Mineralogic evolution in hydromorphic sandy soils and podzols in “landes du médoc”, France

Geoderma, 19 (1977) 339--359 339 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

MINERALOGIC EVOLUTION IN HYDROMORPHIC SANDY SOILS AND PODZOLS IN "LANDES DU MI~DOC", FRANCE

D. RIGHI and F. DE CONINCK Laboratoire de P~dologie, E.R.A. n ° 220 du C.N.R.S. "P~dologie des Pays Atlantiques'" Facult~ des Sciences 86022 Poitiers (France) Geologisch Instituut, Krijgslaan 271 B-9000, Gent (Belgie) (Received November 22, 1976; accepted September 30, 1977)

ABSTRACT Righi, D. and De Coninck, F., 1977. Mineralogic evolution in hydromorphic sandy soils and podzols in "Landes du M~doc", France. Geoderma, 19: 339--359. Soils of the nearly level "Landes du M~doc" in southwestern France have a pattern of alternating bodies of hydromorphic podzols (Haplaquods) and low humic hydromorphic soils (Psammaquents). The soils are formed in a sedimentary mantle of coarse, quartzose sands with a slight microrelief consisting of low, elongated ridges and shallow, intervening troughs. The water table is at shallow depths throughout the plain, even at the surface in places. The podzols on the crests of the low ridges have distinct A: and cemented B2h horizons. Podzols persist down the sides of ridges but going downslope first lose the A 2 horizon and then the cementation of the Bh horizon. Soils in the shallow troughs have A 1 and Cg horizons without B horizons. The fine silt (2--20 urn) and clay (0--2 um) fractions of the parent sands contain primary trioctahedral chlorite, mica, feldspars, and quartz, with the last mineral predominant. During soil development, the first three minerals undergo weathering at different rates and to different extents. Chlorite is most strongly weathered, followed in order by plagioclases and K-minerals. In the fine silt fraction, weathering seems to occur mostly by fragmentation of particles. In the clay fraction, the phyllosilicates successively form irregularly interstratified minerals with contractible but not expandable vermiculitic layers, interstratified minerals with contractible and expandable smectitic layers, and finally smectites. The extent to which the silicate minerals are weathered becomes progressively greater from the low humic hydromorphic soils to the podzols with friable Bh horizons to the podzols with cemented Bh horizons. Smectite is present only in the A 2 horizons of these last podzols. The aluminum released by weathering of silicate minerals is translocated in part in the form of organo-metal complexes into the Bh horizons of the podzols. Greatest concentrations of A1 are associated with coatings of monomorphic organic matter on mineral grains in the cemented Bh horizons, in which some A1 has also crystallized into gibbsite. That mineral was not detected in friable B horizons of podzols nor in the low humic hydromorphic soil. Contrary to expectations, the mobile A1 did not enter interlayer spaces of expanding 2:1 clay minerals.

340 INTRODUCTION The Landes du Medoc region of southwestern France is a nearly flat plain mantled by sedimentary, coarse quartzose sands of Pleistocene age. The land surface consists of a succession of low ridges and shallow troughs. Ridges are elongated in a NE--SW direction, have an average width of about 10 m and stand about 5 dm above the trough bottoms. Troughs are also approximately 10 m wide, on the average. The microrelief governs positions of the water table with reference to soil profiles, both on ridges and in troughs, the water table being at the surface in numerous swampy areas and shallow lagoons. The pathways of soil development are strongly affected by the position of the water table with reference to the soil surface. The purpose of this study was to determine the differences in mineralogy within and among profiles, to identify changes that had occurred in mineralogy insofar as possible, and to relate mineralogical changes to macromorphology and micromorphology. Because the sand fractions consist of almost pure quartz, the mineralogies of only the clay (0--2 pm) and fine silt (2--20 pm) fractions were determined. MATERIALS AND METHODS

Soil profiles Two sets of profiles were selected to illustrate the characteristic sequences of the region. The first set, identified by the letter L, consists of four profiles in a toposequence extending from the crest of a low ridge into the adjacent trough. The second set, identified by the letter B .1 consists of five profiles picked to represent the different positions on ridges and in troughs. They thus form a synthetic rather than actual toposequence. Each of the nine profiles is identified by an arabic numeral followed by the letter of the set to which it belongs, thus 1L (= 1 LAG). Horizon designations follow the conventions given by the "C.P.C.S." 1964--1967. The soil pattern and the nature of the profiles are directly related to the microrelief and the depth to the water table. Podzols (Spodosols) with A: horizons and cemented Bh horizons (alios) .2 occupy the highest parts of the low ridges. Next going d o w n the sides of ridges are podzols w i t h o u t A2 horizons but with cemented Bh horizons. These are replaced further downslope by podzols with friable Bh horizons. Softs of the shallow troughs are low humic hydromorphic types (Psammaquents), where mean depth to the water table is virtually zero; water is at the surface 6 to 8 months a year. All podzols have water tables near the surface in winter and are considered a hydromorphic variety (Haptaquods) *~ The capitals L and B correspond respectively to previous publications on the same soils. .2 Alios: local term for cemented Bh horizon.

the notations

LAG and BER, used in

341 Differences in macromorphology of the profiles can be found within a distance of metres. Paralleling such distinctions are marked differences in micromorphology (Righi and De Coninck, 1974). The differences in consistence between the cemented and friable Bh horizons are associated with differences in the microfabric. The cemented horizons have thick cutans of m o n o m o r p h i c organic matter, whereas the friable ones consist of aggregates of mineral particles and partly altered plant remains loosely bound together by polymorphic organic matter (De Coninck et al., 1974; Righi and De Coninck, 1974). Nine horizons were sampled to represent the four profiles of the L sequence. These are as follows: 3L: podzol with best drainage; A~I, A2, B22h (cemented), and Cg 2L: podzol with intermediate drainage; B1 and B2h (cemented) 4L: podzol with intermediate drainage; B2h (friable) 1L: low humic hydromorphic soil, wettest of the sequence A~I and Cg Small irregular iron nodules are scattered through the B and Cg horizons of all four profiles, and numerous nodules are present in the A1 ~ horizon of the last profile. The Cg horizons of the first and last profiles are taken to represent the parent materials from which the four were formed. Thirteen horizons were sampled to represent the five profiles of the B set. These are as follows: 1B: podzol with best drainage: A,,, As, and B22h (cemented) 5B: podzol with intermediate drainage; AI~ and B2h (cemented) 4B: podzol with intermediate drainage; A~, B2h (friable), and B3 7B: podzol with intermediate drainage; A~, B2h (friable), and B2h (cemented pockets) 6B: low humic hydromorphic soil -- wettest of the set A ~ and Cg A few mottles are present in the B3 horizon: of profile 4B and more are evident in the sampled horizons of profile 6B. The Cg horizon of the last profile of the set is taken to represent the parent materials from which the five soils were formed. Among the samples from the nine soil profiles, the ranges in particle-size distribution, pH and CEC (cation exchange capacity) are small. All horizons are very coarse textured as indicated by the following ranges: coarse sand (2.0--0.2 mm), 80--90%; fine sand (0.2--0.05 mm), 3--12%; coarse silt (0.05-0.02 mm), 0--1.5%; fine silt (0.02- 0.002 mm), 0--3%; and clay (less than 0.002 mm), 0.5--3%. The pH in water is between 4 and 5.5. The CEC is mostly less than 10 mequiv./100 gm soil, and base saturation is less than 15%. Methods

Organic matter in samples was first destroyed by treatment with H20~, after which the clay (0--2 pm) and fine silt (2--20 pm) fractions were separated by dispersion with NH4OH, sedimentation, and flocculation with KC1. The procedure of De Coninck and Herbillon (1969) was followed in the

342

dithionite-citrate treatment of the samples, after which A1 and Fe were determined by atomic absorption. Amounts of the two elements extracted are identified in tables and text as Fe-extr and Al-extr. The Fe-extr comes from organo-mineral complexes or iron hydroxides in the form of mottles, nodules or both. The Al-extr comes from organo-mineral complexes and is considered " a m o r p h o u s " (Righi et al., 1976). The X-ray analyses were made on samples of the fractions after dithionitecitrate treatment. The 0--2 pm fraction was saturated with either K ÷ or Mg2~ and oriented specimens were used. The 2---20 pm fraction was X-rayed with random orientation. For glycol treatment, slides were kept at room temperature for 24 h in an atmosphere saturated with ethylene glycol vapour. The apparatus was a C.G.R. instrument with CuKa radiation. For total chemical analyses, subsamples were fnsed with strontium diborate and then dissolved in HC1. All elements were determined by atomic absorption (Jeanroy, 1972, 1973). The difference between total Fe and Fe-extr (dithionite-extractable) is identified as Fe-lat. A few subsamples were given citrate treatment according to the m e t h o d of Tamura (1958). Between 100 and 200 mg of sample were boiled in 50 ml of 1N trisodium citrate for one hour. Amounts of Fe, A1, Mg, and K extracted by the treatment were determined by atomic absorption. After dithionite-citrate treatment, some clay samples were digested in 0.5N HC1 overnight at 80 ° C. Approximately 150 mg of each sample were placed in 100 ml of acid. Microprobe analyses were made with the Cameca apparatus in the CNRS-BRGM laboratory at Orleans. RESULTS

Low humic hydromorphic profiles (1L and 6B) X-ray diffraction Patterns were obtained for the fine silt (2--20 umi and clay (0--2 t~m) fractions of the two sampled horizons from each profile. Patterns for the fine silt fractions of the corresponding horizons are much alike (Figs. 1, 3). For the Cg horizon specimens, the spacings are characteristic of primary chlorite, mica, feldspars, and quartz. Chlorite was identified from spacings at 14.2, 7.1, 4.1 and 3.55 A plus weak odd and strong even secondary reflections. The same minerals axe indicated by the patterns for the A~I horizons with stronger reflections for quartz and weaker ones for the other minerals. The X-ray patterns for the clay fractions are similar for the Cg horizons of the two profiles (Figs. 2, 4), The unheated K ÷ and Mg2÷ saturated samples have identical patterns. These have spacings of chlorite, mica, quartz and feldspars as well as a weak 4.80A spacing. Broad reflections occur at 10 and 14 A, ;-ut saturation with glycol does not shift the latter. Gradual collapse accompanies

343

.) J JJ

4~5 333 355

•

J

7

110

k

14

Fig. 1. Sequence L (LAG); X-ray patterns of Na*-saturated samples of the fine silt (2-20 t~m) fractions.

heating to 550°C. This collapse in combination with the 4.80 A spacing suggests transformation of some chlorite into an irregular mixed-layer chlorite-vermiculite mineral. Digestion of the clay fraction of the Cg horizon of profile 1L in HC1 (Table Ia) eliminated the chlorite reflections b u t did not affect those of mica. The X-ray patterns of the clay fractions of the A1, horizons differ somewhat from each other and also from those of the Cg horizons. In the :IL profile, quartz is the only mineral identifiable in the A,, horizon. Others present in the Cg horizon of that profile were not found. A weak reflection also occurs at 14 A, which does not change with glycol saturation but collapses partially u p o n heating. Behaviour of the clay fraction in the A,~ horizon parallels that for the same fraction in the Cg horizon of profile 1L. In contrast to that profile, the clay fraction of the A, ~ horizon of profile 6B has the same minerals as does the Cg horizon, although the mica lines are weaker.

Chemical composition Chemical analyses were made of the fine silt and clay fractions of the A1, and Cg horizons of both profiles, whereas dithionite-citrate treatment was

344

Mg'*O

Mg** G 1LAG Cg

/\

M g +÷ K'250 o

3LAG A2

K'400 o

K*

j;~.

-

II / ~ \

K'250 o

K'550 o

K'400 o

4~

~5

7

I0

I~ K'550 o

dA I

x25

~

l0-

_ 14

i"4g'* O Mg ~* Mg'" /.LAG

3,$5

4;:S

5

B2h

,

~--

7

3 L A G B22h

10

-~

.... J'x~-"

14

3.33 355 4.25

5

7

~*

10 14

Fig. 2. S e q u e n c e L ( L A G ) ; X-ray patterns o f s o m e samples o f the clay ( 0 - - 2 pro) fractions w i t h different treatments: Mg~+G: saturated + glycol solvated; Mg2*: saturated; K*: saturated; K ÷ 2 5 0 ° : saturated + h e a t e d at 2 5 0 ° C ; K ÷ 4 0 0 ° : saturated + h e a t e d at 4 0 0 ° C ; K ÷ 550 ° : saturated + h e a t e d at 5 5 0 ° C.

given to the fine silt fraction of the 1L profile and to the clay fractions of both profiles. Treatment of the fine silt fraction from the A,, and Cg horizons of the 1L profile with dithionite-citrate extracted only traces of A1. In contrast, appreciable amounts of Fe were extracted, presumably from the mottles (Table II). In total composition, the fine silt fraction was lower in all elements except Si in the A11 horizon than in the Cg horizon in each profile (Table III). Moreover, comparisons of the t w o horizons in each profile suggest that some fine

345

T A B L E Ia HC1 t r e a t m e n t ; a m o u n t s e x t r a c t e d as p e r c e n t a g e o f clay Horizon

A1203

Fe203

MgO

CaO

Na~O

K~O

1L

6.36

2.07

0.79

0.15

0.15

0.23

Cg

T A B L E Ib Citrate t r e a t m e n t ( T a m u r a ) : a m o u n t s e x t r a c t e d as p e r c e n t a g e o f clay Horizons

Fe203

A1203

MgO

K20

3L 4B

0.32 1.49

tr. 1.67

0.49 0.85

0.16 0.02

B22h A,

T A B L E II A m o u n t s o f Fe a n d A1 e x p r e s s e d as Fe203 a n d A l : O 3 e x t r a c t e d w i t h c i t r a t e - d i t h i o n i t e a n d o f F%O3-1at (in p e r c e n t a g e s ) Horizons

Clay ( 0 - - 2 u ) A1203

F i n e silt ( 2 - - 2 0 #)

Fe:O3

Fe203-1at

A1203

Fe:O3

1.9 2.0 0.6 0.5 6.8 3.7

2.2 8.9 2.7 0.5 5.1 6.0

1.67 3.49 0.67 1.00 2.11 4.55

tr. tr. tr. tr. 0.25 tr.

0.22 0.72 0.07 0.05 3.75 1.56

8.5 4.7

5.4

2.05

--

--

3.7

2.02

--

--

2.1 1.8

1.49 1.33

--

--

--

--

1.8

0.57

--

--

1.2 2.9 1.5 0.9 2.1 4.8 1.9 0.4 1.4

1.04 0.59 1.04 1.16 0.65 0.39 0.32 0.74 0.37

-tr. 0.34 0.28 tr. 0.22 ----

-0.14 0.44 0.28 0.08 1.16 ----

Sequence LAG: 1L 1L 3L 3L 3L 3L 2L 2L 4L

A,, Cg A,, A2 B22h Cg B, B~h B2h

Sequence BER: 6B 6B 7B

At, Cg A,

7B 4B 4B 4B 5B 5B

B2h A, B2h B3 A,, B2h

IB 1B 1B

A,~ A2 B22h

3.1

2.1 2.6

7.5 3.8 9.0 8.6 1.9 20.0 2.3 0.3 23.5

346

silt has disappeared from the AI~ horizons, possibly through either or both of fragmentation and chemical weathering. Contents of TiO2 are lower in the A1 horizons than in the Cg horizons, which lends support to the idea that some fine silt has been lost through weathering or physical breakdown, Ti being generally included in slightly weatherable minerals. This possibility is "also supported by the indications from X-ray patterns of less chlorite, mica, and feldspars in the AI~ horizons. In the 1L profile, the clay fraction of the A~I horizon is appreciably lower in Fe-lat (the difference between total and dithionite-extractable iron) and A1 than the corresponding fraction of the Cg horizon. Total K is slightly lower. These differences suggest greater weathering of chlorite than of K-minerals (micas and feldspars) within the profile. Furthermore, the fine silt fractions are lower in chlorite and K-minerals in the AI~ horizon than in the Cg horizon. Lower levels in both clay and fine silt fractions suggest complete decomposition of some chlorite and perhaps a little of the K-minerals during soil development. Feldspars and micas are grouped together as K-minerals because disappearance of the mica reflections does not guarantee absence of that mineral. After the dithionite treatment, a subsample of the clay fraction from the Cg horizon of plofile 1L was digested in HC1. All of the Mg, two-thirds of the Fe-lat, less than half of the A1, and only 5% of the K were extracted by the digestion. Subsequent X-ray patterns indicated that the chlorite has been dissolved while the mica rays are not affected. The amounts of Fe and Mg dissolved in HC1 indicate a trioctahedral form of chlorite with an Fe/Mg ratio of a b o u t ~ .3. The clay fractions are lower in Fe-lat, Mg, Ca, K and Na b u t higher in Si in the A ~ and Cg horizons in profile 6B than in corresponding horizons of profile 1L (Tables IV and VI). Differences between the t w o horizons of the former profile are small. Amounts of Fe and A1 extracted by the dithionitecitrate treatment are also small for both horizons. These data indicate smaller amounts of weatherable minerals in the clay fraction of profile 6B than profile 1L. At the same time, the chemical composition and X-ray patterns suggest that some of the fine silt has been broken down physically into clay particles to offset the chemical weathering of chlorite, mica and feldspars in the clay fraction.

Podzol profiles with best drainage (3L and 1B) X-ray diffraction (Figs. 1, 2, 3 and 4) The patterns for the fine silt and clay fractions are closely similar for the Cg horizons of profiles 3L and 1L (Fig. 1). The same minerals make up the Cg horizons in the two profiles, although the proportions m a y not be identical. Upon heating of the clay fraction of the Cg horizon of profile 3L, the 14 A spacing contracts a little more, indicating some transformation of chlorite. The fine silt fraction (Fig. 1) of the B22h horizon of profile 3L consists of chlorite, mica, feldspar and quartz plus gibbsite (shown by intense reflections

A,, Cg A,~ A2 B22h Cg B2h B2h

6B 6B 7B 7B 7B 4B 4B 4B 5B 5B 1B 1B 1B

A,, Cg A, B2h friable B2h c e m e n t e d A, B2h B3 A,~ B2h A,, A: B22h

S e q u e n c e BER:

IL 1L 3L 3L 3L 3L 2L 4L

Sequence LAG:

Hr)rizons

87.11 77.81 87.60 78.93 52.09 88.29 86.44 81.55 90.39 72.39 91.86 84.42 68.77

87.71 75.26 90.28 84.35 48.91 66.06 74.14 84.38

SiO 2

3.63 11.70 5.74 9.98 17.05 4.52 5.43 8.76 3.68 12.82 2.32 7.23 15.86

4.38 12.05 3.32 8.13 24.60 14.45 10.41 6.58

Al:O~

0.43 1.29 0.48 0.54 11.50 0.46 1.04 0.47 0.24 2.58 0.30 0.39 1.09

0.77 2.56 0.28 0.49 5.58 5.63 3.78 0.89

Fe20 ~

0.17 0.51 0.13 0.24 0.39 0.18 0.17 0.25 0.07 0.32 0.19 0.11 0.30

0.20 0.50 0.20 0.18 0.61 1.34 0.49 0.21

MgO

0.40 1.16 0.53 0.71 0.60 0.65 0.35 0.81 0.24 1.19 0.26 0.48 0.99

0.57 1.05 0.18 0.33 0.81 1.31 0.57 0.74

CaO

0.69 1.90 1.22 1.42 1.13 0.70 0.76 1.12 0.78 1.26 0.46 0.77 1.95

1.04 2.64 0.71 1.31 1.35 2.65 1.18 1.22

Na20

T o t a l c h e m i c a l c o m p o s i t i o n o f t h e silt f r a c t i o n ( 2 - - 2 0 p m ) (in p e r c e n t a g e s )

T A B L E III

0.62 2.19 1.72 1.88 1.35 1.15 1.15 1.82 1.35 1.83 0.83 3.22 2.59

1.02 2.49 1.22 3.16 1.53 2.92 1.38 1.18

K~O

0.30 0.76 0.38 0.54 0.78 0.35 0.47 0.76 0.46 0.96 0.46 0.73 0.80

0.60 1.13 0.62 0.91 1.05 1.67 1.26 0.82

TiO~

5.82 2.24 2.98 4.77 15.10 4.01 4.34 3.79 3.79 6.41 2.86 1.77 7.36

4.25 1.44 2.90 1.65 15.70 3.12 6.19 3.87

1000°C H:O +

99.17 99.56 100.78 99.01 100.22 100.31 100.06 99.03 101.80 99.76 99.54 99.12 99.71

100.54 99.12 99.71 100.51 100.14 99.15 99.13 99.89

z

---3

5~

348

at 4.80 A and 4.35 A). In the B22h horizon of profile 1B, quartz, mica, feldspars and probably some chlorite (shown only by spacings of 7.1 A and 3.55 ?~) and some gibbsite are present (Fig. 3). Patterns for the clay fractions from the B22h horizons of both profiles show weak intensities for the silicates and obvious ones for quartz and gibbsite. Reflections at 4.80 A and 4.35 A disappear after the clay is heated to 250°C. Small differences in the clay fractions from the A2 horizons of the two profiles are indicated by the X-ray patterns. For that horizon in profile 3L (Fig. 2), the clay has a broad reflection centered around 12.5 )~ and a clear 10 A spacing. Glycol saturation shifts the broad reflection to a higher value

\

/

I

dA

t J 333 3.ss

l 42s

,

l 7

I t0

J ~4

Fig. 3. S e q u e n c e B ( B E R ) ; X - r a y p a t t e r n s o f Na÷-saturated samples of the f i n e s i l t ( 2 - - 2 0 pm) fractions.

349 b u t does not change the 10 A spacing. Heating causes a gradual collapse back to the 10 A spacing b u t leaves a broad asymmetric reflection extending to 12 A. These changes in the X-ray patterns with the various treatments indicate mixedlayer minerals. Some mica layers persist intact, whereas the K ÷ has been removed from others in some mineral grains. Similarly, certain chlorite layers persist intact and others have lost O H - . The clay fraction in the As horizon of profile 1B (Fig. 4) has minerals with 10 A and 14 A spacings, the latter swelling to 16.5 A with glycol saturation and collapsing to a broad reflection between 10 A and 12 A with K s saturation and gradually contracting further to 10 A upon heating. These data indicate the presence of a mica and of smectite, the latter probably having been derived from the former. The clay fraction of the A2 horizon of profile 1B is thus much like that of the same horizon of profile 3L.

Chemical composition The nature of the fine silt and clay fractions in the different horizons of the two profiles indicates appreciable weathering of minerals and some redistribution of weathering products. The fine silt fractions of both B22h horizons are high in total A12Oa and also have large ignition losses (Table III), which provide further evidence for the presence of gibbsite, the more so because almost no amorphous A1 is present. Amounts of Fe-lat, Mg, Ca and Na in that fraction, however, are lower in the B22h than in the Cg horizon of profile 3L. Quantities of these elements in the A2 and AI~ horizons are still lower. Furthermore, very little A1 was extracted from the silt fractions of the B22h and A2 horizons of profile 3L by dithionite-citrate treatment. The chemical composition of the clay fractions from the Cg horizons of profiles 3L and 1L is very similar (Table IV) to further show that the parent materials were also very similar. For profiles 3L and 1B, a striking feature is the high content of A1 and the large ignition loss of the clay fractions from the B2~h horizons (Table IV). Contents of A1 are 30 and 35%, much higher than those of the A~ and Cg horizons (Table !I). Moreover, the Al-extr is high or very high in the B22h horizons, ranging up to almost 70% of the clay fraction in the B22h of lB. The Al-extr is considered to be " a m o r p h o u s " . A m o u n t s of Fe-extr in the clay fractions of those horizons are also high, b u t Fe is believed to come from mottles and nodules. The amounts of A1, Fe, Mg, and K extracted b y citrate (Tamura) from the clay fraction of the B2~h horizon of profile 3L are low although expanding 2:1 minerals are present and much A1 has accumulated. The SiO2/K20 ratios (Table VI) are low in the clay fractions of the B22h horizons. Possible explanations are: (1) some early illuviation of micaceous clays into the horizons (semi-quantitative determinations with a microprobe through the organic coatings show more K in layers right next to the quartz grain than elsewhere); (2) b r e a k d o w n of silt-sized micas into particles of clay size; or (3) some of both.

,\

~25

5

"\ /~ "-

. K*

~/~g" /.

?k . ,

"T

-T0

/~ ........ ~ / ~

,'\j

\/, Mg++G

=.5,oo

~, /'/'j~ / K+ / ~ j / x~/!'W K+250°

~.

K'250 o _ ~..-;~....... --/\~t,_--, ~ K'/.O0 o / ......... r, /~ / / K + 5 5 0 °

, 6BER Cg " J

I, 6BER All ....~ '\\ '\

.25

..... J ~J \ \ j

_

/..25

5

7

'~ 5BER B2h

5BER All

7

10

l&

K+250 o

K+

K+250 °

/ ~Mg++

I/~- Mge

14

A/ ~' J W

)',~J

10

~

--~

K÷250o

il ,

3.55

a+~

t

i

~.25

),A,~

\,_ ..... j' \

rig. 4. S e q u e n c e B ( B E R ) ; X-ray p a t t e r n s o f s o m e s a m p l e s o f t h e c l a y ( 0 - - 2 ~ m ) f r a c t i o n s vith d i f f e r e n t t r e a t m e n t s : Mg2+G: s a t u r a t e d + g l y c o l s o l v a t e d ; Mg 2÷ : s a t u r a t e d ; K+: a t u r a t e d ; K * 2 5 0 ° : s a t u r a t e d + h e a t e d at 2 5 0 ° C; K + 4 0 0 ° : s a t u r a t e d + h e a t e d at 4 0 0 ° C; P 5 5 0 ° : Saturated + h e a t e d at 5 5 0 ° C .

~

.g"

d4

H

MI**G

L

~

\

i

7

\". . . . . . .

J~

/

- .... .

--

\-J"

K+250~

"<

K* 550 °

K÷400 °

K+250 °

~ " Mg++

~b i~

\

.J

....

i

JD/t

"~

'

L/i ,"

-~

_ J"

1BER B22h

%

1BER A2

i'~ t Mg ++G

A,, Cg A,1 A2 B22h Cg B, B2h B2h

6B 6B 7B 7B 4B 4B 4B 5B 5B 1B 1B 1B

All Cg A, B2h c e m e n t e d A1 B2h B3 A11 B2h A1, A~ B22h

S e q u e n c e BER:

IL IL 3L 3L 3L 3L 2L 2L 4L

Sequence LAG:

Horizons

59.69 61.69 60.15 38.70 51.62 44.88 45.57 70.85 21.97 67.25 72.19 25.98

66.97 51.15 73.99 70.97 24.43 52.93 59.62 35.23 52.21

SiO 2

16.71 18.05 17.89 27.98 19.85 27.29 27.63 11.84 32.40 6.35 11.15 30.20

14.29 19.17 8.86 13.54 35.87 18.57 18.70 27.22 21.56

A1203

3.59 3.13 3.17 2.24 3.49 2.54 2.06 2.75 5.19 2.22 1.14 1.77

3.87 12.39 3.37 1.50 7.21 10.55 2.94 7.45 5.72

Fe203

0.42 0.45 0.62 0.31 0.80 0.35 0.30 0.41 0.18 1.14 0.36 0.14

0.65 0.79 0.28 0.41 0.29 0.88 0.56 0.47 0.47

MgO

0.42 0.45 0.66 0.55 0.16 0.20 0.22 0.49 0.24 1.02 0.24 0.20

0.39 0.49 0.29 0.38 0.45 0.46 0.53 0.35 0.35

CaO

0.35 0.40 0.56 0.30 0.31 0.27 0.26 0.38 0.13 0.41 0.48 0.25

0.41 0.91 0.25 0.58 0.16 0.49 0.51 0.28 0.40

Na20

1.63 1.97 1.56 1.81 2.06 1.58 1.53 1.60 1.00 0.75 2.31 1.29

2.57 2.53 2.13 3.79 2.74 2.64 2.21 2.18 2.19

K20

Total c h e m i c a l c o m p o s i t i o n o f t h e clay f r a c t i o n (0--2 urn) e x p r e s s e d as p e r c e n t a g e s

T A B L E IV

0.38 0.75 0.96 0.25 1.40 0.57 0.32 1.50 0.31 0.77 3.57 0.17

1.38 -3.11 -0.84 -2.38 ---

TiO 2

15.83 13.46 13.71 27.43 19.70 21.66 21.71 9.75 37.71 19.12 7.50 38.22

9.90 10.77 6.63 6.02 27.56 13.78 12.92 24.92 15.83

1000°C H20 _+

99.03 99.35 99.27 99.60 99.39 99.35 99.60 99.60 99.13 99.06 99.03 99.22

100.43 -98.91 -99.58 -100.37 ---

CO ¢.n

352

Podzol profiles with intermediate drainage ('~L, 4L, 7B, 4B and 5B) All of these profiles fall between the two extremes represented by the podzols with best drainage (3L and 1B) and the low humic hydromorphic soils (1L and 6B). The five profiles are intermediate in drainage, in position of water table, and in characteristics. The B2h horizon is weakly cemented in profile 2L, strongly cemented in profile 5B, cemented in spots in profile 7B, and friable in the other two. The B~ horizon is transitional between the A2 and B~ h horizons in profile 2L, whereas none of the other profiles has a distinct A2 so the A~ horizon rests on the B2h horizon.

X-ray diffraction Patterns of the silt fractions were obtained for one horizon in each of profiles 2L and 4L (Fig. 1), two in each of profiles 5B and 7B (Fig.3), and three in profile 4B (Fig.3). The silt fractions of all B2h horizons contain quartz and all have some mica, chlorite, and feldspar although the reflections were weak for those minerals. Gibbsite, shown by a clear 4.80 A spacing, was present only in the cemented B2h horizon of profile 5B. The B2h horizon of profile 4L had reflections at 10 A and 14 A, the latter being broad and then collapsing partially upon heating. The Ai horizons of profiles 4B, 5B and 7B contained quartz and traces o f feldspar in the fine silt fractions, whereas the B3 horizon of profile 4B contained those minerals plus mica and a weathered chlorite. The X-ray patterns of the clay fractions are given in Fig. 4 for the A ~ and B2h horizons of profile 5B and for the A~ horizon of profile 7B. Strong reflections for quartz and gibbsite and weak ones for mica and chlorite are evident in the pattern for the B2h horizon of profile 5B. For the A~ horizon, the pattern indicates a complex 10 A and 14 A mineral plus a mica. Although only the tracing for one horizon in profile 7B is given in Fig. 4, all horizons b u t one have reflections at 14.2 A, 10 A and 7.1 A. No swelling occurred in the clay fraction of the B3 horizon of profile 4B, b u t there was a gradual contraction to 10 £ upon heating. In contrast to the B and A~ horizons of profile 7B, the A1 horizon of profile 4B did contain a swelling mineral that contracted appreciably upon heating. The X-ray data thus indicate possible occurrence of partially weathered chlorite in the clay fractions of the A~ and B3 horizons of profile 4B and of minerals with mica layers and others with complexes of 10 A and nonswelling 14 A layers in the B2h horizons of all profiles.

Chemical composition The total chemical composition is given in Table III for the silt fractions of the B2h horizons of profiles 2L and 4L; the AI, B2h and B3 horizons of profile 4B; and two portions of the B2h horizon of profile 7B (see also Table V). The amounts of Fe, Mg, Ca, Na and K are lower in the silt fractions of these

353

horizons than in the Cg horizons of profiles 1L and 6B, suggesting moderate weathering of chlorite, mica and feldspars. Aluminum is also lower except in the B2h horizon of profile 5B and the cemented portions of the corresponding horizon of profile 7B. The cemented horizons and portions of horizons are appreciably higher in Al, Fe, and ignition loss, to indicate accumulation of amorphous A1 and Fe compounds. The composition of the clay fraction (Tables IV and VI) tends to parallel that of the fine silt, although there are differences. The contents of total A1 are higher in all Bh horizons than in the A horizons or the Cg horizons of profiles 1L and 6B. Largest amounts are in the cemented horizons or portions of horizons. Ignition loss is also greater in the Bh horizons, again being largest in the cemented layers and pockets. This loss is consistent witl~ the greater amounts of Al-extr, the portion removed by dithionite-citrate treatment (Table II). The amounts of Al-extr differ among the Bh horizons, being highest in those that have some degree of cementation. Thus, for example, the Bh horizon in profile 2L has more Al-extr and more cementation than the friable, corresponding horizon of profile 4L. Similarly, much Al-extr also marks the cemented B2h horizon of profile 5B. GENERAL DISCUSSION

Chemical and mineralogical aspects of weathering in profiles The inferences offered in this section a b o u t the weathering of minerals during development of the soils are based on comparisons of the A and B horizons of the nine profiles with the Cg horizons of the low humid hydromorphic profiles (1L and 6B) and the Cg horizon of 3L. Those Cg horizons are taken to represent the parent materials from which all nine profiles have been formed. The homogeneity of the parent material is confirmed by the identical distribution of the different granulometric fractions and b y the similar chemical composition of the Cg horizons. As indicated in an earlier section, all of the soils are sandy and the sand fractions are almost pure quartz. Proportions of silt and clay are low, but those fractions contain minerals subject to weathering. The comparisons are consequently restricted to the fine silt (2--20 pm) and clay (0--2 pm) fractions. The two fractions in the Cg horizons of profiles 1L and 6B contain quartz, feldspars, micas, and primary trioctahedral chlorite. In those horizons, the minerals are largely unweathered; X-ray patterns indicate no alterations, except the chlorite which shows a weak weathering in the Cg horizons. The silicate minerals in both size fractions have undergone some weathering in the A and B horizons of the profiles, indicated by their X-ray patterns and chemical composition. The intensity of weathering of the minerals parallels the degree of soil development. The contents of Fe, Mg, Ca, Na and K in the fine silt fraction become progressively lower from the deeper horizons to the soil surface in these

354

profiles. Contents are lower in the A horizons than in the B horizons and those are lower than in the Cg horizons. Amounts of Fe and Mg drop off more than do those of Ca and Na, and these latter more than K. These relationships suggest that chlorite is most susceptible to weathering, followed in order by the plagioclases and the K-minerals (micas and K-feldspars). An exception to this general trend occurs in the cemented B2h horizons. Silicate minerals seem to have undergone little more weathering in those horizons than in the Cg horizons. The mineral grains have monomorphic coatings of organic matter which could have protective effects (Brydon et al., 1968; Souchier, 1971). Further indications of the parallel trends in the weathering of minerals and development of soils in the Medoc region can be drawn from the SiO2/Fe:O3-1at and the SIO2/A1203 ratios of the clay fractions, given in Table VI. The SiO2/Fe203-1at ratios are 39.0 and 106 in the Cg and AI~ horizons of profile 1L, the low humic hydromorphic soft. The ratios in profile 3L, a podzol with a cemented B2h horizon, are 31.4 in the AI and 307 in the Cg horizon. The SIO2/A1203 ratios for the same horizons of the two profiles are 4.5 and 7.9 and 4.8 and 14.1. The highest SiO2/Fe203-1at and SIO2A1203 ratios for horizons of the nine profiles are 560 and 18.0, respectively, for the A ~ horizon of profile 1B, a podzol with an evident As and a cemented B2h horizon. The wider spread within profiles in the silica--iron than in the silica--aluminum TABLE V Molecular ratios: SIO2/K20, SiO:/Fe203-1at and SiO2/Al~O3 of the fine silt fraction (2-20urn) Horizons

SiOJK20

1L 1L 3L 3L 3L 3L 2L 4L 6B 6B 7B 7B 7B 4B 4B 4B 5B 5B 1B 1B 1B

146.0 41.6 150.0 46.6 40.5 36.6 123.0 140.0 241.6 65.0 80.5 65.5 60.7 122.5 120.0 72.1 187.1 63.1 170.0 41.2 42.2

A,, Cg A,, A~ B22h Cg B2h B2h A,, Cg Al B,h friable B,h cemented A, B~h B3 A,, B2h A,, A2 B22h

SiO2/Fe203-1at

486.6 125.0 1500.0 700.0 81.0 55.0

735 360 1370 1500 150

SIO2/A1203 36.5 10.4 50.0 17.5 3.4 7.8 12.3 23.3 41.4 11.8 24.2 13.2 5.3 36.7 28.8 17.1 41.6 9.6 69.5 28.0 7.6

355

TABLE VI Molecular ratios: SiO2/K20, SiO2/Fe,O,-lat and SiO2/A1203 of the clay fraction (0--2 pro) Horizons

SiO~/K20

SiO2/F%O3-1at.

SiO2/A1203

1L 1L 3L 3L 3L 3L 2L 4L 6B 6B 7B 7B 4B 4B 4B 5B 5B 1B 1B 1B

41.1 31.5 53.5 29.5 13.8 31.4 25.2 38.7 58.2 49.0 62.5 33.7 39.1 44.1 47.5 69.4 32.7 140.0 50.0 30.7

106.7 39.0 307.5 196.6 30.8 31.4 44.6 68.5 110.0 128.7 250.0 106.6 215.0 125.0 108.6 295.0 150.0 560.0 240.0 215.0

7.9 4.5 14.1 8.9 1.1 4.8 2.2 4.2 6.0 5.8 5.7 2.3 4.3 2.8 2.8 10.2 1.1 18.0 11.0 1.4

A,I Cg A,, A~ B~2h Cg B2h B2h A,, Cg AI B2h A1 B~h B3 A,, B2h A,, As B22h

ratio is interpreted as due to greater weathering of chlorite than of mica and feldspars in the clay fraction. The alterations of silicate minerals during development of the soils seem to differ for the fine silt and clay fractions. The X-ray patterns of mica and chlorite in the fine silt fraction (2--20 pm) do not indicate any transformations of those minerals. If alterations had occurred, they would normally result in some interstratified minerals, and those were not detected. Lack of evidence for chemical weathering of the fine silt suggests the possibility of physical breakdown of some silt into clay. It does seem that some silt has disappeared within the profiles. Physical breakdown is the likeliest possibility. Comparisons of X-ray patterns of the clay fractions (0--2 pm) in the A, B and Cg horizons clearly suggest transformations of mica and chlorite into complex and irregularly interstratified minerals consisting of micaceous layers, contractible but not expansible 14 A (vermiculitic) layers, and chlorite layers. Such changes could occur through removal of K ÷ from micas and of hydroxide layers from chlorites. The podzols with A2 horizons and cemented B horizons also have swelling minerals in the clay fractions of the former horizons. Those minerals seem to have been formed in place, probably from both mica and chlorite. The occurrence of smectite in eluvial horizons of podzols has been widely reported (Franzmeier et al., 1963; Gjems, 1963, 1967; Malcolm et al., 1969; Coen and Arnold, 1972).

356

Among the podzols studied, the more distinct the A2 horizon, the better the evidence for swelling minerals in the clay fraction of that horizon. According to Gjems (1967) and Kodama and Brydon (1968) the amounts of expansible minerals in such A2 horizons reflected the intensity of weathering; the more weathering the more expansible clay. If that interpretation is valid, the degree of profile development in the podzols of Medoc and the degree of weathering of minerals originally present are comparable. Not a single feature indicated the presence of secondary chlorite. Citrate treatment (Tamura, 1958) extracted only small quantities of A1 (Table Ib) and did not change the X-ray patterns. Moreover, the tendency of 14 A minerals to collapse on heating becomes progressively greater from the Cg to the A horizons. If secondary chlorite had been formed, the reverse should be true. Although it has been widely believed that "free" A1 tends to enter open spaces in 2:1 layer minerals rather than form hydroxides (the anti-gibbsite effect .... see Jackson, ~.963), the findings of Robert and Razzague-Karimi (1974) indicate that the opening of the lattice structure and entry of A1 do not occur in the presence of complexing organic matter. Rather, there is some destruction of the lattice structure and the export of A1 from soft horizons. The clay fraction in some horizons of soils of the Medoc does contain swelling 2:1 clay minerals as well as considerable amounts of A1 b u t no secondary chlorite could be detected.

Behaviour of elements released during weathering The K, Na, Mg and Ca released by the weathering seem to have been removed from the soil profiles. The removal is ascribed to a combination of percolating water and a slowly circulating water table. Actual removals were not monitored, but differences in the composition of horizons and the evidence of translocation of A1 indicate that the more soluble elements could have been removed completely. Evidence has already been presented to show that the amounts o f A1 released are greater; as the horizons in the podzol profiles become more distinct. At least a part of this A1 is mobilized in the form of organometal complexes. A portion of those complexes is transported o u t of the soil in the water table (Righi et al., 1976). The other part accumulates in the B2h or spodic horizons. The marked accumulation of A1 in the m o n o m o r p h i c coatings is illustrated for a quartz grain in Fig. 5. It is believed that the A1 assumes an amorphous form as the complexes break d o w n and that such A1 as welt as the complexed form are extracted b y the dithionite-citrate treatment. It is also believed that AI continues to be released from complexed forms as organic matter is progressively mineralized. The AI may then remain amorphous, as it has in the B2h and B3 horizons of profile 4B with 9.0 and 8.6% Al-extr, respectively. Alternately, some of the released A1 could crystallize into gibbsite. Its occurrence in tropical hydromorphic podzols has been reported earlier (Turenne, 1975). Among softs of the Medoc that were studied, gibbsite appeared only in the cemented Bh horizons. Those have cutans of monomorphic organic matter

357

quartz grain

coating

~,.

(40 u )

..,i void

.I ~%. i i -i ~ J i

t

%,j. ,,. %.

.,. t

I

..".... "%1.

//

•

;~ \

::

'..... :

.-.. " . , , , . . .

:

.~

' /'\' i! :

~

~- _ ~ -At

""

..

• Fe

oo..o j- ......

... '- . . . . . .

"~

K

Si

Fig. 5. E l e c t r o n i c m i c r o p r o b e traverse o f a m o n o m o r p h i c organic c o a t i n g in a c e m e n t e d B2h h o r i z o n to give s e m i - q u a n t i t a t i v e d i s t r i b u t i o n o f A1, K, F e and Si.

as well as the largest concentrations of A1. Gibbsite occurs in the fine silt as well as the clay fractions o f some such Bh horizons. A m o u n t s of gibbsite are greater in profiles o f the L sequence than in those of the B set. Reasons for the difference are n o t k n o w n but t h e y may lie in the rates of circulation o f the water tables, which may cause differences in the amounts of A1 exported.

Clay migration in podzols Some observations made in the study indicate that small amounts of clay minerals occur in the B horizons of the podzols. The SiO2/K20 ratios (Table VI) o f the clay fractions are lower in the B horizons than in the A and C horizons o f some profiles. Moreover, those are n o t offset by higher ratios of the fine silt fractions.

358

Semi-quantitative determinations with a microprobe of the distribution of elements within the monomorphic organic coatings show the highest amounts of Kclose to the quartz grains and a decrease outward from that grain in most cases. More than one mechanism may be responsible for such distribution. The quartz grains may have had thin coatings of K-minerals prior to the onset of soil development. Alternately, some clay minerals may have been transferred to the B horizon before organic matter was translocated. The K data do show that clay minerals are not present throughout the cross-section of the organic cutan. Apparently, no clay minerals were being translocated with the organic matter and the organometal complexes. CONCLUSIONS

The data on the mineralogy and chemical composition of the fine silt and clay fractions in the different horizons of the profiles of "Landes du Medoc" permit the drawing of certain conclusions. These are: (1) The intensity of weathering of silicate minerals parallels the degree of profile development. The minerals are least weathered in the low humic hydromorphic profile and most weathered in podzols with distinct A2 and cemented Bh horizons. The first has the least and the last has the most pronounced horizonation. (2) Among the silicate minerals, chlorite weathered most easily, followed next by plagioctases, and last by the K-minerals (mica and K-feldspars). The K-bearing minerals are the most resistant among the silicates in these soils. (3) The weathering of chlorite and mica seems to have gone through three stages. The first stage is the formation of irregularly interstratified minerals with vermiculitic 14 A layers. The second stage consists of interstratified minerals with some expanding layers. The third stage consists of expandable or swelling minerals such as smectite. The swelling clay minerals occur only in the surface horizons of the podzols with the most pronounced development. (4) In the podzols, the aluminum released by weathering of silicates is translocated in the form of organo-metal complexes. This A1 accumulates in part in amorphous form in the Bh horizons. The largest accumulations were found in the Bh horizons with coatings of monomorphic organic matter on the mineral grains. (5) Contrary to expectations, the mobilized AI did not result in the conversion of expanding 2:1 clay minerals into secondary chlorite. Instead, part of the A1 crystallized into gibbsite. That mineral was found in both the fine silt and clay fractions of the cemented B2h horizons of podzols.

REFERENCES

Brindley, G.W., 1961. Chlorite minerals. In: G. Brown (Editor), The X-ray Identification and Crystal Structures of Clay Minerals. Mineralogical Society (Clay Minerals Groups), London, pp. 242--298.

359

Brydon, J.C., Kodama, H. and Ross, G., 1968. Mineralogy and weathering of clays in orthic podzols and other podzolic soils in Canada. Trans. 9th Int. Congr. Soil Sci., 9th, Adelafde, III, pp. 41--51. Coen, G.M. and Arnold, R.W., 1973. Clay mineral genesis of some New York spodosols. Soil Sci. Soc. Am. Proc., 38 (2): 342--350. C.P.C.S., 1963--1967. Classification des sols. I.N.R.A., Versailles. De Coninck, F. and Herbillon, A., 1969. Evolution min~ralogique et chimique des fractions argileuses dans des alfisols et des spodosols de la Campine (Belgique). Pedologie, XIX (2): 159--272. De Coninck, F., Righi, D., Maucorps, J. and Robin, A.M., 1974. Origin and micromorphological nomenclature of organic matter in sandy spodosols. In: G.K. Rutherford (Editor), Soil Microscopy. Proc. 4th Int. Working Mtg. Soil Micromorphology. Kingston, Ont., pp. 263--280. Franzmeier, D.P., Whiteside, E.P. and Mortland, M.M., 1963. A chronosequence of podzols in northern Michigan, III. Mineralogy, micromorphology and net changes occurring during soil formation. Mich. Agric. Exp. Sta., Q. Bull., 46 (1): 37--57. Gjems, O., 1963. A swelling dioctahedral clay mineral of a vermiculite--smectite type in the weathering horizons of podzols. Clay Miner. Bull., 183--193. G]ems, O., 1967. Studies on clay minerals and clay-mineral formation in soil profiles in Scandinavia. Rep. Norv. Forest Res. Inst., 81 (XXI): 299--415. Gjems, O., 1970. Mineralogical composition and pedogenic weathering of the clay fraction in podzol soil profiles in Zalesine, Yugoslavia. Soil Sci., 110 (4): 237--243. Jackson, M.L., 1963. Interlayering of expansible layer silicates in soils by chemical weathering. Clays Clay Miner., Nat. Conf., l l t h , pp. 29--46. Jeanroy, E., 1972. Analyse totale des silicates naturels par spectrophotom~trie d'absorption atomique. Application au sol et ~ ses constituants. Chim. Anal., 54 (3): 159--166. Jeanroy, E., 1973. Analyse totale par spectrophotom~trie d'absorption atomique des sols, roches, minerals, ciments apr~s fusion au m~taborate de strontium. Analysis, 2 (10--11): 703--712. Kodama, H. and Brydon, J.E., 1968. A study of clay minerals in podzols in New-Brunswick, eastern Canada. Clay Miner., 7 : 295--309. Malcolm, R.L., Nettleton, W.D. and McCracken, 1~ J., 1969. Pedogenic formation of montmorillonite from a 211--212 intergrade clay mineral. Clays Clay Miner., 16: 405-414. Righi, D. and De Coninck, F., 1974. Micromorphological aspects of humods and haplaquods of the "Landes du M~doc" France. In: G.K. Rutherford (Editor), Soil Microscopy. Proc. 4th Int. Working Mtg. Soil Micromorphology. Kingston, Ont., pp. 567--588. Righi, D., Dupuis, T. and Callame, B., 1976. Caract~ristiques physico-chimiques et composition des eaux superficielles de la nappe phr~atique des Landes du M~doc (France). Pedologie, XXVI (1): 27--41. Robert, M. and Razzague-Karimi, 1974. Evolution des micas triocta4driques en presence d'acides organiques. Bull. Groupe Franqais Argiles, XXVI (2): 307--317. Souchier, B., 1971. Evolution des sols sur roches cristallines ~ l'~tage montagnard (Vosges). M~m Serv. Carte G~ol. Als. Lorr., 33, 134 pp. Tamura, T., 1958. Identification of clay minerals from acid soils. J. Soil Sci., 9(1): 141-147. Turenne, J.F., 1975. Modes d'humification et diff~renciation podzolique dans deux topos~quences guyanaises. Th~se doctorat des Sciences, Universit~ de Nancy, 181 pp.