Micromorphologic Evidence of Adhesive and Cohesive Forces in Soil Cement at Ion

8/4/2019 Micromorphologic Evidence of Adhesive and Cohesive Forces in Soil Cement at Ion

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MICROMORPHOLOGIC EVIDENCE OF ADHESIVE AND COHESIVE FORCES IN SOIL CEMENTATION

O.A. CHADWICK and W.D. NETTLETONJet Propulsion Laboratory, California Institute of Technology, MS 183-501 ,4800 Oak Grove Dr., Pasadena, CA 91109; National Soil Survey Laboratory,Federal Bldg., Rm. 345 , 100 Centennial Mall North, Lincoln, NE 68508

ABSTRACTCemented soil horizon differentation is based on the material that isdominantly responsible for restricting root growth and retarding porosity.Chemical bonding properties of the cementing material determine whether itcoheres to itself or adheres to the s-matrix. the relative amount ofadhesion or cohesion of soil cement can be inferred most clearly from fabricarrangements of strongly cemented horizons. Ionicly bonded calcite andgypsum produce open-porphyric related-distribution-patterns in which theprimary grains float in the expanding crystal matrix. In contrast,covalently bonded silica, iron, aluminum, and organic matter produce close-porphyric related-distribution-patterns in which isotopic cutans surroundgrains and fill voids and channels. Whole-horizon chemical data andquantification of cement on thin sections indicate that adhesive cementationrequires less cement than cohesive cementation.

1. INTRODUCTIONFour types of chemically cemented soil horizons are recognized by Soil

Taxonomy based on the dominant type of cementing agent: petrogypsic, gypsum;petrocalcic, calcium carbonate; placic, iron, aluminum, and organic matter;duripan, silica (Soil Survey Staff, 1975) . Cemented horizons develop inresponse to positive feedback reactions that lead to accumulation ofpedogenic compounds until the pores in the s-matrix are plugged by plasma(Torrent and Nettleton, 1978) . Sharp contrasts in micromorphology are notedwhen placic and duric horizons are compared with petrocalcic and petrogypsichorizons. Placic and duric horizons usually have plectic or close-porphyricmicrofabrics with isotic plasma (De Coninck and McKeague, 1985; Brewer etal., 1983; McKeague and Guertin, 1982) , whereas petrogypsic and petrocalcichorizons usually have open-porphyric microfabrics with crystic plasma (Allen,1985; Brewer et al ., 1983; Nettleton et al ., 1982) .

In this paper, we argue that differences in cemented-horizonmicrofabrics are due to the amount of adhesion that occurs between the s -matrix and cementing compounds, and that differences in cemented-horizonplasmas are due to the type of chemical bonding occuring within the cementingcompounds. Our hypothesis is that covalent bonding favors adhesion ofsilica, iron, aluminum, and/or organic cementing compounds to the existing s -


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matrix which results in close-porphyric fabric and isotic plasma. Incontrast, ionic bonding favors cohesion of the calcite or gypsum cement toitself which results in open-porphyric fabric and crystic plasma.

2. MATERIALS AND METHODSWe selected typical examples of soils with strongly cemented or

indurated horizons from the National Soil Survey Laboratory (NSSL) database(Table 1). The soils were chosen as modal examples of each type of cementedhorizon; they contained a minimum amount of accessory cementing agents, suchas calcium carbonate in the petrogypsic or duripan horizons. The amount ofcementing agent in crushed samples of the cemented horizons was measuredusing the following chemical methods (Soil Conservation Service, 1972):gypsum by water extract procedures (6Fla,6Flb); calcium carbonate bymanometric measurement of C02 evolved after treatment with HC1 (6Eld); ironby atomic absorption spectrometry on a dithionite-citrate xtract (6C2a); andorganic carbon by FeS04 titration on an acid-dichromate digestion (6Ala).Silica was measured by atomic absorption spectrometry after duripan sampleswere boiled for 2.5 minutes in 0. 5 NaOH (Torrent et al ., 1980) and placicsamples were incubated for 2 hours with 8OoC 0.5 NaOH.TABLE 1Soils Used in This Study and Their MicromorphologySeries Class f cation Horizon Microfabric PlasmaChesterton fine-loamy,mixed Btqml plectic, isotic(S76CA73-2 mesic Typic Durixeralf close-porphyricRedding fine,mixed,thermic Btqm2 plectic, isotictaxadj Abruptic Durixeralf close-porphyricKruzof thixo ropic Bsm plectic, isotic(S67Ak51-2) Cryic Placohumod close-porphyricShinaku sandy-skeleta1,mixed Bsm plectic, isotic(S62Ak0-3) frigid Placic Haplaquod close-porphyricSierocliff loamy-skeletal, 2Bkqml open-porphyric crystic(S66NV9-24) carbonatic mesicXerollic PaleorthidUnnamed coarse-loamy,gypsic BYm open-porphyric crystic(S74NM35-9) thermic Petrogypsic

(S66CA37-2)

Gyps orthid

The areal extent of cement as observed from thin sections was quantifiedusing a video-based computer image analysis system. For each cementedhorizon, and 8 to 9 cm2 area o n thin sections was divided into 3 categories:grains (> 20 pm), cemented plasma, and voids. Categories were quantifiedbased on operator-defined gray-level thresholding and density slicing of a


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digitized video image. Errors associated with the technique are estimated at10 to 15% primarily because of the following uncertainties: 1) measurementsof void space formed by grain plucking during thin section preparation, 2)overestimation of cemented plasma due to measurement of clay-size s-matrix,3) gray level confusion between cemented plasma and lithofragments. Thinsections were prepared by the method of Innes and Pluth (1970) and describedusing terminology suggested by Brewer et al. (1983) and Bullock et al.(1985).

3. RESULTSThe noncalcareous duripans in the Chesterton and Redding taxadjunct

soils from California have plectic or close-porphyric microfabrics withskeleton grains coated and bridged by grayish-yellow to reddish-brown isoticplasma (Table 1; Fig. 1A). The cemented plasma is about 38% by volume, 3.5to 11.2% Si02 and about 4% Fe203 by mass (Table 2). Voids are about 10% byvolume.

The placic horizons in the Kruzof and Shinaku soils from Alaska haveplectic or close-porphyric microfabrics with skeleton grains coated andbridged by yellowish-brown, reddish-brown, and dark-brown isotic plasma(Table 1; Fig. 1B). The cemented plasma is 30 to 40% by volume, about 32%Fe203, 2 to 6% organic carbon, and about 6% Si02 by mass (Table 2). Smallsilica/aluminum ratio values indicate that the silica may be released fromallophane. Voids are 3 to 9 % by volume.

The petrocalcic horizon in the Sierocliff soil from Nevada has an open-porphyric microfabric with crystic plasma (Table 1; Fig. 1C). The cementedplasma is about 83% by volume, 72% CaC03 by mass (Table 2). Voids are about3% by volume.

The petrogypsic horizon in the unnamed soil from New Mexico has an open-porphyric microfabric with crystic plasma (Table 1; Fig. 1 D ) . The cementedplasma is about 97% by volume, 89% CaS04.2H20 by mass (Table 2). Voids areabout 2% by volume.

The amount of cement commonly measured in cemented horizons wasevaluated using samples for the NSSL database: placic, 24 f 4% (n=5) Fe203;petrocalcic, 68 f 19% (n=ll) CaC03; petrogypsic, 8 6 f 7% (n=2) CaS04.2H20.No additional data on the silica content of duripans are available. Inagreement with data presented in Table 2 , these data indicate that placichorizons have less cementing material than petrocalcic and petrogypsichorizons.


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F i g . 1. Pet r og r aph ic micrographs o f cemen ted ho r izon s . A : Redd ing taxad junc tBtqm2, p a r t l y c r o s s e d p o l a r i z e d l i g h t ; B : Shinaku Bsm , p a r t l y c r o s se dp o l a ri z e d l i g h t ; C : S i e r o c l i f f Bkqml, c r o s s e d p o l a r i z e d l i g h t : D : unnamedBym, c ro s s ed p o l a r i z ed l i g h t .


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211TABLE 2Microscopic and Chemical Analyses of Cemented Horizons

Type Microscopic Analysis Chemicalof CementedSeries Horizon Plasma Grains Voids% -%

Chesterton Duripan 38Redding Duripan 38taxadjKruzof Placic 30

Shinaku Placic 41

Sierocliff Petrocalcic 83Unnamed Petrogypsic 97

52 10 3.5 Si0251 11 11.2 Si0261 9 31.2 Fe2035.0 Si026.1 Org. Carb.

6. 7 Si022.1 Org. Carb.1 2 89.0 CaS04.2H20

56 3 32.0 Fe203

14 3 72 CaC03

4. DISCUSSION AND CONCLUSIONSThe micromorphology of cemented pans indicate two different styles of

soil cementation. In the placic and duripan horizons, the s-matrix isretained as an integral part of the cement because it acts as a sorptionsurface for aluminum and iron oxyhydroxy compounds, organic carbon, and/oropaline silica. In contrast, in the petrocalcic and petrogypsic horizons,the s-matrix is expelled because it has little chemical affinity with theprecipitating calcium carbonate and gypsum crystals. Measurements on thevolume and mass of cementing compounds indicate that less iron and silica isrequired to cement a horizon because the s-matrix is part of the cement.More calcium carbonate and gypsum is required to cement a horizon because thes-matrix is not an integral part of the cement.

The placic and duripan horizons are formed by sorption reactions betweenthe soil solution constituents and the s-matrix. In acid soils, ironoxyhydroxy and organic compounds coat most aluminosilicate clays and primaryminerals while in calcareous soils, silinol and aluminol groups are exposedon uncoated mineral surfaces (Sposito, 1984). In both cases, exposed -OH or-COOH groups on soil surfaces can form covalent or quasi-covalent bonds withhydrated iron, aluminum, silica or organic compounds in the soil solution (DeConinck, 1983; Chadwick et al. , 1987). Thus, the cement accumulates byadhering to s-matrix components. The partly hydrated, covalently bondedcements are poorly crystalline which results in isotic plasma.


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The petrocalcic and petrogypsic horizons are formed by growth andinterlocking of authigenic crystals that have little affinity for the surfacereactive groups on the s-matrix surfaces. The 02- ortion of the C032- andS042- is more reactive with a strong electron acceptor like Ca2+ than therelative weak electron acceptor -OH and -COOH groups on the s-matrixsurfaces. Cementation occurs as crystals cohere to each other throughchemical intergrowth and physical interlocking. The s-matrix is not held inplace by adhesion and is displaced by the increasing volume of crysticplasma.

5. REFERENCES

Allen, B.L. 1985. Micromorphology of Aridisols. In: L.A. Douglas and M.L.Thompson (Editors), Soil Micromorphology and Soil Classification. SoilSci. SOC. Am. Spec. Pub. No. 1 5, Madison, WI, pp. 197-216.Brewer, R . , Sleeman, J.R., and Foster, R.C. 1983. The fabric of Australiansoils. In: Soils an Australian Viewpoint, Division of Soils, CSIRO,CSIRO: Melborne/Academic Press: London, pp. 439-476.Bullock, P. , Federoff, N. , Jongerius, A, , Stoops, G. , Tursina, T., nd Babel,U. 1985. Handbook For Soil Thin Section Description. Wain Research,Mount Pleasant, Wolverhampton, W V , England, 47 pp.Chadwick, O. A. , Hendricks, D.M., and Nettleton, W.D. 1987. Silica in duricsoils: I. A depositional model. Soil Sci. SOC. Am. J . , 51: 975-982.De Coninck, F. 1983. Genesis of Podzols. Acad. Analecta (R. Acad. Belg.),De Coninck, F. and McKeague, J.A. 1985. Micromorphology of Spodosols. In:L.A. Douglas and M.L. Thompson (Editors). Soil Micromorphology andClassification. Soil Sci. SOC. of Am. Spec. Pub. 15, Madison, WI, pp.Innes, R.P. and Pluth, D.J. 1970. Thin section preparation using an epoxyimpregnation for petrographic and electron microprobe analysis. Soil Sci.

SOC. Am. Proc., 341483-485.McKeague, J.A. and Guertin, R.K. 1985. Fabrics of some Canadian soils inrelation to particle size and other factors. Soil Sci., 133:87-102.Nettleton, W.D., Nelson, R.E ., Brasher, B.R., and Derr, P.S. 1982.Gypsiferous soils in the Western United States. In: J.A. Kittrick, D.S .Fanning, and L.R. Hossner, (Editors). Acid Sulfate Weathering, Soil Sci.SOC. of Am . Spec. Pub. No. 1 0 , Madison, WI, pp. 147-168.Soil Conservation Service. 1972. Soil Survey Investigation Report No. 1. SoilSurvey Laboratory Methods and Procedures for Collecting Soil Samples.USDA, Lincoln, NE, 63 pp.Soil Survey Staff. 1975. Soil Taxonomy: A basic system for making andinterpreting soil surveys. USDA-SCS Agric. Handb. 436. U.S. GovernmentPrinting Office, Washington, D. C. , 754 pp.Sposito, G . 1984. The Surface Chemistry of Soils. Oxford University Press,New York, 234 pp.Torrent, J . and Nettleton, W.D. 1978. Feedback processes in soil genesis.Geoderma, 20:281-287.Torrent, J . , Nettleton, W.D., and Borst, G. 1980. Genesis of a TypicDurixeralf of Southern California. Soil Sci. SOC. Am. J . , 44:575-582.

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Micromorphologic Evidence of Adhesive and Cohesive Forces in Soil Cement at Ion