THE FLOW CHARACTERISTICS HALLOYSITE SUSPENSIONS

Transcription

1 Clay Minerals (1995) 30, THE FLOW CHARACTERISTICS HALLOYSITE SUSPENSIONS OF B. K. G. THENG AND N. WELLS* Manaaki Whenua - Landcare Research, Private Bag 11052, Palmerston North, New Zealand, and *2 Hudson Street, Island Bay, Wellington, New Zealand (Received 12 July 1994: revised 23 November 1994) A B S T R A C T: The rheology of aqueous suspensions of Na halloysites with different particle shape has been investigated using a Haake rotary viscometer. Three halloysites from New Zealand were used: Matauri Bay (MB), Te Akatea (TA) and Opotiki (Op) which are mainly composed of thick long tubules, short thin laths, and spherules, respectively. Suspensions of the materials subsequently prepared from the MB and TA samples show a pseudoplastic consistency under shear, characterized by a steep initial rise in shear stress as shear rate increases, followed by a linear increase in stress when a certain shear rate is exceeded. The Op halloysite, on the other hand, shows Newtonian flow behaviour for which shear stress increases linearly with shear rate throughout. For MB and TA, the shear stress developed during rotor acceleration is larger than that produced during deceleration, and the corresponding flow curves enclose a hysteresis loop. As suspension ph increases, the pseudoplastic character decreases as does the amount of hysteresis. Above ph 7.5, flow approaches Newtonian and hysteresis is absent. Plots of Bingham yield value against ph at different ionic strengths (0.003, 0.03 and 0.3 M NaC1) intersect at ph 6.0 for MB and at ph 7.1 for TA. These values are identified with the point of zero charge (PZC) of the particle edge surface. The flow characteristics of halloysites may be explained in terms of the influence of particle shape, ph, electrolyte concentrations, and layer composition on particle interactions. The rheology of kaolinite suspensions has been widely investigated, and at acid ph values such suspensions have been shown to exhibit pseudoplastic behaviour. That is, as the rate of shear is progressively increased, the resultant shear stress initially rises, then declines, and finally becomes independent of shear rate (Michaels & Bolger, 1962; Flegmann et al., 1969; Jasmund & Lagaly, 1993). On the other hand, the rheology of halloysite suspensions has not been well documented, although available information (Kent et al., 1968) does indicate that suspensions of halloysite of tubular morphology should also exhibit pseudoplastic flow under shear. Like kaolinite, halloysite is a 1:1 type layer silicate mineral in which each layer within the crystal structure is composed of a silica-based tetrahedral sheet and a gibbsitic octahedral sheet. However, whereas the layers in kaolinite are contiguous, those in halloysite are normally separated by a monolayer of water molecules (Grim, 1968). The two species also differ in particle shape. Kaolinite occurs almost invariably as pseudohexagonal platelets while halloysite can assume a tubular, spheroidal, or tabular morphology due, apparently, to different concentrations of Fe in its surface layers (Soma et al., 1992). Particle shape, per se, is an important factor in influencing the flow behaviour of suspensions, but additionally is known to affect the reactivity of halloysite towards inorganic anions (Theng et al., 1982) and its propensity for intercalating uncharged polar organic compounds (Churchman & Theng, 1984). The rheological behaviour of kaolinite suspensions and their sensitivity to ph and ionic strength have been interpreted in terms of particle interactions (van Olphen, 1977; Jasmund & Lagaly, 1993), and on this basis, Rand & Melton (1975; 1977) have suggested that at a certain ph, identifiable with the point of zero charge (PZC) of the kaolinite crystal edge, the suspension yield stress does not vary with ionic strength. The The Mineralogical Society

2 100 B. K. G. Theng and N. Wells availability of halloysites with different morphology and chemical composition would enable particle interactions to be more clearly defined than is possible with kaolinite. In an attempt to clarify these points further and supplement the limited information on halloysite suspension rheology, we have investigated the flow characteristics of s u s p e n s i o n s of halloysites with different morphology as a function of clay concentration, ph and ionic strength. MATERIALS AND METHODS Three halloysites from New Zealand with differing particle morphology were used: Matauri Bay (MB), Opotiki (Op), and Te Akatea (TA). Their sampling locality, mode of formation, bulk and surface layer composition, and Fe content before and after d e f e r r a t i o n have b e e n g i v e n e l s e w h e r e (Churchman & Theng, 1984; Soma et al., 1992). Scanning electron microscopy (Fig. 1) shows that FIG. 1. Scanning electron micrographs of sheared halloysite suspensions showing differences in the morphology and size distribution of particles. (a) Matauri Bay (MB) halloysite; (b) Opotiki (Op) halloysite; (c) Te Akatea (TA) halloysite. Scale bar = 2 Ilm.

3 the MB sample consists mainly of long thick I tubules with an average length and width of 1.67 and 0.22 Ixm, respectively. The Op halloysite is 30 I mostly composed of spherules with an average diameter of 0.42 lam. The sample of TA halloysite is largely made up of short slender laths, averaging 0.51 and 0.08 Ixm in length and width, respectively, 25 forming blocky aggregates. Figure 1 also gives a good indication of the particle size distribution for each sample.,.,, t~ 20 The raw clays were ultrasonically dispersed in o_ water at ph 10 and the <2 I.tm fraction was /-, separated by sedimentation under gravity. The clay suspensions were coagulated at ph 6 and at an ~ 15 electrolyte concentration of 1 M NaC1. Excess salt was removed either by washing with distilled water and centrifuging, or by dialysing against distilled 9 water until free of chloride. A portion of the salt- u) lo free TA halloysite was deferrated by treating with sodium citrate-dithionite-bicarbonate according to the method of Whitton & Churchman (1987). Suspensions of different strengths were prepared 5 from the respective stocks either by evaporation under ambient conditions, or by dilution with water. The ph and ionic strength were varied by adding o HC1/NaOH and weighed amounts of solid NaC1, respectively. Rheological measurements were carried out at ~ using a Haake programmable rotary viscometer, model RV3, details of which have been described by Wells & Theng (1985). The bellshaped concentric cylinder rotary attachment with a 0.4 mm gap was used for the measurements; this was dipped into the suspension, accelerated at constant rate to 800 rpm and decelerated to zero rpm at the same rate. An acceleration rate of 50 rpm -a was used and the top speed achieved corresponded to a shear rate of 4300 s -1. The shear stress developed was plotted against shear rate using an X-Y recorder. RESULTS Figure 2 shows representative plots of shear rate (D) against shear stress (z) for suspensions of the three Na halloysites used. For the MB (ph 5.6) and TA (ph 6.0) samples the flow curves are typical of clay materials with a pseudoplastic consistency (van Olphen, 1977). At low rates of shear, such systems exhibit non-newtonian flow which is characterized by a progressive decline in viscosity (z/d) as D increases. Above a certain value of D the flow Flow of halloysite 101.J LO Shear rate, D (s -1) 20% 15% MB 10% CTA 10%./ ~/~B 5% B 3% ~OD 11% FIG. 2. Representative flow curves for suspensions of sodium MB (ph 5.6), TA (ph 6.0) and Op (ph 5.8) halloysites at the specified w/v concentrations. curve becomes linear, the slope giving the plastic viscosity (r/pl) of the suspension. The intercept of the linear portion of the curve with the stress axis (D = 0) is referred to as the Bingham yield (stress) value, denoted by ZB. In contrast to MB and TA, the Op sample (ph 5.8) exhibits true Newtonian flow, that is, shear stress is proportional to shear rate over the entire range of D. This behaviour extends even to fairly thick slurries with a solid concentration as high as 60% w/v (not shown). It is interesting to note that the curves for MB halloysite at 3 and 5% w/v show an inflection in shear stress at high rates of shear ( s-i). This point marks tbe transition from laminar to turbulent flow which sets in when the Reynolds number attains a certain (critical) value (van Olphen, 1977). This feature will not be further considered except to point out that the less concentrated the suspension, and the higher the ph (cf. Fig. 4), the lower is the

4 102 B. K. G. Theng and N. Wells o3 O- E ph L) U) 0 O~ ft. /oj ~o p, r i i I i Suspension Concentration (%w/v) FIG. 3. Relationship between solid concentration, expressed as weight of clay per 100 ml suspension, and plastic viscosity for MB, TA and Op halloysites..~ 6 shear rate before turbulent flow sets in because the suspension viscosity is lower. Figure 3 shows that the plastic viscosity for all three halloysite suspensions increases linearly with clay concentration within the range investigated (~<25% w/v). For the Op sample this relationship holds, up to a concentration of ~ 40% w/v, beyond which the viscosity increases exponentially (not shown). All three lines converge to a viscosity of 1 mpa s when extrapolated to zero concentration. This limiting value equals the viscosity of water at 20~176 the suspending medium used in this study. The influence of ph on flow, illustrated by the behaviour of MB halloysite, is shown in Fig. 4. As would be expected by analogy with kaolinite suspensions (Flegmann et al., 1969; Jasmund & Lagaly, 1993) the system becomes progressively less pseudoplastic as ph increases. Indeed, at ph 7.5 and above, flow approaches that of a Newtonian fluid. Figure 4 also shows that at ph ~< 6.0 the curves obtained on accelerating the rotor do not coincide with, but lie above, those produced on rotor deceleration. The hysteresis loop enclosed by the acceleration ('up') and deceleration ('down') branches of the flow curves, decreases in size as suspension ph increases. The amount of hysteresis becomes vanishingly small at ph 7.5 and above. Suspensions of TA halloysite show a similar behaviour to those of MB; for the Op sample, however, the acceleration and deceleration pathways coincide irrespective of ph (not shown). o 0 looo 2oo0 zooo 40oo Shear rato, D (s -1) FIG. 4. Influence of ph on flow characteristics for suspensions (10% w/v) of MB halloysite. Figure 5 shows plots of Bingham yield value against ph at three concentrations of NaC1 for MB halloysite. The family of curves intersect at ph 6.0. By analogy with kaolinite (Rand & Melton 1975), this point may be identified with the point of zero charge (PZC) of the crystal edge surface. Figure 6 gives similar plots for suspensions of TA halloysite. Here the point of intersection is at ph 7.1 for both the original and deferrated materials. Being Newtonian in character, the flow curves for Op halloysite do not yield ~B values. DISCUSSION Effect of clay concentration The flow curves for suspensions of Na MB and TA halloysites at ph 5.6 and 6.0, respectively, are pseudoplastic in character. At comparable solid concentrations (e.g. 10% w/v), this feature is more pronounced for MB than TA halloysite (Fig. 2). Such a flow behaviour is typical of flocculated clays with a platy particle morphology exemplified

5 Flow of halloysite '~ 1.2 Q. MB Q "o 0.E O.e E ~ 0.4 f-._c CO 0.~ 02 4 L, 6 J, 8 i 110 ph FIG. 5. Relationship between ph and Bingham yield value at different electrolyte concentrations for (4.9% w/v) suspensions of MB halloysite. O, 3 x 10-3 M NaC1; /k, 3 x 10-2 M NaCI; [S], 3 x 10-1 M NaC1. ~ 1.0 > 0.8 "0 0 "~ o.6 E -~0.4 r- CO ph FIG. 6. Relationship between ph and Bingham yield value at different electrolyte concentrations for (9% w/v) suspensions of TA halloysite. O, 3 x 10-3 M NaCI; /k, 3 x 10-2 M NaC1; I--l, 3 x 10-1 M NaCI. Solid symbols refer to deferrated samples. by kaolinite, illite and smectite (Goff et al., 1983; Jasmund & Lagaly, 1993). On the other hand, suspensions of Op halloysite behave much like a system of dispersed (deflocculated) particles showing true Newtonian flow characteristics. These observations may be explained in terms of the effect of shape on particle association in suspension. As discussed below, the PZC of the edge surface of MB and TA particles is at ph 6.0 and 7.1, respectively (Figs. 5 and 6). Under the ph conditions used in this instance, the particles of MB would tend to associate in an edge-to-edge (e-e) fashion while those of TA would occur mostly in an edge-to-face (e-f) arrangement (Rand & Melton, 1975; van Olphen, 1977). Neither association would apply to particles of Op halloysite because of their spheroidal morphology; rather, they would associate like a string of beads irrespective of ph. In line with this view, scanning electron micrographs (Fig. 1) of dilute suspensions of the three halloysites indicate that the long, thick tubules of MB tend to form 2-3 p.m platy micro-aggregates while the short, thin laths of TA are associated into 3-4 ~tm blocky units, and the spherules of Op into 3-4 gm globular clusters. These micro-aggregates, in turn, apparently form a continuous network structure. It is suggested that, in the case of MB and TA halloysites, this network structure progressively breaks down as the rate of shear increases, leading to an overall decrease in viscosity. There is a certain shear rate at which structural disruption is complete, and above which the viscosity remains constant. The threshold value of D at which flow changes from non-newtonian to linear would clearly depend on the solid concentration in suspension. It is possible that in the breakdown of the network structure there is a competing build-up at the same time but that the rate of disintegration exceeds that of reconstruction. For Op halloysite, which exhibits Newtonian consistency throughout, no specific value of shear rate has to be exceeded before (laminar) flow is induced. This is because the clusters of spherules making up the network can presumably roll over one another like ball bearings. By the same token, the Bingham yield value (Zn) would represent the work required to 'disentangle' the network of micro-aggregates while the plastic viscosity (r/pl) is a measure of the resistance to flow of the micro-aggregate units. The linear increase in rh,i. with clay concentration (Fig. 3) is consistent with this postulate. Differences in slope between samples reflect the influence of size and

6 104 B. K. G. Theng and N. Wells shape on the flow propensity of the respective micro-aggregates. For a given suspension concentration, the larger, blocky micro-aggregates of TA apparently offer greater resistance to flow than their smaller, platy MB counterparts. The gradient is least for Op halloysite as the globular clusters of spherules can slide past each other with relative ease. Effect of ph The influence of ph on flow, exemplified by the behaviour of MB halloysite (Fig. 4), may similarly be rationalized in terms of particle interactions as affected by surface charge characteristics. X-ray photoelectron spectroscopy has provided good evidence to show that halloysites carry a negative charge on the basal or 'face' surface of their crystals, arising from the non-stoichiometric replacement of A13+ by Fe 3+ in the octahedral sheet (Soma et al., 1992). Since this charge is intrinsic to the layer structure, its sign and magnitude are independent of ph. By contrast, the charge on crystal edges varies with the ph of the ambient solution or suspension. The amphoteric nature of the edge surface may be ascribed to the protonation and deprotonation of hydroxyl groups coordinated with exposed AI ions under acid and alkaline ph conditions, respectively. This process is illustrated in the following scheme: /o.~ /~ alkaline side point of zero acid side of PZC charge (PZC) of PZC where the vertical line denotes the extent of the coordination sphere of A1. At the PZC the edge surface is essentially uncharged while at ph values above and below the PZC it will be negatively and positively charged, respectively. Assuming as before, that the edge surface of MB has a PZC of 6.0 (Fig. 5) we would expect e-e association to predominate at and near ph 6. As the ph decreases below this value the magnitude of the positive charge on particle edges increases progressively (Theng et al., 1982) and e-f association becomes important. Accordingly, the shear rate that needs to be exceeded before linear flow sets in, as well as the corresponding yield value, show a progressive increase with decreasing suspension ph (Fig. 4). The parallel increase in the extent of hysteresis lends further support to this interpretation. Inasmuch as (hysteresis) loop size is a measure of the cohesive strength of the network structure (Wells & Theng, 1985), the data of Fig. 4 indicate that the lower the ph, the more cohesive the structure becomes. At the same time the propensity of the network to re-form, after being disrupted by sheafing, is diminished. The amount of hysteresis decreases with successive cycles of rotor acceleration and deceleration (not shown). Even at ph 3.5, hysteresis becomes vanishingly small after six cycles of shearing. In this regard, suspensions of non-spheroidal halloysites behave similarly to those of imogolite (Wells et al., 1980). However, when such a system is allowed to 'rest' for 18 h or longer, there is an almost complete recovery of consistency indicating that (flocculated) halloysite suspensions are essentially thixotropic. At ph = PZC when the charge on the particle edges is zero, e-e association by van der Waals forces is apparently the preferred mode of particle interaction (Rand & Melton, 1975). The flow curve at ph 6.0 indicates that the resultant network structure is less cohesive than that formed by e-f association involving electrostatic attraction between particles. At this ph the system is relatively easy to disrupt by sheafing. However, it can also recover quickly giving a minimal amount of hysteresis. At ph 7.5 and above, the particles tend to repel each other since the charge on both the edge and face surface is negative. As a result, the system deflocculates, loses cohesion, and exhibits a near- Newtonian flow behaviour with no detectable hysteresis and a very small yield value. Taking into account that for TA halloysite the PZC of edge surface is at ph 7.1 (cf Fig. 6), the effect of ph on flow characteristics follows a similar pattern to that shown by the MB sample. In the case of Op halloysite, however, Newtonian flow obtains under both acid and alkaline conditions because changes in surface charge characteristics do not materially affect the mode of association between spheroidal particles. Effect of electrolyte concentration Besides being dependent on ph, the Bingham yield value (~B) is sensitive to variations in ambient

7 Flow of halloysite 105 electrolyte concentration (Figs. 5 and 6). For MB and TA halloysites the plots of ph vs. ~ at different concentrations of NaC1 intersect at ph 6.0 and 7.1, respectively. As will be shown below, this point represents the PZC of the edge surface. At a low electrolyte concentration (e.g. 3 x 10-3 M NaC1), and below the edge PZC, e-f association would predominate. Since the extent of this association increases as ph continues to fall, the yield value of the corresponding (flocculated) suspensions would rise accordingly. Although this expectation is borne out by experiment, the increase in zb only extends to ph "~ 4. The apparent decline in Zn below this point may be ascribed to the acidinduced dissolution of octahedrally coordinated A1 exposed at crystal edges. The A13 ions released into solution migrate to basal surfaces where they displace Na + ions from exchange sites (Grim, 1968). As a result, the charge on crystal faces becomes less negative, and e-f attraction is reduced. The maximum in yield value close to ph 4 therefore reflects the fullest extent of e-f association (Rand & Melton, 1975). At a high electrolyte concentration (> 10-1 M NaC1) the electrical double layers on particle surfaces are suppressed, while van der Waals attraction between faces increases. The overall effect is a reduction in e-f association. Thus at a given ph below the edge PZC, yield values decline as the ambient electrolyte concentration increases from to M NaC1. Above the PZC of the edge surface, both edges and faces are negatively charged, and the particles would repel each other. By suppressing the electrical double layers, electrolyte addition would reduce particle-particle repulsion, especially between faces (Heath & Tadros, 1983). This and the general increase in van der Waals attraction would promote face-to-face (f-f) association. Raising the electrolyte concentration at a given ph above the edge PZC would, therefore, increase yield values. At the point where the curves intersect, the yield value is independent of electrolyte concentration. Following Rand & Melton (1975), this point may be identified with the PZC of the edge surface. Further, e-e association would predominate since on either side of the PZC the edges would repel each other. The depression of the edge PZC from the value of 9.4 for gibbsite (Hingston et al., 1972) may be ascribed to the presence of tetrahedrally coordinated Si in the halloysite structure in so far as the PZC of silica is 2-3 (McBride, 1989). The question remains why the edge PZC for MB is more than one ph unit lower than for TA which is comparable with the value of 7.3 that Rand & Melton (1975) found for Na kaolinite. Although the origin of the edge charge in both MB and TA is associated with exposed A1 (O, OH), the MB sample contains very little (0.18% w/w) structural Fe. On the other hand, (deferrated) TA halloysite has 2.48% w/w Fe in octahedral positions. Furthermore, the isomorphous substitution of Fe 3+ for A13+ is apparently nonstoichiometric in that approximately two A13+ ions are lost for every Fe 3+ ion gained (Soma et ai., 1992). It seems probable, therefore, that for TA halloysite, the depressing effect of structural Si on the edge PZC is tempered by the presence of both Fe and vacancies in the octahedral sheet. Since only a small amount (0.31% w/w) of Fe is external to the structure of TA halloysite, deferration does not affect the position of the edge PZC (Fig. 5). However, the presence of non-structural Fe exerts a noticeable influence on Bingham yield values. Only at an electrolyte concentration of M NaC1 do the curves for the original and deferrated samples coincide. The nature of the iron compounds removed by citrate-dithionite-bicarbonate treatment is not known. To have the effect described, however, they are likely to be of short-range order with a large surface area, resembling ferrihydrite in characteristics. Also, these compounds are probably closely associated with halloysite particle surfaces. By analogy with natural ferrihydrites (Schwertmann & Fechter, 1982), non-structural Fe compounds would be positively and negatively charged below and above the edge PZC of TA halloysite (ph 7.1), respectively. As such, their occurrence on basal surfaces would reduce f-f repulsion below ph 7.1 while above this ph, double layer repulsive interactions between faces would increase. In keeping with this suggestion, the yield values of the deferrated samples are lower below the edge PZC, and higher above it, as compared with the original materials. Only at an NaC1 concentration of 3 x 10-1 M, when van der Waals forces become dominant, does the distinction in flow behaviour between the deferrated and original samples vanish. Variations in PZC values have also been observed for kaolinites (Rand & Melton, 1977). The present study indicates that small differences in

8 106 B. K. G. Theng and N. Wells chemical composition as well as structural defects are important contributory factors. Because of their spheroidal morphology, particles of Op halloysite do not appear to have any edges in rheological terms. However, there are surface irregularities where A1 (O, OH) groups are exposed. The existence of such peripheral defects has been indicated by phosphate sorption which occurs specifically at the edge surface of halloysites (Theng et al., 1982). On this basis, the edge area of Op is estimated to be only about 50% smaller than that of MB. ACKNOWLEDGMENTS This project was partially funded by the New Zealand Foundation of Research, Science and Technology. We thank Mr G.D. Walker for the scanning electron micrographs and Miss J.L. Williams for typing the manuscript. REFERENCES CHURCHMAN G.J. & THENG B.K.G. (1984) Interactions of halloysites with amides: morphological factors affecting complex formation. Clay Miner. 19, FLEGMANN A.W., GOODWIN J.W. & OqTEWILL R.H. (1969) Rheological studies on kaolinite suspensions. Proc. Brit. Ceram. Soc. 13, GOFF D.I.A., MCPHERSON R. & FERGUSON J.A. (1983) Rheology of some Victorian clays. J. Aust. Ceram. Soc. 18, GRIM R.E. (1968) Clay Mineralogy, 2nd Edition. McGraw-Hill, New York. HEATH D. & TADROS Th.F. (1983) Influence of ph, electrolyte, and poly(vinyl alcohol) addition on the rheological characteristics of aqueous dispersions of sodium montmorillonite. J. Colloid Inte~ Sci. 93, HINGSTON F.J., POSNER A.M. & QUIRK J.P. (1972) Anion adsorption by goethite and gibbsite. I. The role of the proton in determining adsorption envelopes. J. Soil Sci. 23, JASMUND K. & LAGALY G., editors (1993) Tonminerale und Tone. Steinkopff Verlag, Darmstadt. KENT R.C., GORDON R.S. & HAHN S.J. (1968) Model for rheology of halloysite suspensions. J. Am. Ceram. Soc. 51, McBRIDE M.B. (1989) Surface chemistry of soil minerals. Pp in: Minerals in Soil Environments, 2rid Edition (J.B. Dixon & S.B. Weed, editors). Soil Science Society of America, Madison, Wisconsin. MICHAELS A.S. & BOLGER J.C. (1962) The plastic flow behaviour of flocculated kaolin suspensions. Ind. Eng. Chem. Fundamentals, 1, RAND B. & MELTON I.E. (1975) Isoelectric point of the edge surface of kaolinite. Nature 257, RAND B. & MELTON I.E. (1977) Particle interactions in aqueous kaolinite suspensions. I. Effect of ph and electrolyte upon the mode of particle interaction in homoionic sodium kaolinite suspensions. J. Colloid Inte~ Sci. 60, SCHWERTMANN U. & FECHTER H. (1982) The point of zero charge of natural and synthetic ferrihydrites and its relation to adsorbed silicate. Clay Miner. 17, SOMA M., CHURCHMAN G.J. & THENG B.K.G. (1992). X- ray photoelectron spectroscopic analysis of halloysites with different composition and particle morphology. Clay Miner. 27, THENG B.K.G., RUSSELL M., CHURCHMAN G.J. & PARFITT R.L. (1982). Surface properties of allophane, halloysite and imogolite. Clays Clay Miner. 30, VAN OLPHEN H. (1977). An Introduction to Clay Colloid Chemistry, 2nd Edition. Wiley-Interscience, New York. WELLS N. THENG B.K.G. (1985) Factors affecting the flow behaviour of soil allophane suspensions under low shear rates. J. Colloid Inteff. Sci. 104, WELLS N., THENG B.K.G. & WALKER G.D. (1980) Behaviour of imogolite gels under shear. Clay Sci. 5, WHI'ITON J.S. t~ CHURCHMAN G.J. (1987) Standard methods for mineral analysis of soil survey samples for characterisation and classification in NZ Soil Bureau. NZ Soil Bureau Scientific Report, 79, 8-10.