INTERACTION OF AMMONIA WITH VERMICULITE

Transcription

1 Clay Minerals (1972) 9, 263. INTERACTION OF AMMONIA WITH VERMICULITE J. L. AHLRICHS,* A. R, FRASER AND J. D. RUSSELL The Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen (Received 5 September 1971) ABSTRACT: The interaction of ammonia with Na-, NH4-, Ca-, Cu-, and Mgforms of two vermiculites has been investigated by infrared spectroscopy and chemical analysis. Both NI-h ions and co-ordinated NH3 are produced in the interlayer space in amounts which depend on the exchangeable cation and the particle size of the vermiculite. With the exception of that in Cu-vermiculite, the co-ordinated NH3 is rapidly displaced by atmospheric water vapour. Nl-h ions are slowly decomposed at normal humidity and more rapidly and completely at high humidity. The co-ordinated NH3 in Cu-vermiculite is converted to NH4. Relative amounts of NH4 and co-ordinated NH3 are influenced by particle size, smaller particles favouring NH4. The stabilities of both species to water vapour increase with particle size. INTRODUCTION The interaction of ammonia with inorganic soil constituents has been the subject of many investigations, reviewed by Mortland (1966). Infrared spectroscopy has been indispensable in identifying the products formed in NHa-treated clays. Using this technique, both ammonium ion and ammonia co-ordinated to exchangeable cations have been identified in montmorillonite and saponite; the relative amounts and stabilities of these species were shown to be a function of the mineral and the exchangeable cations (Russell, 1965). Since Mortland et al. (1963) observed only NH4 in NHs-treated vermiculite, and it was postulated (Mortland, 1966) that the NH, would be unavailable, it seemed worthwhile to re-investigate the NH~-vermiculite system, considering also the effect of particle size and physical form of the specimens on the products and their stability. MATERIALS AND METHODS The vermiculites used were from Loch Scye, Caithness (Aitken, 1965) and from Llano County, Texas. The latter specimen had a significantly higher Fe content * Visiting research worker from Department of Agronomy, Purdue University, Lafayette, Indiana, U.S.A.

2 264 J. L. Ahlrichs, A. R. Fraser and J. D. Russell than material from the same locality characterized by Shirozu & Bailey (1966). The K content was also high and was reduced from about 6 to 0.1~ w/w by refluxing for 1 week in 0"2 M BaC12 with daily replacement of the solution. Aqueous dispersions (about 1~/o w/v) of the vermiculites were prepared by the alkylammonium method of Walker & Garrett (1967). The vermiculites were saturated with Na, NH4, Ca, Cu and Mg by two methods. In one, the alkylammonium-vermiculite suspensions were washed in the centrifuge with 1 M solutions of the appropriate chorides then water, till they were chloride-free. Films were prepared by drying aqueous suspensions of these homoionic vermiculites on polyethylene sheets on a fiat surface. The air-dry films, peeled from the polyethylene, were fragile and open in texture, and will be referred to as 'porous'. In the other method, the alkylammonium-vermiculite suspensions were first dried down on polyethylene sheet then saturated with the appropriate cation by washing with chloride salt solutions followed by water. These films were tough, well-oriented and transparent when peeled off, and will be referred to as 'oriented' Contents of the various exchange cations are shown in Table 1. The c.e.c, of the Loch Scye and Llano vermiculites would appear to be respectively about 172 and 139 me/100 g air-dry sample, corresponding to 215 and 160 me/100 g ignited material (1000~ Na saturated). The higher exchangeable cation content of the Cu-vermiculites is thought to be due to adsorption of (CuOH) + ions. TABLE 1. Exchangeable cation contents (me/100 g air-dry sample) of homoionic saturations of vermiculite from Loch Scye and Llano Cation Loch Scye vermiculite Llano vermiculite Na* NH4t Ca~ Cuw Mg *, Flame emission of HF/H2SO4 digested mineral; t, Colorimetrically; :~, Flame emission, Ca replaced by Mg; w D.C. arc; 82 Atomic absorption, Mg replaced by Ca. The vermiculite films (1-2 mg/cm 2 and of reasonably uniform thickness) were placed in an evacuable pyrex tube through which anhydrous NHa gas was passed at a rate of ml/min for 2 h (porous films) o r 16 h (oriented films or flakes). The films were then transferred from the tube, either immediately or after evacuation at 0-01 mm Hg for 1 h, to flasks containing concentrated HC1 for chemical determination of total N (NH8 + NH~ +) by the colofimetric method described by Fraser & Russell (1969). Vermiculite films were also treated with NH3 at the same flow rate, in an evacuable infrared cell similar to that described by Angell & Schaffer (1965). Infrared spectra of the films were recorded successively in an atmosphere of NH3, in vacuum following the NH3 treatment, and in air after exposure of the

3 Interaction of ammonia with vermiculite 265 films to atmospheric water vapour using a Grubb Parsons $4 double-beam spectro, meter, from cm -1 with NaC1 optics, and from cm -1 under the higher resolution of a 2500 lines per inch diffraction grating. Ammonium contents of NH~-treated films of known weight and area were calculated from the absorbance of the NH4 deformation vibration at 1430 cm -1 using the analysed NH~-vermiculites as standards. RESULTS AND INTERPRETATION The investigation of the interaction between NH3 and vermiculite was carried out on both porous and oriented films of Loch Scye and Llano vermiculites. Results for the porous films were generally more reproducible. Infrared absorption spectra indicate that flowing NH3 displaces H20 from Vermiculite interlayers though not so completely as from montmorillonite or saponite (Russell, 1965). Since NH4 ions are formed in all the samples, it is pertinent to consider the effect of external conditions on NH~ ions at exchange sites in NH4- vermiculite. It has previously been shown that the majority of NH~ ions in vermiculite are inaccessible to D20 (Farmer, Russell & Ahlrichs, 1968). Consequently the NH4 bands at 3250, 3060, 2870 and 1430 cm -1 in the spectrum of NH4-vermiculite Waveleng'th (/zm) 3'0 3,4 3' t--r----t r ] r 1 T i" ] T r ] ] ] 50% r.h. ~ _ I J. L I I J._. l J ZOO Frequency (r -]) FIG. 1. Infrared absorption spectra of NH4-vermiculite (Llano) in vacuum, hydrated in air at about 50~ relative humidity, and in dry NH3 gas.

4 266 J. L. Ahlrichs, A. R. Fraser and J. D. Russell at 50% r.h., are affected very little on removing under vacuum the water of hydration absorbing at 3400 and 1630 cm -1 (Fig. 1). The effect produced by NH3 on the NH4 bands although small is more obvious in the spectrum, in the slight weakening of NH4 bands at 3060 and 2870 cm -1, the appearance of broad absorption below 3000 cm -1 and an inflexion near ]500 cm -x (Fig. 1). These new features indicate that in an atmosphere of NH8 a small proportion of NH4 ions becomes strongly hydrogen bonded to NHa (Russell, 1965; Corset, Huong & Lascombe, 1968b). Sorption of NH, Porous Films. Spectra of Na-, Mg-, Ca- and Cu-vermiculites in NH8 (broken lines) and in vacuum following NH~ treatment (full lines Figs. 2 and 3) clearly show that considerable amounts of NH4 absorbing at 3250, 3080, 2870 and 1430 cm -1 Wavelength (/zm) Wavelength (/zm) 3' ' Frequency (crn-)) Frequency (cm-l) b-~o. 2. FIG. 3. FxG. 2. Infrared absorption spectra of Llano vermiculite, saturated with the cations indicated, in dry NH3 gas (broken curves) and subsequently in vacuum (solid curves). b-~o. 3. Infrared absorption spectra of Loch Scye vermiculite, saturated with the cations indicated, in dry NH3 gas (broken curves) and subsequently in vacuum (solid curves).

5 Interaction of ammonia with vermiculite 267 are present in all but the Cu-saturated specimens. The spectrum of NH,-vermiculite in NHs is included for comparison. The amounts of NH, formed (Table 2) have been calculated from the absorbance of the 1430 cm -~ NH~ band. Some of the results are similar to those for montmorillonite and saponite (Russell, 1965). With Mg the amount of NH~ formed approaches the cation exchange capacity, 84% for Loch Scye and 97% for Llano. With Ca, it is about 60% in Loch Scye and 80% in Llano, in agreement more with Ca-montmorillonite (72%) than with Ca-saponite (34~/o). With Cu the amount of NH, formed in vermiculite (6-13 mmol/100 g) is less than that formed in montmorillonite (16 mmol/100 g), but with Na it is up to eight times as high. In an atmosphere of NH3, molecular NH3 co-ordinated to exchangeable metal cations is present in the interlayers for all ion saturations, as shown by the absorption bands near 3370, (3330 for Cu), 1610, and cm -x (broken lines Figs. 2 and 3). This is substantiated in Cu-vermiculite by the pronounced blue coloration, and by the appearance in the spectrum of bands at 3330, 3270, 3185, 1610 and 1217 cm -a, reasonably close to those quoted for ammino-copper (II) salts (3320, 3250, 3170, 1643, 1287 cm -x, Powell & Sheppard, 1956). In spectra of Mg- and Ca-vermiculites, bands at about , 1610, and 1217 cm -1 for Mg and , 1610, and 1156 em -~ for Ca are assigned to NH~ co-ordinated to Mg and Ca respectively, in agreement with observations by Russell (1965), and Corset, Huong & Lascombe (1968a). NHs is presumably also co-ordinated to Na but the NH.~ band expected at about 1090 crn -1 is obscured by intense Si-O vibrations. The considerable decrease in the intensities of 1217 and 1156 cm -1 bands on evacuation indicates that much of the ammonia co-ordinated to Mg and Ca is unstable in vacuum. A slight sharpening of the deformation vibration may indicate, as suggested for Ca- and Mg-saponite (Russell, 1965), a decrease in co-ordination number. The stretching band of the NH3 retained by the Ca- and Mg-vermiculites after evacuation occurs at 3360 cm -1 in Loch Scye samples and at 3370 cm -1 in Llano samples. These low frequencies suggest that the NH3 molecules in vermiculite are more strongly hydrogen bonded to surface oxygens than they are in montmorillonite (3400 cm -1) or saponite (3390 cm-~). The increasing strength of hydrogen bonding, montmorillonite < saponite,( vermiculite, follows the increasing extent of tetrahedrally-derived negative charge. Ammonia retained by Na-vermiculites absorbs at 3390 cm -1, some cm -~ higher than the NH3 retained by Ca and Mg. A similar, though less marked effect was observed in saponite, where, in Na-saponite, NH3 absorbed at 3395 cm -x compared with 3390 cm -~ for Mg. No such effect occurred in montmorillonite. Less NH3 is retained against evacuation by Na-vermiculite than by Na-montmorillonite and Na-saponite; in mmol/100 g, 85 for montmorillonite, 119 for saponite, 21 for Loch Scye, and 48 for Llano vermiculite; in mol NHs/cation, 1 for montmorillonite and saponite but only for the vermiculites. Whereas in montmorillonite and saponite, the amount of NH3 retained against evacuation follows the sequence Na > Ca ~> Mg (Russell, 1965), in both vermiculites the order is Ca 2> Mg > Na. Oriented Films. Results for oriented films of the various forms of the two

6 268 J. L. Ahlriehs, A. R. Fraser and J. D. Russell vermiculites are generally similar to those for the porous films, although the replacement of H20 by NH3 is much slower in the oriented films. The principal difference between the two types is that generally the oriented films, irrespective of exchangeable cation, retain more NH~ against evacuation than do the porous ones. This observation includes the NH4-vermiculites, the absorption band of NH~ at 3370 cm -1 being easily discernible in spectra of evacuated oriented films: ammoniated porous films of the NH4-vermiculites retain no NH,~ against evacuation. Less NH4 is produced in the oriented films for all cationic forms, other than the Llano Cu-sample, in which 40 mmol NH4/100 g is produced compared with only 6 in the porous film (Table 2). This contrasts sharply with the oriented Loch Scye Cu-sample in which no NH~ could be detected. The tendency for Llano vermiculite to retain more NH3 than the Loch Scye sample is more marked for oriented films. TABLE 2. Maximum N sorbed following NH3 treatment, and NH3 and NIq4 (mmol[100 g sample) retained against evacuation (0.02 mm Hg for I h), by porous films of Loch Scye and Llano vermiculites saturated with various exchangeable cations Loch Scye vermiculite N held against evacuation Llano vermiculite N held against evacuation Cation MaximumN* NI-I3+NH4~ NH4~ NH3w Maximum N* NH3+NHat NH4.~ NH3w Na NI-I Ca Cu Mg *, Maximum N was determined colorimetdcally on samples immediately after removal from the NH3 stream; t, NI-I3+NH4 was determined colorimetrically; :~, NH4 was estimated from the optical density of the 1430 cm-1 infrared absorption band of the NH4 ion; w NH3 was obtained by difference. Decomposition of NH~ by water vapour Porous films. Following ammonia treatment and evacuation to remove physically adsorbed NI-I3, vermiculite films were exposed to the laboratory atmosphere (50~176 r.h.) and their infrared spectra were recorded at intervals. The decomposition of the NH~ formed in the vermiculites is shown in Fig. 4 (Loch Scye) and Fig. 5 (Llano), NH4 being calculated from the absorbance of its 1430 cm -1 absorption band. The NH4 initially formed in Loch Scye Ca- and Mg-samples (Fig. 4) decomposes~ rapidly during the first 24 h and then much more gradually thereafter, with Mg showing greater instability. In the Llano Mg-sample (Fig. 5) decomposition is similar to that in the Loch Scye Mg-sample, but in the Llano Ca-sample, it virtually ceases after 24 h. The NH4-contents of the vermiculites after 12 days' exposure to 50~/o r.h. were determined by analysis. Estimates from the 1430 cm -1 NH4 band intensity were within + 10~/o of the analytical values except for the Ca-vermiculites

7 Interaction of ammonia with vermiculite 269 for which they were 33~ higher and the Cu-vermiculites for which they were 39~ lower. The high values in Ca-vermiculites suggest that calcium carbonate, which also absorbs near 1430 cm -1, had been formed. This will be discussed more fully in a later section. The low values in the Cu-vermiculites are due to the presence of co-ordinated NH3. Excellent agreement (within +4~176 was obtained between the total N by analysis and the sum of NH4 estimated from the 1430 cm -1 NH4 band and co-ordinated NH~ estimated from its 1217 cm -1 band. When the NH3-treated Cu-systems are exposed to air, spectral changes indicate conversion of co-ordinated NH, to NH4 + ions. This is a slower, more continuous process in the Llano sample (Fig. 5) than in that from Loch Scye (Fig, 4). The additional NH4 formed in NH4- saturated vermiculites is lost rapidly and completely in 2-4 days for the Loch Scye sample, resembling the behaviour of NH4-montmorillonite, but is incompletely lost from the Llano sample. Again decomposition over 2-4 days is rapid, but ceases after 4 days leaving a stable mmol of additional NH~, a value confirmed by analysis. Of the 73 mmol NH4 formed in Loch Scye Na-vermiculite, 53 mmol are lost rapidly in 2 days, and only a further 7 after 12 days. This behaviour is again similar to montmorillonite. The Llano Na-sample retains a stable mmol NH4 after 2-3 days. The stability of the NH4 formed in vermiculite is lower, the higher the relative humidity of the environment to which the vermiculite is exposed: NH3-treated Mg-saturated Loch Scye vermiculite films containing some 166 mmol NH4, lose, after 12 days' exposure, 100 mmol NH4 at 50~/o r.h., 120 mmol at 80~/o r.h., and 140 mmol at 98% r.h ' o _o 120 % o Mg z O n Ca Mg o o.~--_r.-~. NH 4-4- h -,6 I I Time (days) Na o I I I I I 8 I0 12 FIG. 4. Decrease, with exposure to air at about 50 % relative humidity of the NH4 formed by NH3 treatment of porous films (full lines) and oriented films (broken lines) of Loch Scye vermiculite containing the indicated exchange cations. NH3 content was estimated from the absorbance of the 1430 cm-t infrared absorption band of the NH4 cation.

8 270 J. L. Ahlrichs, A. R. Fraser and J. D. Russell Oriented Films. Oriented films of NH3-treated Ca- and Mg-vermiculites on exposure to air at 50~/o r.h. show an intitial increase in NH, content (broken curves, Figs. 4 and 5). Accompanying changes in the infrared spectrum indicate loss of co-ordinated NH8 and adsorption of water. Although results were somewhat variable, there is some evidence that the initial increase in NH4 shown by oriented films of Ca- and Mg-vermiculites only occurs if the flow rate of NH3 during the initial treatment is high. This produces high levels of co-ordinated NH,~ in preference to NH~ +. Lower NH3 flow rates result in higher initial levels of NH4 which do not increase on exposure to air. The subsequent steady loss of NH4 from the Ca-vermiculite films after about 1 day's exposure to air is in contrast to the apparent behaviour of the porous films and indicates that very little carbonate is produced. This was substantiated by agreement between NH,-content from analysis and from the 1430 crn -1 band intensity. The stability of the NH4 formed in Na-vermiculites is greater for oriented than for porous films, presumably due to slower diffusion of molecules and ions in the former. Effect of COz. When porous films of NH3-treated Ca-vermiculites are exposed to air, NH4 present is less easily decomposed by water vapour than it is when the films are exposed to CO2-free air. For example, following NH8 treatment and evacuation, total NH3 + NH4 sorbed was 142 mmol/100 g by analysis. After exposure to air at 50% r.h. for 24 h, this value decreased to 107 due mostly to loss of NHs, but in a CO=-free atmosphere at the same humidity the NH4 content fell to 39. Spectra of the films exposed to air show the formation of broad absorption near 1430 cm -1 thought to be due to carbonate. No such effects were observed for oriented films of Ca-vermiculite. The reason for this may be linked to the more rapid diffusion of Ca(OH)2 in the porous films to particle edges where reaction "<;- - - "~"... c~ ~ ~ Cu Y I 1 I I I I I I I ] [ I I0 12 Time (c~sys) FIG. 5. Decrease, with exposure to air at about 50~ relative humidity of the NH4 formed by NH3 treatment of porous films (full lines) and oriented films (broken lines) of Llano vermiculite containing the indicated exchange cations. NH4 content was estimated from the absorbance of the 1430 cm -1 infrared absorption band of the NH4 cation. NH 4

9 Interaction of ammonia with vermiculite 271 with CO2 can take place. It has previously been shown that the NH4 formed in Ca-montmorillonite by NH~ treatment is stabilized when Ca(OH)2 is converted to carbonate in the presence of relatively high levels of CO2 (Russell, 1965; Du Plessis & Kroontje, 1966). Effect of particle size on sorption and desorption o[ NH~ The infrared spectrum of a large single crystal of hydrobiotite (17 mm diameter disk) following NH.~ treatment is in good qualitative agreement with that of the vermiculite films. Both co-ordinated NH.~ and NH, are present, the latter taking about 7 days to reach its maximum value over the whole flake compared with 2-16 h for the films. In contrast, ion exchange was extremely slow when a similar flake was immersed in NH,C1 solution, the amount of NH~ introduced even near the edge being substantially less than that formed by NH3 treatment. The rate of movement of the NH, boundary during NH~ treatment of the large hydrobiotite flake was about 0"4 mm in the first hour. The NH, formed in the large single crystal by NH~ treatment was stable even after 14 days in water. A comparison was also made between different forms of Loch Scye Mg-vermiculite. Single crystal flakes ( mm), oriented and porous films, and <2 ~m powder, were treated with flowing NH.~ for 16 h, then the NH~ + NH, contents were measured before and after exposure to air at 50% r.h. for 2 h. The flakes and the oriented film contain mmol NH3 + Nt-I4 losing 5~176 and 30~176 respectively after 2 h in air. Porous films and powder contain between 150 and 200 mmol NH3 + NH4 and lose about 30% and 12%. In a separate experiment, flakes were shown to lose over 95~ of the NH3 + NH, after 15 days at 100~ r.h. Although the implications are not absolutely clear, it would appear that oriented films behave like the flakes in sorbing more NH~ than the porous film or the powder. Comparing the flakes with the powder, it would seem that the sorbed NHa is more easily lost from the latter. These observations agree qualitatively with unpublished data (M. H. Stone) on the effect of particle size (I mm-20 ~m) of hydrobiotite on its sorption of NH~: it was noted that there was a decrease in lotal NH2 sorbed (whether as NH3 or NH4 or both) with decreasing particle size. DISCUSSION The results of the present study establish that NH, is formed in vermiculite treated with NH3 confirming observations by Mortland et al. (1963), and that like montmorillonite and saponite (Russell, 1965), vermiculite can absorb NH3 by co-ordination to exchange cations. The similarity extends to the relative amounts of NH~ and NH, on Ca-, Mg- and Cu-saturated species, but while K- and NH,-montmorillonite and saponite retain NH3 against evacuation probably at lattice edges and imperfections, NH,-vermiculites in the form of porous films do not. The implications are that the vermiculites have fewer structural imperfections or that

10 272 J. L. Ahlrichs, A. R. Fraser and J. D. Russell the association of NH~ with such features is weaker than it is in montmorillonite and saponite. The two vermiculites differ from each other in several respects, but especially in the greater capacity of the Llano specimen to sorb and retain NH3 (Table 2). The retention appears to have an inverse relationship with cation exchange capacity and, when montmorillonite and saponite are also considered, it attains a maximum value for Ca, Mg, Na and possibly also Li saturations in the c.e.c, range me/i00 g. This suggests that in this range there may be an optimum balance between the number of interlayer cations and the space available to accommodate NH,~ molecules. In relation to cation exchange capacity, the levels of formation of NH4 in Ca-,Mg-. and Cu-vermiculites are comparable with those in montmorillonite and slightly higher than those in saponite. But the levels formed in the Na-vermiculites are anomalously high (up to 60~/o of the c.e.c, in the case of the Llano sample). If the mechanism proposed by Mortland et al. (1963), and Mortland & Raman (1968), to account for the formation of NH4 applies to Na (i.e. [Na(H~.O)]+ + NH3~NaOH + NH,+), the NaOH produced--59 and 80 mmol/100 g for the two vermiculites studied--should migrate to particle edges, and on exposure to air should be converted to Na2CO3 which would absorb near 1400 cm -1. There is no spectroscopic evidence for this; the intensity of the 1425 cm -1 NH, deformation band is consistent with chemical analysis and the band is not broadened; it is difficult to rationalize this with the finding that carbonate is formed in Ca-vermiculite from the less basic Ca(OH)2. High levels of NH4 and therefore of strong base also, have been reported in Na- and Li-nontronites (Mortland & Raman, 1968), and an explanation was sought in terms of the tetrahedrally derived charge of nontronite producing a stronger polarizing effect on co-ordinated water molecules. But this cannot be the only explanation, since in Na-saponite very low levels of NH4 are produced. For the above equilibrium to favour the NH~ ion, the latter must be effectively removed from the reaction, perhaps by immobilization at sites where it is then inaccessible, due to collapse of the basal spacing on NH.a treatment. From Figs. 4 and 5 this effect is apparently greater in the Llano specimen, although it is the Loch Scye material which has the greater fixing capacity for K, and also the greater inaccessibility of NH, to D20. Neither vermiculite contains appreciable F precluding an explanation along the lines proposed by Newman (1969), but it may be that the greater capacity of the Llano vermiculite to trap NH4 stems from the fact that its NH~-form is more fully collapsed at normal humidity (d001 = 10.5 A) than is the Loch Scye sample which contains a few layers that are capable of expansion (do01 = 11"5 A). Contrary to the predictions of Mortland (1966), the additional NH4 formed in vermiculite is not completely fixed in the interlayers, but undergoes decomposition on exposure to air. At humidities such as might be encountered in soil, the decomposition of NH4 in porous films of Mg-vermiculite goes almost to completion in about 18 days. The rate of release of NH4 as NH,~ is approximately 160 mg NH3

11 Interaction of ammonia with vermiculite 273 nitrogen/100 g/day, and from oriented films is a more gradual 50 mg/100 g/day over a period of at least 50 days. Small flakes of vermiculite react with NH3 as completely as clay size material, and in an ammoniated soil could therefore provide a useful reserve of ammonia N for a considerable period. Only in very large flakes does the NH4 become unavailable, although in this instance the fact that the flakes were of hydrobiotite may explain the increased stability. ACKNOWLEDGMENTS The authors would like to thank Dr V. C. Farmer for his continuous interest in the work and M. H. Stone for making his results available. REFERENCEg AITKEN W.W.S. 0965) Mineralog. Mag. 35, 151. ANGELL C.L. & SCHAFFER P.C. (1965) J. phys. Chem., Ithaca, 69, CORSET J., HUONG P.V. & LASCOMBE J. (1968a) Spectrochim. Acta, 24A, CORSET J., HUONG P.V. & LASCOMRE J'. (1968b) Spectrochim. Acta, 24A, Du PLESSlS M.C.F. & KROONTJE W. (1966) Proc. Soil Sci. Soc. Am. 30, 693. FARMER V.C., RUSSELL J.D. & AHLRICHS J.L. (1968) Trans. 9th Int. Conf. Soil Sci., Adelaide, 3, 10l. FRASER A.R. & RUSSELL J.D. (1969) Clay Miner. 8, 229. MORTLAND M.M. 0966) Agricultural Anhydrous Ammonia, (M.H. McVickar, W.P. Martin, I.E. Miles and H.H. Tucker, editors), Chap. X, p Soil Science Society of America. Madison, Wisconsin, U.S.A. MORTLAND M.M, FRIPIAT J.J., CHAUSSIDON J. & UYTTERHOEVEN J. (1963) J. phys. Chem., Ithaca, 67, 248. MORTLAND M.M. & RAMAN K.V. (1968) Clays Clay Miner. 16, 393. NEWMAN A.C.D. (1969) d. Soil Sci. 20, 357. POWELL D.B. & SHEPPARD N. (1956) 3". chem. Soc RUSSELL J.D. (1965) Trans. Faraday Soc. 61, SmROZU H. & BAILEY S.W. (1966) Am. Miner. 51, WALKER G.F. & GARRETT W.G. (1967) Science, N. Y. 156, 385.