DIVISION S-9—SOIL MINERALOGY

Mineralogy of Aridisols on Dissected Alluvial Fans,Western Mojave Desert, CaliforniaMostafa K. Eghbal* and Randal J. Southard

ABSTRACTThere are relatively few well-documented reports about the min-

eralogy of Aridisols. A Durorthid and two Haplargids were sampledin the Mojave Desert, California. The soils formed in alluvial depositsestimated to be >300 000, 200 000, and 100 000 yr old, respectively,and were examined for factors influencing variation in mineralogy ofthe clay, silt, and sand fractions. Smectite is the major constituent ofthe clay fraction in all three soils, whereas mica and kaolinite are lessabundant. A broad 1.55-nm d spacing was most common in the Mg-saturated clay samples, indicating a low-charge and low-crystallinitysmectite. The clay mineralogy of a buried soil underlying all threesoils is also dominated by smectite, with minor amounts of kaoliniteand mica. The similarity in clay mineralogy of these different-agedsoils suggests that, even during wetter climates of the Pleistocene,leaching did not remove enough silica to create an unstable environ-ment for smectite. The fine and medium silt fractions in all three soilsare composed of quartz, feldspar, smectite, mica, kaolinite, and ver-miculite. Treatment of the silt fraction with NaOH, which dissolvesnoncrystalline and short-range-ordered silica and aluminosilicates, re-leased considerable smectite. This suggests that the smectite is com-posed of pedogenic microagglomerates and is not truly silt sized. AMg-saturated x-ray diffraction maximum at about 1.9 nm, which de-creased to about 1.6 nm after the NaOH treatment, was observed insome of the silt samples. The presence of the 1.9-nm peak is probablya result of noncrystalline aluminosilicates in the interlayers of smec-tite, rather than a result of interstratification of mica-smectite.

SOILS IN ARID REGIONS generally have smectite asthe dominant soil clay mineral or have enough

smectite to dominate their properties (Alien and Fan-ning, 1983; Nettleton and Brasher, 1983; Gile andGrossman, 1979; Borchardt, 1989; Buol, 1965; Galet al., 1974). The clay fraction of soils with duripansand high amorphous silica content are also typicallydominated by smectite (Boettinger, 1988; Flach et al.,1974; Torrent et al., 1980). Buol (1965) reported thatmontmorillonite does not weather to kaolinite or toother more resistant minerals in soils as long as gyp-sum and calcite remain in the soil profile. The lowrainfall in arid regions may not fully leach silica, cre-ating an environment likely to preserve smectite andmay actually aid in its formation (Gal et al., 1974).

Smectites generally occur in the clay fraction, oftenas fine clay (Borchardt, 1989). Smectites have alsobeen reported in the sand and silt fraction in soils withhydrothermally altered parent material (Graham et al.,1988; Curtin and Smillie, 1981). Boettinger (1988)found sand- and silt-sized smectites in Durorthids and

M.K. Eghbal, College of Agriculture, Isfahan Univ. .of Technol-ogy, Isfahan, Iran; and R.J. Southard, Soil and Biogebchemistry,Univ. of California, Davis, CA 95616. Contribution from theDep. of Land, Air and Water Resources, Univ. of California,Davis. Received 13 Mar. 1992. *Corresponding author.

Published in Soil Sci. Soc. Am. J. 57:538-544 (1993).

Haplargids on granitic pediments in the western Mo-jave Desert. The smectites were single crystals, notaggregates of clay-sized particles, and apparently notof hydrothermal origin.

Durorthids and Haplargids occur in deposits of dif-ferent ages on dissected alluvial fans in the westernMojave Desert (Eghbal and Southard, 1993a). Thenoncrystalline and short-range-ordered silica and al-uminosilicates in duripans and duric horizons of thesesoils cement clay-sized particles together, forming mi-croagglomerates (Eghbal and Southard, 1993b). Thepurposes of this study were to investigate the differ-ences in the mineralogy of the Aridisols forming indeposits of different ages and to evaluate the effect ofthe noncrystalline and short-range-ordered silica andaluminosilicates on the mineralogy of the sand and siltfraction.

MATERIALS AND METHODSStudy Site

The study area is in the western Mojave Desert (35°10'N,117°55'W), approximately 100 km north of Los Angeles and20 km northeast of Mojave, on a dissected alluvial fan. Thesediments of the alluvial fan are derived from the Mesozoicgranitic rocks of the Rand Mountains. The climate is warmand arid with a mean annual air temperature of 18 °C (Valverdeand Hill, 1981). Mean annual precipitation, falling betweenNovember and March, is estimated at 110 mm, based on datafrom stations in nearby Mojave and Cantil (Valverde and Hill,1981; National Oceanic and Atmospheric Administration, 1982).Vegetation in the study area consists mostly of creosote bush[Larrea tridentata (Sesse & Mocino ex DC.) Cov.] and fid-dleneck (Amsinckia tessellata).

A trench, 40 m long and 3 m deep, exposed three soils, aloamy, mixed, thermic, shallow Typic Durorthid (Alko series)and two fine-loamy, mixed, thermic Typic Haplargids (Neur-alia and Garlock series). These soils occur in alluvial depositsestimated to be >300 000, 200 000, and 100 000 yr old, re-spectively (Eghbal and Southard, 1993a). A buried soil,>783 000 yr old, underlies the three soils at a depth of 1.5 to2 m. The Alko soil occurs on the shoulder slope and has acalcareous duripan with a laminar cap at a depth of 10 cm andanother less calcareous duripan with no laminar cap at a depthof 40 cm (Table 1). The Neuralia soil, on the side slope, iscalcareous throughout and the Garlock soil, on the toe slope,is noncalcareous to a depth of 65 cm (Table 1). The landsurface on which these soils occur has a 3% slope to the north.

Laboratory StudyThe soil samples from each horizon were air dried, crushed,

and sieved to remove the >2-mm fraction. Approximately 30g of < 2-mm fraction were used for particle-size determination(Gee and Bauder, 1986) and fractionation, following removalof organic matter with NaCIO (commercial bleach) and of sol-uble salts and CaCO3 with pH 5.0 buffered NaOAc (Jackson,Abbreviations: XRD, x-ray diffraction; CBD, cirrate-bicarbon-ate-dithionite; TO, turbid grains.

538

EGHBAL & SOUTHARD: MINERALOGY OF ALLUVIAL ARIDISOLS 539

Table 1. Selected morphological, physical, and chemical properties for representative horizons of the three soils exposed in theexcavated trench.

Depthcm

0-11-10

Lam. Cap§10-25§40-100

190-215

0-11-10

30-65§30-65§80-115

145-160

0-11-6

50-65135-150150-180

HorizonColor

(moist) Texturet Gravel^or.

Clay SiO2__ „

A12031,_-1

SKVA1A——————————————————— /f ————————————— '" "' —— •• ^ —— 0

Alko, Typic DurorthidAlA2

BqkmBqk

BtqkmlBtqkbl

AlA2BkBtk

Btqk2Btqkbl

AlA2Bt

Btqk2Btqkbl

10YR5/610YR5/410YR8/210YR5/6

7.5YR4/67.5YR4/4

10YR5/410YR5/410YR5/6

7.5YR5/67.5YR4/47.5YR5/4

10YR4/410YR4/45YR4/4

7.5YR5/67.5YR5/4

grcoslsi—

coslgrcosl

sclNeuralia, Typic

grcoslsisiscl

grlcosscl

Garlock, Typicgrlcos

siscl

vgrslcosl

155

——20—

Haplargid305

1310338

Haplargid153

—503

1122—134

29

111117211120

1013241612

14.421.637.936.428.034.8

12.514.612.915.416.930.9

10.211.318.941.146.4

6.310.94.09.99.2

16.3

6.38.07.58.98.3

13.7

6.27.6

15.213.912.4

2.32.09.53.73.02.1

2.01.81.71.72.02.2

1.61.51.23.03.7

t cosl = coarse sandy loam, grcosl = gravelly coarse sandy loam, grlcos = gravelly loamy coarse sand, scl = sandy clay loam, si = sandy loam,vgrsl = very gravelly sandy loam.

t Field-estimated volume.§ Each portion of the Bqkm/Bqk and Bk/Btk horizons was sampled separately. Lam. Cap = upper laminar portion of the duripan.

1979). Clay samples were saturated with Mg and K and sol-vated with glycerol on porous ceramic tiles (Whittig and Al-lardice, 1986) prior to XRD analysis. Silt samples were crushedfiner than 0.18 mm and mounted on ceramic tiles for XRDanalysis. Silt samples containing phyllosilicates were treatedfor 2.5 min in boiling 0.5 M NaOH as described by Jacksonet al. (1986) to remove the noncrystalline and short-range-ordered silica and aluminosilicates. The NaOH-treated sampleswere analyzed by XRD before and after separating the clay-sized particles. The XRD analyses were made with a DianoXRD 8000 x-ray diffractometer (Diano Corp., Woburn, MA)producing Cu-Ka radiation at 50 kV and 15 mA. Simulationsof the XRD patterns were performed using the NEWMODprogram (Reynolds, 1985). Some of the chosen parameters forthis simulation were: Mg for the saturation cation, two layersof glycerol, dioctahedral smectite and trioctahedral mica. Var-ious concentrations of Fe and K were used, but the locationof the major peaks did not change significantly. To estimatethe degree of cementation in some horizons, particle-size dis-tribution was determined after the removal of cementation bya modified NaOH method: 100 mL of boiling 0.5 M NaOHwere poured on a 10-g sample in a centrifuge bottle and heatedin boiling water for 5 min; the supernatant was discarded aftercentrifugation, and the treatment was repeated twice. The soilswere separated into sand (2-0.05 mm), silt (50-2 /wn) andclay (<2 /u.m) by sieving, sedimentation, and centrifugationusing the methods outlined by Jackson (1979). Grains of thevery fine sand fraction from selected horizons were immersedin refractive index oil (n = 1.544) and analyzed using a pe-trographic microscope. To quantify noncrystalline and short-range-ordered aluminosilicates, oven-dried samples were groundto pass an 80-mesh sieve (<0.18 mm) and treated first withCBD, then with 0.5 M NaOH for 2.5 min (Jackson et al.,1986). The alkaline extracts were analyzed for Si and Al withan ARL-3510 ICP spectrometer (Applied Research Laborato-ries, Sunland, CA).

RESULTS AND DISCUSSIONClay Mineralogy

The clay-mineral assemblage of the Alko, Neuralia,and Garlock soils in the study area is dominated by smec-

1.5 nm

10 14

2 ThetaFig. 1. The x-ray diffraction patterns of the <2-jun fraction

of the A2 horizon from the Neuralia soil: (a) Mg saturated,(b) K saturated, (c) Mg-glycerol solvated, and (d) K-saturated, 550 °C.

tite, with minor amounts of mica and kaolinite. Figure1 is a representative XRD pattern obtained for the clayfraction of these soils. The Mg-saturated smectite dif-fraction maximum is at approximately 1.5 nm and isquite broad (Fig. la). The higher than usual spacing andbroad peak may indicate the presence of fairly low-chargeand low-crystallinity smectite. At 25 °C with K satura-

540 SOIL SCI. SOC. AM. J., VOL. 57, MARCH-APRIL 1993

tion (Fig. Ib), the high background on the > 1.3-nm sidemay indicate the presence of small amounts of hydroxy-interlayered vermiculite or smectite. Hydroxy-interlay-ered 2:1 minerals do not occur in the clay fraction of thelower horizons of these soils. The 1.5-nm peak shiftedto a series of poorly defined peaks >1.7 nm followingglycerol treatment (Fig. Ic), and is a further indicationof low charge and low crystallinity of the smectite. All2:1 clay minerals collapsed to 1.0 nm when heated to550 °C (Fig. Id).

The clay mineralogy of the three soils in the studyarea is similar (Fig. 2), even though the soils span anage range estimated to be from 100 000 to >300 000yr. The lack of variation in clay mineralogy suggeststhat, even during wetter periods of the Pleistocene,leaching did not occur to the extent of removing enoughsilica to favor kaolinite stability.

There is no difference between the clay mineralogy ofthe buried soil (XRD patterns not shown) and the threesoils that overlie it. The dominant clay mineral in theburied argillic is smectite, with minor amounts of micaand kaolinite. This may indicate that the weathering re-gime intensity and duration, when this soil was at thesurface, was similar to the weathering regime during thelate Pleistocene and Holocene.

The similarity of the mineralogy among these soilsmay also suggest that they were formed during "soil-forming intervals" (Morrison, 1967), when temperatureand precipitation were optimum for soil formation. Mor-rison (1967) characterized these intervals by unique cli-matic conditions resulting in accelerated soil formationover a relatively short time span. The climates betweenthe soil-forming intervals were too dry and apparently

1.5 nm

0.72

14

2 ThetaFig. 2. The x-ray diffraction patterns of the < 2-jum fraction,

Mg saturated, of the (a) Btqk horizon (25-40 cm) of theAlko soil, (b) Bk horizon (30-65 cm) of the Neuralia soil,and (c) the Bt horizon (50-65 cm) of the Garlock soil.

not conducive to significant soil formation. Morrison(1964) found soils on the same deposit in the CarsonDesert, Nevada, that had nearly identical profile mor-phology and clay mineralogy regardless of whether theywere in a buried or relict position. The clay mineralogyof the soils in the study area may be interpreted to implythat soil-forming intensity during the intervals was thesame. Although the soil-forming interval concept mayexplain the similarity of the clay mineralogy, the mor-phological differences among the three profiles cannotbe explained this way. Birkeland and Shroba (1974) be-lieved that sufficient soil and stratigraphic evidence ex-ists to suggest that the concept of soil-forming intervalscannot be applied to many soils in the western USA.

The biotite in the clay fraction is probably derivedfrom comminution of inherited biotite in the silt and sandfractions. The mode of kaolinite formation, however, isnot clear. Kaolinite could be inherited from the parentmaterial. Geomorphic studies of an alluvial fan in thestudy area suggests that sediments are transported fromthe granitic pediments to the east. Boettinger (1988),however, found no kaolinite in the clay, and only smallamounts in the silt fractions of soils forming on graniticpediments, approximately 35 km south of our study area.The minor amounts of kaolinite in the soils of this studycould also be relict from localized feldspar weatheringduring wetter Pleistocene climates. The possibility of aneolian addition of kaolinite cannot be completely dis-counted, although a local source for kaolinite is not ob-vious in this environment.

Fine and Medium SiltThe fine and medium silt fractions (2-20 /j.m) of the

three soils are composed of quartz, feldspar, mica, ka-olinite, smectite, vermiculite, and hydroxy-interlayeredvermiculite or smectite (Table 2). Data in Table 2 arebased on interpretation of x-ray diffractograms like thosein Fig. 3. Some of the fine and medium silt samplesshowed an XRD maximum around 1.9 nm (Fig. 3a) thatexpanded to 2.1 nm following glycerol solvation (Fig.3b). The 1.9-nm peak collapsed only to 1.4 nm after Ksaturation (Fig. 3c). The nature and mode of formationof the 1.9-nm compound is not clear, but its presencemay be explained by (i) interstratification of mica andsmectite, or (ii) interlayering of smectite by noncrystal-line aluminosilicates.

Modeling the diffraction patterns with NEWMOD(Reynolds, 1985) suggested that the 1.9-nm materialconsists of a regular interstratification of one mica layerand two smectite layers. However, illite-smectite inter-grades with >40% smectite are almost always randomly,not regularly, interstratified (Reynolds and Hower, 1970;Inoue and Utada, 1983). Moreover, our data indicate thatNaOH treatment alters the diffraction characteristics ofthis compound (Fig. 4a), which would be unlikely if itwere interstratified mica-smectite.

A second possible explanation for the presence of the1.9-nm peak is the interlayering of smectite with non-crystalline aluminosilicate compounds. The buried Btqkhorizons in all three soils show a very distinct 1.9-nmpeak in the Mg-treated samples (Fig. 3a). Aluminumextracted by 0.5 M NaOH increases in the buried Btqkhorizons (Table 1). Other horizons with high extractedAl also have an XRD peak :>1.8 nm (Table 2). The Al

EGHBAL & SOUTHARD: MINERALOGY OF ALLUVIAL ARIDISOLS 541

Table 2. Mineralogical data for silt fractions of selected horizons of the Alko, Neuralia, and Garlock soils.f

Depth

1-1040-100

170-190190-215230-270

1-1030-65

1-650-65

135-150135-150§150-180150-180§150-18011

Horizon

A2Btqkm

BCBtqkblBtqkb3

A2Bk

A2Bt

Btqk2Btqk2BtqkblBtqkblBtqkbl

FinePrimaryminerals

Q, F, M, AQ,F,MQ,F,MQ, F, MQ, F, M

Q, F, MQ, F, M

Q, F, M, AQ,F,M

Q, F, M, A

and medium silt (2-20 ftm)Secondaryminerals

Alko, Typic DurothidSt, K, V, HIVSSt, K, V, fflVSS, K, V, HIVSSt, K, V, HIVSS, K, V, HIVS

Neuralia. Typic HaplargidSt, K, V, HIVSSt, K, V, HIVS

Garlock, Typic HaplargidS, K, V, HIVS

S.K.VSt, K.V

Q, F, M, A S, K, VQ, F, MQ, F, MQ, F, M

St, K, V, M-V, HIVSV, M-V, K, S, HIVS

S, K, V, M-V

CoarsePrimaryminerals

Q, F, M, AQ, F, AQ, F, A

Q, F, A, MQ, F, A, M

Q, F, A, MQ, F,A

Q, F, A, MQ, F,A

Q, F, A, MQ, F, A

Q, F, A, Mnana

silt (20-50 fim)Secondaryminerals

V, K————

_—

V,K—

St, K, V, HIVS—

S, K,Vnana

t Q = quartz, F = feldspar, M = mica, A = amphibole, S = smectite, K = kaolinite, V = vermiculite, HIVS = hydroxy-interlayered smectiteor vermiculite, M-V = 1:1 mica-vermiculite, na = not analyzed.

t Smectites with >1.8-nm d spacing.§ Samples treated with NaOH and then fractionated.II Samples treated with NaOH but not fractionated.

1.9 nm 1.6 nm

14

2 ThetaFig. 3. The x-ray diffraction patterns of the 2- to 20-ju.m fraction

of the buried Bt horizon (150-180 cm) from the Garlocksoil: (a) Mg saturated, (b) Mg-glycerol solvated, and (c) Ksaturated.

2 ThetaFig. 4. The x-ray diffraction patterns of the (a) Mg-saturated

and (b) Mg-glycerol-solvated 2- to 20-/im fraction of theburied Bt horizon (150-180 cm) from the Garlock soil afterthe NaOH treatment.

extracted by NaOH is generally higher in soils of thisstudy than those reported by Boettinger (1988) andChadwick et al. (1987) for duripans and duric horizons.The treatment of the fine and medium silt fraction withNaOH (without separating the clay-size material) re-duced the 1.9-nm spacing of the Mg-saturated sample to1.6 nm (Fig. 4a). The 1.6-nm spacing probably belongsto smectite, because it increased to 1.9 nm after glycerol

solvation (Fig. 4b). The shift to the 1.6-nm spacing afterNaOH treatment may be a result of the removal of non-crystalline and short-range-ordered silica and aluminos-ilicate compounds. One possible explanation is that thesecompounds occur in the interlayers, which could in-crease the spacing to 1.9 or 2.0 nm, but not affect swell-ing. Upon removal of these compounds the spacingdecreased to 1.6 nm with Mg saturation. The removal

542 SOIL SCI. SOC. AM. J., VOL. 57, MARCH-APRIL 1993

of the aluminosilicate may be incomplete, as is indicatedby the higher than usual 1.6-rim spacing.

Rich (1968) described the optimum conditions for Alinterlayer formation in expansible phyllosilicates as:moderate pH (4.6-5.8), frequent wetting and dryingcycles, and low organic matter content. Interlayeredsmectites are more often reported in Ultisols and Alfi-sols. There is relatively less information available oninterlayer formation under alkaline condition. Whittig(1959) reported the presence of a vermiculite-chloriteintergrade in a solodized-solonetz soil with pH of 7.7.The structural composition of the Al-interlayer materials,especially in neutral-to-alkaline conditions, is not known.Barnhisel and Rich (1965), however, reported that thebayerite structure is formed in an alkaline medium,whereas gibbsite is favored in an acidic medium. Theyworked with a pure Al system, not mixed with Si. Insoils of this study, solution levels of silicic acid are toohigh to allow pure alumina compounds to form.

The x-ray analysis of the fine and medium silt frac-tions, after the 2.5-min treatment with 0.5 M boilingNaOH and separation of the clay-size material, showsthat most of the smectite was removed, as indicated bythe loss of the 1.9-nm peak (Fig. 5a). The measured claycontent of this horizon also increased from 12 to 40%when the < 2-mm fraction was treated with NaOH (Table3). The clay minerals in the fine and medium silt frac-tions may be a constituent of particles similar to mi-croagglomerates described by Chadwick et al. (1989)and Southard et al. (1990), rather than true silt-sizedsmectites, as proposed by Boettinger (1988). Somesmectite still remains in the silt fraction, as indicated by

1.01 nm

10 14

2 ThetaFig. 5. The x-ray diffraction patterns of the 2- to 20-/un fraction

of the buried Bt horizon (150-180 cm) from the Garlocksoil after the NaOH treatment: (a) Mg saturated, (b) Mg-glycerol solvated, and (c) K saturated. The clay-sized particleswere separated from the sample after the treatment.

the broad background centered around 1.8 nm with glyc-erol solvation (Fig. 5b). The 2.5-min treatment withboiling NaOH may not have been sufficient to removeall the cement, and some microagglomerates may haveremained after the treatment. The presence of silt-sizedsmectite, however, cannot be ruled out completely byour data.

With the absence of the dominant 1.9-nm smectitepeak after the NaOH treatment, other peaks of 2.5-,1.8-, 1.4, 1.23-, and 1.01-nm spacing became more ob-vious (Fig. 5a). Removal of the noncrystalline and short-range-ordered silica and aluminosilicates reveals thepresence of vermiculite as indicated by a 1.4-nm peak(Fig. 5a) that collapsed with K treatment (Fig. 5c). The2.5- and 1.23-nm peaks (Fig. 5a) may indicate the pres-ence of a vermiculite-mica interstratification. The cal-culated XRD pattern (NEWMOD, Reynolds, 1985)suggests a 1:1 ordered interstratification of vermiculite-mica.

Coarse SiltMajor components of the coarse silt fraction (20-50

ju.m) in all three soils are quartz, albite, and hornblende(Table 2, Fig. 6a). Mica is present in small amounts ina few of the samples. In addition to quartz (0.334 and0.427 nm) and feldspars (0.32 nm), the coarse silt frac-tion of the Btqk and Btqkbl horizons of Garlock alsocontains smectite, kaolinite, and vermiculite (Table 2,Fig. 6b). The presence of smectite in the coarse silt frac-tion is shown by the 1.8-nm peak of the Mg-saturatedsample (Fig. 6b), and expansion to a broad peak centeredaround 2.1 nm following glycerol solvation (Fig. 6c).Treatment of the Garlock Btqk horizon (135-150 cm)whole soil material with boiling 0.5 M NaOH, followedby fractionation, produced a coarse silt fraction with nosmectite (Fig. 6d). The clay content of this horizon in-creased from 16.1 to 31.8%, and silt content decreasedfrom 7.4 to 1.7% after the NaOH treatment (Table 3).The smectite in the coarse silt fraction most likely occursin silica-cemented microagglomerates.

Sand Grains in the Cemented HorizonsPreliminary field work showed that the Alko, Neur-

alia, and Garlock soils have cemented to partly cementedgravelly horizons in their profiles (Table 1). In the Alkosoil, an indurated horizon (Btqkm) occurs below a highlycalcareous duripan with laminar cap (Bqkm/Bqk). InNeuralia and Garlock soils, the Btqk horizons are onlypartly cemented. Minerals observed in the very fine sandfractions of selected Btqk horizons (Table 4) were pri-marily quartz, albite, biotite, hornblende, and opaqueminerals. Some grains were unidentifiable and were calledTG.

Table 3. Comparison of the particle-size distributions in twocemented horizons of the Garlock soil.

Untreated NaOH treatedDepth Horizon Sand Silt Clay Sand Silt Clay

135-150 BtqkISO-180 Btqkbl

76.571.9

7.416.4

16.111.7

66.551.5

1.78.2

31.840.4

EGHBAL & SOUTHARD: MINERALOGY OF ALLUVIAL ARIDISOLS 543

1.8 nm 0.334

2 ThefaFig. 6. The x-ray diffraction patterns of the 20- to 50-/un

fraction of the (a) Bk horizon (30-65 cm) from the Neuralliasoil and Btqk horizon (135-150 cm) of the Garlock soil (b)Mg saturated and (c) glycerol solvated before NaOHtreatment, and (d) after NaOH treatment.

Treatment of the very fine sand fraction from the Btqkhorizon of the Garlock soil with boiling 0.5 M NaOH(modified method) reduced the TG content from 72 to11% (Table 4). The increase in the clay content anddecrease in the silt and sand content after NaOH treat-ment (Table 3) indicate that the TG are probably mi-croagglomerates, products of silt and clay cementation.Thus, the noncrystalline and short-range-ordered silicaand aluminosilicates play an important role in the sta-bility and formation of the microagglomerates in the veryfine sand fraction. Some grains may be unrecognizabledue to a coating of Fe oxides. The CBD treatment of the

very fine sand fraction from the Btqk horizon in Garlock,however, did not decrease the TG content significantly(Table 4).

The TG content of the very fine sand fraction increasesdownslope toward the Garlock soil as the quartz andfeldspar content decreases. A similar trend in the min-eralogy of the very fine sand was observed in the buriedBt horizon along the trench (Table 4). The increase inthe TG content along the trench is probably due to var-iation in the weathering environment. The second duri-pan (Btqkm) of the Alko soil and horizons below it havebeen practically isolated after the formation of the secondduripan (Eghbal and Southard, 1993b). On the other hand,the Btqk horizons in the Neuralia and Garlock soils arenot indurated and have undergone further weathering sincethe time of formation of the second duripan in the Alko.The cementation of the buried Bt horizon by silica in-creases from the Alko to the Neuralia and Garlock, asindicated by the increase in the NaOH-extractable SiO2and the SiO2/Al2O3 ratio (Table 1). The location of eachsoil on the landscape also influences the weathering en-vironment. The higher silica content of the Garlock soilmay be due, in part, to lateral water flow during wetperiods of the present climatic regime or of previousPleistocene climates.

SUMMARYAlthough the ages of the granitic alluvial deposits in

which the Alko, Neuralia, and Garlock soils formed areestimated to range from 100 000 to >300 000 yr old,the clay mineralogy is similar. Smectite is dominant,with minor amounts of kaolinite and mica. The similarityof the clay mineralogy in these soils may indicate thatthe intensity of soil-forming processes influencing clayformation has been similar. The lack of variation be-tween the clay mineralogy of the buried soil (783 000yr old) and soils that overlie it may also indicate that theclimate at the time when this soil was at the surface wassimilar to climatic conditions during the late Pleistoceneand Holocene.

Smectite occurs in silt fractions mostly as a componentof microagglomerates, but the presence of silt-sizedsmectites cannot be completely ruled out. The microag-glomerates were destroyed when treated with NaOH. Some

Table 4. Mineralogy of the very fine sand fraction of the gravelly Btqk(m) and buried Btqk horizons from the Alko, Neuralia,and Garlock soils.

Sample! Horizons Quartz Feldspar Biotite TGt Amphibole Opaque Glass OthersTotalgrains

cm

1 (100-150)2 (80-115)3 (135-150)3 (135-150)§3 (135-150)11

Cemented horizonsBtqkm2Btqk2Btqk2

21187

368

30325

428

119

153

17

3236721167

Buried horizons

11 = Alko, 2 = Neuralia, 3 = Garlock soils.j TG = turbid grains.§ NaOH-treated sample.H Citrate-bicarbonate-dithionite treated sample.

11022322790568157

1 (190-215)2 (145-160)3 (150-180)

BtqkblBtqkblBtqkbl

282118

281815

859

325154

211

111

020

142

12007931349

544 SOIL SCI. SOC. AM. J., VOL. 57, MARCH-APRIL 1993

Mg-saturated smectites in silt fractions produce a 1.9-nm peak before NaOH treatment and a 1.6-nm peak aftertreatment. The NaOH treatment appears to have two ef-fects: (i) destruction of microagglomerates and releaseof clay-sized material, and (ii) reduction of spacing insmectites from 1.9 to about 1.6 nm. The presence of the1.9-nm peak is probably a result of NaOH-soluble non-crystalline aluminosilicates in the interlayers, rather thana result of mica-smectite interstratification