-treated samples, those digested with alkali gave rise to IR patterns which differed slightly from those of the original material. In particular, the frequency of the main Si--O absorption was reduced from 1032 cm -~ for Na-substituted nontronite up to 1022--1020 cm-1 A small but significant change also occurred in the MSssbauer spectra. After 24-hr. NaOH treatment the Q.S. of both doublets and the proportion T A B L E II M o s s b a u e r p a r a m e t e r s and Si--O v i b r a t i o n f r e q u e n c y of variously t r e a t e d s a m p l e s of n o n t r o n i t e Treatment
M1
(trans) site
I.S.
Q.S.
I"
(mm/s)
(mm/s)
(ram/s)
(1)
0 . 2 9 (2)
0 . 4 0 (2)
(1)
0.29 (1)
(1)
(1)
-0.39 0.1 N HC1 (series 1, 2, 3) 0.38 0.3 N N a O H , 24 hr. 0.38 E v a p o r a t e d to dryness with NaOH 0.35
M2 (cis) site . . . . . . . . . . . . . . . . . . . . . I.S.
Q.S.
I"
(mm/s)
(ram/s)
(mm/s)
65 (7)
0.39 (1)
0.68 (1)
0.39 (2)
35 (7)
1,032
0 . 3 8 (2)
67 (4)
0.38 (1)
0.69 (1)
0.38 (2)
33 (4)
1,032
0 . 3 3 (1)
0 . 3 8 (2)
60 (3)
0.37 (1)
0.76 (1)
0 . 3 8 (2)
40 (3)
1,020
0 . 4 2 (1)
0 . 3 8 (2)
45 (4)
0.34 (1)
0.76 (1)
0 . 3 8 (2)
55 (4)
1,008 (broad)
I.S. relative to m e t a l l i c Fe.
%
Si--O ( c m -1 )
%
99 of Fe 3÷ in M1 (trans) relative to that in M2 (cis) sites were slightly increased (Table II). The changes we observed in the IR and MSssbauer spectra were similar in trend to those reported by Russell (1979) for nontronites evaporated to dryness with alkali-metal hydroxides, though they were much smaller in magnitude. We repeated Russell's experiments with nontronite from Washington and, to facilitate direct comparison with some of the data in Russell's paper, also with nontronite from Garfield. The changes observed were greater than those recorded with samples of series 4, b u t much smaller than those reported by Russell. The main Si--O absorption of the Garfield nontronite was reduced to only 998 cm -1 on treatment with 15 mmol/g 0.5 N NaOH, compared with 985 cm -1 attained in Russell's experiment with only 5 mmol/g solution. The MSssbauer spectra that we recorded show a small increase in quadrupole splitting of both doublets, but this cannot be compared directly with the spectra presented by Russell, because he did not report numerical values and did not resolve the spectra. The separation of the t w o unresolved c o m p o n e n t s of the specimen of the Garfield nontronite which we examined after reaction with NaOH was 0.51 + 0.06 mm/s, which is considerably less than that of the correspondingly treated sample shown in fig. 2 of Russell's (1979) paper. The MSssbauer parameters of the NaOHtreated Washington nontronite are listed in Table II.
Scanning electron microscopy (SEM) Fig. 1A is a micrograph of the original nontronite and Fig. 1B of an aliquot of the sample heated with 0.1 N HC1 for 24 hr. at 88°C (series 1 ). Chemical analysis of this sample showed that ~ 50% of the Fe, Mg and A1 ions had passed into solution, yet the morphology of the remaining solid does n o t differ appreciably from that of the starting material. The surfaces are rougher and the edges frayed, b u t no p r o f o u n d changes in morphology could be detected. Fig. 1C shows a micrograph of a sample heated 24 hr. with 0.3 N NaOH at 88°C (series 4). The particles are much more corroded than those of the corresponding acid-treated sample and some framboidic clusters appear, probably of colloidal material. DISCUSSION Three different courses of acid or alkali attack may be envisaged: (1) Incongruent dissolution o f the t y p e postulated b y Osthaus (1954) in which ~ 4 0 % of the tetrahedral Fe and more than half the tetrahedral A1 still persist after all the octahedral Fe and A1 ions have been dissolved. (2) Congruent dissolution, in which all the cations are dissolved at the same rate, although some m a y be precipitated and/or readsorbed by the remaining solid. (3) Incongruent leaching extending through only a few unit cells.
100
Fig. 1 Scanning electron micrographs of: (A) Na nontronite; (B) nontronite after reaction with 0.1 N HCl (24 hr., 88°C); and (C) nontronite after reaction with 0.3 N NaOH (24 hr., 88°C). Original magnification: 16,300. Acid attack T h e chemical analyses can clearly d i f f e r e n t i a t e b e t w e e n m e c h a n i s m (1) and (2), b u t u n d e r the drastic c o n d i t i o n s o f a t t a c k prevailing in the experim e n t s , which cause appreciable dissolution o f the mineral at all stages, small differences in the relative p r o p o r t i o n s o f e l e m e n t s in solution, such as are e x p e c t e d t o result f r o m m e c h a n i s m (3) w o u l d be obscured. T h e results show t h a t selective dissolution o f the t y p e envisaged b y Osthaus, in which farreaching changes o c c u r in the o c t a h e d r a l sheets, while the rate o f dissolution o f t e t r a h e d r a l l y c o o r d i n a t e d ions is m u c h slower, does n o t o c c u r u n d e r the c o n d i t i o n s o f the e x p e r i m e n t s . These conclusions are s u p p o r t e d b y the IR and MSssbauer spectra. If the o c t a h e d r a l sheets were dissolved b e f o r e the t e t r a h e d r a l or if one of the cation species o c c u p y i n g o c t a h e d r a l or t e t r a h e d r a l positions were dissolved m u c h m o r e readily t h a n the others, some changes in the e n v i r o n m e n t o f the re-
101
maining ions would be expected. MSssbauer spectra are sensitive indicators of the environment of Fe atoms. The spectra of the acid-treated samples are identical with those of the original material, indicating that selective dissolution had not occurred in the bulk specimen. Similarly the IR spectra of the acid-treated samples were indistinguishable from those of the original mineral, demonstrating that no appreciable structural changes had taken place. It should be emphasised that the conclusions reached apply only to the conditions of the experiments. There is abundant evidence in the literature for incongruent dissolution of aluminosilicates by water, on mild acid treatment or on reaction with complexing solutions (e.g., Carroll and Starkey, 1971; Robert and Veneau, 1978; Yariv and Cross, 1979). It is probable that on drastic attack, as on mild acid treatment, the outer layers are dissolved incongruently, b u t that complete dissolution of these layers ensues rapidly. The differences in atomic ratios of the elements in solution are small relative to the overall concentration and the modified layers, present at any moment, constitute only a minor fraction of the total solid. Alkali attack
The question whether alkali dissolution is congruent or incongruent under the conditions of the experiments cannot be answered by the methods of investigation used. Changes were observed in the IR and MSssbauer spectra of the alkali-hydroxide-treated specimens, showing that either the entire sample or a major part was affected by the treatment. The changes were minor for the samples of series 4 and only slightly larger for those of series 5. Russell (1979) observed much larger changes for similarly treated samples. The difference may be due to some of the unspecified experimental conditions, such as the rate of drying or the time which elapsed before the spectra were recorded. Whatever the magnitude of the change, treatment with alkali hydroxides leads to a decrease in the frequency of the Si--O absorption and an increase in Q.S. of the Fe 3÷ doublets. Similar effects were observed when smectites were heated with alkali halides (Heller-Kallai, 1975). An increase in the Q.S. of the Fe 3÷ doublets also occurs on heating smectites alone, but this is associated with an increase, not a decrease, in the frequency of the Si--O absorption. Heller-KaUai (1975) and Russell (1979) concluded that on alkali halide or alkali hydroxide treatment, respectively, deprotonation of structural hydroxyl groups occurred; Russell found a correlation between the shift in the Si--O frequency and the content of octahedral Fe 3÷ and inferred that deprotonation is specifically associated with FeOH groups. Heller-Kallai showed, however, that while deprotonation does indeed commence with FeOH groups, it occurred also when a montmorillonite with negligible Fe content was heated with KBr. Therefore it seems possible that the correlation observed by Russell may be due to kinetic effects and the deprotona-
102
tion of other h y d r o x y l groups may also occur on treatment with alkali hydroxides, though less rapidly. It appears that alkali hydroxides, like the alkali halides, may act as proton acceptors in reactions with smectite and, depending on experimental conditions, will cause different amounts of deprotonation of structural h y d r o x y l groups. Fe 3÷ in nontronite occupies M1 and M2 sites nonpreferentially. NaOH treatment seems to lead to an increase in the population of M1 relative to that in M2 sites (Table II), but the effect is probably only apparent. Deprotonation eliminates the difference in Q.S. due to the cis--trans arrangement of the OH groups and Fe 3÷ in M2 sites may give rise to a doublet with Q.S. similar to that of Fe 3+ in M1 sites. In summary, the experiments described indicate that reactions of nontronite with alkali hydroxide, in contrast to those with acids, affect the entire structure and lead to deprotonation of h y d r o x y l groups, the extent of the reaction depending on the experimental conditions. The question whether dissolution is congruent or incongruent, or whether it occurs throughout the structure or is confined to reaction rims could not be answered by the methods of investigation adopted.
REFERENCES Brindley, G.W. and Youell, R.F., 1951. A chemical determination of tetrahedral and octahedral aluminum ions in a silicate. Acta Crystallogr., 4: 495--496. Carroll, D. and Starkey, H.C., 1971. Reactivity of clay minerals with acids and alkalies. Clays Clay Miner., 19: 321--334. Churchman, G.J. and Jackson, M.L., 1976. Reaction of montmorillonite with acid aqueous solutions: solute activity control by a secondary phase. Geochim. Cosmochim. Acta, 40: 1251--1259. Dudas, M.J. and Harwarc~, J.E., 1971. Effect of dissolution treatment on standard and soil clays. Soil Sci. Soc. Am. Proc., 35: 134--140. Heller-Kallai, L., 1975. Interaction of montmorillonite with alkali halides. Proc. Int. Clay Conf., Mexico City, pp. 361--372. Mathers, A.C., Weed, S.B. and Coleman, N.T., 1954. The effect of acid and heat treatment on montmorillonoids. Clays Clay Miner., Proc. 3rd Natl. Conf., pp. 413--420. Osthaus, B., 1954. Chemical determination of tetrahedral ions in nontronite and montmorillonite. Clays, Clay Miner., Proc. 2nd Natl. Conf., pp. 404--417. Osthaus, B., 1955. Kinetic studies on montmorillonites and nontronite by the aciddissolution technique. Clays Clay Miner. Proc. 4th Natl. Conf., pp. 301--321. Robert, M. and Veneau, G., 1978. Stabilit~ des min~raus phylliteux 2/1 en conditions acides -- R61e de la competition octa~drique. Int. Clay Conf., Oxford, pp. 385--394. Ross, G . J , 1969. Acid dissolution of chlorites: release of magnesium, iron and aluminum and mode of acid attack. Clays Clay Miner., 17: 347--354. Rozenson, I., 1975. Reduction--oxidation reactions in dioctahedral clays. M.Sc. Thesis, Hebrew University, Jerusalem (in Hebrew). Rozenson, I. and Heller-Kallai, L., 1976. MSssbauer spectra of dioctahedral smectites. Clays Clay Miner., 25: 94--101. Russell, J.D., 1979. An infra-red spectroscopic study of the interaction of nontronite and ferruginous montmorillonites with alkali metal hydroxides. Clay Miner., 14: 127--138. Yariv, S. and Cross, H., 1979. Geochemistry of Colloid Systems. Springer, Berlin, pp. 313--318.
95
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
N O N T R O N I T E A F T E R ACID OR ALKALI ATTACK L. HELLER-KALLAI and I. ROZENSON
Department of Geology, The Hebrew University, Jerusalem (Israel) (Received June 30, 1980; revised and accepted February 2, 1981)
ABSTRACT Heller-Kallai, L. and Rozenson, I., 1981. Nontronite after acid or alkali attack. Chem. Geol., 32: 95--102. X-ray diffraction patterns, infra-red and MSssbauer spectra of samples of nontronite reacted with 0.1 N HCI at 70--90°C for various lengths of time were indistinguishable from those of the starting material. After appropriate corrections for exchange of interlayer cations, constant Al/Mg and Al/Fe ratios of the solid were maintained. It was concluded that dissolution may be incongruent in a narrow reaction rim, but that the structure of the bulk material is not affected by acid attack. In contrast, minor changes were observed in the infra-red and MSssbauer spectra of samples reacted with 0.3 N NaOH solution. These were attributed to partial deprotonation of the entire structure. Scanning electron micrographs showed that alkali attack changed the morphology of the clay appreciably while acid attack did not.
INTRODUCTION
The question whether clay minerals dissolve congruently or incongruently on acid attack has been debated for many years. Osthaus' (1954, 1955) conclusions that octahedral cations of montmorillonite and nontronite are extracted in preference to tetrahedral ones on drastic acid attack has found wide acceptance, though other investigators reported that the rates of dissolution of tetrahedral and octahedral cations from these minerals are equal (e.g., Mathers et al., 1954). Similar controversies pertain to the results of acid attack of chlorites (Brindley and Youell, 1951; Ross, 1969). Churchman and Jackson (1976) presented evidence to show that A1 dissolved o u t of montmorillonite was partially adsorbed on to the clay interlayers. The presence of a metastable Si-rich secondary phase, "depleted montmorillonite", was also inferred and attributed to preferential leaching of Mg and A1. Many of the previous studies of acid attack were designed to establish the reaction kinetics and emphasis was placed on the chemical aspects of the reactions and on determination of the thermodynamic constants. Churchman and Jackson (1976) showed that at least in the pH range they investigated (1.5--4.2) it is invalid to treat acid dissolution as a single-step process. Kinetic studies of alkali attack are impracticable, since precipitation of 0009-2541/81/0000---0000/$02.50 © 1981 Elsevier Scientific Publishing Company
96 hydroxides occurs at the high pH-values of the experimental solutions. Dudas and Harward (1971) studied the effect of a combined NaOH or KOH and sodium dithionite--citrate bicarbonate extraction of clay minerals, but in such combined treatments it is difficult to assess the role of alkalinity alone. Russell (1979) showed that interaction of alkali-metal hydroxides with dioctahedral smectites leads to deprotonation of OH groups. In the present study, attention was focussed on possible changes in the structure of clay minerals after acid or alkali attack, without consideration of the reaction kinetics. Nontronite was chosen for detailed investigation. The relatively drastic conditions of attack adopted render the results of the acid attack directly comparable with those reported by Osthaus.
EXPERIMENTAL Materials
Nontronite from the State of Washington (Source Clay Mineral) was used for the experiments. The structural formula deduced after correction for the presence of quartz and iron oxides is:
(Ca0.34 Mg0.14 ) (SiT.~s A10.Ts) (A10.ss Fe2.TS Mg0.33 Ti0.0s )O20(OH)4 The a m o u n t of Fe203 in the sample is 1.2% of the total, or 5.5% of the iron present (Rozenson, 1975). Some experiments were also carried out with nontronite from Garfield, Washington (American Petroleum Institute). Procedure
In each experiment 40 mg of sample were mixed with 20 ml of solution. Series 1: Nontronite was heated with 0.1 N HC1 in test-tubes immersed in an oil bath at 88 + 2°C for periods of time ranging up to 1 hr. Series 2: Nontronite was heated with 0.1 N HC1 in sealed ampoules maintained at 70 + 2 ° C for periods of time ranging up to 91 hr. Series 3: Similar to series 2, but the temperature was maintained at 90 -+ 2 ° C and heating times ranged up to 140 hr. Series 4: Nontronite was heated with 0.3 N NaOH in test-tubes immersed in an oil bath at 88 + 2°C for periods of time ranging up to 24 hr. After various periods of heating the samples were centrifuged and washed with distilled water. The supernatant solutions of all the samples of series 1 were analysed for A1, Mg and Fe by atomic absorption spectroscopy and those of series 2 and 3 for A1 and Fe. In the course of the experiments of series 1 and 4 some evaporation occurred and the solutions became more concentrated with time. Since no attempt was made to establish reaction kinetics or any thermodynamic constants, this does not affect the validity of the results. Series 5: Samples were covered with 15 mmol/g 0.5 N NaOH solution
97
and evaporated to dryness, following the procedure outlined by Russell (1979). Infra-red (IR) spectra were recorded of all the samples. Selected specimens were subjected to X-ray diffraction (XRD), MSssbauer spectroscopy and scanning electron microscopy (SEM). The MSssbauer spectra were recorded and resolved into two Fe 3+ doublets, as previously described (Rozenson and Heller-Kallai, 1976). RESULTS AND INTERPRETATION
Chemical analyses The fraction of Fe, Mg and A1 which remained in the solid state after various reaction times was deduced from duplicate analyses of the supematant solutions of the samples of series 1. The results are shown in Table I. To obviate possible changes introduced in pretreatment of the nontronite, this was used in its natural state, without cation exchange or removal of iron oxides. To avoid the necessity of introducing arbitrary assumptions, only the ratios of elements remaining in the solid state were considered. The original sample contained exchangeable Mg and adsorbed Fe. It is evident that the A1/Mg ratio increased gradually in the course of the first few hours, indicating that the exchange of Mg ions was not instantaneous, but required several hours for completion. The high value observed after 24-hr. treatment is probably due to experimental error. The reproducibility of the analysis was lower than for the less reacted samples. Moreover, since the amount of Mg in the solid phase is low, the error in the A1/Mg ratio is large. The fact that the A1/Fe ratio did not vary appreciably with time and, in particular, did not increase with time, indicates that under the prevalent experimental conditions TABLE I
Dissolution of nontronite on acid attack (series 1 ) Time (hr)
0 0.5 1.5 2 6 12 18 24
% e l e m e n t in s o l u t i o n
% e l e m e n t in solid phase .1
AI
A1
-0.49 0.71 0.71 1.27 1.47 1.63 1.91
Fe
-1.47 2.28 2.44 4.00 4.92 5.92 7.02
Mg
-0.28 0.35 0.37 0.51 0.55 0.58 0.67
4.02 3.53 3.31 3.31 2.75 2.55 2.39 2.11
, I (% in original solid) -- (% in s o l u t i o n ) . ,2 I r r e p r o d u c i b l e results.
Fe
15.13 13.66 12.85 12.69 11.13 10.21 9.21 8.11
R a t i o o f e l e m e n t s in r e m a i n i n g solid
Mg
1.08 0.80 0.73 0.71 0.57 0.53 0.50 0.41
Al/Mg
A1/Fe
3.72 4.41 4.50 4.63 4.78 4.77 4.78 5.14 *2
0.27 0.26 0.26 0.26 0.25 0.25 0.26 0.26 *=
98 dissolution of iron oxides and of the mineral itself occurred simultaneously. Chemical analyses of A1 and Fe in the supernatant solutions of selected samples of series 2 and 3 were carried out to monitor the extent of the reaction. The A1/Fe ratio in the solid phase remained approximately constant for all the samples of series 2. Samples of series 3 showed a similar A1/Fe ratio in the initial stages of the reaction, but after 15 hr. or more the results were erratic. After 50-hr. reaction time in sealed ampoules at 90°C - 65% of the A1 ions and after 140 hr. over 70% had passed into solution. The results obtained with series 4 were irreproducible. At the prevalent high alkalinity polymerisation, precipitation and/or adsorption of some of the products is expected to occur.
X-ray diffraction, infra-red and MSssbauer spectroscopy All the acid-treated samples but one gave rise to XRD patterns, IR and MSssbauer spectra which are indistinguishable from those of the original sample. The single exception is the sample which was heated for 40 hr. at 90°C in a sealed ampoule. This drastic treatment led to the formation of a new unidentified phase with a featureless absorption in the OH stretching region and a shoulder extending from 940--910 cm -~. This sample will be excluded from the following discussion. All the alkali-treated samples of series 4 remained fully expandable with glycol. Unlike the acid-treated samples, those digested with alkali gave rise to IR patterns which differed slightly from those of the original material. In particular, the frequency of the main Si--O absorption was reduced from 1032 cm -~ for Na-substituted nontronite up to 1022--1020 cm-1 A small but significant change also occurred in the MSssbauer spectra. After 24-hr. NaOH treatment the Q.S. of both doublets and the proportion T A B L E II M o s s b a u e r p a r a m e t e r s and Si--O v i b r a t i o n f r e q u e n c y of variously t r e a t e d s a m p l e s of n o n t r o n i t e Treatment
M1
(trans) site
I.S.
Q.S.
I"
(mm/s)
(mm/s)
(ram/s)
(1)
0 . 2 9 (2)
0 . 4 0 (2)
(1)
0.29 (1)
(1)
(1)
-0.39 0.1 N HC1 (series 1, 2, 3) 0.38 0.3 N N a O H , 24 hr. 0.38 E v a p o r a t e d to dryness with NaOH 0.35
M2 (cis) site . . . . . . . . . . . . . . . . . . . . . I.S.
Q.S.
I"
(mm/s)
(ram/s)
(mm/s)
65 (7)
0.39 (1)
0.68 (1)
0.39 (2)
35 (7)
1,032
0 . 3 8 (2)
67 (4)
0.38 (1)
0.69 (1)
0.38 (2)
33 (4)
1,032
0 . 3 3 (1)
0 . 3 8 (2)
60 (3)
0.37 (1)
0.76 (1)
0 . 3 8 (2)
40 (3)
1,020
0 . 4 2 (1)
0 . 3 8 (2)
45 (4)
0.34 (1)
0.76 (1)
0 . 3 8 (2)
55 (4)
1,008 (broad)
I.S. relative to m e t a l l i c Fe.
%
Si--O ( c m -1 )
%
99 of Fe 3÷ in M1 (trans) relative to that in M2 (cis) sites were slightly increased (Table II). The changes we observed in the IR and MSssbauer spectra were similar in trend to those reported by Russell (1979) for nontronites evaporated to dryness with alkali-metal hydroxides, though they were much smaller in magnitude. We repeated Russell's experiments with nontronite from Washington and, to facilitate direct comparison with some of the data in Russell's paper, also with nontronite from Garfield. The changes observed were greater than those recorded with samples of series 4, b u t much smaller than those reported by Russell. The main Si--O absorption of the Garfield nontronite was reduced to only 998 cm -1 on treatment with 15 mmol/g 0.5 N NaOH, compared with 985 cm -1 attained in Russell's experiment with only 5 mmol/g solution. The MSssbauer spectra that we recorded show a small increase in quadrupole splitting of both doublets, but this cannot be compared directly with the spectra presented by Russell, because he did not report numerical values and did not resolve the spectra. The separation of the t w o unresolved c o m p o n e n t s of the specimen of the Garfield nontronite which we examined after reaction with NaOH was 0.51 + 0.06 mm/s, which is considerably less than that of the correspondingly treated sample shown in fig. 2 of Russell's (1979) paper. The MSssbauer parameters of the NaOHtreated Washington nontronite are listed in Table II.
Scanning electron microscopy (SEM) Fig. 1A is a micrograph of the original nontronite and Fig. 1B of an aliquot of the sample heated with 0.1 N HC1 for 24 hr. at 88°C (series 1 ). Chemical analysis of this sample showed that ~ 50% of the Fe, Mg and A1 ions had passed into solution, yet the morphology of the remaining solid does n o t differ appreciably from that of the starting material. The surfaces are rougher and the edges frayed, b u t no p r o f o u n d changes in morphology could be detected. Fig. 1C shows a micrograph of a sample heated 24 hr. with 0.3 N NaOH at 88°C (series 4). The particles are much more corroded than those of the corresponding acid-treated sample and some framboidic clusters appear, probably of colloidal material. DISCUSSION Three different courses of acid or alkali attack may be envisaged: (1) Incongruent dissolution o f the t y p e postulated b y Osthaus (1954) in which ~ 4 0 % of the tetrahedral Fe and more than half the tetrahedral A1 still persist after all the octahedral Fe and A1 ions have been dissolved. (2) Congruent dissolution, in which all the cations are dissolved at the same rate, although some m a y be precipitated and/or readsorbed by the remaining solid. (3) Incongruent leaching extending through only a few unit cells.
100
Fig. 1 Scanning electron micrographs of: (A) Na nontronite; (B) nontronite after reaction with 0.1 N HCl (24 hr., 88°C); and (C) nontronite after reaction with 0.3 N NaOH (24 hr., 88°C). Original magnification: 16,300. Acid attack T h e chemical analyses can clearly d i f f e r e n t i a t e b e t w e e n m e c h a n i s m (1) and (2), b u t u n d e r the drastic c o n d i t i o n s o f a t t a c k prevailing in the experim e n t s , which cause appreciable dissolution o f the mineral at all stages, small differences in the relative p r o p o r t i o n s o f e l e m e n t s in solution, such as are e x p e c t e d t o result f r o m m e c h a n i s m (3) w o u l d be obscured. T h e results show t h a t selective dissolution o f the t y p e envisaged b y Osthaus, in which farreaching changes o c c u r in the o c t a h e d r a l sheets, while the rate o f dissolution o f t e t r a h e d r a l l y c o o r d i n a t e d ions is m u c h slower, does n o t o c c u r u n d e r the c o n d i t i o n s o f the e x p e r i m e n t s . These conclusions are s u p p o r t e d b y the IR and MSssbauer spectra. If the o c t a h e d r a l sheets were dissolved b e f o r e the t e t r a h e d r a l or if one of the cation species o c c u p y i n g o c t a h e d r a l or t e t r a h e d r a l positions were dissolved m u c h m o r e readily t h a n the others, some changes in the e n v i r o n m e n t o f the re-
101
maining ions would be expected. MSssbauer spectra are sensitive indicators of the environment of Fe atoms. The spectra of the acid-treated samples are identical with those of the original material, indicating that selective dissolution had not occurred in the bulk specimen. Similarly the IR spectra of the acid-treated samples were indistinguishable from those of the original mineral, demonstrating that no appreciable structural changes had taken place. It should be emphasised that the conclusions reached apply only to the conditions of the experiments. There is abundant evidence in the literature for incongruent dissolution of aluminosilicates by water, on mild acid treatment or on reaction with complexing solutions (e.g., Carroll and Starkey, 1971; Robert and Veneau, 1978; Yariv and Cross, 1979). It is probable that on drastic attack, as on mild acid treatment, the outer layers are dissolved incongruently, b u t that complete dissolution of these layers ensues rapidly. The differences in atomic ratios of the elements in solution are small relative to the overall concentration and the modified layers, present at any moment, constitute only a minor fraction of the total solid. Alkali attack
The question whether alkali dissolution is congruent or incongruent under the conditions of the experiments cannot be answered by the methods of investigation used. Changes were observed in the IR and MSssbauer spectra of the alkali-hydroxide-treated specimens, showing that either the entire sample or a major part was affected by the treatment. The changes were minor for the samples of series 4 and only slightly larger for those of series 5. Russell (1979) observed much larger changes for similarly treated samples. The difference may be due to some of the unspecified experimental conditions, such as the rate of drying or the time which elapsed before the spectra were recorded. Whatever the magnitude of the change, treatment with alkali hydroxides leads to a decrease in the frequency of the Si--O absorption and an increase in Q.S. of the Fe 3÷ doublets. Similar effects were observed when smectites were heated with alkali halides (Heller-Kallai, 1975). An increase in the Q.S. of the Fe 3÷ doublets also occurs on heating smectites alone, but this is associated with an increase, not a decrease, in the frequency of the Si--O absorption. Heller-KaUai (1975) and Russell (1979) concluded that on alkali halide or alkali hydroxide treatment, respectively, deprotonation of structural hydroxyl groups occurred; Russell found a correlation between the shift in the Si--O frequency and the content of octahedral Fe 3÷ and inferred that deprotonation is specifically associated with FeOH groups. Heller-Kallai showed, however, that while deprotonation does indeed commence with FeOH groups, it occurred also when a montmorillonite with negligible Fe content was heated with KBr. Therefore it seems possible that the correlation observed by Russell may be due to kinetic effects and the deprotona-
102
tion of other h y d r o x y l groups may also occur on treatment with alkali hydroxides, though less rapidly. It appears that alkali hydroxides, like the alkali halides, may act as proton acceptors in reactions with smectite and, depending on experimental conditions, will cause different amounts of deprotonation of structural h y d r o x y l groups. Fe 3÷ in nontronite occupies M1 and M2 sites nonpreferentially. NaOH treatment seems to lead to an increase in the population of M1 relative to that in M2 sites (Table II), but the effect is probably only apparent. Deprotonation eliminates the difference in Q.S. due to the cis--trans arrangement of the OH groups and Fe 3÷ in M2 sites may give rise to a doublet with Q.S. similar to that of Fe 3+ in M1 sites. In summary, the experiments described indicate that reactions of nontronite with alkali hydroxide, in contrast to those with acids, affect the entire structure and lead to deprotonation of h y d r o x y l groups, the extent of the reaction depending on the experimental conditions. The question whether dissolution is congruent or incongruent, or whether it occurs throughout the structure or is confined to reaction rims could not be answered by the methods of investigation adopted.
REFERENCES Brindley, G.W. and Youell, R.F., 1951. A chemical determination of tetrahedral and octahedral aluminum ions in a silicate. Acta Crystallogr., 4: 495--496. Carroll, D. and Starkey, H.C., 1971. Reactivity of clay minerals with acids and alkalies. Clays Clay Miner., 19: 321--334. Churchman, G.J. and Jackson, M.L., 1976. Reaction of montmorillonite with acid aqueous solutions: solute activity control by a secondary phase. Geochim. Cosmochim. Acta, 40: 1251--1259. Dudas, M.J. and Harwarc~, J.E., 1971. Effect of dissolution treatment on standard and soil clays. Soil Sci. Soc. Am. Proc., 35: 134--140. Heller-Kallai, L., 1975. Interaction of montmorillonite with alkali halides. Proc. Int. Clay Conf., Mexico City, pp. 361--372. Mathers, A.C., Weed, S.B. and Coleman, N.T., 1954. The effect of acid and heat treatment on montmorillonoids. Clays Clay Miner., Proc. 3rd Natl. Conf., pp. 413--420. Osthaus, B., 1954. Chemical determination of tetrahedral ions in nontronite and montmorillonite. Clays, Clay Miner., Proc. 2nd Natl. Conf., pp. 404--417. Osthaus, B., 1955. Kinetic studies on montmorillonites and nontronite by the aciddissolution technique. Clays Clay Miner. Proc. 4th Natl. Conf., pp. 301--321. Robert, M. and Veneau, G., 1978. Stabilit~ des min~raus phylliteux 2/1 en conditions acides -- R61e de la competition octa~drique. Int. Clay Conf., Oxford, pp. 385--394. Ross, G . J , 1969. Acid dissolution of chlorites: release of magnesium, iron and aluminum and mode of acid attack. Clays Clay Miner., 17: 347--354. Rozenson, I., 1975. Reduction--oxidation reactions in dioctahedral clays. M.Sc. Thesis, Hebrew University, Jerusalem (in Hebrew). Rozenson, I. and Heller-Kallai, L., 1976. MSssbauer spectra of dioctahedral smectites. Clays Clay Miner., 25: 94--101. Russell, J.D., 1979. An infra-red spectroscopic study of the interaction of nontronite and ferruginous montmorillonites with alkali metal hydroxides. Clay Miner., 14: 127--138. Yariv, S. and Cross, H., 1979. Geochemistry of Colloid Systems. Springer, Berlin, pp. 313--318.