Clay Differentiation in Aridisols of Northern MexicoJ. Ducloux,* J. P. Delhoume, S. Petit, and A. Decarreau

ABSTRACTAridisols with smectite B horizons are usually classified as Argids

although the argillic characters may not always be present. In theChihuahuan Desert of northern Mexico, some Aridisols have clayeyhorizons that do not seem to result from clay illuviation. Our purposewas to determine the origin of the clay enrichment and the mineralogi-cal relationships between the parent materials and the B horizons.The study area is the Mapimi Playa, derived from a lutite lagoonalsediment parent material. A soil sequence from Orthids to Argids wasidentified. Four profiles were analyzed. The particle-size distributionwas determined by the sedimentation and centrifugation methods. Theclay fractions were studied by x-ray diffraction and peak decomposi-tion, infrared spectroscopy, and chemical analyses. Soil water extractswere analyzed with an absorption spectrometer. From the Orthids tothe Argids, clay enrichment in B horizons is controlled by the <0.2-umclay size fraction and small amounts of palygorskite. The B horizonscontain prominent smectite, which has more Mg than the lutite smectite(parental material). The existence of the AlMgOH infrared vibrationsin the soil clay suggests the neogenesis of a smectite different fromthe lutite smectite. The composition of the soil water extract suggeststhat Mg-smectite and palygorskite may form in Mapimi soil profiles.The Argids of Mapimi Reserve result principally from clay neogenesisunder current climatic conditions.

SOILS OF ARID LANDS ARE PRODUCED primarily bymechanical weathering and wind transfer (Cooke

and Warren, 1973). It is thought that their clay mineralsare commonly inherited from the parent materials. Thepredominant clay mineral is smectite (Nettleton and Pe-terson, 1983), subordinately kaolinite and illite, andsometimes palygorskite-sepiolite groups occur in "calcicsoils" (McGrath and Hawley, 1987). Aridisols undergopedogenic processes, with redistributions of CaCCh andsalts on recent surfaces, and clay and silica on old stablelandforms (Nettleton and Peterson, 1983). Aridisols mayJ. Ducloux, S. Petit, and A. Decarreau, Universite de Poitiers, URA 721CNRS, 86022 Poitiers C6dex, France; and J.-P. Delhoume, ORSTOM,93140 Bondy, France. Received 28 July 1993. *Corresponding author.

Published in Soil Sci. Soc. Am. J. 59:269-276 (1995).

contain argillic horizons, apparently Pleistocene relicts,with weak and discrete clay skins in nonswelling horizons(Nettleton et al., 1969). Sometimes, argillans on pedsurfaces are lacking (Gile and Grossman, 1968; Alien,1977) and their destruction is attributed to authigeniccalcite crystal growth (Alien and Goss, 1974). However,clay transformation is also possible in response to envi-ronmental conditions (Gal et al., 1974; Alien and Fan-ning, 1983; Eghbal and Southard, 1993).

In the Chihuahuan Desert, we described a soil sequencealong a slope characterized by a progressive increase inclay content from Orthids to Argids. Initially, the genesisof the argillic horizons was difficult to explain throughclay inheritance or illuviation. Therefore, the objectivesof this study were to establish the nature and mechanismsresponsible for this clay enrichment and their relation-ships to the parent materials.

MATERIALS AND METHODSStudy Area

The study area is in the Chihuahuan Desert, playa (bolson)of Mapimi, in the Reserva de la Biosfera, states of Durango-Chihuahua-Coahuila (Mexico), approximately 150 km northfrom Torreon (Coah) (Fig. 1). The present climate is arid(precipitation averages 264 mm annually, falling mostly assummer rain), with cool winters and hot summers, and a meanannualairtemperatureof20.8°C (Cornet, 1988). Thedominantvegetation consists of creosote bush [Larrea tridentata (Sesse& Mocino ex DC.) Cov.], Prosopis glandulosa, with Hilariagrass [Hilaria mutica (Buckley) Benth.] (Montana and Breimer,1988; Delhoume, 1988) and various Cactaceae. The parentmaterials consist of the Formation Las Quiotentas del Terciario(Oligocene-Miocene), which can be divided into three facies:(i) a lutite facies; (ii) a rudite facies; and (iii) a volcanic facies.The rudite and volcanic facies occur as lenses and disconnectedbodies within the lutite facies (Bartolino, 1988). The volcanic

Abbreviations: XRD, x-ray diffraction; TEM, transmission electron mi-croscopy; I/S, illite/smectite mixed layers; FTIR spectra, Fourier transforminfrared spectra; IR vibration, infrared vibration.

270 SOIL SCI. SOC. AM. J., VOL. 59, JANUARY-FEBRUARY 1995

MAPIMI^v_MIEXIO

Fig. 1. Location and details of the study area: B = basaltic peak(Cerro S. Ignacio); R = rhyolitic bodies; L = conglomerate hills(lomas). Arrows indicate the water flow direction. Dashed linesindicate the mapped zone. Transect C-V is the cross section shownin Fig. 3.

facies occurs in the Cerro San Ignacio (basaltic inselberg),which is the dominant topographic feature (1475-m elevation)of the studied area. From the cerro, the alluvial-fan piedmonts(upper bajadas) extend from 1 to 2 km in length. They developon the lutite facies covered by thin alluvial-colluvial fans ofapproximately 1 to 2% slope, which disappear on lower baja-das; differential erosion has exposed discontinued cuestas ofrudite facies called lomas. At the footslope of the cuestas, thereverse slope (less 1%) induces hollows formation due to theinflux of runoff water (Delhoume, 1988). These hollows, whichcollect = 550 mm of water from rainfall and runoff (Delhoume,1991), are Hilaria grass covered. Downslope, the playa withalluvial flats extends 8 to 9 km from the cerro at an elevationof =1100 m (Fig. 2). As indicated by Breimer (1988) andDelhoume (1988), the soils are principally luvic Yermosolsand luvic Xerosols, salty-sodic and -calcic phases, and Solon-chaks in the playa (Food and Agriculture Organization, 1974) -Calciorthids, Haplargids, and Natrargids, respectively (Soil

wS. Ignacio Peak

1475m

lomasplaya

1110m

Slope %

Soils

25-6

Regosols

0.5

——Xerosols --Yermosols •

Xerosols—— Yermosols —

Survey Staff, 1975). This study was carried out on an area ofa few square kilometers including bajadas and two lomas atthe feet of which extended two Hilaria grass covered depres-sions (Fig. 1).

Field MethodsThe distribution of the soils was investigated on the upland

of the first two cuestas and mapped according to observationsmade using pits bored every 100 m. The soil profiles have 5-to 7-cm-thick A horizons extending to more or less clayey Bhorizons, weathered lutite C2 horizons with a blocky structure,and a massive Cl lutite rock. The spatial distribution of thesoil horizons was based on their texture and structure (Fig.3). Four soil profiles were selected to describe the typicalsequence. The first profile (I), located in the drainage areabetween the lomas, has a low clay content and the lutite isclose to the soil surface. The following profiles (J, M, andC) show a clay content increasing progressively in their Bhorizons. The B horizons of the M and C profiles can beinterpreted as argillic horizons (clay increasing, upper limit,thickness, and structural characteristics) in spite of the nonexist-ence of clay skins. The C profile is an isothermic TypicPaleargid, the M and I profiles are isothermic Typic Haplargids,and the J profile is a Typic Camborthid (Soil Survey Staff,1975).

Laboratory MethodsMicroscopic observations were conducted from thin sections

of oriented horizon samples imbedded in synthetic resin anddiamond polished with 1-u.m powder. Bulk samples were airdried and gently crushed to pass a 2-mm sieve. Particle-sizedistribution was determined on <2-mm samples after removalof organic matter with H2O2. The <2- and <l-um fractionswere extracted by sedimentation after dispersion with a 102 gL~' sodium hexametaphosphate solution. Three clay size frac-tions were then separated with ultracentrifugation from the<l-Hm samples: 1 to 0.2 |im (coarse), 0.2 to 0.1 urn (fine),and <0.1 p,m (very fine). All the clay fractions were analyzedby the following: (i) x-ray diffraction of oriented clay sedi-mented onto glass slides, after Ca, K, and Li saturation(Greene-Kelly, 1957) followed by (001) peak decomposition(Lanson and Champion, 1991; Lanson and Besson, 1992),i.e., each (001) peak is filled by one or several theoretical (001)reflections with gaussian or laurentzian shape, and diffraction ofdisoriented clay powder (CoKa, 40 kV, 40 mA); (ii) infraredspectra obtained on a Fourier transform spectrometer (Nicolet510 FT, Nicolet Instruments, Madison, WI) at ambient temper-ature, with sample KBr disks (1:100 dilution); (iii) chemicalanalyses with an absorption spectrophotometer after fusion with

Fig. 2. Sloping profile of the study area; the vertical lines indicateArgid locations.

Fig. 3. Clay content (<2 um) isolines along the C-V cross sectionparallel to the lomas. Profiles C, M, J, and I are the locations ofthe analyzed profiles.

DUCLOUX ET AL.:.CLAY DIFFERENTIATION IN ARIDISOLS 271

strontium borate and acid dissolution; (iv) TEM observations onclay subfraction suspensions. Extractions of soil solutions weremade with a soil/water (v/v) ratio of 1:5; Si, Al, and Mg weredeterminated with an absorption spectrophotometer.

RESULTSSpatial Distribution of Clayey Horizons

The clayey horizons extend parallel to the uplands ofthe lomas, but break off between the lomas (Fig. 1 and3). The greater clay contents are located in the flat zoneswhere the clayey horizons are the thickest and nearthe soil surface. In the uplands, the total clay contentdecreases. At the reverse slope of the loma, the transitionfrom the clayey soils to soils developed on the lomaconglomerates is ~ 1 m wide. Toward die drainage area,the clay content also decreases, and the clayey B horizonsbecome thinner (Fig. 3). The clayey B horizons alwayscontain a higher content of fine (0.2-0.1 \im) and veryfine (<0.1 urn) clay fractions (Table 1). Moreover, thehigh content of fine clay is associated with the highestcontent of total clay (r2 = 0.79) while the increase infine clay seems to develop at the expense of the veryfine clay. In the Cl and C2 horizons of the fresh andweathered lutite, the very fine clay size fraction is low.

Microscopic Study of the Soil HorizonsThe A horizons show a weak pedal microorganization.

The matrix with angular cracks contains abundant skele-ton fragments of quartz, plagioclase feldspars, basalt,and carbonates. The cambic and argillic B horizons arecomposed of rounded peds 50 to 100 u.m in width associ-ated with detrital minerals and rocks like those of theA horizons. All the material is fractured, which generateslarger angular peds. The carbonate fragments alwaysoccur in the peds, never in the voids. Any clay transloca-tion was identified as void and grain argillans. The C2and Cl horizons show a structure composed primarilyof rounded peds 50 to 100 |im in width. These peds are

characteristic of the lutite facies and developed duringthe diagenetic desiccation cycles (Freytet and Plaziat,1982). Sometimes the C horizons contain lenticular gyp-sum crystals previously reported by Bartolino (1988).Under TEM, illite was clearly identified in the coarseclay (1-0.2 jim) fractions as weakly smoothed euhedralparticles. In the fine fractions, smectite showed velumaspects. In the B21 horizons of the Profiles C and M,palygorskite was identified as fibers in the fine and veryfine clay fractions.

Clay Mineralogy of the LutiteThe XRD patterns of the lutite clay (C horizons of

Profiles I and M, and subordinately B22, B, C2 horizonsof Profiles M, I, and C, respectively) showed intensediffraction lines at 1.4 nm (Fig. 4), which shifted to1.68 nm after glycol solvation. After the Greene-Kellytest, the expansion reached 1.7 nm, indicating a beidellitetype smectite. The K saturation and heating at 105°Cproduced a collapse at 1.21 to 1.18 nm, indicating thepresence of I/S. On the powder patterns, the (06,33)peak is located at 0.150 nm, indicating a dioctahedralstructure.

The decomposition treatments of the XRD patternsindicate the presence of a mixed-layer, 1.10- to 1.13-nmphase, expandable to 1.3 nm, and slightly more abundantin the coarse clay fraction. In the Cl horizon of Profile J,which has a conglomerate bed, a significant illite componentappeared at 1.0 nm in all the clay fractions. In othersamples, the lutite clay is a mixture of a dioctahedralsmectite always predominant in all the clay fractions, withtetrahedral substitutions and small quantities of I/S. Tracesof kaolinite were also detected, principally in the coarseclay fraction.

Clay Mineralogy of the B HorizonsThe patterns of the B21, B22, and B3 horizons of

Profile C, of the B21 horizon of Profile M, and the Cl

Table 1. Selected physical and chemical properties of the soils studied.

Particle-size distribution

Clay

Profile

C

M

I

J

Horizon

AB21B22B3C2A

B21B22C2ABClABCl

Depth

cm0-56-50

50-7575-100

100-1800-66-28

28-52100-120

0-55-24

60-900-77-23

23-50

i1

0.62.35.33.82.80.22.12.30.40.10.70.10.12.00.0

0.2-0.1um

1.66.94.94.62.81.59.14.61.00.92.20.91.93.60.6

1-0.2um

9.912.415.717.16.99.46.28.13.97.5

11.53.1

19.213.64.6

<2um

14.646.941.634.329.720.431.827.416.716.321.613.927.431.411.3

Silt0.002-0.05

mm

53.032.133.535.835.549.532.830.058.651.256.847.452.544.650.1

Sand0.05-2

mm

32.421.024.729.134.730.135.442.624.731.721.036.420.035.238.6

CaCOj

16.220.520.521.717.715.118.019.928.216.626.024.915.520.925.9

pH

8.78.99.08.38.28.68.38.99.28.68.47.98.38.58.7

Structuret

1 fabk2-3 vc p. 2 c abk

IcabkmassivemassiveI f s b k

1 c p. 2 m abkmassivemassiveI f s b k2cabkmassiveZmsbk

2-3 m abk. 1 c pmassive

t Grade, class, and type of structure (Soil Survey Staff, 1951).

272 SOIL SCI. SOC. AM. J., VOL. 59, JANUARY-FEBRUARY 1995

Profile C

1.4 1.0

Profile J

A-Hor.

1.4 1.0

1.4 0.7

Profile I

A Hor.

1.4

Fig. 4. X-ray diffraction patterns of the fine (0.2-0.1 fan, noted*)and very fine (<0.1 fun) clay size fractions of Ca-saturated air-driedsamples.

horizons of Profiles I and J (Fig. 4) exhibited broadpeaks near 1.4 nm, with a peak at 1.05 to 1.06 nm dueto palygorskite clearly evidenced in the very fine clayfraction. After glycol solvation and the Greene-Kellytest, the expansion was up to 1.78 to 1.80 nm. The(06,33) peak showed two lines at 0.150 and 0.151 nm.The (001) peak at 1.0 nm of illite was also evident.

The XRD patterns of the clayey horizons show amixture of phases. The major mineral is smectite, whichhas a 1.4-nm peak expandable to 1.7 nm after glycolsolvation. From 1.1 to 1.0 nm, a shoulder indicatesmixed-layer phases and illite. The decomposition treat-ments indicate that smectite is prominent in all the sam-ples, particularly in the very fine clay fractions of theclayey horizons. Illite is always present in small quanti-ties, principally in the coarse clay fraction. Mixed-layerI/S seems to be more abundant in the coarse and fineclays than in the very fine fraction. Kaolinite is presentin traces, particularly within the coarse clay fraction.

According to the (06,33) reflection, all the 2:1 clayphases are dioctahedral. Some fibrous clay at 1.06 nm(palygorskite type) was clearly evidenced, after decom-position treatments, in the B21 horizons of Profiles Cand M.

Profile M Clay Mineralogy of the A HorizonsThe XRD patterns of the A horizons (Profiles C, M,

J, and I) show a prominent expandable 1.4- to 1.7-nmphase similar to that of smectite from the B horizons.However, the wider (001) peak indicates a very smallamount of stacked layers of clay particles. Illite is abun-dant in the coarse clay fraction. Some I/S phase appearsat 1.2 nm, principally in the fine and very fine clayfractions. Kaolinite is present principally in the coarseclay fraction.

On the scale of the study area, all the soil horizonsand particularly the clayey horizons consist mainly ofsmectite. The relative intensity of the 1.4-nm peak in-creases from the coarse to the very fine clay fractions.The surface horizons contain a micaceous mineral princi-pally in the coarser clay fractions; mica amounts decreasewith depth and are absent in the lutite horizon, exceptin the detrital conglomerate beds (Profile J). Mixed-layerI/S minerals are prominent in the coarse and fine claysize fractions of the B horizons, and occur in the lutitefacies. Trace amounts of palygorskite were identified inthe B horizons. Quartz and calcite are present particularlyin the coarse clay fraction.

Chemical Composition of ClayThe chemical compositions of some fine and very fine

clay fractions are given in Table 2. All the samples showa similar composition typical of smectites, with MgOcontents about 4 to 5%. The CaO content varies withcalcite concentrations. The relatively high K^O contentsindicate the presence of micaceous clay, particularly inthe coarse clay size fractions.

Structural formulae (Foster, 1961) were calculated formonomineral samples (from the XRD data) of the lutiteclay (1-0.2 nm, Cl and C2 horizons of Profiles I andM, respectively) and are shown on Fig. 5. The formulaewere characterized by high Al^ and A1IV levels, andcorrelatively the Mg level was relatively low. The totalcharge was artificially high, because a part of the Cafrom calcite was integrated into the formulae as an ex-changeable cation.

The soil clay formulae are theoretical because thedifferent clay size fractions are mixtures. However, de-spite the presence of depleted Mg clays (illite and mixed-layer I/S), the Mg contents increase from the parentmaterial to the clayey B horizons (Table 2). A linearregression exists between Al^ and Mg^ contents fromthe formulae: KMg = 1.532 - 0.848 XM, r2 = 0.68.Octahedral Mg is correlated negatively with octahedralAl, showing a progressive Al to Mg substitution from thelutite clay to the clayey B horizons. From a mineralogicalviewpoint, it expresses the progressive destabilization(solubilization) of the lutite clay and the recrystallizationof clays richer in Mg.

Fourier Transform Infrared SpectraAll IR spectra were studied in detail to observe subtle

but significant variations in band intensity. We discuss

0.7

DUCLOUX ET AL.: CLAY DIFFERENTIATION IN ARIDISOLS 273

Table 2. Chemical composition of clay size fractions.

Profile Horizon Fractiont SiOi TiO2 A1203 Fe203 MnO MgO CaO Na2O

t b = 0.2-0. l-(im (fine) clay fraction; c = <0.1-(im (very fine) clay fraction.t nd = not determined.

K20 H20

c

M

I

J

AB21B22

B3C2A

B21B22C2ABClABCl

bcbcccbbcbbbbbbb

49.448.147.948.748.348.450.248.051.147.750.250.149.048.350.646.0

0.80.60.50.20.30.30.60.80.50.40.50.50.50.50.50.7

16.616.916.315.916.016.217.314.616.518.317.016.516.118.019.019.4

5.75.55.75.45.45.96.25.15.35.26.05.45.65.86.25.8

m

0.10.10.10.10.10.10.10.10.10.00.20.10.10.20.10.1

4.94.84.75.04.94.54.53.64.23.74.33.84.24.04.33.6

1.71.73.02.12.31.92.27.31.92.11.93.53.41.92.42.0

0.30.20.30.20.10.10.20.30.20.70.20.10.30.40.10.2

3.02.62.72.32.32.43.22.42.22.23.02.51.83.54.03.9

17.619.518.219.019.919.518.020.020.520.418.520.0nd|18.9nd

18.5

only one series of spectra corresponding to the M soilprofile. The prominent IR bands are characteristic ofsmectites. Some samples contain calcite and/or quartz.In the OH stretching region (Fig. 6a), the spectra showan intense absorption band near 3625 cm"1 that does notshift significantly in shape or position for all the samples.It occurs in the frequency range of A12OH vibrations insmectites (Farmer, 1974). The broad band centered at3420 cm'1 results from interlayer water (Farmer, 1974).A slight shoulder at 3697 cm"1 results from the occur-rence of small quantities of kaolinite (Farmer and Russell,1964) as evidenced by XRD. In the OH bending region(Fig. 6b), the 916 cm"1 band is assigned to A12OH

4.5 2.5

Fig. 5. Palygorskite (P)-smectite (S) stability fields. Small black circlesare surface horizons; asterisks are clayey B horizons; stars arecambic and lutite horizons.

3800

AhOH3625

3600 3400

1200 1000 800 600Wavenumber (cm-1)

B21 horizon

C2 horizon

3200

B21 horizon

C2 horizon

400

Fig. 6. Infrared spectra in (a) the OH-stretching region and (b) theOH-bending region of Profile M, very fine clay size fractions.

274 SOIL SCI. SOC. AM. J., VOL. 59, JANUARY-FEBRUARY 1995

vibrations (Farmer, 1974). Another significant OH vibra-tion band is at 839 cm"1, which is attributed to AlMgOH(Russell et al., 1970; Farmer, 1974). On some spectra,a sharp band at 872 cm~' is attributed to calcite (Gadsden,1975); when occurring as a broad shoulder, this lastband could be assigned to AlFeOH vibrations (Russellet al., 1970; Cracium, 1984). Quartz is responsible forthe small doublet located at 800 and 780 cirr1. Fromthe lutite toward the upper part of the soil profile, the839/916 band intensity ratio increases. This agrees withthe chemical data, which gave evidence for Mg enrich-ment of the soil samples.

DISCUSSIONField and Soil Structure Data

The landscape of the study area (Fig. 2) is slightlyconcave and indicates a monogenic origin. The soilsoccur on the same geomorphic surface that Delhoume(1988) and Breimer (1988) attribute to the last Pluvial.The soil mapping showed that Orthids are transitionalto Argids, which are always located proximal to theuplands of the lomas. It would be difficult to acceptdifferent ages for Orthids and Argids developed on thesame geomorphic surface. Consequently, the soils moreprobably developed during the Holocene. This hypothesisis consistent with the results of Nettleton and Peterson(1983), which indicate that cambic horizons have formedin moderately calcareous parent materials in New Mexicoduring the last 2200 to 5600 yr, and those of Gile (1975),which indicate that argillic horizons in the same areaare about 7500 to 10000 yr old.

The spatial distribution of B horizons showed that theyare thick in the Hilaria zones and thinner in the drainagezones (Fig. 3), without the thinning being related to adownward slope in topography. The thinning is not in-duced by erosion effects, but more certainly by internalprocesses such as clay illuviation or clay neogenesis.Moreover, the clayey B horizons showed a decrease ofvery fine clay compared with the fine clay. This evolutioncan partly be due to crystal growth of clay developingin the poorly drained Hilaria zones where abundant runoffwater accumulates. Moreover, carbonates are present inall the soil horizons in spite of a weak leaching inthe A horizons, and optical microscopy showed detritalcalcite grains with corroded and dissolved surfaces. Thelutite microstructure is still present in the B horizons.Therefore, the soils appear pedologically young, possiblybeing formed during the Holocene.

Origin of Clay MineralsPalygorskite and Smectite

Palygorskite has been described in various environ-ments, such as hydrothermal (Furbish and Sands, 1976),marine, lagoonal, playa (Parry and Reeves, 1968), cal-crete (Van den Heuvel, 1966; Gardner, 1972; Goudie,1972; Watts, 1980) and in many arid-zone soils (Singerand Norrish, 1974; Bachman and Machette, 1977; Yaa-lon and Wieder, 1976). One of the hypotheses concerningclays and particularly the origin of palygorskite is eolian

dust (Gile, 1970; Yaalon and Ganor, 1973). Wind trans-fer in the Mapimi area is currently important, and paly-gorskite might come from the playa. However, this min-eral was not detected in either surface or C horizons,but only in the B horizons, signifying that palygorskiteis probably not detrital in origin.

The other hypothesis for palygorskite in B horizonsis neogenesis. Singer (1989) noted palygorskite in soilsaffected by fluctuating groundwater, with pH between 7.5and 8. Palygorskite is considered an authigenic mineralformed either by alteration of montmorillonite (Yaalonand Wieder, 1976) or by neoformation (Millot et al.,1977) in calcareous environments. Singer (1989) notedthat the frequent association of montmorillonite and paly-gorskite is "a result of the proximity in their stability fields,but does not necessarily imply a solid-phase transformationof one into another." The plotting of the chemistry ofwater-extract cations on the stability diagram of palygor-skite-smectite (Fig. 5) (Elprince et al., 1979) confirmedSinger's observation. Activity coefficients y of Mg2+

were calculated with the Debye-Hiickel formula andwere between 0.7 and 0.8. The water extracts from theclayey B horizons and from lutite are located near thestability limit for smectite and palygorskite (Fig. 5),signifying that they may be currently forming in theMapimi soils. According to the current hydric variationsin the Chihuahuan Desert inducing small variations insoil water chemistry, it would be possible to precipitatealternatively palygorskite or partially magnesian smec-tite. Presently, we feel that in Mapimi soils palygorskiteformation is not necessarily related to a calcificationstage as indicated by Yaalon and Wieder (1976), butcan occur in a moderate calcareous material or perhapsduring a decalcification stage (Nahon and Ruellan, 1975).

Illite and Illite/Smectite MineralsAn illitic mineral has been detected in the coarser clay

fractions, principally in surface horizons, subordinatelyin the upper clayey horizons, and sometimes hi sedimen-tary conglomerate beds. The work of Nettleton et al.(1973) and the observations of Mahjoory (1975) de-scribed the formation of micas hi surface horizons ofdryland soils in environments similar to the Mapimi site.However, micas coarser than the smectite clays couldindicate a possible detrital origin by eolian transfer fromrhyolitic bodies located 2 to 3 km southeast of the studyarea.

Mixed-layer I/S were also observed in the clayeyhorizons. The presence of I/S interlayers presupposesrelationships between the two minerals, and two forma-tion routes may exist, either from smectite or from illite.The experimental extraction of K from mica mixed layersmay produce interstratification of expanded or collapsedlayers, depending on treatments (Shawney and Reynolds,1985). Also, I/S layers should result from interstratifica-tions of small particles (Nadeau et al., 1985) or smectite-to-illite transformation by K+ fixation (Nettleton et al.,1973). The presence of I/S principally in the fine andcoarse clay fractions contributed to the acceptance ofthe hypothesis of Shawney and Reynolds (1985).

DUCLOUX ET AL.: CLAY DIFFERENTIATION IN ARIDISOLS 275

The occurrence of I/S mixed layers in small quantitiesin the lutite facies could not explain their increasedabundance in the clayey horizons. In fact, we can considerthat detrital illite particles of the clayey horizons wereless weathered than those of A horizons due to K+

extraction.Moreover, experimental studies (Mamy and Gauthier,

1976; Eberl, 1984; Andreoli et al., 1989) have shownthe influence of wetting and drying cycles on smectite-illite transformations. The severe climatic fluctuationsthat exist in northeastern Mexico are important aspects ofsoil moisture and temperature regimes and may contributeto the I/S formation from smectite. On the other hand,Jones (1983) has shown that the formation of I/S intergradesin preexisting smectite seems to compete with the neofor-mation of fibrous clays. This fact could explain the smallamounts of palygorskite in the Mapimi soils althoughtheir geochemical conditions of formation are joinedtogether.

Lutite Smectite and Soil SmectiteThe clays of the lutite facies are composed of dioctahe-

dral smectite formed in lagoonal environments accompa-nied by gypsum crystallization (Bartolino, 1988), andcharacterized by tetrahedral Al-Si substitutions (beidel-lite like). In the octahedral composition and relativetetrahedral charge diagram of smectite (Borchardt,1989), they are at the limits of montmorillonite-beidellitefields (Fig. 7). The clay plots near the middle of themontmorillonite field strongly indicate a relative MgVI

enrichment and a constant octahedral Fe content. Thiscan be interpretated as the transformation of the lutiteclay, and the neoformation of a new kind of smectite inthe B horizons with a trioctahedral tendency (montmoril-lonite like). Borchardt and Hill (1985) considered thatmontmorillonite can crystallize from solution if the pHis >6.7. In the water extracts, the pH was «7.8 to 8.4.Montmorillonite might be expected to form pedologicallyfrom solutions rich in Si, Al, and Mg (Nettleton andBrasher, 1983).

In the (4S1-M+-3R2) diagram (Meunier et al., 1991)where M+ = Na+ + K+ + 2Ca2+, 4Si = Si/4, and3R2 = (Mg + Fe2+ + Mn)/3, the clay plots are locatedbetween the di- and trioctahedral areas, indicating com-positions of either an intermediate phase or a mixtureof di- and trioctahedral smectites. The FTIR data argueclearly for the intermediate phase. The FTIR patternsshow a relative increase of the AlMgOH absorption bandwith the increase of Mg, but did not reveal the appearanceof a MgsOH vibration band typical of trioctahedral clays.This tendency to a "saponitization" is consistent with theobservations of Droste (1961) and Papke (1970), whodescribed the appearance of saponite in playa soils andsediments in environments similar to those of MapimiReserve.

CONCLUSIONSMapimi Aridisols are distributed along a topographical

sequence controlled by current transfer and accumulationof water. Associated with the same Holocene geomorphic

0.9

uu-+ 0.8

|| O.T

0.6-

'%***

0.1 0.2 0.3 0.4Aliv/Aliv + Mgv

Fig. 7. Location of the samples in the octahedral composition andrelative tetrahedral charge diagram. The small square correspondsto the whole diagram (M = montmorillonite; B = beidellite; N =nontronite). The arrow indicates the evolutionary pathway fromthe lutite to the horizon clays.

surface, Orthids develop on the runoff zones, and Argidsappear in the flat zones. The clay enrichment of Argidsis due to the increase of the fine size clay fractionsof additional smectite, probably formed from dissolvedsedimentary smectite. Soil smectite is richer in Mg thansedimentary smectite. Indeed, the geochemical condi-tions must be sufficiently different from the sedimentaryconditions to provoke the dissolution of the lutite clayand the crystallization of a MgAl clay with small amountsof palygorskite. The soil water extracts support the possi-bility of the pedogenic processes. Even in the event of aclay translocation—past or present—in spite of pedogenicenvironmental conditions unfavorable to this process,Argids of the Mapimi Reserve result principally fromclay neogenesis under current climatic conditions.

276 SOIL SCI. SOC. AM. J., VOL. 59, JANUARY-FEBRUARY 1995