Micromorphology and Mineralogy of Illuviated Clay in a Davidson SoilDidiek H. Goenadi and Kirn H. Tan*

ABSTRACTClay illuviation was studied in a highly weathered soil of the

Southern Piedmont by macro- and micromorphological techniques.Five pedons of a Davidson soil (Rhodic Paleudult) were sampledfrom four counties for preparation of thin sections and analysis ofsoil matrix, illuviated clay and argillans. The related distributionpattern of the soil fabric in the epipedon was different than that inthe argillic horizon or saprolite. A single-spaced porphyric fabricwas found in the A horizon, which had no illuvial clays. The argillichorizon exhibited an open-spaced porphyric fabric with illuvial clayranging in concentrations from 2.4 to 5.6%. A double-spaced por-phyric fabric was noticed in the saprolite, which contained 1.4%illuvial clay. X-ray diffraction analysis of clays in the soil matrixvs. argillans showed strong evidence for kaolinite as the major min-eral eluviated from the upper horizon. The data also suggested that,in the argillic horizon, kaolinite was formed in situ through alter-ation of K-feldspar minerals. The relatively high contents of ka-olinite and iron-oxide minerals, together with the presence of prom-inent argillans in the argillic horizons, indicated that the soil hasundergone an intensive weathering and leaching process.

THE DAVIDSON SOIL in the Southern Piedmont, GA,has been developed on basic rock residuum

(Giddens et al., 1960). It is classified as a clayey, ka-Dep. of Agronomy, Miller Plant Science Bldg., Univ. of Georgia,Athens, GA 30602. Contribution of the Univ. of Georgia, Collegeof Agriculture, Agric. Expt. Stn., College Station, Athens, GA. Thisresearch was supported by state and Hatch funds allocated to theCollege Station, and conducted at the College Station in cooperationwith the Agency for Agric. Research and Development (Badan Lit-bang Pertanian), Indonesia. Received 7 Dec. 1987. 'Correspondingauthor.Published in Soil Sci. Soc. Am. J. 53:967-971 (1989).

olinitic, thermic, Rhodic Paleudult (Soil Survey Staff,1975). Although a number of investigations have beenconducted (Nyun and McCaleb, 1955; Rich et al., 1959;and McCaleb, 1959), many questions about the gen-esis of the soil remain unanswered. Perkins et al. (1971)reported that the soil has been formed by laterizationand podzolization processes. More recent data indi-cated that the soil appeared to be more closely relatedto an Oxisol (Goenadi and Tan, 1988), however. Thepresence of an argillic horizon was part of the reasonsfor disqualifying placement of the Davidson soil inthe Oxisol order. Perkins et al. (1971) believed thatthe increase in clay content in the Bt horizon was dueto in-situ formation rather than to eluviation-illuvia-tion processes. However, adequate data were notavailable to support the hypothesis above, especiallyfor Davidson soils developed from basic/ultrabasicrocks in Georgia.

Clay formation is a time-dependent process relatedto weathering and soil formation. High annual rainfalland warm temperatures tend to induce the alterationof weatherable minerals, forming the finer particles ofthe soil. These two factors are perhaps the most activefactors in weathering and soil formation in the South-ern Piedmont of Georgia. The presence of clay skinsand increased clay contents in the argillic horizon areusually considered as strong indications for clay illu-viation, although Nettleton et al. (1969) reported thatclay skins could be absent in the Bt horizons of somesoils in arid and Mediterranean climates of the south-western USA. No micromorphological data are avail-able yet to verify the occurrence of clay skins or ar-gillans in the argillic horizons of Davidson soils.

968 SOIL SCI. SOC. AM. J., VOL. 53, MAY-JUNE 1989

Therefore, the purpose of this investigation was tostudy clay illuviation in a Davidson soil from theSouthern Piedmont, GA, using both macro- and mi-cromorphological techniques.

MATERIALS AND METHODSThe Southern Piedmont is part of the Piedmont Plateau

which together with the Appalachian mountains makes upthe pre-Cambrian or crystalline area. The elevation of thePiedmont Plateau area in Georgia, below the Appalachianmountains, ranges from about 366 m above sea level at thenorthern to about 103 m at the southern boundary where itmerges with the southern Coastal Plain Province (Giddenset al., 1960; Perkins and Shaffer, 1977). Geologically, theSouthern Piedmont is rather complex and consists of gran-ite, marble, hornblende gneiss-gabbro, diorite, metavolcan-ics and metasediments of the pre-Cambrian period. Biotitegneiss, feldspathic-biotite gneiss, and amphibolite-horn-blende gneiss were reported as the common soil-formingrocks in the area under investigation (Georgia GeologicalSurvey, 1976). Annual rainfall in the sampling area variedfrom 1170 to 1270 mm, and the average number of frost-free days ranged from 210 to 230 d/yr (Perkins and Shaffer,1977).

Five pedons were sampled at four different locations (Put-nam, Jasper, Oglethorpe and Lincoln counties) as reportedearlier (Goenadi and Tan, 1989). Oriented clod samples weretaken from each major horizon and the saprolite for prep-aration of soil thin sections, using the procedure as describedby Fitzpatrick (1984). Soil fabric analysis was conducted us-ing the method and terminology as proposed by Stoops andJongerius (1975). The term "related distribution pattern"(RDP) was used to indicate the distribution pattern of theplasma and coarse particles (Brewer, 1976) in a soil fabric.The presence of voids, as a consequence of coarse grainspacking, and the concentration of pedpgenetic products re-sulting from weathering processes are implied in the RDPconcept. Stoops and Jongerius (1975) introduced a specifictype of RDP called c//related distribution, enabling a mi-cromorphological classification of soil fabrics based on theratio of coarse c and fine particle/concentrations. Five basictypes were recognized: (i) monic, (ii) gefuric, (iii) chitonic,(iv) enaulic and (v) porphyric c/f related distribution. Theporphyric c//related distribution pattern was subdivided intoclose porphyric (coarse grains have points of contacts), sin-gle-spaced porphyric (distance between coarse grains is lessthan their diameters), double-spaced porphyric (distance be-tween coarse grains is one to two times their diameters), andopen-spaced porphyric (distance between coarse grains ismore than twice their diameters).

Illuvial clay was calculated using the point count methodwith the aid of a grid placed in the ocular of a ,polarizingmicroscope set at a magnification of 125X (McKeague etal., 1967). The number of squares in the grid occupied byoriented clay were counted under cross-polarization for 100fields. The results presented were the average of three rep-licates. Extra clods and soil samples were collected for de-termination of soil bulk density and particle size distributionanalysis (Soil Survey Staff, 1984).

Argillans or clay films of the argillic horizons were col-lected following the procedure described by McDaniel andNielsen (1985). Clay films viewed under a binocular micro-scope were carefully scraped from ped surfaces. Interiors ofpeds were also sampled to obtain matrix material. Miner-alogical analysis of both soil matrix material and clay filmswere performed by x-ray diffraction analysis. Quantificationof clay minerals present in the samples was conducted usingChung's method (Chung, 1974) with corundum as internalstandard. Scanning electron microscopy of mineral grains

Table 1. Macromorphological characteristics of the soil investigated.

Horizon

ApBt3Bt4

Bt5

C

Depth,cm0-16

74-105105-142

142-203

203+

Texture

LCC

C

CL

Colorf(moist) Structure

2.5YR 3/2 IfgrJ2.5YR 3/6 3fabk2.5YR 3/6 3fabk

mottles:7.5YR 5/62.5YR 3/6 3mabkmottles:

7.5YR 5/610YR 5/6Mottled: —

2.5YR 4/810YR 5/6

SYR 7/8-8/4

Consistence

fr§fifi

fi

vfi

Clayskin

Nonemplmp

mp

None

t Munsell color notation.j 1 = weak, 3 = strong, f = fine, m = medium, abk = angular blocky, gr

= granular.§ fr = friable, fi = firm, vfi = very firm.H mp = many, prominent.

Table 2. Bulk density (BD) and particle size distribution of the soilinvestigated.

Horizon

ApBt3Bt4Bt5C

Depth

cm

0-1674-105

105-142142-203203+

BD

Mg m 3

1.701.461.471.361.32

Sand

436875590

345

Silt

"•&

309349397357313

Clay

255564547553342

were conducted using the Philips 505 (Philips, Inc., Neth-erlands) SEM at 20 kV. Prior to analysis, the samples werecoated once for 4 min with a Au-Pd alloy in a HummerSputter coater (Technics, Inc., Alexander, VA). Because ofmany similarities between the five pedons analyzed, onlyresults from the most representative pedon are presented.

RESULTS AND DISCUSSIONMacromorphology and Physico-ChemicalCharacteristics

The macro or field morphology showed strong evi-dence for the presence of an argillic horizon. The il-luvial (Bt3, Bt4, and Bt5) horizons were different fromthe eluvial (Ap) horizon in texture, structure, and moistconsistence (Table 1). The clay content of the Bt ho-rizons was more than twice that in the Ap horizon(Table 2). The textural data were in the range of thosereported by Rich et al. (1959) for the Davidson soils.On the other hand, the bulk density of the Ap horizonwas higher than that of the argillic horizon which couldbe due to compaction or to a more dense close-pack-ing of the coarser materials, mainly quartz, in the Apthan in the argillic horizons. The amount of grains pervolume of soil in the Ap horizon increased, whereasthat of plasma decreased. Since plasma is lighter inweight than the coarse grains, an increase of the latterper volume basis may result in an increase in bulkdensity.

MicromorphologyThe different types of soil fabrics characterizing the

pedons were reported earlier (Goenadi and Tan, 1989).Matrix color of thin sections, RDP, amount of illuvialclay and the ratio between coarse and fine (2 /tm) frac-

GOENADI & TAN: MICROMORPHOLOGY AND MINERALOGY OF ILLUVIATED CLAY IN A DAVIDSON SOIL

Table 3. Micromorphological characteristics.

969

Thin sectionfHorizon matrix color

Ap 10YR 3/6 and10YR 4/6

Bt3 10YR 4/8 andSYR 5/8

Bt4 10YR 4/8 andSYR 6/6

BtS 2.5YR 4/8 andSYR 7/6

C 10 YR 5/8 and7.5YR 7/8

f Munsell color notation.$ c = % coarse fraction, f = %

Illuvialclay

None

2.4

5.6

4.9

1.4

fine fraction (<2

RDP

Single-spacedporphyricOpen-spacedporphyricOpen-spacedporphyricOpen-spacedporphyricDouble-spacedporphyric

Aim).

c/f*

40

15

5

5

30

tions are listed in Table 3. The c/j£x) related distribu-tion pattern is denned as the distribution of materialfiner than a specified limit (x) with respect to coarsermaterial.

Soil fabric analysis showed the presence of argillansin the Bt horizons lining voids and pores (Fig. la).They were somewhat microlaminated in the BtS ho-rizon (Fig. Ib). Illuviation of clay is clearly demon-strated by the data in Table 3. The concentrations ofilluviated clay counted by the grid method increasedfrom none in the Ap to 5.6% in the Bt4 horizon. Onthe basis of RDP, the fabric of the Ap horizon ap-peared to meet the criteria for a single-spaced por-phyric fabric (Table 3), whereas the argillic horizonsand the saprolite exhibited more an open-spaced, anddouble-spaced porphyric fabric, respectively. The c/fvalues, decreasing from Ap to the Bt4 horizons, werein agreement with the distribution pattern of coarseand fine materials as expressed in terms of c/f relateddistribution pattern.Mineralogy of Soil, Clays and Argillans

The mineral composition of the soil matrix in theAp horizon was dominated by quartz (Table 4). Nextin abundance were goethite, kaolinite and magnetite.The soil matrix of the BtS and C horizons was dom-inated by kaolinite and magnetite, enriched by goe-thite, ilmenite, and hornblende. The source of the ironoxide minerals was believed to be hornblende. Horn-blende was detected in significant amounts in the BtShorizon, and was third in abundance in the mineralsuite of the C horizon. The decrease in hornblendecounts from the C to the BtS horizon correspondedwith a significant increase in goethite, hematite andmagnetite counts. Clay skins or argillans showed somevariations in mineralogy compared to that of soilmatrices. Kaolinite and magnetite were also detectedTable 4. Mineralogical composition of soil matrix and argillans.

Fig. 1. Soil thin section micrographs of: (A) Bt4, and (B) BtS ho-rizons. Ar = argillans, Q = quartz, F = feldspar, v = void. Planepolarized light.

in large amounts in clay skins or argillans, but goethiteconcentrations tended to increase in argillans from Bt3to BtS horizons. Another difference was that lepido-crocite appeared to be concentrated more in argillansthan in soil matrix, especially in the Bt3 horizon.

The relatively high kaolinite content detected in theBtS and C horizon matrices suggested that kaolinitein these horizons did not only come from the upperhorizons, but also from in situ alteration of feldsparminerals. Alteration of K-feldspar into clay can be seenin Fig. Ib, where the presence of the original rockstructure boundary was still noticeable in the middlepart of the micrograph. Scanning electron microscopyindicated that most, if not all, of the light mineralfraction (containing the feldspar minerals) was com-posed of feldspar altered into kaolinite (Fig. 2a). Thestructural boundaries of the feldspar minerals are still

Sample

Ap matrixBt3 argillanBt4 argillanBtS argillanBtS matrixC matrix

Kt

139282282377231309

H

404896726783

K-spar

———_62

Q

3005143

1018334

Gi

192632——14

Goekg-i& "O

18490

12312915799

Hm

6957923692—

Mag

127142205145229166

Lep

46241

36—_—

Ilm

786395

14298

105

Hb

———41

128

f K = kaolinite, H = halloysite, K-spar = K-feldspar, Q = quartz, Gi = gibbsite, Go = goethite, Hm = hematite, Mag = magnetite, Lep = lepidocrocite,Ilm = ilmenite, and Hb = hornblende.

970 SOIL SCI. SOC. AM. J., VOL. 53, MAY-JUNE 1989

0.336

Bt5«V"'

;———'

X-RAY DIFFRACTION

0.359 0.719 nm

light minerals

i.427

30" 25 20 1526

10 5°

Fig. 3. X-ray diffraction analysis of: (a) the light mineral fractionof the Bt5 horizon; and (b) weathered rock from the C horizon,showing the presence of kaolinite and quartz. Degrees 20; Cu Karadiation at 30 kV and 15 mA.

Fig. 2. Scanning electron micrographs of: (a) the light mineral frac-tion of the BtS horizon; white bar = 1 mm; (b) a single grain athigher magnification (white bar = 10 fim), showing kaolinitepackages.

visible, but at higher magnification (Fig. 2b) the ka-olinite packages can be noticed very clearly. X-ray dif-fraction analysis of the light minerals provided sup-porting evidence by yielding diffractograms (Fig. 3)with strong peaks for kaolinite at 0.719 and 0.359 nm.Quartz was the other abundant mineral in the lightfraction (0.336 and 0.427 nm). These results are incontrast with those of Calvert et al. (1980), who re-ported a direct transformation of feldspar into gibb-site, tubular halloysite and amorphous materials. Thedifferences in results are perhaps attributed to differ-ences in soils used. Whereas Calvert et al. (1980) haveused a Pacolet soil (Typic Hapludult), the present au-thors have analyzed a Davidson soil (Rhodic Paleu-dult). Direct transformation of feldspar into tubularhalloysite has also been reported in deeply weatheredsoils of the Sierra Nevada, California (Southard andSouthard, 1987).

ConclusionsThe relatively large contents of kaolinite and iron

oxide minerals, together with the low concentrationof weatherable minerals in the C horizon were be-,lieved to be indications that an intense weatheringprocess has taken place. The direct transformation ofK-feldspar into kaolinite in the C horizon and thepresence of prominent argillans in the argillic horizonswere additional evidence for an intense weathering andleaching process in the soil. On the other hand, theabsence of a true eluvial (E) horizon suggested only a

mild influence of a podzolization process. Davidsonsoils were formerly classified as Reddish Brown La-teritic soils (Giddens et al., 1960; Rich et al., 1959),which by definition were not characterized by E ho-rizons in their profiles. High concentrations of ses-quioxides and decreasing quartz contents in the Bthorizons were indications that desilification was per-haps a major process in the genesis of this Davidsonsoil.

The results obtained suggested that the Bt horizonsof Davidson soils were true argillic horizons. Thesehorizons possessed >1% illuvial clay. The data indi-cated that the high clay content in the argillic horizonswas not attributed only to illuviation, but also to di-rect transformation of primary minerals into clay.

ACKNOWLEDGMENTSGrateful acknowledgements are extended to Mr. Robert

Wilkes, Soil Correlator, USDA-SCS, Soil Survey Div., Ath-ens, GA and to Mr. Louie Frost, Soil Scientist, USDA-SCS,Elberton, GA, for identification and sampling the pedons.

PARFITT & KIMBLE: CONDITIONS FOR FORMATION OF ALLOPHANE IN SOILS 971