Mineral transformations in some sandy soils from Alberta, Canada

Geoderma, 61 (1994) 17-38 Elsevier Science B.V., Amsterdam

17

Mineral transformations in some sandy soils from Alberta, Canada J.M. Arocena, S. Pawluk and M.J. Dudas Department of Soil Science, Universityof Alberta, Edmonton, Alta. T6G 2E3, Canada (Received November 27, 1991; accepted after revision March 26, 1993)

ABSTRACT Three sandy Luvisols from Alberta were studied through mineral investigations and geochemical prediction based on the ionic composition of the in situ soil solution to evaluate the stabilityand formation of minerals. The transformation of phyllosilicatesin this study appears to be dominated by the vermiculitization of mica and to a lesserextent of chlorite.Hydroxy intcrlayercd phyUosilicate is also obscrvcd throughout the sola. There are good agreements between geochemical predictions and mineral determination for calcite,dolomite, goethite, hematite and Icpidocrocite. These observation suggest that a geochemical model using data from the composition of in situ soilsolution can bc used to evaluate the stabilityand formation of the low quantities of the c o m m o n carbonates, oxides and oxyhydroxidcs. The unique micro- or paleo-cnvironments are believed to bc responsible for the disagreement between the model and the mineral observation for gypsum while the "anti-gibbsite"effect of organic components may bc responsible for the non-formation of gibbsite.

INTRODUCTION

Mineral transformation is most often studied by mineralogical characterization using X-ray and total elemental analysis, electron microscopy, among others. In recent years, the use of geochemical models in soil mineralogy has gained acceptance and when combined with conventional observations offers an alternate and complementary approach for evaluation of mineral transformations. The use of models is especially appropriate for minerals that readily attain equilibrium with the in situ soil solutions. Gibbsite and (proto)imogolite are examples of such minerals which can achieve equilibrium with in situ solution within 24-48 hours of residence time (Turner and Brydon, 1965 ); ferfihydrite (Spiers, 1990) and imogolite (Dahlgren and Ugolini, 1989) are further examples. The carbonates and sulfates are also reported to attain equilibrium conditions over a short period of time (Miller, 1989). Transformation of phyllosilicates, however, is different because equilibrium conditions require 1200 days for kaolinite (May et al., 1986) and 0016-7061/94/$07.00 © 1994 Elsevier Science B.V. All fights reserved. SSDI O016- 7 061 (93)E0044-V

18

J.M. AROCENA ETAL.

about 1-4 years for the 2:1 phyllosilicates (e.g., Kittrick, 1971 ). Moreover, the uncertainty about the chemical composition of the 2:1 phyllosilicates makes it very difficult to choose the appropriate constants for the various thermodynamic parameters. In such situations, the use of XRD, wet chemistry and other conventional measurements are still the normal practices. The objective of this study is to characterize the nature of minerals in some sandy soils from Alberta, Canada with emphasis on Fe and A1 oxides and phyllosilicates. The transformations of these minerals will be studied in relation to physical mineral determination and the use of a geochemical model based on the composition of in situ soil solution. MATERIALS AND METHODS

The study area, physical and chemical analyses Three sandy soils developed on Quaternary sandy deposits of alluvial and aeolian origin were selected for the study. The site, profile descriptions and genesis of these pedons as revealed by the microfabric, leachate and soil composition are given by Arocena et al. ( 1992 ). Particle size separation was conducted on samples < 2 mm in size after ultrasonic dispersion for 6 minutes at 400 watts. Clays were separated from sand and silt through successive dispersion and gravity sedimentation. The fine earth fraction and selected clay separates were treated with dithionitecitrate-bicarbonate (Mehra and Jackson, 1960), sodium pyrophosphate (McKeague, 1967) and ammonium oxalate (Schwertmann, 1964) and the extracts were analyzed for Fe, A1 and Si by atomic absorption spectrophotometry. Cation exchange capacity (CEC) of the clays was determined following the methods of Alexiades and Jackson (1965 ). Total elemental composition of the different particle separates was determined according to the microwave digestion procedure proposed by Warren et al. (1990).

Mineralogy Mineral identification was conducted by XRD analyses with a Co-Ka radiation generated at 50 kV and 25 mA using a goniometer equipped with a curved crystal monochromator. Diffraction was carried out by step-scanning at 0.05 ° 20 intervals every 2 seconds. Untreated clay fractions (fine < 0.2 a m and coarse >0.2 a m ) and silt samples w e r e C a 2+ and K + saturated. Subsamples of coarse clay from the B horizons were successively extracted with 0.33M dithionite-citrate-bicarbonate (DCB) followed by 0.33M sodium citrate (Na-cit) solutions and were also C a 2÷ saturated. Potassium-saturated clays were scanned from 3-36 ° 20 after equilibrating at 0% relative humidity

MINERAL TRANSFORMATIONS IN SOME SANDY SOILS FROM ALBERTA, CANADA

19

(RH) and from 3-19°20 after equilibrating at 54% RH as well as after heating to 300 and 550°C. Calcium-saturated clays were likewise scanned from 3-36 ° 20 at 54% RH and from 3-19 ° 20 after glycerol (Gly) and ethylene glycol (EG) solvations. Mica was identified by the stable 1.0 nm basal spacing in all the XRD treatments while smectitic (low-charge) clay was recognized by the swelling of the basal spacing to about 1.7 nm with EG and Gly solvations of Ca-saturated samples and the identification of vermiculitic (high-charge) clay was based on the expansion of the basal spacing to about 1.7 nm with EG but not with Gly. The distinctions between vermiculite and smectite was supplemented by intercalation of the DCB and Na-cit treated clays with nc = 6 to 18 alkylammonium hydrochloride compound to measure the charge density of expanding 2:1 type phyllosilicates (Lagaly and Weiss, 1969 and Olis et al., 1990). Vermiculite has a layer charge of > 0.6 eq ( + ) (Si, A1)40/~ as compared to < 0.6 for smectite (Bailey, 1980). The vermiculite content was determined from DCB treated clays following the method proposed by Alexiades and Jackson (1965) and the mica content was calculated from the total K20 content. Mineral analysis of the sand and silt fractions was determined by powder diffraction after the fractions were packed in an aluminum sample holder against a filter paper to minimize preferred orientation and were scanned from 3 to 90°20. Iron-rich minerals were concentrated using a modified Frantz isodynamic separator operated over a range of magnetic flux densities from 0.2 to 1.7 tesla. In situ XRD analyses on selected pedofeatures were conducted by peeling part of a thin section from the glass slide and mounting it on a transmission type microcamera equipped with a 50/tm collimator (Wicks and Zussman, 1975; Arocena et al., 1992). The camera was exposed to fine focus Co-Ka radiation generated at 30 kV and 20 mA. Exposure times varied from 8-12 hours. Sub-microscopic observations were conducted on selected pedofeatures using a scanning electron microscope equipped with an energy dispersive X-ray analyzer (EDS). Heavy mineral components of the sand fractions were mounted on glass slides for identification under a polarizing microscope.

Soil solution and mineral stability In situ soil solution referred to in this study is the "stationary" interstitial water at field moist condition. The average water content during sampling 1 was about 90 g water kg- 1 soil for pedon and about 180 g water kg- ~ soil for pedon 2. In situ soil solution was extracted by immiscible displacement using a 1:1 ratio of field moist soil to tetrachloroethylene and centrifuged at 14,500 rpm for 1 hour. The solutions were analyzed for pH, electrical conductivity and cation and anion content. Cations other than A1 and Si were determined

20

J.M. AROCENA ET AL.

using an atomic absorption spectrophotometer and anions other than carbonate by high performance liquid chromatography. Aluminum was determined colorimetricaUy by the aluminon-method (Barnhisel and Bertsch, 1982), Si by the blue silicomolybdous acid method (Hallmark et al., 1982 ) and carbonate by titration against 0.01N HCI. The activities of the ionic components were calculated from the measured concentrations of the different ionic constituents using the computer program SOLMINEQ-PC (Kharaka et al., 1988 ). The saturation index ( SI ) for each mineral was calculated as the quotient between the log of the ionic activity product and the log of the solubility product constant. The SI values of zero, positive and negative denote equilibrium, oversaturation and undersaturation with respect to the mineral of interest, respectively. All the thermodynamic calculations were assumed on a closed and isolated system at 25 °C temperature and 1 arm pressure. RESULTS

Minerals and their distribution The classes of minerals identified in the pedons are carbonate, oxides, sulfate and silicates (Table 1 ). The carbonate mineral present in pedons 1 and 2 is calcite. The oxides present are mainly those of Fe and comprise goethite, hematite, lepidocrocite and ferrihydrite. Goethite occurs with quartz and phyllosilicate (s) as ascertained by in situ Laue photographs of Fe-rich nodules in thin sections (Fig. 1, top). Lepidocrocite is present in the very fine clay fraction and in some nodules in the Btf horizons (Fig. 1, bottom). The existence of ferrihydrite is apparent from the high Feo/Fed ratios (Schulze, 1981; De Geyter et al., 1982) of samples particularly from the B horizons (Table 3 ). In pedon 3, the presence of gypsum was highly localized in some of the indurated nodules in the B horizon. The euhedral crystals are smooth and free of any corrosion (Fig. 2 ). The silicates in all the three pedons are similar (Table 1 ) and composed of quartz, feldspar and phyllosilicates and for this reason only the X-ray patterns for pedon 1 are discussed. Quartz and feldspars are present in the sand and silt fractions with quartz also identified in the clay separates as well. There are several phyllosilicates such as micaceous minerals, as suggested by the stable dool spacing near 9.97 nm for all the X-ray treatments. The observed doo2 reflection at around 0.498 nm further suggests that muscovite is present in the clay and silt fractions from the C horizons (Figs. 3 and 4). Chlorite is also identified in the clay and silt fraction of the C horizon (Figs. 3 and 4). The X-ray patterns of the chloritic mineral separated from the bulk of the clay and silt samples by magnetic separation suggests it to be clinochlore (Fig. 5A, B).

MINERALTRANSFORMATIONSIN SOMESANDYSOILSFROMALBERTA,CANADA

21

TABLE 1 Occurencc of minerals in three sandy Luvisols from Alberta Minerals

Occurrence a

Horizon

Fraction or pcdofeatures

Carbonates calcite

C

sand fraction

Sulfate gypsum

B (pedon 3 )

indurated nodules

Oxides hematite goethite lepidocrocite

C Ac, B, C B

Fe-rich nodules Fe-rich nodules fine clay

Silicates tectosilicates quartz feldspars

Ae, B, C Ae, B, C

sand, silt and clay sand and silt

phyllosilicates muscovite vermiculite smcctite kaolinite chlorite

Ae, B, C Ae, B, C C Ae, B, C C

silt and clay silt and clay silt and clay silt and clay silt and clay

nesosilicates garnet, sillimaniteand andalusite

Ae, B, C

sand

aSimilar for the three pedons except the presence of gypsum in the B horizon of pcdon 3.

The other phyUosilicates arc the 2:1 types with variable basal spacings. The first kind is the high charge component as suggested by the reflection at around 1.4 nm that does not swell with Gly solvation but expands to about 1.7 nm upon EG solvation (Fig. 3). Upon intercalation with different lengths of alkylammonium molecules, the observed relationship between d0ol spacings and the number of carbon atoms in the alkylammonium molecules in the interlayer shows a positive regression line (Fig. 6A ). The calculated charge density of this mineral is about 0.71 eq( + ) (Si, A1)40/~ ~which suggests a high charge species or vermiculite according to the AIPEA classification (Bailey, 1980). This phyllosilicate is present in the Ae horizons and estimated to range from 250-300 g kg -1 clay (Table 4). Another kind of phyllosilicate shows expansion of the basal spacing to about 1.7 nm for the Ca-saturated clay after Gly and EG solvation and K-saturated sample at 54% RH; this suggests the presence of smectite (Fig. 3 ). Upon intercalation with different chains of alkyl ammonium molecules, this mineral

22

J.M. AROCENA ET AL.

Fig. 1. In situ Laue photographs of goethite (GT) occurring in association with quartz in Ae horizon of pedon 3 (top) and lepidocrocite (LP) in association with goethite in Btf horizon of pedon 3. Goethite has X-ray reflections at around 0.498, 0.418, 0.267, 0.257, 0.245 and 0.224 nm; quartz has reflections at 0.422 and 0.334 nm and lepidocrocite has reflections at 0.615,334 and 0.245 nm. Each pattern is obtained after 12 h exposure to fine focus Co-Ka radiation. (Note: the location of the decimal point of the given d-spacing indicates the corresponding Debye-Scherrer ring. )

MINERALTRANSFORMATIONSIN SOMESANDYSOILSFROMALBERTA,CANADA

23

Fig. 2. Scanning electron micrographs of euhedral gypsum crystals from the indurated nodules in the Btf horizon of pedon 3.

formed a step-wise mono, duo and pseudo-trilayering of alkylammonium molecules (Fig. 6B). The calculated charge density is about 0.38 eq( + ) (Si, A1)40/~ t and is consistent with the AIPEA classification (Bailey, 1980). Smectite was identified in the clay fractions (Fig. 3 ) as well as traces in the silt fractions (Fig. 4) of the C horizons. The third kind of 2:1 phyllosilicate with variable basal spacing has a reflection near 1.40 nm at 54% RH; fails to expand with either Gly or EG solvation but gradually collapses to a range of spacings around 1.10-1.30 nm upon 550°C heat treatment (Fig. 3). After six successive 30 minute citrate treatments (Wada et al., 1987) to remove possible interlayer components, this mineral expands to about 1.60 nm after EG solvation but not after Gly solvation (Fig. 7); this suggests that the mineral is a high charge phyllosilicate. Intercalation of the citrate-treated samples with dodecylamine hydrochloride expanded the basal spacing to about 1.8 nm. The estimated charge density based on door spacing at nc=12 (Olis et al., 1990) is 0.71 e q ( + ) (Si, AI)40/~ 1 and is similar to the high charge component identified earlier in the Ae horizons. The calculated (Alo-Alp)/Sio values for B horizons are about 2 (Table 2 )

24

J.M. AROCENA ET AL.

and may reflect the presence of proto-imogolite (Farmer, 1982 ). Estimates of the amount of this mineral in clay samples based on Sio (Partitt, 1990) generally show an enrichment in the B horizons compared to other horizons. Values range from 3.3 to 6.44 g kg -1 clay in the B horizons and from 1.24 to 3.8 g kg-m clay in the Ae and C horizons (Table 2). The presence of kaolinite is evident from the doublet around 0.357 and 0.354 nm (Fig. 8) as suggested by Bradley (cited in Dixon, 1989) and the disappearance of the 0.712 reflection with 550°C heat treatment. The nesosilicates make up most of the heavy fractions (S.G. >2.85 g cm -3) of the sand. These are largely garnets, sillimanite and andalusite.

Composition of the in situ solution Chemical analyses of the natural solutions show higher acidity in the Ae horizons compared to B horizons (Table 3). In the Ae horizons, the range in

~

1,70

" 1"0(X)700 j~.

"

.

~.~

1.40 j 1,00

Ae

K-550°C

.500

~ ,

,

'

A

~

'~

j

K - ~O°C

2.56 ....

K - 550°C

~

.~.500

K - 300°C

K - 54% RH

- 0% RH t

~

I

St

K - 54% RH

Ca - G l y c e r o l

~~.

I

I

;.8'~i d -

I

I

I

. .0.512 . . . . . . . . . . . . 0.370

spacing (nm)

I

Ca - Glycerol

Ca - Glycerol

a - Ethylene Glycol

I

1.401.00

K - 550°C

~

K - 300°C

K - 54% RH

~

Sfj

~

~_Ca - E t h y l e n e Glycol

I

0.290

2.56

0.856

0.512

0.370

d - spacing (nm)

0.290

2.56 .....

O.d 56. . . . .0.512 .

c;.3'~0 ~'0.290

d - spacing (nm)

25

M I N E R A L T R A N S F O R M A T I O N S I N S O M E S A N D Y SOILS F R O M ALBERTA, C A N A D A

1.70 l 1.00

C1

~

,

~

,

I

~

~

0,500

,

I

,

I

j ~

,

~

K - ~O°C

,

C2k

K- 55O%

I

'~

t .00 0.7OO

K 5500C

K - 550°C ,

1.70

i ~

\

K 300=C

A

j~.soo

I

K - 3OO%

1 , 1 , 1 ,

K - 54% RH

j

~

K- 54% RH

~

% ~

K -

54% RH

I J l , l =

K- 0% RH

I

•~

Ca - Glycerol ,

,

i

I

~

I

,

i

I

,

Ca - Glycerol

~ I

,

I

I

~

I

,

I

,

I

,

I

,

~

,

I

,

I

[

a - EthyleneGlycol

,

I

i

I

i

I

i

I

i

(

Ca - 54% RH

,

I

t

I

Ca - Glyce~

~

= - EUwle~ Glycd

1

Ca - 54% RH

'o. 12 'o.3 0 d - s p a c i n g (nm)

d - s p a c i n g (nm)

d - spacing (nm)

Fig. 3. XRD patterns under seven treatments of the clay fractions from pedon 1. The presence of vermiculite is indicated by the expansion of the dool spacing of the Ca-saturated sample to about 1.7 nm with EG but not with Gly solvation (Ae horizon). Smectite is indicated by the expansion of basal spacing to 1.7 nm with EG and Gly solvations (C1 and C2k horizons). The hydroxy interlayered 2:1 phyllosilicate is indicated by the reflection at about 1.4 nm that gradually collapses to about 1.1-1.3 nm with heating to 500°C (BI~, Bt, C1 and C2k horizons). Chlorite is suggested by the 1.4 nm reflection that is unaffected by any treatment (Bfj, Bt, C1 and C2k horizons), while mica is identified by the 1.0 nm reflection that is unaffected by any treatment (in all the horizons). Kaolinite is indicated by the disappearance of the 0.712 nm reflection upon 500 °C heat treatment and the doublet at around 0.357 nm.

pH (3.9-4.6) corresponds to the buffering capacity of most organic acids (Shoji et al., 1988; Ugolini et al., 1988). The amounts of cations are also slightly higher in the Ae horizons than the B horizons regardless of season and are consistent with the values reported in the literature (e.g., Coen and Arnold, 1972; David and Driscoll, 1983; Chesworth and Macias-Vasquez, 1985;

i

'

I

i

2.56

~

I

'

I

'

'

Ca

'

I ~

i

,

- 54~

NIt

1

a - Ethylene Glycol

Ca - Glycerol

i

K - 54% RH

I

K - 300°C

K - 0% RH L

I

0.500

K - 550°C

Ae

0.8'56 o.5'12 0.3~0 .... 0.290 d - spacing (nm)

I

1.00

~-i

,

i

,

i

0.700

~

,

i

,

~

,

i

,

i

d - spacing (rim)

0.37~0 0.290

Ca - 54% RH

,

Ca - Glycerol

K - 54% RH

K - 31~°C

K - 550°C

a - Ethylene Glycol

i

5~"

0.856 ~ 0.512

~

~

2.56

,

~

1.00

I

'i

i

i

tOO

i

,

i

,

i

,-~

3 0 0 ° C

,

i

,

t

i

0.5'12

0.370 '0.2;0

,

Ca - Glycerol

a - Ethylene Glycol

t

-

K - 54% RH

K

Ca - 54% Rtl l

,

.500

Bt K - 550°C

d - s p a c i n g (nm)

2.56' '0.8'56

,

~ i

~,,~

1,40

, --I

~

~

,

,

I

I

~

I

I

I

~

,

i

,

t

,

i

t

-

I

,

~

I

I

L

Glycerol

K - 54% RH

C a

,

a - Ethylene Glycol

I

i

K - 300°C

C1

o 5 ' 1 U 0.370 . . . . . . . 0.290

I

o,s00

d - s p a c i n g (rim)

2.56 . . . . . . . . 0.856

I

~

11.00

,."° I o.~00

1.70

P

I

]

/

Fig. 4. X R D patterns under seven treatments of the silt fractions from pedon 1. Refer to Fig. 3 for the criteria used for the identification of the phyllosilicates.

,

1.70

re Z > re > r"

©

>

MINERAL TRANSFORMATIONS IN SOME SANDY SOILS FROM ALBERTA, CANADA ......

27

C~ . . . . . . . . . . . . . . . . .

A

clay,

pedon

C1,

Ch Ch - chloflte Mi - mica Fd - feldspar Iz - quartz

j |

: : : : : : : : : : : : : : : : : : : : : : : : : : : : : B

silt, C1, pedon

Ch

C~ ,2:_ Otz

Ch

Ch

2.56

Fd

0.856

0.512

,, : ,, : ,, : : : ,, : 0.290 0.239 0.204

0.370

d - spacing (nm)

Fig. 5. XRD diffractograms of magnetic fractions isolated from the silt (A) and clay (B) fractions of the C horizon, pedon I. The samples show the presence of chlorite with quartz, mica and feldspar as impurities in the separation. We used a magnetic flux density of 0.2 T to concentrate the chloritic material. ,

•

26

,

.

,

•

,

,

,

.

,

•

,

A

2.2

1.8

~

:ool spac.Jng=0.888+0.0948n¢

r = 0.g88 vcE"

1.4 I

~

I

=

"

'

"

I

~

I

~

I

~

I

*

I

10'

"

'

"

' 14

"

'

"

1'8

2.2

1.8

1.4

number of ~rbon (n~

Fig. 6. (A) Linear relationship between dool spacing and nc in alkylammonium hydrochloride compound for the high charge phyllosilicate with a calculated charge density of about 0.71 eq ( + ) (Si, Al)4Oi-o' (Ae horizon, pedon 1 ). (B) Step-wise formation of mono-, duo- and pseudo-trilayering of alkylammoninm molecules in the interlayer of smectite with a charge density of about 0.38 eq( + ) (Si, A1)40~ 1 (C2k horizon, pcdon 1 ).

28 A

J.M. AROCENA ET AL. 14rim

C

14nm

UT - EG

B UT - Gly

D

CIT - Gly

dooI spacing (nm)

Fig. 7. X-ray diffractograrns of clay samples from the Btf horizon of pedon 1; untreated (A) ethylene glycol and (B) glycerol solvated and for citrate-treated (C) ethylene glycol and (D) glycerol solvated. The basal expansion with EG but not Gly indicate the presence of vermiculite. The citrate treatment seems able to remove the interlayer component. Legend: UT=untreated; CIT = Nacitrate treated; EG = ethylene glycol; Gly = glycerol.

Manley et al., 1987). The dominant cations are calcium ( 14 to 54 mg 1-l in the Ae and 5 to 15 mg 1- l in the B) and potassium ( 10-37 mg 1-1 in the Ae and 2 to 10 mg 1-~ in the B). The relative quantities of cations follow the trend Ca 2+, K + > Mg 2+, Na +, Fe 3+, or A13+. Generally, the concentration of anions is higher in the Ae horizon compared to the B horizon with C1- and S O 2 - as dominants; this is similar to results published earlier (e.g., Coen and Arnold, 1972; David and Driscoll, 1983; Chesworth and Macias-Vasquez, 1985; Manley et al., 1987). Bicarbonate ions are the exception where, amounts in the B horizons (30 mg 1- ~) are almost double those of the Ae horizons ( 15 mgl-1). DISCUSSION

Thefate of the phyllosilicates Transformation of phyUosilicates is similar among the three pedons and for the discussion purposes, emphasis will be on pedon 1. There is a consistent

29

MINERAL TRANSFORMATIONS IN SOME SANDY SOILS FROM ALBERTA, CANADA

TABLE 2 Extractable Fe, AI and Si and allophane (proto-imogolite) content (g kg -I clay) in the coarse clay fractions Horizon

Alo

Sio

Fed=

Aid

Sia

All a

Feo/Fed

A1/Sie

5.4 34 13

8.9 38 9.3

7.6 9.2 0.8

14 58 45

13 55 16

14 13 5.0

38 64 1.3

0.7 0.6 0.3

0.4 1.7 4.7

2.3 1.3 0.3

5.0 20 11

5.5 20 4.8

2.9 2.1 1.5

7.0 87 35

6.6 30 8.5

4.4 7.0 6.5

14 21 10

0.7 0.4 0.3

0.9 2.7 1.9

0.9 2.0 2.9

8.9 100 8.0

6.6 40 7.6

1.9 5.3 3.7

5.8 11 8.9

11 53 18

0.6 0.8 0.2

1.3 2.4 1.0

Fep =

Alp

Sip

C2k

3.5 15 5.8

5.7 22 5.5

7.2 6.9 0.0

Pedon 2 Ae Btf IIC2k

3.5 10 4.3

3.0 14 1.9

Pedon 3 Ae Btf C2

5.7 23 3.0

4.1 27 4.4

Pedon 1 Ae

B~

Feob

15 118 53

12 55 12

aPyrophosphatc. bOxalate. CDithionitc-citrate-bicarbonatc. dAllophanc content estimated using Parfitt (1990). e (Alo-- Alp)/Sio. i

i

i

i

i

C2, pedon 1

0.354(chlorite)

I

0.398

i

I

,

i

I"

=

I

0.370 0.346 0.325 doo1 spacing (nm)

=

I

0.306

Fig. 8. The identification of kaolinite in the presence of chlorite is inidicated by the doublet around 0.357 and 0.354 nm. The graph is an enlargement of the selected area shown in the Ca54% RH XR D diffractogram from the C2 horizon ofpedon 1.

30

J.M. AROCENAET AL.

TABLE 3

Composition of in situ soil solutions (mg 1- ~) from some sandy soils of Alberta Horizon

pH

Ca

Mg

Na

K

C1-

SO~-

HCO£

Si

AI

Fe

PO]-

NO~-

4.1 5.6

14 5.2

3.5

1.6

2.0 2.3

16 9.6

32 35

23 25

17 30

11 8.3

7.3 2.8

2.7 0.9

27 0.4

0.6 0.5

4.6 5.6

45 13

7.7 1.9

2.6 1.6

37 3.9

12 8.8

16 1.8

20 30

4.5 1.3

2.3 0.2

0.3 0.6

28 0.4

tr tr

54 15

8.0 2.9

3.4 1.5

21 3.6

15 13

37 2.0

15 30

8.7 6.0

3.2 1.4

3.5 1.7

18 1.1

4.4 2.1

4.5 5.5

17 9.4

4.2 2.7

1.7 1.3

10 2.2

30 30

8.1 4.1

15 27

17 7.0

1.7 2.7

1.5 7.1

4.0 0.4

0.3 0.3

4.3 5.8

24 14

4.3 2.9

1.3 1.4

14 2.4

11 6.5

7.9 1.0

14 31

0.6 0.6

2.3 3.1

0.2 0.1

1.9

tr

tr tr

21 9.1

4.4 4.3

1.3 1.5

12 9.5

11 8.0

8.1 0.8

16 22

9.3 4.8

2.8 4.4

5.3 0.8

tr 0.4

4.3 0.1

Pedon 1 Summer 1989 Ae

BO Fall 1989 Ae B0

Spring 1990 Ae Bfj

3.9 5.5

Pedon 2 Summer 1989 Ae

Btf Fall 1989 Ae

Btf

Spring 1990 Ae

Btf

4.1 5.3

decrease in the intensities near the 1.0 nm region for clays from the C horizon towards the soil surface (Fig. 3). The reflection around 1.7 nm of EG solvated clays is absent for the C horizons but is very strong for the surface horizon. The observed X R D patterns indicate a decrease in the amount of mica and an increase in the amount of high charge component (vermiculite) from the C to Ae horizons. Estimates of the quantities of these two phyllosilicates also show higher vermiculite and lower mica contents in the sola compared to C horizons (Table 4). These observations are similar to those reported in many studies involving the transformation of mica to vermiculite in Podzolic soils (Ross et al., 1982; Farmer et al., 1985; Righi and Lorphelin, 1986; De Coninck et al., 1987). Weathering results in the removal of K + from mica (Scott and Amonette, 1988 ) particularly on the edges and along cracks where the mineral shows preferential cleavage (Barnhisel and Bertsch, 1989; Fanning et al., 1989). The transformation is a degradation process referred to as vermiculitization of mica. Chlorite, which occurs in the C horizons (reflection near 1.4 rim) is appar-

31

MINERALTRANSFORMATIONS IN SOME SANDY SOILSFROM ALBERTA,CANADA TABLE 4 Mica and vermiculite contents ( g k g - ~ clay) of the clay fraction Horizon

Ae B Bb C

Pedon 1

Pedon 2

Pedon 3

Mica

Vt"

Mica

Vt

Mica

Vt

100 190 nd c 320

270 170 220 120

210 170 nd 430

300 110 240 160

200 140 nd 280

250 70 190 110

=High charge component (vermiculite). bNa-citrate treated, *Not determined.

ently absent in the A and B horizons (Fig. 3). The weathering of chlorite to vermiculite has been suggested by earlier workers (Rabenhorst et al., 1982; Adams and Kassim, 1983). The production of organic acids in the organic and eluvial layers is believed to be the driving force in these degradation processes (Tongokonov et al., 1987; Ugolini et al., 1987; Kodama and Schnitzer, 1977). In Podzolic soils, the liberal amounts of Fe and A1 present in the soil solution may be incorporated as interlayer components through a process that may lead to the formation ofinterlayered phyllosilicates (Farmer et al., 1985; Harris et al., 1988 ). This phenomenon is probably occurring in pedons under study as suggested by the following observations: ( 1 ) failure of the K-saturated clays from the B horizons to completely collapsed to 1.0 nm after heat treatment at 550°C, and (2) by the extractions of AI and Fe by Na-citrate treatments of the clays from B horizons (Table 5) equivalent to about 50% and 10-20%, respectively, of that removed by dithionite-citrate-bicarbonate. The extracted A1 is believed to be an hydroxide in the form of "gibbsite islands" that act similar to the gibbsite sheet in chlorite (Pawluk and Brewer, 1975 ). A similar suggestion is proposed for Fe (Carstea et al., 1970; Ghabru et al., 1990) and in addition, Fe is proposed to originate from the tetrahedral layer where it may substitute for Si (Cardile, 1989). However, another possible explanation for the observations mentioned above is the incomplete removal of OH-sheets from chlorite (Rabenhorst et al., 1982; Adams and Kassire, 1983 ). The extent of this phenomenon is not believed to be great because of the low amount of Mg extracted by Na-citrate from the clays. Moreover, the low amount of clinochlore in the solum cannot account for the high amount ofinterlayered high charge component. The charge density of the interlayered phyllosilicate from the different horizons throughout the three pedons is 0.710.72 eq( + ) (Si, A1)40/~ ~ (Table 5).

32

J.M. AROCENAET AL.

TABLE 5 Charge density (eq ( + ) (Si, A1)40/~ ~) and amounts (g kg-~ clay) of AI and Fe extracted by Nacitrate and citrate--dithionite-bicarbonate from clays Horizon

Charge

Al-cit a

Fe-cit

AIdb

Fedb

Pedon 1 Ae Bt~matd Bfjnod s C2k

0.71 0.71 nd c 0.71

3.8 27 16 18

1.5 13 8.7 8.4

11 55 26 16

13 39 69 45

Pedon 2 Ae Btfmat d Btfnod e IIC2k

0.71 0.72 0.72 0.72

3.6 15 10 nd

1.4 12 7.9 nd

6.6 33 26 8.5

7.0 30 70 35

Pedon 3 Ae Btfmat a Btfnod c C2

0.71 0.71 0.71 0.72

4.6 30 7.1 nd

1.2 15 4.5 nd

12 55 31 12

15 41 76 53

aNa-citrate. bCitrate--dithionite-bicarbonate. CNot determined. dMatrix of the B horizons (i.e., excluding the nodules). Clndurated nodules present in the B horizons.

Kaolinite is considered stable because of its near constant distribution within the profdes but it may also have undergone physical translocation from the Ae to the B horizons during the process of lessivage.

Thefate of carbonates and oxides: conventional and theoretical evaluations Dissolution of primary carbonates from the solum is regarded as the first stage of mineral transformation and is likely similar to the decarbonation processes in other soils that developed from calcareous materials. The initial removal (i.e., none detected by conventional observations) of carbonates and sulfates in the solum is predicted by the negative values for SI (based on the calculated ionic activities of the natural solutions in Table 6) of calcite, dolomite, siderite and gypsum in the Ae and B horizons (Fig. 9). The presence of gypsum in the indurated nodules might be due to the microbial activity in certain microsites where higher activity of SO42- and Ca 2+ can exist (Rosanov, 1961; Zinder and Brock, 1978; David et al., 1982) although the possible formation in an earlier environment cannot be discounted. The formation of ferrihydrite as evaluated using the equation of Fox ( 1988 ):

33

MINERAL TRANSFORMATIONS IN SOME SANDY SOILS FROM ALBERTA, CANADA TABLE 6 Calculated activities ( - l o g of molar concentration) of natural soil solutions" Horizon H + Ca 2+ Mg 2+ Na + K +

C1- SO 2-

HCO~"

H4SiO4

AP + Fe 2+ Fe 3+ PO 3-

NO~-

Pedon 1

Summer 1989 Ae Bi~

4.1 5.6

3.6 4.0

4.0 4.3

4.1 4.0

3.4 3.6

3.1 3.0

3.8 3.7

3.6 3.5

3.3 3.4

3.9 5.4

4.4 4.9

12 13

15 14

5.0 5.1

4.6 5.6

3.1 3.6

3.6 4.2

4.0 4.2

3.1 4.0

3.5 3.6

4.0 4.8

4.7 3.3

3.7 4.2

4,4 6,6

5.4 5.0

13 14

14 14

tr tr

3.5 3.5

4.0 4.0

3.9 4.2

4.4 4.0

3.4 3.5

3.6 4,8

4.3 3.4

3.4 3.6

4.2 5.5

4.3 4,6

12 13

16 14

4.2 4.5

4.5 5.5

3.5 3.7

3.9 4.0

4.2 4.3

3.6 4.3

3.1 3.1

4.2 4.5

3.8 3.6

3.1 3.5

4.5 5.3

4.7 4.0

12 12

15 14

5.3 5.3

4.3 5.8

3.3 3.5

3.8 4.0

4.3 4.2

3.5 4.2

3.5 3.8

4.2 5.1

3.7 3.6

4.6 4.6

4.4 5.7

5.5 5.8

15 14

16 tr

tr tr

3.4 3.7

3.8 3.8

4.3 4.2

3.5 3.6

3.5 3.7

4.2 5.2

3.6 3.9

3.4 3.7

4.3 4.7

4.1 4.9

13 13

tr 14

4.2 5.8

Fall 1989 Ae Bi~

Spring 1990 Ae Bt]

3.9 5.5

Pedon 2

Summer 1989 Ae Btf

Fall 1989 Ae Btf

Spring 1990 Ae Btf

4.1 5.3

'~,_.alculated from measured concentrations given in Table 3 using SOLMINEQ-PC (Kharaka et al., 1988) at 25°C and 1 atm pressure.

a(Fe a+) (a(OH))2.35= i0-31.7 is thermodynamically possible in the Ae and B horizons (Fig. 9) and is consistentwith the high Fco/FCd ratios.Hematite, goethite and lepidocrocitc have negative values for SI in the Ae horizons but not in the B horizons. Hematite shows higher SI values compared to those for goethite and Icpidocrocite but empirical observations show higher proportions of goethite than hematite. The high gocthite/(goethite + hematite) ratio of 0.60 in Fc-rich nodules and the calculated AI for Fe substitution in goethite that can reach as high as 28 mol% AI reflects high AI activity (Campbell and Schwertmann, 1984) and the presence of organic ligands in the system (Schwertmann, 1985 ). Schwcrtmann and Taylor (1989) have suggested that the "anti-hcmatitic" effect occurs because organic compounds can tic up available Fc 3+. Reduction of Fc 3+ following oxidation of ligands also lowers the proportion of Fc 3+ in the system (Cornell et al., 1989a). The positive value of SI for lepidocrocitc in the B horizon may indicate the occurrence of periodic reducing conditions in that

34

J.M. AROCENA ET AL.

8 • (A) Ae, pedon 1 4

•

II

i l l

I -8

-4 -" "l

-

q

r

.=_o O9

,ll I

-12 Ca DI Sd Gy Fh G't Hrn Lp Gb Im

Ca DI Sd Gy Fh Gt Hm Lp Gb Im

6 --(C) Ae, pedon 2,

E

~[! .....

2

I=

I F

"~ -2 minT_. -6 ml i

•

==-L II

"I "

-10 Ca DI Sd Gy Fh GR Hm Lp Gb Im

09

-4

-8 Ca DI Sd Gy Fh Gt HmLp Gb Im

LEGEND:.

Ca-calcite; Dl-dolomite; Sd-siderite; Gy-gypsum; Fh-ferrihydrite; Gt-goethite;Hm-hematite; Lp-lepidocrocite; Gb-gibbsite; Im-imogolits

I Summer89 [] Fag89 [ ] Spring9o

Fig. 9. Saturation indices of the different minerals in (A) Ae and (B) Bfj horizons of pedon 1 and (C) Ac and (D) Btfhorizons ofpcdon 2.

microenvironmcnt (Campbell and Schwertmann, 1984; Loveland and Clayden, 1987 ) and a system that lacks the inhibiting effects of bicarbonate ions on lcpidocrocit¢ formation (Corncll et al., 1989b) or it could have originated in a reductomorphic paleoenvironmcnt. Gibbsite has a positive SI in the Ae and B horizons (Fig. 9) but XRD observations failed to detect this mineral. This could be attributed to several factors among them is the instability of gibbsite in acid complexing environments (Righi and Lorphelin, 1986) where active formation of intcflayered phyUosilicates involve the incorporation oral (OH)3 into the interlayers. The "anti-gibbsit¢" effect of soluble organic matter may also have prevented A1(OH) 3 crystallization. The formation and stability of imogolit¢ was evaluated using the equation given by Dahlgren and Ugolini ( 1989 ): (aA13+ ) 2a(H4SiO4)/(all + ) 6= 1012 The calculated SI values are consistently positive in both Ae and B horizons

MINERAL TRANSFORMATIONS IN SOME SANDY SOILS FROM ALBERTA, CANADA

35

(Fig. 9). Generally, SI values arc higher in the B horizons (SI=4-7) compared to the Ae horizons (SI=0.4-3). Ovcrsaturation of solution with respect to imogolite in the Ae horizons is likelyin contrast to the findings of Dahlgren and Ugolini (1989) because they used leachate solutions while in the present study we used immiscibly displaced solutions which were "resident" in the horizons at the time of sampling. These solutionsnormally have longer equilibrationtime than the leachates.The findings support the initial proposition of Farmer et al. (1980) and Farmer ( 1982 ) that proto-imogolite sols can form in the eluvialhorizons. The higher SI values of imogolite in the B horizons connote additional formation probably from AI and Si releasedby microbial activity as originally suggested by Buurman and Van Rceuwijk (1984). SUMMARY The mineral transformation in three sandy soils from Alberta was investigated through empirical observations and geochemical evaluation based on the ionic composition of the in situ water chemistry. The fate of the phyllosilicates in this study as evaluated by mineral observations appears to be dominated by the vermiculitization of mica and probably to a lesser extent of chlorite. There is also a probable formation of inteflayered phyllosilicate through the formation of "gibbsite islands" or from the incomplete removal of the OH-sheets from clinochlore. Thermodynamic evaluation of the in situ soil solutions from the Ae and the B horizons shows that they are saturated with respect to ferrihydrite and imogolite and in agreement with the measured chemical parameters related to the presence of the same minerals. The agreement between geochemical evaluation and empirical observation is also true for the absence of calcite and dolomite in the Ae horizons and the presence of goethite, hematite and lepidocrocite in the B horizons. The results suggest that geochemical models using in situ soil solution can be used to evaluate the stability of the low quantities of the common oxides and oxyhydroxides which normally require purification and pre-concentration treatments before any fruitful mineral identification is possible. The disagreement between the model and the mineral characterization for gibbsite and gypsum can be attributed to several reasons such as the instability of gibbsite in acidic complexing environment and "anti-gibbsite" effect of organic components on gibbsite formation while the existence of gypsum may reflect the influence of unique micro- or paleoenvironments. ACKNOWLEDGEMENTS The authors wish to acknowledge the Natural Science and Engineering Research Council of Canada for providing financial support; G. Braybrook for SEM work, M. Abley and P. Yee for assistance in chemical analyses.

36

J.M. AROCENA ET AL.

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MINERAL TRANSFORMATIONS IN SOME SANDY SOILS FROM ALBERTA, CANADA

37

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J.M. AROCENA ET AL.

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