EFFECT OF EXCHANGE IN MICAS

Transcription

1 Clay Minerals (1970) 8, 361. THE SYNERGETIC ON THE CATION EFFECT OF EXCHANGE IN MICAS HYDROGEN IONS OF POTASSIUM A. C. D. NEWMAN Rothamsted Experimental Station, Harpenden, Herts. (Received 8 June 1970) ABSTRACT: Interlayer K in eleven trioctahedml micas, with compositions ranging from fluorphlogopite to biotite, was replaced by treating cut flakes with aqueous solutions of Na salts at controlled ph values. Replacement in natural micas was faster at ph 4.5 than at ph 9, and there is a co-operative action or synergy of H + and Na + in replacing K +. The synergetic effect of H + tends to be greatest in micas that lose most net negative charge when K is replaced,and increases when Fe2+ in biotite is oxidized by heating. Possible relationships between the synergetic effect and the chemical composition of micas are discussed in relation to the structure and chemical changes in the aluminosilicate layers when K is exchanged, and it is concluded that incorporation of protons into the structure is responsible for the synergetic effect. INTRODUCTION When interlayer potassium in mica is replaced by a hydrated cation, the cation exchange is accompanied by a phase transition, and as potassium is replaced an expanded vermiculite-like product is formed that is contiguous with unchanged mica. This process has some special properties. First, an optical discontinuity occurs at the interface between the expanded phase and unexpanded mica, so that the replacement of potassium in a mica flake can be directly observed under the microscope (Mortland, 1958; Bassett, 1959) and the rate of replacement readily measured (Rausell- Colom et al., 1965). Secondly, when mica is equilibrated with a solution of a replacing cation, the maximum concentration of potassium attained in solution is almost independent of the amount of mica potassium replaced (Newman, 1969). Although primarily a reaction of cation exchange, replacement of potassium is accompanied by secondary reactions in the aluminosilicate framework, and Newman & Brown (1966) suggested that, as a consequence of these reactions, the replacement of interlayer potassium may be ph-dependent. However, in a recent report of optical measurements on a biotite exchanged in solutions of different acidities, Wells & Norrish (1968) concluded that H ions have no specific role in accelerating K replacement. This conclusion seemed to conflict with observations that the exchange para-

2 362 A. C. D. Newman meters for micas were greater in acidic than in neutral solutions (Newman, 1969), and to resolve this disagreement, I measured the rates of K replacement at different acidities, using the optical method and micas that previously showed enhanced replacement at acid ph. Materials MATERIALS AND PROCEDURES Eleven micas were used; five phlogopites and five biotites previously investigated (P1, P3, P4, P5, P6; B1, B3a, B4, B6, L1; Newman, 1969), and a synthetic fluorphlogopite containing 9 % fluorine, of which there was too little to analyse further chemically. Table 1 gives the structural formulae of the ten natural micas. TABLE 1. Structural formulae of micas per O20(OH, F)4 P1 P3 P4 P5 P6 BI B3a B4 B6 L1 Si 5"85 5" "76 5" "48 Al 2'10 2"34 2"30 2"27 2"18 2"23 2"74 2"51 2"42 2"52 AI Ti "21 0"33 Fe3+ 0"03 0" "01 0"12 0" "36 0"34 0"19 FeZ Mg Ca 0"04 0" "06 0"06 0" " Mn 0"01 0"01 0"01 0"0 0'0 0" "03 0" K 1"84 1" " " "75 1" Ha OH " F Flake preparation To measure potassium replacement reliably, flakes of mica must be cut so that the disturbed region at the edge is as narrow as possible, yet a very sharp edge is not suitable because exchange is then inhibited (Newman & Brown, 1969). The most suitable edges were cut with a fine high-velocity air jet containing suspended alumina powder; the S.S. White Airbrasive unit, operated by air at 2 Kg cm -2 pressure and loaded with 10/~m Al2Os powder produced a suitable jet. A piece of mica about 1 mm thick cut by knife from a hand specimen and fixed to cardboard with water-soluble glue, was advanced very slowly under the jet so that air pressure did not build up on the partly-cut mica. Several parallel cuts about 1-2 mm apart were made in one direction, and after fixing to a fresh piece of card, more parallel cuts were made at right

3 Synergetic effect of hydrogen ions on cation exchange of K in micas 363 angles to the first set. The glue was dissolved in hot distilled water and adhering alumina removed from the mica by washing on a sieve with distilled water. Some flakes were suitable for immediate use; others were cleaved under water with a needle to flakes about tan thick. Flakes were selected in which the disturbed regions at the edges were less than 3 ~m wide. Potassium replacement The rate at which the zonal interface moves depends on temperature, the replacing cation, and the concentrations of potassium and the replacing cation in the external solution (Rausell-Colom et al., 1965). These variables were kept constant by making all rate comparisons in 0"25M NaCI solution at its boiling point, using a large reaction volume (500 ml) so that for 1 mg mica sample there was an insignificant accumulation of potassium in the external solution; loss of solvent was prevented by fitting the reaction flask with a reflux condenser. Solutions of different ph were obtained by adding small amounts of HC1 or NazCO3; the ph values used were 3"4, 4"5, 7, 9 and 10, and there was little change in ph (<0.1 unit) during the experiments. Some experiments were also done with 0"125M Na2SO4, with 0"0003M H~SO4, and with the two reagents combined; these solutions had ph values of 5" , 3.6 and 4.2 respectively. Up to five mica flakes were refluxed with solution in one flask, and at the end of the reaction period the near-boiling solution was poured off. The flakes remaining in the flask were immediately cooled, transferred with cold solution to a cavity microscope slide closed with a cover slip to prevent evaporation. When the microscope measurements were completed, the flakes were returned to the reaction flask containing fresh boiling solutions for further exchange of potassium. Microscope measurements The width of the replacement region at many places along a straight edge was measured in transmitted light, using an eyepiece graticule calibrated for 400 times magnification. For measurements between a sharp zonal interface and a good edge, the uncertainty was about 1 division of the graticule or 2-5 #m. Occasionally, when the optical discontinuity at the interface was weak, observations were made with crossed polars and a sensitive tint plate; under these conditions, the apparent magnification decreased by only 1%. For each mica and reactant solution, the width of the replacement zone was measured after reaction times ranging from 1 hr to approximately 6 days; most micas showed some replacement after 1 hr but phlogopites containing much fluorine (P1, P6 and fluorphlogopite) were much less reactive and a replacement zone in these was only evident after 24 hr. RESULTS The width of the replacement zone (8) was plotted against the square root of the total reaction time (t~); if a simple diffusion process is rate-limiting the graphs should be linear, but, as found by Wells & Norrish (1968), this behaviour was never observed (Figs 1 and 2). The parameter relating 8 to t~ seems to be time-dependent, but

4 364 A. C. D. Newman I I I I I E ::k...g 150 m IO ~---../-, I i I I I [Time (hr)] y2 FIG. 1. Rate of boundary movement in phlogopite 1 during reaction with 0"25M NaCI. O:pH3"4;O:pH4"5; D:pH7; II:pH9. the reason for this is not known. Rates of replacement for different micas and conditions were therefore compared on an empirical basis, and the rates were expressed as the square of the gradient of the 8/t89 graph at the same specified time, 36 hr (Table 2); this parameter has the dimensions of a diffusion coefficient (12s-1). Although the time chosen was arbitrary, partly dictated by the properties of the micas investigated, comparisons of the gradients at other times placed the rates of replacement in the same order. Potassium replacement in all the micas except the fluorphlogopite was faster in the more acid solutions, although the difference was much greater with P5, B1, B3a and B4 than with the others; the influence of hydrogen ions on the equilibrium concentrations of potassium released to solution by the same micas (Newman, 1969)

5 Synergetic effect of hydrogen ions on cation exchange of K in micas 365 was in the same relative order. These results conflict with the report of Wells & Nonish (1968), who concluded that 'the hydrogen ion, rather than having some special significance in the replacement of interlayer potassium, behaves remarkably like a metal cation'. If Wells & Norrish were correct, the effect of hydrogen ions in the presence of sodium ions would be the replacement by hydrogen ions alone superimposed on replacement by sodium,.the total being the sum of the parts. To test this, micas P3, B1, B3a and B4 were reacted with 0"125M Na2SO4 alone, with 0"0003M H~SO~ alone, and with a solution containing both reagents; in each mica flake the replacement from reaction with the combined solutions was greater than the sum of the replacements in the single reagents (Table 3). These observations convincingly demonstrate how H + ions, in concentrations too small to replace I l I / I 400 / E 300,/ ~O 100 /.J i I I I [Time (hr)] ~'2 FIG. 2. Rate of boundary movement in biotite 1 during reaction with 0"25M NaCI. O:pH3"4; O:pH4"5; [3:pH9; I[:pHl0.

6 366 A. C. D. Newman TABLE 2. Rate parameters* for the replacement of K in micas at various ph values, mm2 see-1 x 10 a MicM" ph 3"4 4" E:~ FP "63 0"73 0" "80 PI 6"13 5"65 4"94 3" "58 P3 73" "3 26"0 1"56 P4 18" ,98 P "4 37"3 3"15 P6 5"65 4"94 3" "72 B "8 32"9 2"73 B3a 72"4 42" "87 3"12 B4 45"1 31"6 -- I1.1 10"4 2"85 B "45 L * Rate parameter is the square of the gradient of the graph of boundary width (~) against (time)89 at t = 36 hr. tfp = fluorphlogopite, others as Table 1. :~E = rate parameter at oh 4.5 rate parameter at ph 9 much mica-k alone, nevertheless substantially accelerate the replacement of K by Na, and show that the conclusion of Wells and Norrish does not apply to the micas studied here. The co-operative action, or synergy, of H + and solution cation in replacing interlayer K suggests that H + ions have a specific influence on the exchange process. TABLE 3. Width of boundaries in micas after 22 hr (/~m) Solutions* Mica Na2SO4 H2SO4 Na2SO4+H2SO4 P3 89 8"7 142 B B3a B * 0"125M Na2SO4, 0"0003M H2SO4 and 0"125M Na2SO M H2SO4

7 Synergetic effect of hydrogen ions on cation exchange of K in micas 367 Origin oj the synergetic effect* INTERPRETATION AND DISCUSSION Mortland & Ellis (1959) reviewed the processes likely to limit the rate at which mica K exchanges and concluded that equations based on diffusion of cations best explained their results; other workers (Reed & Scott, 1962; Rausell-Colom et al., 1965; Chute & Quirk, 1967) likewise used diffusion equations to account for the rate of K exchange. If a diffusion explanation is correct, it is not immediately obvious why relatively minute concentrations of H ions should so influence the diffusion of the replacing cation and potassium. An explanation solely in terms of interionic effects in the interlayer space must be rejected because the effect of H ions is greater in some micas (e.g. B1, B3a) than in others (e.g. P1, P3). Strong fields within the interlayer region interact with the diffusing species and make migration much slower than in free solutions. Structure determinations on vermiculites (Mathieson & Walker, 1954; Shirozu & Bailey, 1966) show that interlayer cations occupy definite positions between the aluminosilicate layers; the interlayer field is therefore not uniform and cations at equilibrium are localized at positions of minimum potential energy. In the non-equilibrium condition of exchange, cations move from one minimum to the next, and the rate of diffusion is dictated by the height of the barriers between minima. It is relevant, therefore, to enquire what factors influence the depth of these minima, and whether they can be altered by the presence of H ions. In general, both the detailed structural arrangement of atoms surrounding the K atom and the charge density of the aluminosilicate layers probably influence the strength of bonding for potassium. Because of detailed structural adjustments these may be related influences; they are, however, discussed separately in the following sections. Structure of potassium sites Nearest neighbours to K are the O atoms of the tetrahedral sheets, six from one sheet and six from the next; usually the tetrahedra are twisted into a ditrigonal configuration so that three O atoms from one sheet and three from the other are closer to K than the remaining six O atoms. The structure of the octahedral layer seems to determine the twist and tilt of the tetrahedra (Bailey, 1966) and therefore the configuration of the O atoms neighbouring K. Although there is no experimental evidence to assess whether the O configuration affects the exchange of interlayer K, the possibility of such an effect ought not to be overlooked. A longer range, but apparently more important, interaction occurs between octahedral OH groups and K atoms. In trioctahedral minerals, H is repelled equally from three adjacent octahedral cations, so that the OH dipole is oriented normal to the basal plane and H repels interlayer K atoms (Bassett, 1960). In dioctahedral micas, the OH dipole is tilted at 74 ~ to the normal in the direction of the vacant octahedral site, * For convenience, the intemity of the synergetic effect (E) is expressed as the ratio of the rate parameter at ph 4"5 to that at ph 9.

8 368 A. C. D. Newman 5OO ob6 I obi I(X3 to ta 5O B3a 9 9 P3 ~o B4 \ 9 P4 Io y- 5-0 I-0 FP I I I I 2 3 O.b F atoms per (OH,F) 4 FIG. 3. Relation between the rate of K-replacement at ph 4.5 and F substitution in trioctahedrai micas. and the interaction with K is weaker (Vedder & McDonald, 1963; Vedder, 1964). These different configurations seem to have a dominant effect on the exchange of interlayer K, for the equilibrium concentration of K from dioctahedral micas is far smaller than that from trioctahedral micas. Fluorine can partially or completely replace OH in trioctahedral micas and usually the greater the extent of this substitution, the smaller the equilibrium exchange of K (Newman, 1969) and slower the exchange (Fig. 3; see also Rausell-Colom et al., 1965). Presumably F does not repel K as does OH, though this simple explanation may eventually prove inadequate, for recent structural investigation shows that K-O distances are lengthened when F substitutes for OH (Rayner, 1968). Exchange of K is also inhibited when biotites are oxidized; when biotites are heated to 400 ~ C or above, Fe 2+ is oxidized and the structural hydrogen content decreases (Rimsaite, 1967), and it has been suggested that this loss of hydrogen from OH groups decreases the repulsion of K (Robert & Pedro, 1968; Barshad & Kishk, 1968).

9 Synergetic effect of hydrogen ions on cation exchange of K in micas LtO Ble eb6 20 td o B3o 9 B4e P4e P6 P5e P3 12 I I I I I Exchange capacity, me/g,air-dry wt. FIG. 4. Relation between the cation exchange capacity of mica alteration end-products and total Fe content of the original micas. Decrease of charge density In trioctahedral micas, the net charge of the aluminosilicate layer decreases when K is exchanged (Newman & Brown, 1966); the decrease, as indicated by the cation exchange capacities of the materials produced, is related to the Fe content of the micas (Fig. 4). Although some relation between the amount of net charge loss and the exchangeability of K is indicated (for example, micas B1, B6 and L1 lose most net charge and exchange K more readily than P1, P4 and P6, which change relatively little), it is subsidiary to the effect of F replacing OH. Effects of H ions on replacement of K Structural environment of K. As a deficit of H in a mica increases its affinity for K, this increased atfmity may be modified in the presence of H ions. The structural expansion that accompanies K replacement allows cation and water to penetrate the structure and reach 02- that would usually be OH- but that lack H, either because adjacent Fe ~+ has oxidized, or because the mineral was deficient in H when formed.

10 370 A. C. D. Newman In the newly-hydrated interlayer region H ions may react with these 02- to form OH-, which will tend to repel K and also make the net negative charge smaller. If H ions can facilitate K exchange in this way, the increased resistance to replacement of K in biotite after oxidation should be apparent only when the supply of hydrogen ions at the site of K exchange is limiting, and in a weakly acidic environment the oxidized biotite might well exchange K as readily as the unoxidized mica. To test this, four similar flakes of biotite 6 (which contains much ferrous iron but little F, and so is susceptible to oxidation by this reaction) were selected, and two were partly oxidized by heating at 450 ~ C in air for 16 hr. Oxidation during heating was judged by the change in colour from grey-green to grey-brown and by the increase in birefringence; oxidation was very inhomogeneous and browner colours formed on the surfaces and at the edges of the flakes (this variegation was also noted by Rimsaite). The flakes also exfoliated and tended to curl and disintegrate when first put in water. The rates of replacement in the oxidized and unheated flakes were compared by refluxing in 0"25M NaCI solutions of ph 4"4 and 9, and measuring the boundary movement optically as before. Replacement at ph 4 was equally rapid in the oxidized and unoxidized flakes, but at ph 9 the width of the zone of K replacement in the oxidized flakes was narrower in the brown regions at the original :300 E 200 >~ 8 C13,I~ x / ~ I00 0 " 0 ~ I I I ) 2 3 [Time (hr)] ~2 l~. 5. Rate of boundary movement in oxidized and unoxidized flakes of biotite 6. Unheated: [~, ph 4.4; X, ph 9. Heated: +, ph 4.4; O, ph 9, oxidized edge; O, ph 9, fresh edge.

11 Synergetic effect of hydrogen ions on cation exchange of K in micas 371 edge of the flakes than at greener edges that appeared to be fresh fractures; the replacement zone at the greener edges was the same width as in the unoxidized flake (Fig. 5). At the browner and apparently oxidized edges, K was replaced much more slowly than in the unheated flakes. The results showed that replacement of K in the oxidized regions was inhibited in alkaline solution but not in acidic, confirming that H ions can assist K exchange from H-deficient sites. However, if this were the only mechanism by which H ions accelerated the exchange of K, the results could imply that micas with similar F contents but different H contents should exchange K at the same rate in solutions of ph 4, for at this ph the exchange rate did not depend on whether the mica was H- deficient. That this is not generally so can be seen by comparing, for instance, micas B1 and P4; both have the same degree of F substitution (1"78 F per (OH,F),) but at ph 3"5, K is replaced nearly nine times faster in B1 than in P4. It is evident, therefore, that factors additional to H-deficiency can also contribute to the faster K-replacement in acid solution. Broadly considered, the larger synergetic effects are associated with those micas that lose more net negative charge when K is exchanged. With micas P1, P3, P4 and P6 for example, the rate of K replacement at ph 4.5 is dosdy related to their F contents (Fig. 3), the H-synergism is small (E is in the range ) and their alteration products have large exchange capacities of about 1"77 me/g (Fig. 4). This behaviour contrasts with that of micas P5, B1, B3a, B4 and B6, for which the rate of exchange is less closely related to the F content but is much increased in acidic solution (E~ ) and the exchange capacities of the alteration products are smaller ( me/g). These trends indicate that H-synergism and decrease in layer charge are related. Protonation of aluminosilicate layers Raman & Jackson (1966) suggested that loss of net layer charge in micas is accompanied by incorporation of protons in the structure. The evidence for proton uptake is largely circumstantial. (1) When interlayer potassium in micas is replaced by another cation, the ph of the reaction solution increases. The change in ph differs from mica to mica: it is least with muscovites and phlogopites containing much F (e.g. P1 and P6), and increases in the order P4, P3, P5, B4, B1, B3a, L1 and B6 (Newman, 1969), which is similar to the order of the synergetic effect of H ions on the rate of exchange (Table 2). (2) Chemical analysis shows that secondary reactions occur during the exchange of K from micas: Fe 2+ is oxidized and the cation occupancy of the octahedral layer decreases, but changes in cation charge from these reactions do not balance the loss of net layer charge (Table 4). To balance structural formulae satisfactorily, changes are assumed to occur in the composition of the 020 (OH,F)4 framework. Direct determination of the structural H contents of exchanged P1 and P3 shows that additional H is introduced into the aluminosilicate layers (Newman, 1967). To complete this evidence, it is desirable that the H content of exchanged biotites

12 372 A. C. D. Newman TABLE 4. Alterations to octahedral charge and net layer charge of exchanged micas, from data given in Newman and Brown (1966) and Newman (1967) Mica A B C D E F G P1 0 0"23-0" "14 1"83 0"31 P '64 1" "88 B4 (M2) 0"03 0'58-0'55 1 "86 2"41 1" B3a (M3) " B1 (M4) LI (M5)* "58 Muscovite (M6) " " Column: A. Increase in positive charge from oxidation of octahedral Fe3.+ B. Decrease in positive charge from loss of octahedral cations C. Net increase in octahedral charge (A-B) D. Net negative layer charge of original mica E. Expected net negative charge of exchanged mica (D-C) F. Determined layer charge of exchanged mica G. Imbalance in net layer charge alteration (E-F) attributed to proton incorporation. * This mica may lose Fe3+ from octahedral sites, but this ejected Fe 3+ is precipitated on the surface of the mica particles and is determined in the total analysis. should also be determined by direct analysis, but this poses a formidable analytical problem. Residual interlayer water may be retained in exchanged micas and vermiculites up to at least 500 ~ C, but at this temperature Fe 2+ is oxidized and structural hydrogen is eliminated (Vedder & Wilkins, 1969). Thermal analysis of exchanged biotites may therefore indicate a substantial excess of hydrogen, only part of which is structural hydrogen. This problem seems insoluble at present. Nevertheless, the balance of evidence favours proton incorporation as one factor in altering net layer charge in micas. As the accelerating effect of H ions on K exchange seems to be linked with the decrease in layer charge, this synergism probably results from protons being incorporated into the aluminosilicate layers. REFERENCES BAILEY S.W. (1966) Clays Clay Miner., 14th Natl. Conf., Berkeley, Calif., 1. BARSHAD I. & Kmrnc F.M. (1968) Science, 162, BASSETT W.A. (1959) Am. Miner. 44, 282, M.A. 14, 390. BASSETT W. A. (1960) Bull. geol. Soc. Am. 71, 449, M.A. 15, 20. CHUTE J.H. & QUIR~ J.P. (1967) Nature (London) 213, 1156 M.A. 19, 10. MATHIESON A.McL. & WALKER G.F. (1954). Am. Miner. 39, 231. MORTLAND M.M. (1958) Proc. Soil Sci. Soc. Am. 22, 503. MORTLAND M.M. & ELLIS B.G. (1959) Proc. Soil Sci. Soc. Am. 23, 363. NEWMAN A.C.D. & BROWN G. (1966) Clay Miner. 6, 297, M.A. 18, 155. NEWMAN A.C.D. (1967) Clay Miner. 7, 215.

13 Synergetic effect of hydrogen ions on cation exchange of K in micas 373 NEWMAN A.C.D. (1969) J. Soil Sci. 20, 357. NEWMAN A.C.D. & BROWN G. (1969) Nature, Lond., 223, 175. RAMAN K.V. & JACKSON M. L. (1966) Clays Clay Miner., 14th Natl. Conf., Berkeley, Calif., 53, M.A. 18, 78. RAUSELL-COLOM J.A., SW~Aa'MAN T.R., WELTS C.B. & NOI~aSH K. (1965) Experimental Pedology (E. G. Hallsworth and D. V. Crawford, Eds.) London (Butterworths), 40. RAYr,~R J.H. (1968) Rothamsted Experimental Station Report for 1968, Pt. I, 65. REv.o, M.G. & Scoa-'r A.D. (1962) Proc. Soil Sci. Soc. Am. 26, 437, M.A. 18, 81. RtMS~a~ J. (1967) Clays Clay Miner., Proc. 15th Natl. Conf., London (Pergamon), 375. ROBERT M. & PEDRO G. (1968) C.R. Acad. Sci. Paris 267D, Snmozu H. & BAILEY S.W. (1966) Am. Miner. 51, VENDER W. (1964) Am. Miner. 49, 736. Vra~DER W. & McDoNALD R.S. (1963) J. chem. Phys. 38, VrZ~DWR W. & WmrdNS R.W.T. (1969) Am. Miner. 54, 482. WELLS C.B. & NOmUSH K. (1968) Transact. 9th int. Congr. Soil Sci. 2, 683.