Genesis of Maryland Soils Formed from Serpentinite1

Genesis of Maryland Soils Formed from Serpentinite1

M. C. RABENHORST, J. E. Foss, AND D. S. FANNING2

ABSTRACTFour pedons developed over serpentinite were characterized. Elu-

vial and illuvial processes have caused formation of weak-to-mod-erately-expressed argillic horizons. Magnesium-rich parent materialsare responsible for the dominance of Mg on the exchange complex,except in surface horizons where H+ prevailed. Also due to the Mg-rich parent materials, extractable Ca/Mg ratios decreased with depthranging from 0.8 to 2.45 at the surface to 0.03 to 0.17 above thebedrock. The pH values increased with depth from 4.9 to 5.6 at thesurface to 6.6 to 6.8 above the bedrock. Total nickel and chromiumconcentrations were between 180 and 18,900 ppm and 400 to 5,850ppm, respectively.

Weathering of serpentine minerals tends toward the formation ofsmectite, which, with vermiculite, dominates the fine (clay) fractions.These are often interstratifled with hydroxyinterlayered minerals.Serpentine minerals, while absent from the <0.2-/un fraction, oc-curred with chlorite and vermiculite in the 2- to 5- and 0.2- to 2-fanfractions. The presence of the silicon-rich minerals, mica, feldspars,and quartz in the upper portions of the pedons indicates that someeolian materials have been added to the serpentinite residuum. Thisis substantiated by elemental analyses of the silt fractions.

Additional Index Words: ultramaflc, Ca/Mg ratios, soil mineralogy,eolian dust, nickel, chromium, soil chemistry.

Rabenhorst, M. C., J. E. Foss, and D. S. Fanning. 1982. Genesisof Maryland soils formed from serpentinite. Soil Sci. Soc. Am. J.46:607-616.

O ERPENTINITES are magnesium-rich, subsilicic rocksO formed primarily through the metamorphic alter-ation of dunite, peridotite, or pyroxenite, and com-posed chiefly of serpentine minerals. The Marylandserpentinite is located in the eastern Piedmont, whereit is commonly associated with mafic and other ultra-mafic rocks. As a group, soils derived from serpen-tinite have been subject to extensive study in relationto their well-documented infertility. Pedological re-search concerning these soils is relatively scant. Wild-man et al. (1968) studied four California soils formedfrom serpentinite, all of which contained argillic ho-rizons. Weathering of the serpentine resulted in for-mation of montmorillonite of fine (<0.2 pirn) clay size,while coarser fractions remained dominated by ser-pentine. In Hokkaido, Japan, Sasaki et al. (1968) foundevidence of weathering and translocation of iron intwo serpentinite-derived soils. Silicate clay translo-cation was not reported, and pedon data indicate thatthese soils were Eutrochrepts. These workers foundthe amount of serpentine in the clay fraction to de-crease with proximity to the soil surface, while a ver-miculite-like, 14A mineral was observed throughoutthe solum. In another study, Nagatsuka (1966) found

1 Contribution no. 5988 and Scientific Article no. A-2930 of theMaryland Agric. Exp. Stn., Dep. of Agronomy, College Park, MD20742. Presented in part before joint meetings S-5, S-9, and S-2,Soil Science Society of America, 4 Dec. 1978. Received 28 May1981. Approved 5 Jan. 1982.2 Research Assistant and Professors of Soil Science, respectively.Senior Author is presently Research Assistant, Dep. of Soil andCrop Sciences, Texas A&M Univ., College Station, TX 77843.

chlorite interstratified with vermiculite in serpentinesoils and suggested the weathering model: antigorite-> 14A expansible -> randomly interstratified chlorite-vermiculite -> chlorite. In addition to finding serpentineand randomly interstratified chlorite-vermiculite intwo serpentine-derived Hapludalfs, Kanno et al. (1965a,1965b) also reported a more highly expansible mineral(presumably smectite) in the clay fraction.

Coombe and Frost (1956) identified three groups ofsoils overlying serpentinite on the Lizard Plateau,England, based on the degree to which extraneous,nonserpentine material had been added. They usedthe silica-rich minerals quartz, feldspar, and mica andsuch accessory minerals of acid rocks as tourmalineand zircon as indicators of the foreign material. Par-ticle-size data suggested eolian transport as the modeof influx. Kanno et al. (1965b) also identified extra-neous acid minerals in Japanese serpentine soils. Up-slope sedimentary rock and volcanic ash were iden-tified as the sources.

The objectives of this study were: (i) to study themorphological, physical, chemical, and mineralogicalproperties of Maryland soils derived from serpentinite,and (ii) to develop a model for the genesis of thesesoils.

MATERIALS AND METHODSField

Utilizing state and county geologic maps (Clark, 1902;Cloos and Cook, 1953; Matthews, 1925; Southwick andOwens, 1968; Weaver, 1968) and county soil survey reportsto locate areas of soils developed over serpentinite, samplesfrom 48 locations were collected using a standard 7.5 cmin diameter bucket auger. Figure 1 shows the location ofreconnaissance sampling sites.

Using peliminary chemical and morphological data col-lected during reconnaissance sampling, four locations wereselected for profile descriptions and detailed sampling.These were chosen to represent the range in location andcharacteristics of soils formed from serpentinite. The lo-cations of these four sites are also shown in Fig. 1. Pedon1 occurs on the Hunting Hill serpentinite body, and Pedon2 occurs on the well known Soldier's Delight serpentinitebody.

At each of the selected sites, pits were opened and de-scriptions were made according to standard procedures.Samples were collected from each of the horizons to thebottom of the pit. In some cases two or more subsamplesof the same horizon were collected. In instances where thebedrock was not encountered within the pit, a bucket augerwas employed to collect samples to the depth where r.ockoccurred.

When possible, rock samples were collected from the baseof the soil profile. When this was not possible, samples fromboulders or outcrops in the immediate sampling vicinitywere collected for analysis.

LaboratorySoil samples were brought to the laboratory and air-dried

at room temperature. Soils were crushed by hand, using awooden mallet and rolling pin, to pass through a 10-mesh

607

608 SOIL SCI. SOC. AM. J . , VOL. 46, 1982

P E N N S Y L V A N I A

Fig. 1—Sampling locations as related to mafic and ultramafic lithology in the study area.

(2 mm) nylon sieve, and fragments >2 mm in diameter wereremoved.

The particle-size distribution was determined on the sam-ples from each of the profiles described. The pipette method,as described by Kilmer and Alexander (1949) and Day(1965), was employed in the determination of the silt andclay fractions. Sand fractions were determined gravimetri-cally after sieving. All samples collected were sent to theUniversity of Maryland Soil Testing Laboratory for deter-mination of extractable Ca and Mg using a double-acid(0.05N HC1 and 0.025N H2SO4) extraction procedure (Ban-del and Rivard, 1975).

The pH of all samples collected was determined on a 1:1soil/distilled water mixture using a Beckman pH me-ter equipped with a glass electrode. Exchangeable Ca2+,Mg2+, K+, and Ni2+ were determined on samples from thefour profiles. Preliminary measurements of Na+ showedonly trace levels to be present; therefore, additional deter-minations were not made. Displacement of the cations usingIN NH4 OAc followed a slightly modified version of Chap-man's (1965) procedure. A Varian Techtron model 1200atomic absorption spectrophotometer was used for deter-mination of the elemental concentrations. Exchangeableacidity of all samples was determined using theBaCl2-triethanolamine method as outlined by Peech (1965).

In the determination of cation exchange capacity (CEC),a slight modification of Chapman's (1965) method was usedfor saturation and displacement of NB^ from the exchangecomplex. The exchangeable NH,+ was determined using theKjeldahl procedure (Bradstreat, 1954). Cation exchange ca-pacity was also calculated by summing the exchangeableH+ as determined by the BaCl2-triethanolamine method and

the exchangeable bases as displaced by neutral IN NH4OAc.Samples from the four serpentinite-derived pedons were

analyzed for total nickel and chromium using a modificationof Pratt's (1965) procedure. One-half gram of finely ground(<60 mesh) soil was weighed to the nearest 0.1 mg into aplatinum crucible. Six milliliters of concentrated HNO3 wereadded and the contents of the crucible were evaporated todryness on a 100°C sandbath. Ten milliliters of concentratedHF and 1 ml of concentrated HC1O4 were added to thecrucible which was evaporated to dryness on a 200°C sand-bath. This step was repeated using 5 ml of HF and 0.5 mlof HC1O4. The residue was dissolved in 25 ml of 6N HC1and transferred to a 250-ml volumetric flask which wasbrought to volume. Concentrations of Ni and Cr in solutionwere measured using atomic absorption techniques.

Coarse (20 to 50 /u,m) and fine (2 to 20 /Am) silts wereseparated and collected by sedimentation techniques. X-rayspectrographic methods were employed to determine thetotal levels of Fe, Zr, Ti, Ca, and K present. A Philips-4position inverted vacuum x-ray spectrograph was used forthe determination following procedures similar to those ofBeavers (1960). Finely ground rock samples from each pe-don were similarly analyzed.

Organic carbon was determined on selected samples usingthe Walkley-Black wet oxidation technique as reported byAllison (1965) and a correction factor of 1.33. Determina-tions of free iron oxides were made on samples from thefour soil profiles utilizing a slight modification of the tech-nique reported by Fanning et al. (1970). One gram of soilwas crushed, using an agate mortar and pestle, to passthrough a 60-mesh sieve. Crushing was necessary due tothe presence of iron oxides in concretionary form. Following

RABENHORST ET AL.: GENESIS OF MARYLAND SOILS FORMED FROM SERPENTINITE 609

Table 1—Morphology of four serpentinite-derived profiles.t

Profile

1

2

3

4

Horizon

AlApA2B2t

B&C

RAlA2BlB21t

B22t

B3Cl

C2

C3

C4RAlA2BlB2t

RAlBlB21t

B22t

B3

R

Depth, cm

0-66-21

21-2626-38

38-60

60 +0-44-16

16-2828-40

40-53

53-7575-107

107-123

123-155

155-170170 +0-22-18

18-2626-43

43 +0-33-27

27-43

43-92

92-104

104 +

Moist color

2.5Y 4/210YR 5/310YR6/310YR5/4

10 YR 5/4

10YR3/310YR 5/310YR 5/610YR 5/4

10YR4/3

10YR 5/67.5 YR 5/6

10YR 5/6

10YR5/6

10YR 5/6

10YR3/110YR4/210YR 5/410YR5/6

10YR4/310YR4/410YR4/4

10YR 4/4

10YR4/4

Mottling

Color

NoneNone10YR5/410YR 5/6

5/87.5YR5/810Y 6/2

NoneNoneNone10YR 5/6

10YR5/67.5 YR 5/610YR 5/310YR6/62.5Y5/2;2.5Y4/210YR 6/67.5YR 4/42.5Y 5/27.5YR4/410YR6/62.5Y 5/22.5Y 5/4

NoneNoneNoneNone

NoneNoneNone

7.5YR5/610YR 5/37.5YR5/610YR 5/3

Pattern_-

flfelf

m2d

_-_

flf

midc2dc2p

clp

clp

eld

---_

---

c2d

c2d

Approx. %}concretions

15151510

5-40

510105

10

00

0

0

0

tr5

10tr

51010

5

0

Texture§

silsilsilsil

sil,sicl, gl

silsilsilsicl

sicl

sicl1

1

sil

sil

silsilsilsil

silsilsil

sil

ang.cobsil

Structure

Imgr2mgr2mgr2fsbk

1 m sbk to2fsbk

I fg r1m sbk2m sbk3fsbk

2m sbk

1m sbk-

-

-

_

ImgrIf-mpl1m sbk2m sbk

I fgr1m sbk2m sbk

1m sbk

-

Clay filmsl

NoneNoneNoneFewDiscontinuous

NoneNoneNoneCommonContinuousCommonContinuousNone-

-

-

_

NoneNoneNoneFewdiscontinuous

NoneNoneFewDiscontinuousFewDiscontinuous

Consistence

FriableFriableFriableFriable

Firm

FriableFriableFriableFriable

Friable

Friable-

-

-

_

Very friableFriableFriableFriable

Very friableVery friableFriable

Friable

Boundary

csasawcw

aw

cscscscs

gs

--

-

-

_

ascscsas

cscscs

gw

t The abbreviations for soil characteristics are from Soil Survey Manual (USDA, p. 139).t Percent concretions expressed on a volume basis.

§ Based on laboratory analyses,t in pores and on structural units.

the sodium-dithionite treatment, the extract was brought tovolume in a 100-ml volumetric flask and analyzed for ironusing x-ray spectrographic techniques.

The mineralogy of coarse (0.2 to 2 /urn) and fine (<0.2/am) clay and fine (2 to 5 /urn) silt was determined using x-ray diffraction methods similar to those of Jackson (1969).X-ray diffraction patterns were recorded using a Philips x-ray diffractometer equipped with a proportional counter anda single crystal monochrometer. Cobalt Ka radiation wasused, the K/3 radiation being eliminated by the monochrom-eter. Using a scanning rate of 2°20 per minute, scans weremade from 4° to 36° 26. Parallel-oriented specimens on glassslides were employed. Samples were prepared and run underthe following conditions: magnesium saturation and glycerolsolvation at 25°C; potassium saturation and glycerol sol-vation at 25°C; potassium saturation and heating to 300°Cfor 2 hours; potassium saturation and heating to 550° for 2hours. X-ray diffraction analyses were conducted on rocksamples from the various sites described. Each sample wasfinely ground using an agate mortar and pestle. Randomlyoriented samples were then analyzed using either a PhilipsDebye-Scherer-type 57.3 mm in diameter powder cameraor a standard diffraction arrangement using a box mountslide as described by Jackson (1969).

RESULTS AND DISCUSSIONThe morphology of the four pedons is described in

Table 1. Classification of these pedons is as follows:pedon 1—fine silty, mixed mesic Aquic Hapludalf;pedon 2—fine silty, serpentinitic, mesic Typic Haplu-dalf; pedon 3—fine loamy, serpentinitic, mesic LithicHapludalf; pedon 4—fine silty, serpentinitic, mesicTypic Hapludalf (Soil Survey Staff, 1975). These soilsare relatively shallow with three of the four profilesbeing less than about 1 m to bedrock. Similar obser-vations have been noted by other workers studyingserpentine soils in temperate regions (Coombe andFrost, 1956; National Cooperative Soil Survev, 1970,1971; Johansen, 1928; Perkins and Winant, 1931;Smith and Matthews, 1975). The textures throughoutthe profiles were primarily silt loam and silty clayloam. This is perhaps due to the fine-grained natureof the parent material. Table 2 presents selected par-ticle-size data from the four profiles. From these dataand the observation of oriented clay films, it can beseen that these serpentinite-derived soils demonstrateweak-to-moderate expression of argillic horizons.

610 SOIL SCI. SOC. AM. J., VOL. 46, 1982

Table 2—Degree of expression of argillic horizons.

Pedon(I)

Illuvial horizon(E)

Eluvial horizon I-E I:E

———————— % clay ————————1234

31.832.820.225.9

14.613.214.417.9

17.219.65.88.0

2.182.481.401.45

Soil pH values for the pedons studied are presentedin Fig. 2. While Wright and Foss (1972) recognizedthe great variability of soil pH in the Maryland Pied-mont, they reported that most of these soils exhibita pH minimum in the C horizon. In contrast to manyPiedmont soils, those formed from serpentinitic parentmaterials have pH values that increase with depth andapproach neutrality in the deeper horizons. The basesaturation data, also presented in Fig. 2, exhibit asimilar trend of increasing values with depth. Thesetrends are in accord with reports on serpentine soilsof Scotland (Wilson, 1969) and Japan (Kanno et al.,1965b; Sasaki, 1968).

Selected chemical data of serpentine soils are pre-sented in Table 3. It can be seen that magnesiumstrongly dominates the exchange complex and reaches

150-

Fig. 2—Percentage base saturation and pH values with depth in fourMaryland serpentine soils.

a maximum level in the B horizon. This maximum isprobably the result of a combination of the downwardleaching of Mg by acid waters and the greater capacityfor cation retention in the B horizon due to the greateramounts of clay.

The weathering of serpentinite, which is low in cal-cium, has resulted in soils that are very low in ex-changeable Ca. The only exceptions are the organic-rich surface horizons of pedons 1, 2, and 3 whichcontain considerably more exchangeable Ca than theunderlying horizons. These higher surface levels may

Table 3—Selected chemical data of four serpentine profiles in Maryland.

Exchangeable cations

Depth, cm Horizon Ca" Mg" K*

- meq/lOOg -

Ni2* H* Ni"

ppm

CECsum

CECNH4OAC

FreeFe

OrganicCT

—— meq/100 g —— ———— % ——— —Profile 1

0-66-21

21-2626-3232-3838-60 (sil)38-60 (sicl)38-60 (gl)

AlApA2B2tB2tB&CB&CB&C

4.22.31.01.41.60.71.10.8

5.43.34.5

13.321.916.634.717.4

0.20.10.10.10.20.20.30.1

trtrtrtrtrtrtrtr

8.87.36.46.57.14.06.55.2

11112233

18.613.012.021.330.S21.542.623.5

11.69.78.7

11.815.811.522.814.9

3.173.753.854.154.663.664.286.38

2.981.040.50nd0.38ndndnd

Profile 20-44-16

16-2828-4040-5353-7575-9191-107

107-123123-139139-155155-170

AlA2BlB21tB22tB3ClClC2C3 ,*C3C4

9.31.20.50.60.70.81.01.31.00.80.60.7

5.65.6

13.625.025.323.923.630.728.817.613.015.0

0.30.10.10.30.30.20.30.30.20.10.10.1

trtrtrtr0.10.10.10.10.20.10.10.1

12.26.66.67.77.27.17.68.38.15.03.93.9

128

10121723343844242519

27.413.520.833.633.632.132.640.738.323.617.719.8

17.711.411.922.120.821.624.725.423.414.810.013.3

2.753.804.255.456.597.39

10.2210.258.496.487.056.91

4.260.84ndndnd0.25ndndndndndnd

Profiles0-22-82-18

18-2626-4341-43

AlA2A2BlB2tB2t

6.00.60.60.40.20.3

7.74.37.5

10.112.713.1

0.40.10.10.10.10.1

trtrtrtrtrtr

31.311.48.35.95.13.8

27

11656

45.416.416.516.418.117.3

34.810.49.89.2

10.79.6

2.473.063.534.163.693.47

13.511.801.38nd0.52nd

Profile 40-33-27

27-4343-7070-9292-100

100-104

AlBlB21tB22tB22tB3B3

0.20.20.20.20.20.20.3

0.93.4

10.58.4

11.424.428.7

0.20.10.10.20.10.10.1

tr0.10.10.10.10.10.2

17.96.96.45.45.76.45.8

5401915173748

19.210.717.314.317.531.235.1

11.67.1

10.510.310.021.724.5

3.425.016.125.264.634.804.12

4.460.59ndnd0.28ndnd

t nd = Not determined.


Co :Mg0.5 1.0 1.5 2.0

Ni2.5 Mg/g x I03

Cr

50-

cm

100-

234

150

Fig. 3—Weak acid-extractable Ca/Mg values as shown with depthin the four pedons examined.

be due to biologic cycling and surface additions ofcalcium-enriched plant materials as suggested by Kannoet al. (1965b) or possibly some lime additions by manon the soils that have been cultivated. This is furtherillustrated in Fig. 3 which depicts ratios of weak acid-extractable Ca/Mg with depth. The very low valuesfor these soils, especially in the lower horizons, cor-respond well to the observations of many other work-ers (Marrs and Proctor, 1976; Martin et al., 1953;Proctor, 1971; Vlamis and Jenny, 1948; Walker et al.,1955). These low Ca/Mg values have been shown tobe diagnostic of serpentinite-derived soils (Rabenhorstand Foss, 1981).

While the levels of nickel and chromium for theseserpentinite-derived soils, as shown in Fig. 4, are gen-erally higher than for soils formed from other parentmaterials, there is considerable variation between thepedons, which probably reflects the amount of nickelsubstitution for magnesium in the serpentine mineralsforming the parent material for each pedon. Totalnickel, which occurs primarily in the structure of ser-pentine minerals, shows an increase with depth in allfour pedons. This is probably due to the greater in-tensity and duration of weathering and consequentrelease and removal of nickel from horizons nearerto the soil surface. Levels of total chromium, whichmost commonly occur in serpentinites as the acces-sory mineral chromite, are more variable with depthand do not show the same regular weathering trendsobserved for nickel.

The profile distributions of ZrO2, TiO2, CaO, K2O,and Fe2O3 (elemental analyses expressed as oxides)for the coarse and fine silts are presented in Fig. 5.Pedon 3 shows typical, although weakly expressed,weathering patterns. The slight decreases in CaO andK2O and slight increases in TiO2 and ZrO2 with prox-imity to the surface are the result of differential weath-ering rates of minerals containing these elements. Theactual levels of CaO and K2O in this profile, however,

0 1 2 3 4 5 10 2 0 0 1 2 3 4 5 6

50-

cm

100-

150-

Fig. 4—Total Ni and Cr values with depth in the four serpentineprofiles studied.

are much higher than for parent rock samples (Table4). This is unexpected for soils developed only onserpentine rocks and, as explained later, appears toreflect eolian additions of different materials. TheFe2O3 shows a more pronounced decrease toward thesurface.

The trends for levels of Fe2O3, ZrO2, and TiO2 inprofile 2, while much more pronounced than in mostprofiles, appear to be in general accord with normalweathering trends (i.e., weathering and removal of themore labile Fe and consequent residual accumulationof Ti and Zr in resistant minerals). Values for bothCaO and K2O are, however, much greater in the upper70 cm of the profile than below this level. Potassium-bearing phlogopite or biotite might possibly be asso-ciated with serpentine, but both are relatively easilyweathered minerals and would not seem to accountfor these higher K2O levels. The addition of eolianmaterial to the surface and incorporation into the up-per portion of the profile is the most likely explanation.Foss et al. (1978) have observed loess deposits on theeastern shore of Maryland up to 150 cm in thickness.It is reasonable that some windblown materials havebeen deposited west of the Chesapeake Bay as well.Loess deposits nearly one m in thickness have beenobserved 15 to 20 km southwest of pedons 3 and 4(Rabenhorst et al., 1982; Rabenhorst, 1978).

The pronounced decrease in CaO and K2O in thelower 20 cm of profile 4 also suggests that some cal-cium- and potassium-bearing minerals from anothersource were incorporated into this soil. Levels of CaOand K2O in rock samples of all four profiles were ator very near zero. Higher levels in the soil profilesindicate that eolian additions have likely occurred forall four profiles and have subsequently been incor-porated throughout the soil in the shallower profiles(1 and 3) and throughout the upper 75 to 100 cm ofprofiles 2 and 4. Since the eolian material would likelybe higher in Ti and Zr and lower in Fe than the sub-silicic ultramafic serpentinite, this would explain thedrastic decreases in ZrO2 and TiO2 with depth in pro-file 2 with corresponding increase in Fe2O3, as wellas the higher levels of K2O and CaO.

Interpretations of x-ray diffraction data are pre-sented in Table 5. Serpentine dominates the parentrock of profile 1 and is accompanied by some chlorite.

612 SOIL SCI. SOC. AM. J . , VOL. 46, 1982

CaO K20 Fe2°3 Ti°2 Zr°29 ! ? ? 0 1 2 3 0 5 10 I S 2 0 2 5 0 0 . 5 1.0 1.5 0 O.05 0.10

2 5 -

50-

0

25

100-

150-

I75J

O

50J

0

25-

50-

75-

100

Fig. 5—Elemental distributions in the coarse (20-50 /un) and fine (2-20 /un) silt fractions with depth in the four profiles examined.

In the soil, however, serpentine occurs at greatly re-duced levels and is virtually absent from the <2-//,mfractions. Chlorite and vermiculite, much of which isinterstratified, are the dominant materials in the <2-/u,m soil fractions. It has been suggested by Hargittand Livesey (1976) and others that trioctahedral chlo-rite forms through the alteration of serpentine (Wilson,1969). Primary evidence for this suggestion was theobservation of a transition from a chlorite-rich "rind"to a serpentine-rich interior in serpentinite boulders.No such rind or transition was observed in the parentrock of this profile. A report by Cleaves et al. (1974)stating that dissolved material from a serpentine wa-tershed was much higher than for other rock typessuggests that most of the soluble weathering products

Table 4—Elemental analyses of rock samples from thepedons studied, t

Site CaO K,O TiO, ZrO, FeA

1234

0.02T

0.01T

Tt00T

——— % ———0.02TT

0.02

0000

11.175.50

10.049.73

TT = trace: <0.01%.

of serpentine are being removed by percolatinggroundwater, leaving previously existing chlorite toaccumulate residually. The increase of vermiculitewith decreasing particle size in conjunction with higherlevels of chlorite in the larger particle-size fractionssuggests that vermiculite is forming at the expense ofchlorite, probably by a simple transformation leavingthe 2:1 layer intact.

While a small amount of smectite has probablyformed from serpentine in the fine clay, the occurrenceof no other minerals in the soil can be traced to theweathering of serpentine. The presence of smallamounts of quartz, mica, and feldspar throughout theprofile, while absent from the parent rock, supportsthe inferences made for elemental analyses that sur-face mineral additions have occurred at this location.

Pedons 3 and 4 have very similar mineralogy, dom-inated by serpentine, particularly in the >2-;nm frac-tions. Smectite is present primarily in the fine clayfraction, indicating a probable origin from serpentineweathering products. This is supported by other work-ers (Wildman et al., 1968; Kanno et al., 1965). Thedegree of interstratification of chlorite, vermiculite,and smectite generally increases as particle size de-creases. The presence of feldspar, quartz, and mica


Table 5—Semiquantitative interpretation of x-ray diffraction patterns of fine silt (2-5 /un), coarse clay (0.2-2 jun), and fine clay(< 0.2 /on) fractions from selected horizons of the profiles studied.

HorizonDepth,

cmSize

fraction, fan

Relative quantity t of minerals§ present

Fl Qr Mi Vr Sm Chl Serp. Other Remarks

Profile 1Ap

B2t

B&C

6-21

32-38

38-60(c)

2-50.2-2

<0.22-5

0.2-2

<0.22-5

0.2-2<0.2

X-

-X-

-X--

XXX

-XXX

-XX-

X-

-X-

-XX_

XXX

xxxxXXXXX

xxxxXXXXXXXX

_-

X--

X-?XX

Rock sample _ _ _ _ _

XXXxxxxXXXXXXXX

XXXXXXXXXXXX

X-

~X-

-XXX_xxxx

„X

-XX

-X---

Reg. ISChl-Vr;l/2ChlISw/VrAll ISReg.ISChl-VrReg.ISChl-Vr;l/2ChlISw/VrAll ISReg. ISChl-VrPartially IS Vr-ChlAll IS

Profile 2A2

Bl

B22t

Cl

C2

C3

C4

4-16

16-28

40-53

75-91

107-123

123-139

155-170

2-50.2-2<0.22-5

0.232<0.22-5

0.2-2<0.2

2-50.2-2<0.22-5

0.2-2<0.22-5

0.2-2<0.2

2-50.2-2<0.2

XX-_XX_X_..X_-_____

_-__

XXXX..XXXX_XX..X_-X_....-_--_

XXX-XXXXXXX_--_--__--___

XXXXXXXXXXXXXXXX_XX-XX-_--_X_

_-XXX_XXXX-XxxxxXXXXxxxxXXXxxxxXXXXxxxxXXXxxxx

XXXXXXXXXXXXXXXX-XXX_XX_XX-XX

Rock Sample _ _ _ _ _ _

XXXXX_XXXXX-XXXXXX-xxxxXXXxxxxXXX_xxxxXXX-xxxxxxxxXxxxx

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All Chl IS1/2 Chl + 1/2 Vr ISAll IS

Most Chl ISAll Chl IS

1/2 Chl ISl/2SmISw/Chl

Vr-Chl-Sm IS

Chl ISw/VrChllSw/Sm

Chl IS

ProfilesA2

B2t

2-8

26-43

41-43

2-50.2-2<0.2

2-50.2-2<0.22-5

0.2-2<0.2

X..-X_-X--

XX-XX-XX-

X_-XXX-XXX~

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?XXXXXXX?XXXX

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Rock Sample _ _ _ _ _ _

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TalcTalc; 1/2 Chl + all Vr ISTalc; some IS of all mineralsTalcTalcMostly ISTalcTalc

TalcProfile 4

Al

B21t

B22t

B3

0-3

27-43

70-92

100-104

2-50.2-2<0.2

2-50.2-2<0.22-5

0.2-2<0.2

2-50.2-2<0.2

X_-XX-XXX----

XXXX-XX-XX----

_..-XXX-XXXX

---

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_-X-XXXX_XXXX

_XXXX

Rock Sample -

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XXXXXXX

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TalcTalcAll IS; v. poor cryst.TalcTalcAll IS; extr. poor cryst.TalcTalc1/2 Chl ISw/Vr + Sm;poor, cryst.TalcTalcAll IS; poor, cryst.

t X — low amount, < 10%; XX — moderate amount, 10-30%; XXX — high amount, 30-70%; XXXX — dominant mineral, > 70%.§ Abbreviations: Fl — feldspar; Qr — quartz; Mi — Mica; Vr — vermiculite; Sm — Smectite; Chl — chlorite; Kl — Kaolinite; Serp — Serptentine; IS — Inter-

stratified; Reg — Regular.

mainly in the 2- to 5-/u.m fraction indicates that someadditions of material have occurred at these sites aswell.

X-ray diffraction patterns of the 0.2 to 2-//.m fractionfor selected horizons of profile 2 are presented in Fig.

6. Previous suggestions that profile 2 has receivedadditions of eolian material are further supported bythe presence of feldspar, mica, and quartz in the upper80 cm of the profile. The dominance of serpentine inthe 2- to 5-ju,m fraction, even in the upper part of the


profile, indicates that the additions were limited anddid not constitute the main portion of the parentmaterial.

The conspicuous absence of serpentine from thefine clay and the dominance of smectite in this fractionis again evidence for the formation of smectite fromserpentine. In the lower one-half of profile 2, smectitehas formed in moderate-to-high amounts in the 0.2-to 2.0-^.m fraction and is present in small amountseven in the fine silt. The increase in <0.2-/x.m chloritetoward the surface and a corresponding decrease inthe <0.2-/u,m smectite, may be the result of the for-mation of hydroxyinterlayered smectite (tending to-ward pedogenic chlorite).

Vermiculite, which is present mainly in the upperone-half of the profile and is concentrated in the coarseclay and fine silt fractions, has probably formed frommica which had been added to the site and incorpo-rated into the profile through such processes as frostaction and biologic mixing.

SUMMARY AND CONCLUSIONSThe weathering of serpentinite in Maryland, along

with other processes of pedogenesis, has resulted inrelatively shallow soils with dominantly silty textures.Figure 7 shows a generalized model of the genesis ofa serpentinite-derived soil in the Piedmont region ofthe eastern United States. The release and translo-cation of iron has resulted in brownish-colored B ho-rizons. Impeded drainage may cause the iron to be-come segregated as mottles or concretions. Theprocesses of eluviation and illuviation of clay haveproduced weakly-to-moderately expressed argillic ho-rizons. As a result of denudation, which counteractsthe processes of profile development and horizon dif-ferentiation, soils occurring on steeper slopes tend tohave less strongly expressed B horizons. The Mg-richparent material has resulted in the dominance of Mg2+

on the exchange complex causing these soils to havefairly high pH values. The greater intensity of weath-ering near the surface has resulted in increased Mg2+

saturation and increased pH with depth. Nickel andchromium levels are generally very high, but theyshow considerable variation from one location to an-other.

Both the elemental analyses of the silt fractions andmineralogical data have indicated that some materialfrom a geologically different source has been addedto the surface and incorporated into the serpentinite-derived soils. This complicates the understanding ofmineralogical transformations occurring in the soil.

Figure 8 shows important mineralogical transfor-mations that appear to occur during the genesis ofthese soils over serpentinite rocks. Serpentine min-erals are generally abundant in the >0.2-/im fractionsof the soil but are entirely absent from the fine clay(<0.2 )u,m) fraction. While some of the weatheringproducts of serpentine are removed from the soil sys-tem by percolating water, others participate in thesynthesis of smectite, which is most prominent in thefiner fractions. Some hydroxyinterlayers may developin the smectites forming pedogenic chlorite. The ironwhich is released from serpentine structures or fromiron-rich accessory minerals remains in the oxide

K-550"

K-300

Mg-25Q ——^

K-550"

K-300

Mg-25" —

K-550" —

K-300 —

Mg-25"

K-550" —

K-300 _

Mg-25° __.

3.25 3.55 **.27 5.0 7.3 \k.23.33 3.62 4.76 7.1 10.0 18.0

Fig. 6—X-ray diffraction patterns of the 0.2-2 /tin fraction for se-lected horizons of serpentine pedon 2.

form. Vermiculite, which is common in these soils,has probably formed by the alteration of chlorite as-sociated with serpentine in the parent rock or by thealteration of mica which has been added to these sites.The fine clay fraction may have been synthesized fromthe weathering products of serpentine and otherminerals.


Fig. 7—Generalized model of the genesis of a soil derived from serpentinite in the Maryland Piedmont.

SERPENTINITEBEDROCK

SERPENTINEMINERALS

if

SMECTITE,

Loss ofSi&Mg

ILoss ofSi&Mg

PEDOGENICCHLORITE

IRON"OXIDES

CHLORITE- -» VERMICULITE

Loss ofMg(OH),

Fig. 8—Mineralogical transformations occurring during the genesisof serpentine soils.


Documents

Genesis of Maryland Soils Formed from Serpentinite1