Etch-pit size and shape distribution on orthoclase and pyriboles in a loess catena

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Geochimm et Cosmochmica Am Vol. 56, pp. 3423-3434 Copyright 0 1992 Pergamon Press Ltd. Printed in U.S.A.

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Etch-pit size and shape distribution on orthoclase and pyriboles in a loess catena D. L. CREMEENS,’R. G. DARMODY,~ and L. D. NORTON’ ‘GA1Consultants, Inc., 570 Beatty Road, Monroeville, PA 15 146, USA 2Agronomy Department, University of Illinois, Urbana, IL 6 180 1, USA 3National Soil Erosion Research Lab, USDA-AR& Purdue University, West Lafayette, IN 47907, USA (Received August 27, 1990; accepted in revised form June 8, 1992 )

Abstract-Etch pits on 50-100 pm orthoclase and pyribole grains from the E, B, and C horizons of a catena or drainage sequence of well-drained, somewhat poorly drained, and poorly drained loessal soils were analyzed using SEM and digital image analysis. The objective was to determine the influence of soil drainage conditions and the associated geochemical environments on dissolution of primary silicates as determined from the distribution of etch pits on the grain surfaces. Etch-pit size frequency distributions indicated that the poorly drained soil contained the smallest and most numerous etch pits for both minerals. This indicated low growth rates for etch pits in the poorly drained soil. Analysis of etch-pit shapes indicated that etch pits on pyriboles were more elongated and irregular in the poorly drained soil, possibly resulting from a greater growth rate; whereas etch pits on orthoclase grains were more elongated and irregular in the well-drained soil. The larger, more elongated and irregularly shaped etch pits on orthoclase from the well-drained soil, indicative of a greater pit growth rate, is consistent with a flushing model of mineral weathering in soils. The etch-pit size distribution on pyriboles is consistent with the flushing concept, but the shape data indicates that other mechanisms of weathering may be involved. In the upper horizons (Eg and Bhg), the feldspars were intensively corroded with etch pits and trenches. In the deeper Bs and C horizons, etching was less intense. BERNER ( 198 1) concluded that pit shapes on feldspars are not sensitive to the type of acids attacking the grain. This conclusion was based on observations that the same type of pitting was found on a variety of K and Na feldspars taken from several different soils where different soils’ acids should have been present. Laboratory studies of pyroxenes and amphiboles produced results similar to those with feldspars. However, occasional etch features found on laboratory-weathered grains, but not on naturally weathered grains, were suggested as evidence of some control by etchant type ( BERNERet al., 1980; BERNER and SCHOTT, 1982). During pyroxene and amphibole weathering, lens-shaped etch pits form, enlarge, and coalesce. Side-by-side coalescence results in saw-tooth lined cracks and microcavities. End-to-end coalescence results in a grooved or striated surface. HALL and MARTIN ( 1986) and LOCKE ( 1986 ) determined that hornblende etching, estimated from petrographic observations, decreases logarithmically with depth in alpine tills and periglacial deposits. Etching decreased with depth to some zero value where it was indistinguishable from fractures. Etching was also determined to be a function of time; thus it was concluded that etching must decrease with time and that most etching occurs in the first 7500 years. APRIL et al. ( 1986) found both depleted hornblende contents and increased etching on grains with proximity to the surface in till soils of Adirondack watersheds. Feldspars showed a similar trend in etching, greater near the surface, but contents were uniform with depth. LASAGAand BLUM ( 1986) proposed that if the saturation state of natural solutions with respect to any one mineral varied within a critical range, then the mineral surface chemistry would retain a record of the chemical history. Surfaces

INTRODUCTION WITH THE USEOF SCANNINGelectron microscopy (SEM) and kinetic theories of dissolution, researchers have concluded that feldspars in the weathering environment undergo preferential dissolution where some kind of crystal imperfection is manifest at the crystal surface (WILSON, 1975; BERNER, 1978). Based on analogies with metallurgy, surface imperfections are characterized by a distorted lattice and a high strain field. SEM observations of crystallographicalIy oriented etch pits on HF-H2S0., treated ( BERNER, 1978; BERNERand HOLDREN,1979) and naturally weathered feldspars (KELLER, 1978; NIXON, 1979; DEARMANand BAYNES,1979; BERNER, 1981) suggested that initial dissolution of feldspars is crystallographically controlled and independent of the etchant. Feldspars found in soils, however, often show a relationship between depth in soil and degree of etching. Plagioclase feldspars from a C horizon of a soil developed in basalt in Scotland showed deep and extensively developed etch marks when viewed with SEM (WILSON, 1975). MERMUT et al. (1986) found various stages of weathering and regular as well as irregular patterns of etch pits on grain surfaces of K-feldspars and plagioclases from the E, Bt, and C horizons of two forested soils formed in glacial till in northern Saskatchewan. Feldspar weathering by chemical dissolution was greatest in the E horizon. In the Bt horizon, they felt that illuvial coatings of clay and Fe-oxides formed a protective barrier. Etch-pitted grains found in the C horizon were interpreted as a sign of previous weathering. A similar conclusion, that the clay formed a protective coating on grains occurring in the Bt horizon, was reached by CRUICKSHANKet al. ( 1990). Feldspars from the Eg, Bhg, Bs, and C horizons of a peaty podzol (poorly drained) formed in fluvioglacial sands in Scotland showed very marked differences in the intensity of etching (FARMER et al., 1985). 3423

3424

D. L. Cremeens, R. G. Darmody, and L. D. Norton

of quartz grains treated under low-temperature laboratory conditions exhibited etch pits postulated as indicative of the silica saturation state of the reactant fluids ( BRANTLEYet al., 1986). Subsequently, etch-pit densities on quartz surfaces that decreased with depth in a soil profile were interpreted to be caused by the increase in Si concentration in pore fluids. A statistical treatment of the distribution of etch-pit density, size, and shape on mineral grains on a landscape scale should provide an indication of the influence of specific geochemical environments on weathering reactions ( DEARMAN and BAYNES, 1979; LASAGA and BLUM, 1986). Therefore, the objective of this research was to determine the distribution and variability in etch-pit size and shape on orthoclase and pyribole grains from soils in a loess catena (a drainage sequence of soils). Ambient geochemical conditions associated with soil drainage may influence the dissolution of the grains in a manner that can be determined from the distribution of etch pits on grain surfaces. Loess soils were evaluated because of uniform textures, mineralogy, and age throughout the deposit. Well-drained, somewhat poorly drained, and poorly drained soils, all of which have developed in the same parent material, under the same climatic conditions, and for the same period of time, are often found closely associated under field conditions. This association of soils based upon drainage or relief is known as a catena (BRADY, 1974). A catena was chosen to encompass a wide range in pedochemical conditions of redox potential (BAAS BECKING et al., 1960; BOUMA, 1983) and leaching potential (CROMPTON, 1969; BERNER, 1978). In particular, well-drained leached environments should be more favorable for mineral weathering than poorly drained, less leached environments (KELLER, 1957; BERNER, 1978). The renewed flushing of weathering products with freshwater in well-drained soils favors the weathering reaction as aqueous products are removed from the vicinity of the weathering grain ( BERNER, I978 ). Poorly drained soils contain a greater amount of water for a longer period of time. Soluble weathering products are not removed from the vicinity of the reaction during most of the year. Somewhat poorly drained soils are intermediate in the amount of contact time and flushing with relatively fresh water. Soil reduction is induced by water saturation if organic matter contents are great enough and soil temperatures allow for microbial activity ( BOUMA, 1983 ). Oxidation of organic matter occurs when an electron acceptor is available. Under soil conditions, this may be 02, N02, Mn, Fe, or S compounds. Reduction of these compounds by organic matter is thermodynamically possible under soil conditions, but most of the reactions are extremely slow. However, many nonphotosynthetic microbes catalytically decompose the unstable products of photosynthesis through these energy-yielding redox reactions. Reduction takes place if the following conditions are met: presence of organic matter, absence of oxygen supply, and the presence of anaerobic microorganisms in an environment suitable for their growth. Saturated soil becomes depleted of oxygen, which is rapidly consumed by aerobic organisms and cannot be replenished fast enough. Reduction of remaining O2 will take place first, followed by NO*, and then Mn. Later, Fe is reduced and becomes the primary electron acceptor for the microbes.

EXPERIMENTAL METHODS Samples Soil core samples were collected from well-drained (Fayette series), somewhat poorly drained ( Stronghurst series), and poorly drained (Traer series) soils in a dissected loessal landscape in northwestern Adams County, Illinois, approximately 27 km north of Quincy, Illinois. The cores were collected within a 2.25hectare area. Annual precipitation in the Quincy area averages 9 14 mm per year and ranges between 584 and 1220 mm (BUSHUE, 1979). More than 50% ofthe annual precipitation occurs from May through September. The mean monthly temperature ranges from -2°C in January to 26°C in July. Soils in this part of Adams County formed mainly in loess and till. The total loess thickness is 3.0-7.5 m (FEHRENBACHER et al., 1986). These loessal materials consist of Peoria Loess above Roxanna Silt ( LINEBACK,1979; WICKHAM,1979). Underneath, the loess is till of either Illinoian or pre-Illinoian age, approximately 6-l 1 m thick. Cores were collected using a hydraulic soil probe. Samples were collected from the E or BE horizons, the upper Bt horizons, and the deepest C horizon occurring in the Peoria Loess material. In standard soil horizon nomenclature, uppercase letters are used to designate the master soil horizons, while lowercase letters are used as suffixes to indicate specific characteristics of the master horizons ( SOILMANAGEMENTSUPPORTSERVICES,1986). E horizons are mineral horizons in which the main feature is the loss of silicate clay, iron, and aluminum, or some combination of these, by translocation, leaving a concentration of sand- and silt-sized particles. BE horizons are transitional horizons dominated by properties of the B horizon but having subordinate properties of the E horizon. In many cultivated lands, the original E horizon is gone, and the BE horizon underlies the A horizon. Bt horizons are subsoil zones of clay accumulation with visible deposits of clay (t-designation). C horizons represent the weathered but pedogenically unaltered protolith or parent material. In the somewhat poorly drained Stronghurst soil, the lower Bt and C horizons were designated with the suffix g, which indicates gray dull colors (gley) resulting from reduction caused by impeded drainage. In the poorly drained Traer soil, the entire B horizon was designated Btg. The average horizon depth was 0.2-0.3 m for the E or BE horizons. 0.4-0.7 m for Bt. and 1.8-2.4 m for C. Additional cores of each soil were collected and used for particle-size analysis and clay mineralogy. Laboratory Analysis The 53-100 pm fraction of each horizon was isolated following sample dispersion and grain cleaning with 2% NaHCOS, wet sieving, drying with 100% ethanol, then dry sieving ( CREMEENSet al., 1987 ) This grain cleaning method was found to be effective without causing significant damage to grain surfaces. Light and heavy fractions were density separated using bromoform (sp.gr. 2.87), then sprinkled on individual SEM stubs and sputter coated with Au-Pd ( DARMODY, 1985). Twenty grains from each mineral for each horizon were viewed with SEM, giving a total of 360 grains analyzed. Ot-thoclase was identified in the light fraction by energy-dispersive X-ray analysis (EDXA) . Pyriboles were identified by compariing SEM imaged grain morphologies with hornblende grains handpicked, using the petrographic microscope, and previously viewed with SEM. Monoclonic amphiboles were differentiated from monoclinic and orthorhombic pyroxenes by the cleavage angle. Thus, pyriboles are defined in this study as undifferentiated inosihcates while viewing with SEM. The intent was to evaluate monoclinic amphiboles (hornblende). FRYE et al. ( 1962) reported that hornblende and hypersthene were the two most common inosilicates occurring in the fine and very fine sand fractions of Peoria Loess sampled approximately 4 km west of the study area. Each micrograph was analyzed to determine etch-pit equivalent diameter (EQD), area, perimeter, Feret’s maximum (Fmax), and Feret’s minimum (Fmin) by means of a digital image analysis procedure (CREMEENSet al., 1988). Etch-pit equivalent diameter is the diameter of a circle with area equal to that of the measured shape A Feret’s diameter is a diameter which can be measured in a horizontal, a vertical, and two diagonal directions. The largest and smallest Feret’s diameters are indicated by Fmax and Fmin, respectively. The

Weathering of silicate minerals in soils Elongation Index (EI), calculated from Fmax/Fmin (BISDOMand SCHOONDERBEEK, 1983), indicates increasing elongation as the index increases. The Smoothness Index (SI), calculated from (area/ perimeter2)* 1080 (BISJXIMand SCHOONDERBEEK, 1983), indicates increasing smoothness or decreasing indentation as the index increases. As the SI increases, the perimeter becomes more regular and smaller, and the shape of the pit becomes more round. Maximum values of the SI would occur for a disc with a circle as a perimeter. The data was evaluated for analysis of variance using a General Linear Models (GLM) program for unbalanced data (SAS INSTITUTE, 1985 ) . The model evaluated etch-pit means of EQD, SI, and El for effects of soils (drainage), horizons (depth), and soil-horizon interaction or comparisons between all nine soil-horizon combinations, as well as among grains using a Type III sums of squares (SS) . Least squares means (LSM) were calculated because of the unbalanced design. Particle-size analysis was done on other core samples by the pipette method (GEE and BAUDER,1986). Samples were pretreated with 30% hydrogen peroxide to destroy organic matter and dispersed with 10% sodium hexametaphosphate. Clay mineralogy determinations were made by X-ray diffraction on oriented samples prepared by pipetting the <2 pm fraction onto glass slides and solvating with ethylene glycol (GLASS and KILLEY,1986). On selected samples, the 20-50 pm fraction was collected by sedimentation following dispersion with 2% NaHCOS, fused into disks, and analyzed by X-ray fluorescence. This data was used to calculate molar oxide ratios of Si02/( MgO + CaO + Na20 + K20) (Rm-resistate maturity) as an indication of the relative degree of weathering ( WAKATSUKIet al., 1977). Quartz-to-feldspar ratios (Q/F), determined by EDXA during SEM viewing transects of the light fraction samples, were also used to determine relative weathering.

3425 RESULTS

Soil Evaluation The soils in this study were formed in Peoria Loess deposited during the Woodfordian Substage of the Wisconsinan Stage ( WILLMANand FRYE, 1970). Table 1 gives select profile characteristics and soil properties for horizons in the three soils. The distribution of matrix and mottle colors in the horizons below the Ap horizon is used to define the drainage characteristics and moisture regimes of the soils (BOUMA, 1983; SOIL SURVEYSTAFF, 1975). Well-drained soils have bright colors (high chroma) and a lack of gray mottles in the upper B horizon. Somewhat poorly drained soils have gray (gleylow chroma) lower B horizons. When the entire B horizon becomes gray, the soil is defined as poorly drained. The grain size distribution is typical for forested soils formed in loess. The sand contents are low and remain nearly constant with depth. The clay distribution reflects the translocation of clay to form the argillic Bt horizon (SOIL SURVEY STAFF, 1975). Clay moves out of the A and E horizons and is deposited in the Bt horizons. The clay distribution shows the characteristic clay bulge of the argillic Bt horizon. The low pH in all samples, especially in the deeper C horizons, indicates that these loess soils have been extensively

Table 1. Soil Profile Characteristics and Physical and Chemical Prooerties of the Soils Grain-sizes Clay** Colort Analvsis Minerals Soil* Depth Horizon (cml Matrix Mottles SSiC &fi(MK&&,Q&$§

____%____

____%___

lOYR4/3 lOYR5/4 lOYR4j4 lOYR4/4

5 5 3 4

86 75 69 62

9 20 28 34

5.8 4.8 4.5 4.5

- - 29 47 24 49 34 17 60 28 12

21.09 19.26 17.39 16.41

3.61 3.45 -

132-176 89-132 lOYR5/4 lOYR5/6 205-220 176-205 lOYR5/3 lOYR5/4

4 3 6 3

67 63 73 68

30 33 24 26

4.8 4.6 5.3 5.2

60 63 30 25 10 12 66 - 24 - 10 -

15.52 15.50 15.00 -

1.91 -

5.8

27 50 23

33 47 20

18.91 18.85

3.55

69 49 75 75 76 76

17.86 16.51 15.27 14.53 14.66 15.06

Fayette AP BE*

O-20 20-31

8tl*

31-69

Bt2 :: ::*

69-89

Stronghurst AP O-18

E* ;:* 8t9 869 691 cg2* Traer AP

lOYR4/2

4

86

10

18-32

lOYR5/2

4

83

13 4.6

32-48 48-68 68-104 104-132 132-165 165-242

lOYR5/4 lOYR5/3 lOYR5/2 lOYR5/2 lOYR6/2 lOYR6/2

O-26

Kgl*

37-73 26-37

Btg2 869 Cal

73-92 92-108 108-115 115-145 145-180 180-248 248-272

Cg2

w cg4* ca5

lOYR6/2 lOYR5/6 lOYR5/6 lOYR5/4

lOYR4/2

32 69 79 29 18 4.6 4.4 2 64 34 4.6 2 66 32 5.0 4 72 24 5.3 2 70 23 5.7 3

88

9

5.1

2.5Y5/2 lOYR6/2 lOYR5/6 lOYR6/4

3

63 81

34 16

2.5Y5/2 2.516/2 2.5Y5/2 2.5Y6]2 2.5Y6/2 2.51612 lOYR6ll

2 2 2 2 2 2 4

64 66 72 78 73 76 73

34 32 26 28 25 22 23

2.5Y5/4 lOYR5/6 7.5YR5/6 lOYR5/6 7.5YR5/6 lOYR5/6 lOYR573

-

34 21 10 17 17 8 18 7 17 7 17 7 -

2.85 1.69

-

19.78

4.6 4.9

72 50 32 17 18 11

18.37 18.78

3.02 3.41

4.7 4.8 5.7 5.7 5.7 6.2 6.3

79 77 77 77 79 76 72

16.72 15.56 14.91 -

-

13 8 15 8 16 7 16 7 15 6 17 7 19 9

2.47 -

*Horizons evaluated in the SEM study tMunsel1 notation, moist colors IS-sand (2-0.05 mm), Si-silt (0.05-0.002 mm), C-clay (t0.002mm). **Ex-expandables (smectite plus vermiculite), M-mica, hydromica, illite K-kaolinite plus chlorite. +tRm-resistate maturity SiO, / (CaO t MgO + KO t NazO). @Q/F-quartz/feldspar ratio as determined by $DXA during SEM analysis.

3426

D. L. Cremeens,

R. G. Darmody,

and L. D. Norton

FIG. 1. SEM observations of orthoclase and pyribole grains. (a) Orthoclase grain from the Bt horizon of the Fayette soil. Note the two elongated etch pits (greater EI) oriented parallel to cleavage planes on the lower central portion of the grain. (b) Orthoclase grain from the C horizon of the Fayette soil. Note the wide range in size, shape, and orientation of etch pits; some pits appear irregular in outline (lower SI). (c) Pyribole grain from the Btg horizon of the Traer soil. Note the elongated etch pits (greater EI), which may have formed by end-to-end coalescence. (d) Pyribole grain from the BE horizon of the Fayette soil. Note the highly indentate, irregular outline (lower Sl) etch pits, which may have formed from side-by-side coalescence. (e) Pyribole grain from the Bt horizon of the Stronghurst soil. Note the narrow

Weathering of silicate minerals in soils leached. The Fayette and Stronghurst soils appear to have been more extensively leached than the Traer soil. Basal loess material in this region is typically calcareous (FRYE et al., 1962). Minimum pH occurs in the upper B horizon in all soils and is associated with the maximum clay content. Expandable minerals (smectite, vermiculite) increased with depth, while mica and kaolinite decreased. This suggests that the expandables are weathering out of the upper horizons at a quicker rate than the micas. Kaolinite is forming as a secondary phyllosilicate weathering product. The resistate maturity (Rm) decreased with depth, indicating more extensive weathering and subsequent leaching in the upper horizons. This depth trend was consistent in all profiles. Comparisons between profiles reveal similar values for similar depths, indicating that the relative degree of weathering of these soils is the same. The Q/F ratio shows a decrease with depth and little difference between the soils. The exception to this was the Cg4 horizon of the Traer soil. The Q/F of 2.41 indicates less weathering in that horizon than similar horizons in the other soils. One distinct depth trend in the grain size, clay mineral, and Rm distributions is how they approach uniform values in the deeper C horizons in all soils. These uniform values are used to define the parent material lithology. Overall, the C horizon materials from each soil were found to be similar in texture and clay mineralogy (Table 2). Thus, it was assumed that loess parent material for these soils was laterally uniform, and orthoclase and pyribole grains in the unweathered loess parent material were initially similar. Therefore, the soil and horizon differences in etch-pit density, size, and shape distribution should be a function of differing intensities and mechanisms of weathering. In the Peoria Loess of Western Illinois, the average Kfeldspar content of the fine and very fine sand fractions ranges from 13-16%, and the average plagioclase content ranges from 6- 11% (FRYE et al., 1962). Samples of the Peoria Loess, collected approximately 8 km SSW of the study area by FRYE et al. ( 1962), showed that heavy minerals accounted for 0.20.9% of the fine and very line sand fraction. In the heavy fraction, 34-68% were hornblende, followed by epidote ( 1725%) and garnet (4-27%) (FRYE et al., 1962). A similar distribution was found by FRYE et al. ( 1962) in samples from north of Quincy, Illinois, approximately 25 km from the study area. SEM Observations

of Grains

Figure 1 shows examples of SEM images of grains from the study. Surfaces of orthoclase grains were more difficult to interpret than surfaces of pyribole grains. Etch pits on orthoclase surfaces were not always easily defined because their perimeters often were not closed, and the pits tended to coalesce. Figure la is an orthoclase grain from the Bt horizon of the well-drained Fayette soil. Two of the etch pits on this grain

3421

Table 2. Average Particle Size a;tdClay Mineralogy of C Horizons* in this S v Clav** Grain-size AnalvsisB Minerals Soilt S Si C csi/msi EXIl._ K Rnrtt

______*_-----

___%____ 11 15.5

0.43 0.1

n

3.4 70.8 25.8 1.6 2.7 2.4 5

64 25 2 1 2

Stronghurst s.d. n

2.7 72.8 24.5 1.1 2.3 2.5 12

0.48 0.1

76 17 7 15.0 2 1
Fayette s.d.

1

0.4 3

Traer 2.5 72.0 25.5 0.48 76 17 7 15.0 0.7 2.5 2.5 to.1 s.d. 2 1 1 0.1 2 *Refers to average of all horizons designated C in core descriptions *Fayette is well-drained, Stronghurst is somewhat poorly drained, and Traer is poorly drained. §S-sand (2-0.05 mm), Si-silt (0.05-0.002 mm), C-clay (CO.002 mm), csi/msi=coarse silt (53-31 p)/medium silt (31-16 /.ua). **Ex-expandibles-(smectite plus vermiculite) M-mica, hydromica, illite, K-kaolinite plus chlorite. l-tRm=resistate maturity (SiO,/CaO t MgO t KzO t Na,O).

are elongated (greater EI) and oriented parallel to cleavage planes, similar to etch pits on feldspars observed by BERNER and HOLDREN ( 1979). The other etch pits on the grain do not appear to be crystallographically controlled. Figure 1b is an orthoclase grain from the C horizon of the Fayette soil. Etch pits on the surface of the grain have a wide range in size and shape. Etch pit orientation is variable, and many pits are irregularly shaped (lower SI) and may have formed from the coalescence of adjacent smaller pits. Figure lc is a pyribole from the Btg horizon of the poorly drained Traer soil. Most etch pits on this grain are uniform in size and shape and oriented with their long axis parallel to the C axis of the grain, consistent with observations reported by BERNER et al. ( 1980). Some of the pits are highly elongated (greater EI) and may have formed from the end-to-end coalescence of adjacent linear pits (BERNER et al., 1980; BERNER and SCHOTT, 1982 ). Figure 1d is a pyribole from the BE horizon of the Fayette soil. Etch pits on this grain have a wide range in size and shape, and some have a highly indentate perimeter (lower SI) which appear to have resulted from side by side coalescence (BERNER et al., 1980; BERNER and SCHOTT, 1982). Figure le is a pyribole grain from the Bt horizon of the somewhat poorly drained Stronghurst soil. The oriented etch pits on this grain have a narrow range in size and shape but some variation in depth. Etch pit depth was not evaluated in this study. Figure 1f is a pyribole grain from the Bt horizon of the Fayette soil. The etch pits on this grain have a strong orientation and a narrow size range except where coalesced. Linear coalescence (EI) results in trench- or groove-like pits. Some pits appear to be growing by a corner-to-corner coalescence. Figure 1g is an orthoclase grain from the Bt horizon

range in shape and size of etch pits. ( f) Pyribole grain from the Bt horizon of the Fayette soil. Note the variations in coalescence. (g) Orthoclase grain from the Bt horizon of the Stronghurst soil. Note the lack of orientation of pits and the wide range in etchrpit size and shape. (h) Orthoclase grain from the E horizon of the Traer soil. Note coalesced pits encircling the grain. Scale bar is 10 grn in each micrograph except in (h), where scale bar is 50 pm.

D. L. Cremeens, R. G. Darmody, and L. D. Norton

3428

of the Stronghurst soil. This grain contains etch pits with a wide variety of size and shape and no apparent orientation. Some pits in the lower central part of the grain appear as grooves or trenches. Figure 1h is an orthoclase grain from the E horizon of the Traer soil. This highly pitted grain has numerous small etch pits, some of which appear to be oriented normal to the long axis of the grain. These pits have coalesced to the point where they form a ring around the grain. Etch-pit Size Distributions

Etch-pit equivalent diameter for orthoclase (Fig. 2) was skewed toward the smaller size classes in all cases. Mean values of EQD occurred between 0.7 and 1.Opm for all horizons in all soils. Significant differences in etch-pit EQD existed among soils and grains but not among horizons alone or due to soil-horizon interaction (Table 3 ). Total number of observed etch pits per horizon and mean number of pits per grain was greatest in the poorly drained Traer soil. Specifically, the Traer soil had significantly smaller and more numerous etch pits than the better drained soils. The smaller, more numerous etch pits occurred in the E and Btg horizons of

the Traer soil, a horizon trend not found in the better drained soils. For the pyriboles, the distribution of etch-pit size (EQD) around the mean was also skewed toward the smaller sizes (Fig. 2). Etch pits on pyriboles, however, tended to be larger than those on orthoclase. The mean values of EQD on pyriboles ranged between 1.1 and 1.6 pm. Etch pits were also more numerous on pyribole grains than on orthoclase grains for all horizons. Etch-pit equivalent diameter of etch pits on pyriboles varied significantly among soils and grains but not among horizons or due to soil-horizon interaction. As with the orthoclase grains, the pyriboles from the Traer soil had the smallest and most numerous etch pits. Overall, the smallest etch pits on pyriboles were in the Traer E and Cg horizons, while the largest etch pits were in the BE horizon of the welldrained Fayette soil. Etch-pit Density Distributions

The distribution of etch-pit density in the catena was determined by dividing the sum of the area of all etch pits on all twenty grains in each horizon by the sum of the imaged

Orthoclase Traer

Stronghurst

Fayette BE

E

E

Bt

Bt

Btg

0.

2ooo.

Pyriboles d3

1

-s g 2000-

Fayette

1

Stronghurst

1

Traer

J

BE

0. 2000.

Bt

Bt

0. C 2000.

Equivalent

Diameter

(microns)

FIG. 2. Etch-pit size-frequency distribution for orthoclase and pyriboles.

3429

Weathering of silicate minerals in soils

clase, there were significant trends in the SI and the EI with depth (horizons) and drainage (soils) (Table 4). Among the soils, the poorly drained Traer soil had significantly greater values of SI for orthoclase, indicating that etch pits were less indentate-shaped than those on grains from the better drained soils. The SI for orthoclase, averaged by horizons, was significantly greater in C horizons, indicating smoother, more regular shapes. Overall, the SI increased with depth and decreased with increasing drainage. The shallower horizons of the better drained soils were associated with significantly smaller values of SI, indicating more irregular or indentate-shaped etch pits, possibly due to increased coalescence ( BERNERand HOLDREN, 1979). Etch pits on orthoclase grains from the well-drained Fayette soil had significantly greater values of EI, indicating more elongated shapes, than those occurring in the more poorly drained soils. E horizons had significantly greater values of EI than Bt and C horizons. More elongated shapes represented by greater values of EI are interpreted as resulting from endto-end linear coalescence ( BERNERand SCHOTT, 1982). Orthoclase grains from the BE horizon of the Fayette soil had etch pits with shapes (SI and EI) indicating more coalescence than grains from deeper horizons and poorer drainage. For pyriboles, both the SI and EI values varied significantly among soils (Table 4). Neither index varied significantly among horizons or between specific soil-horizon combinations. Maximum SI values (less indentate) occurred in the Fayette soil, and minimum SI values (more indentate, more irregular) occurred in the Traer soil. This indicates possibly more side-by-side coalescence of etch pits in the more poorly

areas of the twenty grains (Table 3 ) . Maximum density of orthoclase grains occurred in the BE and E horizons, with the exception of the Traer soil, where maximum density occurred in the Btg horizon. Minimum densities were found in the C horizon in all soils. BRANTLEYet al. ( 1986) attributed the decreasing etch-pit density, with depth, on saprolitic quartz grains to increasing concentrations of silica in pore solutions. Horizons in the Traer soil had a greater etch-pit density than corresponding horizons in the better drained Stronghurst and Fayette soils. This may be the result of the greater amount of solution-grain contact time for the Traer soil. For the pyriboles, maximum etch-pit density occurred in the Bt( g) horizons of the Fayette and Traer soils. This is not consistent with the trend in density with depth for the orthoclase grains, or with quartz grains reported by BRANTLEY et al. ( 1986). The etch-pit density maximum for pyriboles in the Fayette and Traer soils is associated with the pH minimum (Table 1). It is also associated with the greatest number of etch pits. In the Stronghurst soil, the maximum density occurred in the E horizon; although the value may not be significantly different from the Bt horizon. For pyriboles, the etch-pit density distribution between soils did not vary to the extent that it did for orthoclase. Etch-pit Shape Factor Distributions

Etch-pit shape is determined by the relative dissolution rates in different crystallographic directions ( BRANTLEYet al., 1986) as influenced by crystal imperfections. For ortho-

Table 3. Distribution of Etch Pits and Equivalent Diameter (EQD) of Etch Pits.* Pvriboles Orthoclase

Fayette5

Total Mean/ Mean Pit** pits arain ED0 dens -lun7818 130~a1.00

Stronghurst

11400 190 a0.95

Soil-t horizon

Total Mean/ pits arain

Mean Pit EOD Dens -lun-

14345 239 13693 240

al.59 al.41

17364 318 18149 289 14739 246

al.41 al.39 al.38

Traer

16617 277

0.81

: C

13256 14807 247 221 7772 129

a0.89 a0.91 a0.97

3449 172 2956 148 1413 71

al.00 al.03 a0.98

3.4 2.6 1.7

4496 5651 4199

al.64 ba1.54 ba1.60

9.6 12.4 8.7

4425 4366 221 218 a0.92 2609 130 al.00

4.3 3.2 2.7

4731 236 5401 270 cbal.40 cba1.42 3561 178 cbal.41

9.1 8.3 4.6

Fayette BE

Et

C Stronahurst

22214

370

225 283 210

1.18

&

Et cg

Traer E 5441 272 co.76 4.8 7467 373 cl.14 7.9 8tg 7426 371 cb0.77 5.9 7767 388 cb1.27 12.9 Ca 3750 188 a0.91 4.2 6979 349 cl.13 5.8 *Distribution histograms are shown in Fig. 2. tTota1 number of pits in a soil, in a horizon in all soils, or in a specific horizon in a specific soil. 6Fayette is well-drained, Stronghurst-somewhat poorly drained. Traer-poorly drained. **Pit Density - sum of the area of all etch pits on all twenty arains in a soecific horizon divided bv the sum of imaaed areas of the twenty grains. I-fFor each mineral, means not preceded by the same letter are significantly different (a = 0.05).

D.L.Cremeens, R.G.Darmody,andL.D.Norton

3430

Table 4. Etch-oit Shaoe Factor Analysis* rthoclase SoilL ? SI EIPyribo:les horizon Fayette Stronghurst Traer

1.990 al.92 al.91

a33.31 a33.19 35.35

2.56 a2.92 a2.99

a34.25 b32.24 c30.05

1.99 al.93

a32.64

a2.87

a32.10

C

al.92

a33.38 a33.70 34.77

a2.76

i!

FaFe

a2.02 ~32.43 al.99 cb33.55 Bt bal.98 cba33.96 C Stronghurst a2.00 ~32.05 E b1.86 c32.97 Bt Cg

Traer E

W CQ

a2.84

a31.79

b2.57 a34.98 b2.50 ba33.44 b2.61 ba34.32 ba2.83 cb33.05 ba2.92 cb32.91

ba1.92

ba34.55

ba3.00 dc30.76

al.95

a35.65

ba2.87

d29.88

al.95

a34.60

a3.18

d29.97

1.85 a35.81 ba2.91 d30.29 *Mean values for all etch oits in a soil. horizon, or soil-horizoh combination: tEI=Fmax/Fmin, SI=(A/P2)*1000 see text for definitions fFor each mineral, means preceded by the same letter are not significantly different (a -0.05).

drained environments ( BERNER and SCHOTT, 1982). There was a trend toward lesser values of SI with depth, although differences in SI among horizons or due to soil-horizon interaction were not significant. Etch pits on pyriboles were elongated on all grains. Pyribole grains from the Fayette soil had significan~y Iesser EI (less elongated) values and significantly greater SI (less indentate) values. This indicates less linear coalescence and less sideby-side coalescence in the well-drained soil (BERNER and SCHOTT, 1982). These shape trends are associated with the large etch pits in the Fayette soil. The most elongated (greater EI) and indentate-shaped (lower SI) etch pits on pyriboles were associated with the greater number and smallest size of etch pits occurring in the Traer soil. For orthoclase, the shape trend was the opposite: The greatest numbers of pits occurred in the Traer soil, but the more elongated and indentate pits occurred on grains from the Fayette soil. A qualitative summary of the statistically significant trends in etch-pit shape with drainage (soils} and depth (horizons) is shown in Fig. 3. This shows the increasingly elongated and indentate-shaped pits in orthoclase as drainage increased. For orthoclase, the trend is consistent with the concept that etch pits coalesce as they grow. For the pyriboles, the pits become increasingly elongated and indentate as the soils become wetter. If the more elongated and irregularly shaped pits resulted solely from linear and side-by-side coalescence, respectively, then this trend would indicate greater rates of etch-pit growth in the wetter soils. However, the smallest pits are from the Traer soil. Etch-pit Size and Shape Association An evaluation was made to determine relationships between etch-pit size (EQD) and the two shape factors, SI and EI. If large etch pits result solely from the coalescence of

smaller pits, (BERNER and HOLDREN, 1979; BERNER and SCHOTT, 1982 ) , then etch-pit size should be a function of EI and Sf. Large values of EI reflect elongate pits (linear coalescence) and smaller values of SI reflect more indentate etch pits (side-by-side and comer coalescence ) . Multiple regression of EQD = aE1 f bS1 for all horizons and all soils indicated that approximately 30-55% of the variation in etch pit EQD was associated with variation in EI and SI. Etch-pit equivalent diameter evaluated against each shape factor individually indicated a tendency toward randomness overall. This suggests that large etch pits may not result solely from the coalescence of smaller pits. Coalescence occurs at all size scales and probably at all different types of imperfections. These results may also indicate that the shape of etch pits is partially a function of the proximity of crystal imperfections. On individu~ grains, there may be a stronger correlation with etch-pit size and one or both of the shape factors. DISCUSSION A hypothesis for surface reaction control of the rate of mineral dissolution in the weathering environment has been developed based on the existence of well-developed etch pits on grain surfaces (WILSON, 1975; BERNER, 198 I). This hypothesis is based on analogies with metallurgy and emphasizes that major dissolution originates only at points of excess energy on the mineral surface such as dislocation outcrops. In discussing the formation of etch pits in the current study, and in particular the nature of the protopit, it must be kept in mind that a large part of the evidence is missing and that

ORTHOCLASE STRONGHURST

a I SI

-c

C

FAYEXTE

PYRIBOLES TRAER

STRONGHURST

‘AlWi-lT

A

---bO

FIG. 3.Qualitative summary oftrendsin etch-pit shape.

Weathering of silicate minerals in soils evidence collected at the macroscopic scale of etch pits (approximately 1 pm) may be inadequate to completely explain mechanisms or conditions that occurred on the scale of angstroms ( MEIKE, 1990). Several types of grain imperfections, across a broad range of size scales, can serve as points of higher free energy on a mineral surface. Relative dissolution rates may be dependent on crystallographic parameters such as dislocation outcrops on grain surfaces, twin boundaries, fractures, microcracks, phase boundaries, exsolution lamellae, etc. Laboratory hornblende dissolution studies by ZHANG et al. ( 1990) indicated that higher dissolution rates occur at cracks, grain boundaries, cleavage planes, fractures, and dislocations. High-energy points on mineral surfaces result from strained (high energy) atomic bonds associated with plastic strain in minerals. There are several mechanisms capable of producing plastic strain in minerals. For feldspars, intercrystalline slip and mechanical twinning are among the more important mechanisms ( TULLIS, 1983). Dislocations (or more specifically, dislocation density) have been suggested as a prime factor in selective dissolution on mineral surfaces ( HELGESON et al., 1984; HOLDREN and SPEYER, 1985), drawing from analogies with metallurgy. In metallurgical work, etchants are used to locate dislocations and determine dislocation density. However, as noted by SCHADLER( 196 1) , it is difficult to prove that all dislocations, or more importantly, that only dislocations form etch pits when treated with a specific etchant. The intersection of multiple slip systems results in zones of tangle dislocations ( TULLIS, 1983). This is thought to be the most likely candidate for enhanced dissolution as the greatest configurational strain energy is obtained from tangles of unorganized dislocations ( MEIKE, 1990), Thus, the postulated effect of dislocation density on dissolution ( HOLDREN and SPEYER, 1985) may be much less significant than previously indicated ( MEIKE, 1990; BLUME et al., 1990; MURPHY, 1989; CASEYet al., 1988). Exsolution is a process whereby an initially homogeneous solid phase separates into two (or possibly more) distinct crystalline minerals without the addition or removal of material to or from the system ( HURLBUT and KLEIN, 1977). Perthites are produced by the subsolidus exsolution in alkali feldspars (YUND and TULLIS, 1983). Perthites may occur with a number of different textures and on a wide variety of scales. Lamellar exsolution is one of the common textures; irregular vein and patch petthites are also common. Exsolution also occurs in amphiboles as unmixing of two amphiboles from a primary homogeneous solid phase occurs as a subsolidus reaction ( GHOSE, 198 1). Amphibole exsolution can occur by two mechanisms: nucleation and growth or spinodal decomposition. In hornblende especially, nucleation and growth is the most common mechanism. Spinodal decomposition results in regularly spaced exsolution lamellae. Stress applied to a crystal is opposed by the restoring forces of atomic bonds. Before the stresses can get large enough to produce larger elastic strains, most natural materials will undergo plastic strain such as fracture, slip, or mechanical twinning (YUND, 1983). MEIKE (1990) described fracture as the microscopic scale representation of a group of dislocations arranged side by side. In feldspars, dislocation glide has been

3431

inferred from optical microstructures such as kink bands, deformation bands, undulatory extinction, and deformation lamellae. Instead, some of these features may be due to microcracking and/or microtwinning (TULLIS, 1983). Microcracking is an important deformation mechanism because the excellent feldspar cleavage makes crack initiation and propagation relatively easy. Transformation twinning occurs after crystal formation as crystals rearrange their structure to one of a different symmetry. Transformation twinning in Kfeldspar occurs as microcline or “tartan” twinning ( HURLBUT and KLEIN, 1977). Various other mechanisms have been proposed to explain the formation and growth of etoh pits on mineral grains. Etch pits observed on mica and micaceous vermiculites after laboratory etching with HF were attributed to spontaneous fission tracks from U impurities (YEE et al., 1974). The geometrical shape of tracks, revealed after etching, may be related to the direction of crystallographic axes and unit cell parameters. Microbial action has been proposed as an influence on the etching of biotite and hornblendes in estuarine sands ( FRANKEL, 1977) and in the formation of grooves on glass shards (Ross and FISHER, 1986). WILSONand JONES( 1983 ) suggested that the excretion of select organic acids by lichens influence the preferential etching of feldspars and other minerals at areas of lamellar microstructure. In a study of the kinetics of potassium release by sand-sized K-feldspars from sandy coastal plain soils, SADUSKYet al. ( 1987) found submicron to micron linear to curvilinear cracks that widened to form oval-shaped etch pits. These workers suggested that large prismatic etch pits resulted from enhanced physical weathering, abrasion, and subsequent removal or plucking of loose pieces resulting from enhanced chemical weathering. Coalescence of the prismatic etch pits produced an angular, highly irregular surface. Based on the above discussion, the most likely candidate for the protopits on pyriboles is strain associated with exsolution lamellae. Where etch pits are uniform in size, shape, and spacing (Fig. 1c, e, and f), exsolution lamellae were probably the loci for dissolution. Where etch pits are more irregularly shaped and spaced, then microcracks and fractures are likely candidates. BERNER et al. ( 1980) suggested that side-by-side alignment of etch pits on augite often occurs along the boundaries of basal lamellae. Coalescence during growth makes determination of the protopit even more difficult. On orthoclase grains exsolution lamellae (perthites) are a likely candidate for etch-pit formation (HOLDREN and SPEYER, 1987). In particular, irregular and patch perthites could account for irregularly shaped and spaced pits (Fig. lb). However, during the EDXA analysis in this study, the Na-rich phase was never detected on the grains. The orthoclase grains were specifically screened by EDXA to include only the K-phase. Obviously, other mechanisms were involved. Microcracks, fractures, and twinning boundaries are likely candidates. Twinning might account for crystallographically oriented etch pits, whereas microcracks might account for the more irregularly shaped pits, (Figure 1g). Laboratory-etched Na-rich plagioclase revealed variation in etching in different twin lamellae and etch pits aligned along twin boundaries ( SEIFERT, 1967 ).

3432

D. L. Cremeens, R. G. Darmody, and L. D. Norton

In the population of atomic bonds existing within a single grain, the bonds that are broken first during dissolution are those with the highest energy regardless of the origin of that energy. As dissolution continues, lower-energy, more stable bonds will be broken; and the etch pit will grow. The energy source for the dissolution reaction is the chemical potential of the surrounding solution. Etch pits will grow until the free energy requirements of the hydrolysis reaction at the edge of the pit are greater than that of the surrounding solution. Within a single phase, the trends in etch-pit shape may indicate a different rate of etch-pit formation and growth or a different mechanism as discussed by SADUSKYet al. ( 1987 ) . In our study, it was assumed that depth and lateral variations in loess lithology were a minimal factor in the etch-pit shape distribution. Instead, the geochemical conditions associated with soil depth and drainage affected the formation and/or growth of etch pits. Organic acids (chelators), catalysts and poisons (LASAGA, 1981; BRANTLEYet al., 1986), and the ambient redox potential all vary with soil drainage. Dissolved mineral constituents in solution, most noticeably silicon, vary with depth in soils ( BRANTLEYet al., 1986). LASAGAand BLUM ( 1986) used Monte Carlo simulation to predict that the shape of etch pits upon formation was partially a function of the degree of undersaturation of the surrounding fluid (chemical potential between the fluid and the grain). The nonuniform surfaces of minerals offer a range of sites and corresponding activation energies available for the adsorptiondesorption processes associated with dissolution. The presence of adsorbed inhibitors or poisons is geochemically important to the rate of dissolution because of the nonuniform nature of the mineral surfaces ( BERNER,198 1; LASAGA, 198 1). A substance such as a phosphate compound, which is strongly adsorbed onto the active sites, will reduce the rate of dissolution significantly. SMECK( 1973 ) studied phosphorus distribution in northeast Illinois soils and found that total P contents were greater in the downslope poorly drained soils than in associated better drained soils. In profile, total P was greatest in the B horizon due to translocation. In the acid soils of the study reported here, a significant portion of the total P will occur as insoluble iron and aluminum phosphates. When a soil is waterlogged or flooded, there is a shift from aerobic to anaerobic microbial carbon mineralization (ALEXANDER,1977). Organic acids accumulate because of the fermentative character of the microflora of wet soils. The distribution of specific organic acids/chelators is also affected by landscape position and soil drainage. In a drainage sequence of Michigan spodosols, VANCE et al. ( 1986) found that the total phenolic content increased going from the somewhat excessively drained to the somewhat poorly drained soils. In particular, protocatechuic acid was found in greater concentrations in the somewhat poorly drained soils than in the better drained soils. Protocatechuic acid is a bidentate ligand with ortho-dihydroxy functional groups that have the potential to form chelate complexes with polyvalent metal ions such as Al and Fe (MANLEY and EVANS, 1986; VANCE et al., 1986). This type of chelate complex may play a vital role in solubilizing and mobilizing Fe and Al. MANLEYand EVANS ( 1986) determined that the tri- and bidentate citric and oxalic acids were more effective in dissolving feldspar than protocatechuic acid and monodentate ligands. However,

the amount of aluminum released on dissolution of feldspars appeared to be related more to concentration of the acid than on its ability to form Al-complexes. Iron mobilization in soils can occur by two mechanisms depending on drainage ( STEVENSON,1982). In aerobic conditions, Fe is complexed (tridentate chelate) and then reduced by polyphenols. Under anaerobic conditions, microbially reduced Fe is chelated by bidentate ligands. Both mechanisms result in a mobile ferrous iron-organic matter complex. Only a few studies have evaluated the chemistry of soil water in loess ( SCRIVNERet al., 1973; KARATHANASIS,1987). These studies found that soil solutions collected by pressure or centrifugation had Al and Si activities controlled by kaolinite or amorphous aluminosilicate precipitates. More important in this discussion is the seasonal dynamics and fluctuations of Si and Al concentrations in the soil solution ( KARATHANASIS,199 1). The crude model of soil as a porous body through which water continually percolates must be replaced with one in which the chemical potential of water changes with time following a series of irregular cycles of varying amplitude and frequency (MARSHALL,1977 ) . During the year, most soils are in the drying part of a cycle much longer than in a wetting part. Exceptions to this are poorly drained soils and those in regions subject to high daily rainfall. In a drying cycle, the geochemistry of moisture films surrounding individual grains may approach conditions to where dilute solution chemistry no longer holds and the precipitation of secondary phases controls dissolution. The upper horizons in well-drained soils would be typical of this environment. When these environments are wetted, it is with relatively fresh water ( BERNER,1978 ) . However, they exist in the drying cycle during the greatest part of the year. Poorly drained soils, on the other hand, do not go through the drying cycle to the same extent. These soils are continuously moist, and the soil solution may be an inexhaustible sink for the constituents of the hydrolysis reactions. This would suggest more etch-pit formation or greater etch-pit growth in the more poorly drained soils. Our evidence shows that poorly drained soils have greater numbers and significantly smaller etch pits than better drained soils for both minerals. CONCLUSIONS The number of etch pits found on a grain surface should be a function related to the rate of formation minus the rate of coalescence (S. L. Brantley, pers. commun.). For both minerals, the smallest pits and the greatest number of pits occurred in the poorly drained soils. This suggests a slower growth rate presumably from less flushing with fresh water. Etch-pit density for orthoclase decreased with depth, probably a function of porewater composition. Maximum values of etch-pit density for pyriboles occurred in the Bt(g) horizon of the well-drained and the poorly drained soils. This density maximum was associated with the pH minimum in these soils. Trends in etch-pit shape seem to indicate greater coalescence in orthoclase from the well-drained soils. The largest and most irregularly shaped and elongated pits were found in the upper horizons of the well-drained soil, suggesting a greater growth rate. Thus, the orthoclase data is consistent with the concept of the flushing model as described by BER-

Weathering of silicate minerals in soils NER ( 1978).

Although the well-drained soil is periodically flushed with relatively fresh water, it is not necessarily always at a lower chemical saturation state. It may be at a lower saturation state only during brief wetting periods, and this may be sufficient to cause weathering according to the flushing model. During drying periods, other mechanisms may predominate. In pyriboles, etch-pit shapes attributed to coalescence are more prevalent in the poorly drained soil. Thus, for pyriboles, the size data is consistent with the flushing model; but the shape data indicate greater growth rates, as expressed by coalescence attributed shapes, in the poorly drained soil. In pyriboles, etch-pit shape may not be determined by coalescence alone, and coalescence may not be entirely a function of etch-pit growth.

Acknowledgments-We thank the Center for Electron Microscopy, Univ. of Illinois, for donated scope time. Stan Livingston and Scott Vance are acknowledged for their contributions toward data gathering, analysis, and presentation. The senior author wishes to acknowledge the contributions of the late Dr. Ivan Jansen and Dr. Samual Canner. Dr. S. L. Brantley and an anonymous reviewer provided helpful comments. Editorial handling: G. Sposito REFERENCES ALEXANDER M.

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