Many soils have clay-enriched horizons that are thought to result from pedogenic processes. Various hypotheses are proposed for expected clay size-distributions in soil profiles. The average size of clay crystallites determined by the amount of broadening of x-ray diffraction peaks ranged from 160 to \u3e2000Aº , but the lack of trends in four loess-derived soils did not permit the hypotheses to be evaluated. An unexpected relationship in a well expressed illuvial horizon was noted: as the percent of accumulated clay increased to the observed maximum the average crystallite size also increased rather than decreased as proposed. This suggests that small-sized illuvial clay particles undergo reorganization to form larger stable crystallites. Knowledge of clay crystallite size is expected to supplement other data in interpreting soil genesis

Arnold, R. W.

English

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Application of Clay Crystallite-Size in Soil Genesis Studies

University of Northern Iowa

Proceedings of the Iowa Academy of ScienceVolume 74 | Annual Issue Article 221967Application of Clay Crystallite-Size in Soil GenesisStudiesR. W. ArnoldCornell UniversityCopyright © Copyright 1967 by the Iowa Academy of Science, Inc.Follow this and additional works at: http://scholarworks.uni.edu/piasThis Research is brought to you for free and open access by UNI ScholarWorks. It has been accepted for inclusion in Proceedings of the Iowa Academyof Science by an authorized editor of UNI ScholarWorks. For more information, please contact scholarworks@uni.edu.Recommended CitationArnold, R. W. (1967) "Application of Clay Crystallite-Size in Soil Genesis Studies," Proceedings of the Iowa Academy of Science: Vol. 74:No. 1 , Article 22.Available at: http://scholarworks.uni.edu/pias/vol74/iss1/22Application of Clay Crystallite-Size m Soil Genesis Studies1 R. w. ARNOLD Abstract. Many soils have clay-enriched horizons that are thought to result from pedogenic processes. Various hypothe-ses are proposed for expected clay size-distributions in soil profiles. The average size of clay crystallites determined by the amount of broadening of x-ray diffraction peaks ranged from 160 to >2000A 0 , but the lack of trends in four loess-de• rived soils did not permit the hypotheses to be evaluated. An unexpected relationship in a well expressed illuvial horizon was noted: as the percent of accumulated clay inol'eased to the obse1ved maximum the average crystallite size also increased rather than decreased as proposed. This suggests that small-sized illuvial clay particles undergo reorganization to form larger stable cry~tallites. Knowledge of clay crystallite size is expected' to supplement other data in interpreting soil gene-sis. Soil genesis is the study of the development of horizons in a soil. It involves, in part, estimating the 01iginal properties of the unconsolidated material .and attempting to reconstruct the possi-ble conditions and events which have given rise to the present-day observable features. The ideal situation exists where initially a soil material is iso-tropic in all measurable properties and the soil horizons result from a rearrangement or removal of various components, such as carbonate and clay. Concepts to explain changes have classically assumed the initial materials to be homog.enous, thereby focusing attention on the kinds of processes that would differentiate the parent material into recognizable soil horizons. Loess approaches the concept of an ideal soil parent material in that it (a) has a mixed mineralogy capable of responding to a weathering environment, ( b) has a medium texture which slows up percolating water but does not unduly restrict the removal of weathering end products, ( c) has been exposed long enough on stable landscapes to reflect the influence of the climate and vege-tation characteristic of a region, and ( d) has less variability of other physical and chemical attributes than many materials in which soils have developed. Starting with such a material, a disparity of clay content be-tween upper and lower horizons in a profile is commonly thought to result from a translocation of the inherited clay from the upper zone to the underlying zone. Processes of clay formation and clay destruction may also account for some of the observed differen-ces in clay content throughout a profile. The weight of several 1 Contribution of the Department of Agronomy, Cornell University, Ithaca, New York, as Agronomy Paper No. 758. 122 1Arnold: Application of Clay Crystallite-Size in Soil Genesis StudiesPublished by UNI ScholarWorks, 19671967] CLAY CRYSTALLITES IN SOIL 123 size fractions or of total clay is usually measured, however, very little is known about the size of clay packets or crystallites as they occur in soils. The technique of estimating average crystallite size based on x-ray peak broadening ( Rosauer and Handy, 1961) can be applied to soil clay to provide additional information on horizon development. H YPOTHES1s CONCERNING CLAY SrzE-DISTRIBUTION Phyllosilicates are reported to range widely in size (Grim, 1953; van Olphen, 1963; Marshall, 1964). For example, kaolinite may have single plates 40A 0 thick and 400-900A 0 wide but more commonly they are about 500 A 0 thick and 3000 A 0 or more wide; some mica ( illite) particles are 30 A 0 thick and 1000-,3000 A 0 wide; and some montmorillinite particles are 20-80 A 0 . thick and 200-300 A 0 wide. A general relationship is that montmor-illinite crystallites are small'er than micas ( illite) and chlorites which in turn are smaller than kaolinite crystallites. Previous investigations (Hutton, 1951; Hanway et al., 1960; Glen et al., 1960; Frye et al., 1962) indicate that the inherited clay mineralogy of loess· in and adjacent to Iowa consists of mont-morillinite, chlorite, hydrous mica ( illite), interlayer variations of the above, and small amounts of kaolin with montmorillinite usually comprising 60-80% of the mixture. In their study of the clay mineralogy of a Tama silt loam from southwestern Wisconsin, Glen et al ( 1960) fractionated the -2p. material into 3 classes: coarse clay, 2-0.2p.; medium clay, 0.2-0.08p.; and fine clay, -0.08p.. The 2:1 layer lattice minerals comprised between 65 and 80% of the clay-sized material with the remainder including amorphous material, kaolinite, and primary minerals of clay size. The fine clay fraction contained 60-70% montmorillinite; the medium clay contained 40-50% montmorilli-nite and 15-30% mica, and the coarse clay contained 10-20% mont-morillinite and 30-40% mica. It is assumed that the original loess deposits contained a mixed clay mineralogy dominated by 2:1 layer lattice minerals and that a broad distribution of sizes of these clays also existed. Changes of a uniform distribution of clay content in an initial homogenous material may arise by (a) translocation of the clay from one zone to another, (b) formation of new clay-sized ma-terial, ( c) destruction of inherited or formed clay, or ( d) some combination of formation, destruction, and translocation (Arnold, 1965). Clay is commonly reported as a weight percent, thus if equal weights of different size fractions are analyzed one would mea-sure many more fine clay particles than medium or coarse clay 2Proceedings of the Iowa Academy of Science, Vol. 74 [1967], No. 1, Art. 22http://scholarworks.uni.edu/pias/vol74/iss1/22124 IOWA ACADEMY OF SCIENCE [Vol. 74 particles, consequently the average size would tend to . be weighted toward the smaller crytallites. An average crystallite-size of the dominant clay present in these soil horizons may be es-timated by measuring the boadening of the 060 peak of the 2: 1 lattice clays. Several postulates concerning the expected average size of clay particles may be made for the above kinds of transformations. If destruction is taken to mean the chemical dissolution and break-down of clay it would be expected that the small size particles with large surface areas per unit volume would be destroyed first, thus the average size of the remaining material would increase. Formation of additional clay may result from the physical break-down of silt or silt-sized aggregates of clay deposited in the ori-ginal material, in which case the initial breakdown would yield relatively large-size clay particles; thus the average size of crys-tallites would increase. Formation may also result by the synthe-sis of amorphous gels which crystallize into detectable particles. The initial development of crystallinity is thought to give rise to small-size particles ( Henin and Pedro, 1965), thus the average size of the bulk would decrease. Dispersion and movement in suspension of small-size clays, whether inherited or formed, would be expected to occur before movement of large particles, thus in the zone of depletion the average crystallite size should increase, whereas in th.e zone of accumulation the average size should decrease. Assuming a homogeneous parent material with respect to kinds, amounts, sizes and distribution of clay minerals increasing values of the average crystallite size may be due to ( 1) physical removal of smaller-size particles, ( 2) chemical destruction of the smaller-size particles, or ( 3) increased breakdown of· silt or silt-size ag-gregates of clay. Decreasing values of the average crystallite size may be due to ( 1) the formation of new crystalline material or ( 2) an accumulation of translocated clays. MATERIALS AND METHODS Four loess-derived soils, differing in their expression of a clay enriched subsoil, were selected for examination. A profile of Mo-nona silt loam from Crawford County, Iowa, has an ill-defined textural horizon extending from 8 to 32 inches; two profiles of Tama silt loam· from Grundy County have weakly expressed tex-tural horizons between 8 and 32 inches; and a profile of Tama silt loam, subsoil v;ariant from Cedar County has a well expressed textural horizon between 32 and 42 inches. Monoliths obtained at each site were kept moist to reduce any effect of drying on clay aggregation. A small matrix sample of soil taken from each 2-inch depth 3Arnold: Application of Clay Crystallite-Size in Soil Genesis StudiesPublished by UNI ScholarWorks, 19671967] CLAY CRYSTALLITES IN SOIL 125 increment of a profile was placed in a glass vial and shaken with distilled water. As soon as particles coarser than about 30µ, had settled to the bottom, several ml of the remaining suspension were removed with a pipette. Aliquots of the fine silt and clay suspension were placed on an unglazed ceramic tile, to which a slight suction was applied to withdraw the excess water, until a relatively thick layer of clay was deposited. The prepared tile was tlrnn air dried. The procedure was adopted to reduce any dis-ag-gregating affects of vigorous physical or chemical dispersion, and to obtain a sample representative of the soil clays in their natural state. The inclusion of some fine silt insured that enough quartz would be present to provide a reference peak in the x-ray dif-fraction paittem. In the three Tama profiles additional samples were taken from the exterior portion of subsoil structural units for comparison with the matrix samples. Tiie air-dried samples were examined with a General Electric XRD-5 x-ray diffraction unit. Diffractions were obtained using Ni filtered CuK cc radiation produced at 50 kv and 16 ma, and counting was done wM1 a scintillation rube. Other operating con-ditions included: a 3 ° beam slit, medium resolution Soller silt, 0.2 ° detector slit, 3 ~econd time constant, reverter wa~; in, linear rnnE'.e of 2000cps, scanning speed ol 2° /min, and a ·~hart speetl of 1 in/min. In large crystals there are numerous parallel atomic planes dif-fracting an incident x-ray beam at a very precise angle which gives rise to a sharply defined diffraction peak. In small crystals the number of atomic planes is reduced, small deviations of the ideal angle do not cancel the diffraction of the x-ray beam, and the diffraction no longer produces a sharp maximum peak. The width of a diffraction peak is measured at half-maximum peak height, graphically corrected for broadening due to the K cc radiation and the instrument geometry, and the mean dimension, D, normal to ,the diffracting planes of the crystal is calculated using the Scherrer equation. This equation, D=-K_A. __ (3 cos () ' expresses the average crystallite size, D, as an inverse function of tlie corrected diffraction broadening, (3, where K is a constant equal to unity, A is the x-ray wavelength, and () is the angle be-tween an atomic plane in the crystal and the incident x-ray beam at which a diffraction maximum will occur. Line broadening of the 060 diffraction peak for the 2: 1 layer lattice clays occurs at or near 62° 28 corresponding to a "d" spac-4Proceedings of the Iowa Academy of Science, Vol. 74 [1967], No. 1, Art. 22http://scholarworks.uni.edu/pias/vol74/iss1/22126 IOWA ACADEMY OF SCIENCE [Vol. 74 ing of approximately 1.50 A 0 (Figure 1). The calculation proced-ure was that of Rosauer and Handy ( 1960) except that in each case the correction for instrument broadening of the diffraction peak was obtained from a reference quartz peak at 60° 20. 63 62 60 59 ~Background Half Maximum Intensity D = 157 A Figure 1. Broadening of 060 diffraction peak of 2: I phyllosilicates in loess-derived soil. RESULTS AND DISCUSSION Most of the calculated crystallite sizes (Table 1) are from single determinations. Very little variation was noted if a sample was scanned twice in succession or if the low, broad peaks were remeasured at a later date, however, if the tile was shifted slight-ly or the sample was rerun a week later, larger variations were observed. The error of estimating size increases as the size of the crystallite increases (Rau, 1965) and based on seven sets of trip-licate determinations the average coefficient of variation was 20.3%. The crystallite b-axis dimensions of these clays are commonly between 200 and 350 A 0 , and assuming an average unit cell to be about 9 A 0 wide these particles have 20 to 40 unit cells connected edge-to-edge. Several measurements of the c-axis of the mont-5Arnold: Application of Clay Crystallite-Size in Soil Genesis StudiesPublished by UNI ScholarWorks, 19671967] CLAY CRYSTALLITES IN SOIL 127 Table 1. Average crystallite size of 2:1 layer lattice clays in four loess-derived soils. Depth Monona Tama I Tama II Tama-Var. (in) Matrix Matrix Exter. Matrix Exter. Matrix Exter. 1 271" 201 b 650 3 207 226 218 203 5 198 291 194 7 199 .. 209 203 9 520 217 358 206 11 408 459 297 276 13 373 472 496 194 218 201 15 297 260 936 291 242 187 17 203 >2000 501 271 631 226 19 258 221 166 172 269 203 21 271 255 643 410 286 225 23 303 431 259 326 207 270 304 25 254 180 196 302 189 194 252 27 245 537 182 343 1704 218 610 29 250 230 >2000 >2000 263 230 31 286 157 726 218 1008 221 520 33 217 219 326 236 276 376 35 203 147 247 206 244 244 37 239 266 245 286 338 524 267 39 257 >2000 175 271 225 225 185 41 289 263 284 520 178 271 43 230 263 466 242 285 45 217 283 333 271 203 199 47 239 938 938 171 173 327 49 178 • Size reported in angstroms (A 0 ) for the b axis. b Dash indicates that the size was not determined for that depth. morillinite component indicated that about 5 unit cells are stacked face-to-face, thus the crystallites are assumed to be tab-ular. Profile distributions of crystallite sizes reveal wide fluctuations over small vertical distances, consequently no overall trends of increasing or decreasing sizes are noted. In all four profiles, one or more samples from depths of 8 to 14 inches have larger sizes tha:6. adjacent samples which may reflect increased rates of wea-thering or translocation in the relatively undisturbed soil imme-diately below the plow layer. The clays on the exterior of subsoil peds are commonly larger than those in the matrix in the Tama II and the Tama variant profiles but the reverse was noted for the Tama I profile. It is apparent that no general interpretations can be made re-garding the proposed hypotheses for expected crystallite size in soil profiles without additional information. Calculations of the-oretical clay changes for the Tama II and Tama variant profiles (Arnold, 1963) provide values for the amourut of clay formed from non-clay components, and the amount of clay lost or gained 6Proceedings of the Iowa Academy of Science, Vol. 74 [1967], No. 1, Art. 22http://scholarworks.uni.edu/pias/vol74/iss1/22128 IOWA ACADEMY OF SCIENCE [VoL 74 by translocation. No consistent trend was found when crystallite size was compared with the ratio formed: illuvial clay or the ratio formed: eluvial clay, however, size does tend to increase with in-creasing amounts of clay lost from a zone irrespective of the amounl of clay fonned in situ. The Tama variant is the only profile with a definite zone of higher clay content. In the horizon from 29 to 43 inches the ap-proximate percent of present clay that is illuvial is 0, 20, 30, 30, 30, 15, 6 and 0 respectively for each 2-inch increment starting at the 29-inch depth (Arnold, 1963). A curvilinear relationship ex-ists between crystallite size and % of present clay accumulated by translocaition if the average size for the three samples with 30% illuvial clay is used. The average size of matrix crystallites would decrease as small size particles are moved into the zone. In this soil the minimum size (ca 180 A 0 ) corresponds to an accumula-tion of 5 to 10% of the clay. With increasing amounts of clay mov-ing into the horizon, the average size tends to increase (to ca 350 A 0 ) rather than to decrease further. The small c1ystallites may overlap and build up packets which are 50% or more wider than the initial average size ( 240-250 A 0 ) • Pawluk ( 1963) has re-ported that fine day-size montmorillinite may be bound inlto larger particles by hydrated alumina hydroxy ions released from feldspars. Soil clay minerals have received and are receiving much atten-tion in research efforts yet little is known about actual sizes and distributions of naturally occurring crystallites. I feel that know-ledge of the size of these particles would greatly supplement in-terpretations of the weathering, formation, and movement of clay minerals in the genesis of soils. Literature Cited Arnold, R. W. 1963. Silans in some Brunizem soils. Unpublished Ph.D. The-sis, Iowa State University Library, Ames, Iowa. ------ 1965. Multiple working hypothesis in soil genesis. Soil Sci. Soc. Amer. Proc. 29:717-724. Fyre, J. C., H. D. Glass, and H. B. Williams 1962. Stratigraphy and min-eralogy of the Wisconsinan loesses of Illinois. Ill. State Geol. Survey Gire. 334, 55p. Glenn, R. C., M. L. Jackson, F. D. Hole, and G. B. Lee, 1960. Chemical weathering of layer silicate clays in loess-derived Tama silt loam of Southwestern Wisconsin. In Clays and Clay Minerals, Proc. Eighth Nat'!. Conf., Pergamon Press, London. pp. 63-83. Grim, R. E. 1953. Clay mineralogy, McGmw-Hill Book Company, Inc., New York, 384 p. Hanway, J. J., R. L. Handy, and A. D. Scott 1960. Exchangeable potassium and clay minerals in selected Iowa soil profiles. Iowa Acad. Sci. Proc. 67: 215-231. Henin, S. and G. Pedro 1965. The laboratory weathering of rocks. Experi-mental Pedology, E. G. Hallsworth and D. V. Crawford, Editors. But-terworths, London. pp. 29-39. Hutton, C. E. 1951. Studies of the chemical and physical characteristics of 7Arnold: Application of Clay Crystallite-Size in Soil Genesis StudiesPublished by UNI ScholarWorks, 19671967] CLAY CRYSTALLITES IN SOIL 129 a chrono-litho-sequence of loess-derived Prairie soils of Southwestern Iowa. Soil Sci. Soc. Amer. Proc. 15:318-324. Marshall, C. E. 1964 .. The physical chemistry and mineralogy of soils. Vol. I. Soil Materials. John Wiley and Sons, Inc., New York, 338p. Pawluk, S. 1963. Characteristics of 14 A 0 clay minerals in the B horizons of podzolized soils of Alberta. Clay and Clay Minerals, XI: 74-82. Rau, R.C. 1963. Mea.surement of crystallite size by means of x-ray diffrac-tion line-broadening. Norelco Reporter 10: 114-118. Rosauer, E. A. and R. L. Handy 1961. Crystallite-size determinations of MgO by x-ray diffraction line broadening. Iowa Acad. Sci. Proc. 68:357-371. van Olphen, H. 1963. An introduction of clay colloid chemistry. lnterscience Publ., New York, 301 p. 8Proceedings of the Iowa Academy of Science, Vol. 74 [1967], No. 1, Art. 22http://scholarworks.uni.edu/pias/vol74/iss1/22

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Application of Clay Crystallite-Size in Soil Genesis Studies

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