ACIDIFICATION AND OCHRE FORMATION IN PYRITIC SOILS

C. Bloomfield Pedology Department Kot hams t e d Exp e rime n t a I S t a t i on Harpenden, Werts. li. K .

Sulphate-reducing microorganisms are widely dispersed in soils, and ferrous sul-

phide, Fes, is consequently a not uncommon constituent of anaerobic soils. The

disulphide, FeS2, often accompanies the monosulphide. Although the determination

of ferrous sulphide is very uncertain, especially in the presence of pyrite (9), it seems that the concentration of FeS in soils seldom exceeds 100-200 ppm, where-

as up to 10% pyrite is not uncommon.

Hart ( 7 ) considered the oxidation of FeS to be the essential first stage in the

acidification that occurs when a pyritic soil is aerated. Elemental sulphur for-

med by the atmospheric oxidation of FeS was considered to be oxidized by Thio- bacillus thiooxidans, thereby making the soil acid enough for Fe3+ to exist in

solution, and to catalyse the oxidation of pyrite. It seems unlikely that com-

plete oxidation of the small amounts of Fes usually present in an anaerobic soil

could have much effect on the pH, and as T. thiooxidans grows optimally at pH

2.0 - 3.5 ( I ) , the soil would need to be quite acid before Hart's sequence could

start.

Moist pyrite exposed to the atmosphere oxidizes chemically, in the first instan-

ce giving SO:- and Fe2+. At around neutral reactions the process is relatively

slow, but it becomes faster as the acidity increases. Relow PH 5 the chemical

oxidation of FeL+ is quite slow, but at pH 3.5 - 4.0 oxidation is catalysed by Thiobacillus ferrooxidans, an autotroph that obtains energy by oxidizing Fe2+ or

So ( I O ) . Yicroorganisms with these properties were found in acid sulphate soils

from Malaya, and in pyrite soils and associated ochre deposits in the I J . K . (5).

So long as the pH exceeds c. 2.7, the action of T. ferrooxidans causes hydrated

ferric oxide to precipitate; the organism is very tolerant of acid conditions,

and it retains its activity beyond the point at which Fe20, dissolves. nespite

the very acid reaction of a fully developed acid sulphate soil, ferric iron tends

to be immobilized by the formation of basic sulphates, and pale yellow deposits

of minerals in the iron and aluminium jarosite-natrojarosite range, (K, N a )

(Fe,A1)3 ( S O L , ) , (OH)b, are often found on exposed surfaces and in old root chan- nels. Where it is exposed t o rain, the outer surface of jarosite deposits often

40

carry loosely adhering dark brown ferruginous material, presumably formed by

hydrolysis of jarosite.

Fig. 1 shows that the rate of oxidation of Dyrite is greatly increased in the

presence of an enrichment culture of iron- and sulphur-oxidizing bacteria from a

Malayan acid sulphate soil. Similar results were obtained with a culture of - T.

ferrooxidans obtained from The National Collection of Industrial Bacteria. In

this experiment samples of pyritic shale were incubated on suction plates as

described previously (5). The iron and sulphate contents of the diffusates were

determined periodically, as well as qulphate extracted from the shale by 2N HC1.

About 20 times as much iron was mobilized in the presence of the bacteria as in

their absence, and the final pH of the diffusate was 1 . 2 units lower. Almost all

the iron mobilized in the uninoculated series persisted in the ferrous form, but

in the inoculated series the ferric form was predominant during all hut the first

few days of incubation. The catalytic effect T. ferrooxidans has on the oxida-

tion of pyrite i s probably mainly a secondary effect arising from the reaction FeS2+Fe3+ + ~ F ~ L + + 2 s o . ~ ~ ~ ~ ~ ~ r , Fhrlich (6) found that T . ferrooxidans increa-

sed the extent of leaching of CU from iron-free CuzS, and Beck & Brown (2) ob- tained evidence of direct oxidation of sulphide in Warburg manometric studies.

Fig.2 gives the obtained with an anaerobic soil from Thailand, pH 7.2,

which contained 2.2% oxidizable sulphur, and no detectable Fes. After 25 days'

incubation enough pyrite was oxidized to bring the pH within the range tolerated

by T. was almost entirely in the ferric form, which shows T. ferrooxidans to have been

active. The final atomic Fe:S ratio of the diffusate was about 1:9, compared

with l : 2 for Fes2, s o that proportionately more iron than sulphate was retained in the soil. About a third of the total sulphate was recovered from the HC1 ex-

tract of the soil, so that considerably more pyrite was oxidized than the sul-

phate Content of the diffusate would suggest, and most of the iron involved was

retained in the soil.

It has been suggested that pyritic soils can be reclaimed economically by drai-

ning and irrigating, thereby promoting oxidation and removal of the oxidation

Products. Bloomfield, couiter & Kanaris-Sotiriou ( 4 ) found large amounts of resi- dual Pyrite in a Malayan acid sulphate soil after 5 years' Severe leaching, which

Suggests that the process i s too slow to be practicable. The difficulty of dea-

ling with the considerable depth of very acid soil that would remain when the

Pyrite has disappeared needs to be considered in this context.

ferrooxidans, and shortly after this the iron contained by the diffusate

41

Hart (7) concluded that liming the soil promotes the oxidation of pyrite, but in

view of the chemical and bacterial mechanisms by which pyrite oxidizes, the op-

posite would be expected. To obtain quantitative information on the rate of oxida-

tion/leaching of pyrite and its oxidation products, and the effect of liming,

duplicate undisturbed cores (100 x 50 cm) of pyritic soil, containing 0.8% total

sulphur, were leached with distilled water, applied weekly to give approximately

twice the average annual rainfall of S.E. England. One core was treated with

14 O00 kg CaC03/hectare at the beginning of the experiment, and two equal ad- ditions were made after 4 and 8 months. The iron and sulphate contents of the

effluents were determined at weekly intervals, and after 12 months the sulphate

and oxidizable sulphur contents of the soils were determined. Fig. 3 summarizes

the results. Liming caused a large decrease in the amounts of Fe and SO:- remo-

ved in the drainage water. Including the zero time sulphate contents of the soil,

the total production in the limed and unlimed cores was equivalent to 4 2 9 and

763 g S, respectively, and liming thus had the expected effect of inhibiting the oxidation of pyrite.

Sixteen percent of the total sulphur was recovered in the drainage from the un-

limed core, and, on this basis, under the climatic conditions of S.E.England,

complete removal of the pyrite would take 12-13 years. However, the experimental

conditions were such that oxygen could diffuse to the bottom of the soil column,

and the average temperature was greater than it would have heen in the field;

as there was no transpiration of water from the soil, leaching was relatively

gieater than it would have been in the field. As all these factors would tend

to increase the efficiency of oxidation/leaching, our results no doubt conside-

rably underestimate the time necessary for complete oxidation and removal of the

pyrite.

Table 1 gives the range of pH values of soil from various depths in the columns

at the beginning and end of the experiment. These are the highest and lowest va-

lues of samples taken at 90' intervals from the circumference of the cores. The

zero-time values relate to soil from corresponding points immediately outside

the plastic retaining cylinder, collected when the cores were taken from the

field. T h e heterogeneity of the soil is well illustrated by these figures.

Although liming had a considerable effect on the pH of the soil, it had surpri-

singly little effect on the pH of the effluent, the final value being 2.9, com-

pared with 2.4 for the effluent from the unlimed core. This suggests that, in the

limed core, the soil surrounding drainage fissures must have been comiderably

42

more acid than the values obtained with bulked samples.

TABLE 1

EFFECT OF 4 2 O00 kg CaC03/HECTARE ON THE SOIL pH AFTER 12 MONTHS' LEACHING

HORIZON PH ZERO TIME LIMED UNLIMED

0-6" 2.7 - 3 . 5 6 . 4 - 7 . 4 2 . 9 - 3 . 7

9-12" 2 .7 - 3 . 2 3 . 8 - 4 . 0 2 . 2 - 3 . 2

15-18" 3 . 2 - 4 . 8 3 . 5 - 5 .1 2 . 3 - 2.7

20-24" 5 . 3 - 6 . 0 5 . 0 - 5 . 6 2 . 9 - 4 . 2

26-30" 5 . 8 6 . 4 5 . 8 - 6 . 4 3 . 6 - 6 . 0

Drains installed in waterlogged soils are often blocked by ochre. Filamentous

iron bacteria have long been recognised as the cause of one kind of such depo-

sits, but as they obtain energy from the oxidation of ferrous iron, and grow only under near neutral conditions, these bacteria commonly occur in wet peaty areas

where the formation of organic complexes inhibits the atmospheric oxidation of

ferrous iron.

The association of pyrite and T . ferrooxidans with a quite distinct kind of ochre

seems to have been recognised only recently. Bloomfield ( 3 ) reported the forma-

tion of bright orange-red granular ochre deposits in drains and ditches in the

pyritic soil of a reclaimed open-cast coal site. The pH of the ditch water was

3 . 7 and it contained almost 1.5 g SO?-/litre. Of ten widely separated ochre si- tes in the U . K . , eight were in pyritic areas; the other two, in pyrite-free are-

as, were filamentous. Ferrous iron- and sulphur-oxidizing bacteria were present

in all the non-filamentous ochres, and in all of the 3 2 soil samples collected

from the various horizons of four profiles at two of the pyritic sites. Almost

all the soil samples were less acid than pH 4 , so that iron could not have mo-

ved through the soil in the ferric form. This, and the association of T. ferro-

oxidans with ochre from inside the drains, shows that the iron must have entered

the drains as FeZ+, and that the oxidation of pyrite in the soils was predominant-

ly a chemical process. In the experiment with soil cores, a significant propor-

tion of the iron in the drainage water was in the ferrous state, but this was

only apparent with water collected shortly after its passage through the soil - if more than a few hours elapsed before collection, only ferric iron was present.

The experiment with the limed soil core showed that water draining from a pyritic

43

soil can be considerably more acid than the bulk of the soil, which probably

explains the activity of T. ferrooxidans in drains where the soil around the

drain is not acid enough for the bacteria to grow. Presumably the pH of soil

lining drainage channels is close to that of water passing through them, so that

some growth of T. ferrooxidans in the soil should be possible; this could explain the relatively short lag before active growth that was observed with most of the

soils when they were incubated in Leathen's Fe2+ medium (8).

In the 1968 amendment of the USDA Soil Classification System, sulphidic materials

are defined as waterlogged soils containing 0.75% or more sulphur, mostly in the form of sulphides, and CaCO3 equivalent to less than 3 times the total sulphur

content. 0.75% inorganic sulphur seems a very high lower limit, and 0.1 might

perhaps be more realistic. Although it is undoubtedly essential to record the

presence of enough CaC03 to prevent excessive acidification, if the group were

divided into "sulphidic" and "calcareous sulphidic" categories, the meaning of

"sulphidic" would conform with normal usage.

In the amendment, two tests are proposed as aids in identifying sulphidic mate-

rials. The first is based on acidification produced when the soil is "air-dried

slowly in shade for about 2 months with occasional remoistening". This is a sa- tisfactory test, but as oxidation stops when the soil is dry, and as T. ferro-

oxidans is inactivated by air-drying, the emphasis should be on atmospheric oxi-

dation of the moist soil, and not on drying as such. Storing moist pyritic soils

for a few weeks in thin polythene bags is often sufficient to demonstrate acidi-

fication. The second test involves measuring the increase in acidity after oxi-

dizing pyrite with H202, but the fact that organic matter reacts similarly is

not mentioned. Organic matter also interferes when FeSz is reduced to HES with

Zn/HCl.

Acknowledgements

I thank Mr. G. Pruden for his practical assistance and Messrs. W.I.Kelso,

G.Smitham, and B.D.Trafford, whose co-operation made possible the experiment

with soil cores.

4 4

F i g . 1 . J f f e c t of T. ferrooxidans o n t h e oxidation o f pyrite

45

sei 1

A \ \ \

- \

- s

- 4

- 3

- 2

\ \ pP \\ 11 T F FU S ATF

\ \ \

i \ t o t a l \ so,-

I

X'

. / . ------

p E 7

l6

I I I I I I o 2s so 7 5 l " ( l 1 2 s

D A Y S

Fig. 2. The oxidation of pyrite i n an o ~ i ~ ~ r ~ i l l v i i c u t r o l soil

46

80n t

7 i1 o

6nn

.i !I o

Jno

o U , lil

6, 3 (1 o

2 o 0

o t n l ,so‘i-s

,,’,

I 1

F e = SO - S P

I. I”II Y ( i n n 1 mm D R A I N A G E )

t o t a l so - 5 L.2

F e I.I..ACtIL I)

li

Fe =,so,- s i

I - 3 . 0

< . I ! ) ( , e r f e c t s o f l i m i n g on the oxidation of p y r i t e and t’tr lcachinv of oxidation p r o d u c t s .

47

REFERENCES

( I ) ALEXANDER, N. 1961. Introduction to S o i l Yicrobiology. Wiley & Sons,

New York.

(2) BECK, J . V . & BROWN, D . G . 1968. Direct sulphide oxidation in the solubili-

sation of sulphide ores by Thiobacillus ferrooxidans. .T.Ract. - 96, 1433-34.

(3) BLOOMFIELD, C. 1967. Rothamsted Report for 1966. p.73.

(4) BLOOMFIELD, C., COULTER, J . K . & KANARIS-SOTIRIOU, R. 1968. Oil palms on

acid sulphate soils in Malaya. Trop.Agric. 45, 289-300. (5) BLOOMFIELD, C. 1972. The oxidation of iron sulphides in soils in relation

to the formation of acid sulphate soils, and of ochre deposits in field drains. 3.Soil Sci., 23, 1-16. -

(6) EHRLICH, H . L . 1962. Observation on microbial association with some mineral

sulphides. In: Biogeochemistry of Sulphur Isotopes. Ed. Jensen, Y.L.,

Proc. Nat. Sci. Found. Symp. 153-168.

(7) HART, M.G.R. 1959. Sulphur oxidation in tidal mangrove soils of Sierra

Leone. Pl.&Soil, 11, 215-236. (8 ) LEATHEN, W.W., McINTYRE, L.D. & BRALEY, S . A . 1951. A medium for the study

of bacterial oxidation of ferrous iron. Science, - 114, 280-281.

(9) PRUDEN, G . & BLOOMFIELD, C. 1968. The determination of ferrous sulphide in

soil in the presence of ferric oxide. A n a l y s t , - 93, 532-534.

(IO) TEMPLE, K.L. & COLMER, A.R. 1951. The autotrophic oxidation of iron by a

new bacterium: Thiobacillus ferrooxidans. ~J.Ract. 62, 605-611. -

48

Summary

I n anaerobic soils t he microbiological reduction of sulphates i n t o sulphides f o l -

lowed by the react ion of s u l p h i d e s w i th i ron , normally r e s u l t s i n the formation of

much more p y r i t e than ferrous monosulphide. Consequentl?/ the oxidat ion of pqr i te

is the main cause of a c i d i f i c a t i o n when such soils become aerobic. %st p y r i t e

ox id i ze s s lowly a t neutral or m i l d l y a l k a l i n e reac t ions , i n the first instance

giving Fe2+ and SO:-, and the rate of oxidation increnses with i w r e a s i n g a c i d i t u .

The process is catalyzed hu Fe3+, and once the pH decreases t o 5.5-4.0 the ra t e

a t which p y r i t e is oxidized is great ly accelerated b.^ the in t e rven t ion of Thiobaci l lus ferrooxidans. Liming i n h i b i t s the oxidation of p y r i t e . As well as

causing the formation of acid sulphate soils, the oxidat ion of pyrite b!/ T.ferro-

oxidans i s a l s o responsible for t he blocking of f i e l d dra ins by a -form of ochre

t h a t is qu i t e d i s t i n c t f r o m the mZl-known form produced b y >filamentous i kon

bacteria.

Re'sumi

Dans les s o k anae'rohies l a re'duction microbiologique de s u l f a t e Ci sxlfure,

suivit p a r l a réact ion de su l fure avec du fer, produit normnlement p l u s de p ? d r i t n

que de mono-sulfure ferreux. C ' e c t a l o m l a p q r i t e q k i est la cause nrincipale

de l ' a c i d i f i c a t i o n quand Zes sols deviennent aérobies. .'n. milieu aérobien e t

humide l a p y r i t e oxide lentement à re'actions neutres ou f a ih l emen t alcalines, donnant primairement ~ e ' + e t sot-. Le t a u s d 'ox ida t ion augmente avec I ' a c i d i t é .

L 'ox ida t ion e s t catalysée par Fe3+, e t Ù pi l 4.0-3.5 fortement accélérde p a r I ' i n t e r v e n t i o n de Thiobaci /lus ferrooxidans. I,e chaulage Jreine l 'oxidat ion de l a

p y r i t e . C ' e s t l ' ox ida t ion de l a p y r i t e par T. ferrooccidans qui est resnonsable

pour l ' é v o l u t i o n des sols sulfate's acides. Le même processus m u s e 2 'ohstruct ion

de tubes de drainage en formant une espèce d 'ocre, bien distincte d e s forrres

d 'ocre p lus connues p r o d u i t par l e s bac té r i e s f i l m e n t e u s e s .

Resumen

6n los suelos anaeróbicon l a reducc-ión microbiológica d e l s u l f a t o a l sul,Furo, y l a reacción s u b s i g u i m t e del su l fu ro con e l hierro, producen normalmente l a p i r i t a en exceso d e l monosulfuro de hierro. A s < e s l a p i r i t a l a cau.sa esencial de l a

ac id i f i cac ión quando esos suelos S B vuelven aeróbicos. P i r i t a s humedas se oxiilan lentamente en condieiones neutrales o l igeramente a l ca l inas , dando p e 2 + y SO:- en SU primera f a s e . La taza de oxiilación numenta con e l a m e n t o del acidez. E l

proceso es catalisado por Fe , y una vez La pH se di:sminu?/e a 3.5-4.0 l a taza

a l a cual l a pirita se oxida, es grandemmte aeelerada por l a intervención de

3 +

49

Shiobacillus ferrooxi;dans. E l encalado impide Za ozidacidn de l a n i r i t a . Enelu-

s i k e l causar Za formacidn de suelos ácidos sulfáticos l a ozidacidn de l a

p i r i t a por T.ferroozidans es también responsahle por ei? bZoceo de tubos de &enatie en e l cmpo por una forma de o m e que es b i en d i s t i n t o de l a formi? hien

conocida producida por las bacterias de h i e r ro filamentosas.

2 usammen fass ung

In anaeroben Baden vorkommende mikrobiologische Reduktion des S u l f a t s in S u l f i d

und d i e nachfolgende Reaktion des S u Z f i d s m i t Eisen idird meistens durch die daraus resul t ierende Pyritenbildung ( d i e aiel mehr s tdrker i s t als die Kisenmonosulfid-

bildungi ge fo lg t . Pgri t i s t a l s o die wichtigste iirnache der Vers6uerung der Baden,

wenn d i e s e Biiden aerob werden. fi'euchtes P y r i t o x i d i e r t k a m merkhar bei: neutraler

oder schwnch alkalischer Reaktion. M i t dem Sauemerden d e r Reaktion hesch leun ig t

s ich aurh d7:e Oxidation, d i e von Fe3+ ka-talgsiert w i r d . Sobnld d i e pH-Weerte his 4 .0 -3 .5 abnehmen, f i n d e t e ine s tarke Reschleunigung der Oxidation s t a t t , u.zw. durch d i e Intervent ion des Thiohaci li?us ferroozi:dans; die BekaZkung h e m t dagegen

die Pyri tozidat ion. Die Pyri tozidat ion durch - Y'.ferrooz%dans i s t n i c h t nur die Ursache d e r Bi ldung sehwefelsaurer ISBden; auch d i e Vers topf iwq der Bntwdsserungs-

rbhren im Felde durch eisenhal t ige Ablagerungen kann diesern W~kroorpanismus

zugeschrieben werden. Diese Ablagerungen sind i n ihrer A r t sehr versehieden lion

den durch die Jadenfiirmigen E i senbak te r i en procluzierten Sedimentc.

50

D I S C U S S I O N VAN V E E N : T h e p r o b a b i l i t y of a chemical r e a c t i o n of Fe3+-ions on p y r i t e has

been mentioned by seve ra l speakers dur inq the ses s ion of t h i s a f t e rnoon . (Fes, + Fe3+ + 2 FeZ+ + 2 S o ) I t w i l l be va luable t o pay a t t e n t i o n t o a l t e r n a t i v e b io log ica l pathways t o exp la in t h i s phenomenon. The fo l lowing f ind inqs a r e i m - po r t an t i n t h i s r e s p e c t : 1) The i s o l a t i o n of a non-phosphorus-lipid f r a c t i o n from t h e c u l t u r e s o l u t i o n of iron-grown Th iobac i l l u s fe r rooxidans c e l l s may suqqes t t h a t this compound may be involved i n t h e primary r eac t ion w i t h p y r i t e ( c f .Can . J . Microbio1.15:259, 1969). Resul t s o f a chemical a n a l y s i s and atomic absorp t ion spectrophotometry revea led t h e presence of i ron mostly i n the f e r r o u s form. 2 ) When t h e forementioned chemical r eac t ion p r e v a i l s over a b io loq ica l one, the ox ida t ion o f p y r i t e would a l s o be continued i n the presence of o t h e r i ron ox id iz - i n g microorganism. Recent ly , i ron ox id iz inq f u n q i , de te rmina ted a s Doratomyces s p e c i e s , have been i s o l a t e d from ac id su lpha te s o i l s i n the l abora to ry of micro- b io logy ,Univers i ty of Agr i cu l tu re a t Wageningen.These orqanisms f a i l e d t o ox id i ze p y r i t e i n c u l t u r e s o l u t i o n s a t pH va lues between 2 and 4 , t h i s i n c o n t r a s t t o i d e n t i c a l s o l u t i o n s conta in ing f e r r o u s su lpha te . I t was concluded t h a t the capa- c i t y of Th iobac i l l u s fe r rooxidans t o ox id i ze p y r i t e depended on spec ia l b io loq i - ca l p r o p e r t i e s . The sugges t ion i n Mr. Bloomfie ld ' s paper t h a t t h e amounts of Fe2+-ions e n t e r i n q the d r a i n s could only be der ived from p y r i t e present i n the s o i l by a chemical process seems t o be ques t ionab le . Iron compounds may be sub- sequent ly oxid ized and reduced ( m i c r o ) b i o l o g i c a l l y , dependinq on dep th , oxyqen d i f f u s i o n , organic ma t t e r conten t and pH of the s o i l . (Oxidizinq orqanism, Fe2+ the t o t a l a c t i v i t y of p l a n t r o o t s and microorganism?

f i rs t i n s t a n c e , Fe". In the s t eady s t a t e the ove ra l l r a t e a t wh ich p y r i t e i s

oxid ized wi l l be con t ro l l ed by the r a t e a t which Fe2+ i s r eox id ized , and provided the pH of the system is low enough f o r FeJ+ t o remain i n s o l u t i o n , i t seems inev i - t a b l e t h a t the r a t e of chemical ox ida t ion of p y r i t e w i l l be increased by any process t h a t ca t a lyzes t h e oxida t ion of Fe2+. This does not exclude the poss ib i - l i t y o f d i r e c t microbial ac t ion on the su lphide of cour se , bu t a p a r t from r e f e - rences 2 and 6 i n my t ex t , I have no knowledqe of any such e f f e c t . formation of Fe2+ by anaerobic b a c t e r i a can reach cons ide rab le propor t ions i n waterlogged s o i l , but as t h e cores were f r e e l y dra ined I t h i n k i t un l ike ly t h a t t h i s process was involved . I t i s poss ib l e t h a t some chemical reduct ion o f Fe" by organic mat te r occurred i n loca l pockets of hiqh a c i d i t y (see Pruden & Rloom- f i e l d , Analyst 94:688, 1969) , b u t I do n o t th ink t h a t t h i s e f f e c t would be s i q n i - f i c a n t i n comparison with t h e amount of py r i t e -de r ived Fe2+.

51

Fe3+ + e - , reducing organism). Are b io loq ica l i n h i b i t o r s used t o exclude

BLOOMFIELD: 1) P y r i t e i s r e a d i l y oxid ized by Fe" t o qive SO:- and, i n t h e

2) The

A FOSSIL ACID SULPHATE SOIL IN ICE-PUSHED TERTIARY DEPOSITS NEAR UELSEN (KREIS NORDHORN), GERM4NY

P. Buurman, N. van Breemen and A . G . Jongmans Department o f So i l Science and Geology Universitg of Agricul ture, Wageningen, The Wetherlands

INTRODUCTION

The occurrence of acid sulphate soils outside the domain of recent marine depo-

sits is well known ( 9 ) .

The purpose of this article is to describe the morphology, mineralogy, and gene-

sis of an acid sulphate soil developed in Tertiary marine sediments near Uelsen,

Germany.

A very similar profile, found nearby at the "Kuiperberg" near Ootmarsum, The

Netherlands (Fig.l), was studied in less detail, and will not be discussed here

(cf.3).

GEOLOGY

The profile is exposed in the Lemke clay excavation, which is situated on the

east flank on an ice-pushed ridge (Fig.]). The deposits worked in the pit are

used by the Lemke brick works. In the surroundings of the burrough of Lemke, se- veral old clay pits can be found, but nowadays only one clay pit with deposits

of Oligocene, Miocene and Pleistocene age is in operation. The Middle Oligocene

(Rupelian) deposits consist of heavy, gray clays with levels of septaria (lenti-

cular lime concretions with diameters up to I metre). Fossils are scarce and badly preserved because of filling with pyrite and mechanical deformation, but

the assemblage is typical for the Middle Oligocene. Gypsum crystals up to seve-

ral cm in length can be found in the exposed parts of the deposit. The contact

of the Oligocene clays with the overlying glauconitic btiddle Miocene deposits

is transgressive. The basis of the Miocene is formed by a phosphorite horizon that

yields many casts of fossils, which are strongly weathered at the exposure.

This horizon (which can be seen at two sites in the pit, denoted by A and B in

Fig.2) is strongly inclined (A) or vertical (B). A small exposure of Pleistocene

sand occurs between A and B. Based on observations and measurements in the field,

an attempt was made to reconstruct the original position of the different strata

(Fig.2). The Tertiary and Pleistocene layers are strongly dipping and even over-

5 2

turned locally, as shown by the fact that Miocene sediments overlay Pleistocene

material.

The tilted deposits are covered by an almost horizontal solifluction layer, mar-

ked by a stone line at its base. The layer is pebbly and even contains some boul-

ders of ice-displaced Scandinavian rocks, mainly granites. The solifluction lay-

er is overlain by a coversand deposit which also contains some pebbles.

In the eastern part of the pit, jarosite mottles have developed in the Miocene and in the underlying Oligocene clay. A representative profile was selected for

detailed description and sampling. The results of morphological studies and che-

mical and X-ray diffraction analyses are reported in the following.

MORPHOLOGY OF THE PROFILE

Macromorphology

A schematic diagram of the profile is shown in Fig.3. The Miocene deposit (ITI), which is found between 80 and 250 cm below the soil surface, has a distinct gree-

nish colour caused by the presence of glauconite. Abundant brown mottling occurs

in the upper part of the layer; yellow jarosite mottles are predominant in the

lower part. The glauconite-rich phosphorite horizon ( I V ) is characterized by the

presence of weathered phosphorite concretions (originally containing francolite,

a carbonate hydroxyl apatite) and has very few mottles. The underlying gray Oli-

gocene clay ( V ) also shows clear jarosite mottling. However, this mottling is

mainly concentrated in irregular pockets of Miocene glauconitic material, which

apparently has been incorporated in the older material by mechanical action, pro-

bably as a result of glaciation. Below 370 cm unmottled gray clay (also with in-

clusions of Miocene material) occurs. A more detailed description of the profile

is given in Appendix 1 .

Micromorphology

Samples for the preparation of mammoth-sized thin sections (8x15 cm) were taken

from the horizons 111-1, 111-2, I V , V- l (Zx), and V-2.

The thin sections were prepared according to the method of Jongerius and Heintz-

berger ( 7 ) . The descriptive terms used are mainly those of Brewer (5).

In 111-1 and 111-2 abundant randomly distributed, well rounded skeleton grains

of glauconite (50-500 pm) occur. Especially in 111-1 many of the glauconite grains

are partly disintegrated and some show a brownish color. These weathering

phenomena become less with increasing depth. The bioporosity is high in 111-3 and

53

TABLE 1. GENERAL CHEMICAL AN0 PHYSICAL DATA

(Contents expressed as % of air-dry soil (<2 mm), except if indicated otherwise)

sample No. 1 2 3 4 5 6 7 8 Y 10 I 1 12') 13+) depth (cm) 0-10 15-20 20-40 40-80 80-140 150-170 210-230 260-280 horizon I-All I-Al2 I-Al3 IIC 111-1

pH (HLO) 5.5 5.4 5.0 4.9 4.3 organic/C 4.1 3.5 1.2 0.9 0.4 free Fe,03 1.9 1.5 1.7 1.9 5.7

Grain size distribution (fractions in um)

>zo00 1000-2000

500- I O00

250-500 100-250 50-1 O0

2-50 <2

14.2 15.0 0.9 1.2

4.8 5.3 20.1 20.4

38.0 37.3 15.8 17.0 15.5 14.5

4.9 4.3

14.1 1.4

5.5 21.7 37.2

17.6 12.1 4.5

0.7 0.6

3.3 14.0

35.4 18.4 20.3

8.0

1 .o 0.2 0 . 4

8. I 16.4 Y .4 31.7

33.8

111-2

4.1

0.4 11.4

0.5 15.4 13.6

3.8 37.1 29.6

111-3 IV 4.1

0.3 13.2

o. 1

0.9

32.0 16.0 3.7 24.1 23.2

4.6 0.3 7.3

0.4 1 . 1

41.7

25.9 4.1

13.7 13.0

300-320 v- I 4.9 0.2

7.3

0.2

0.5 15.4

10.0

2.5 22.1

49.3

330-350 v- 1

5.3 0.3 7.8

0.7

0.9 16.9

13.7 3.0 18.7 46. I

360-370 380+ 380+ v-1 v-2 v-2 5.9 5.9++) 5.9++)

0.9 1.7 1.9 6.2 2.1 1.7

- 11.8 - 1 . 1 0.1 0.9

2.6 0.3 1.5

16.7 9.9 13.9

14.1 11.3 13.8 4.6 3.6 7.8 19.1 27.5 13.0 41.8 47.3 49.1

(Cation exchange characteristics (in me/100 g soil)

CEC pH 8 . 2

effective CEC

Ca

Mg Al

L cations CEC clay (in me1100 g clay)

2 1 . 8 2 4 . 7

2 0 . 6 2 2 . 1

4 . 2 4 . 6

0 . 7 1 . 7

13.8 1 4 . 2

1 8 . 7 20 .5

2 4 . 8 2 9 . 6

1 8 . 9 2 5 . 8 3 1 . 5 2 6 . 2 38.8 3 7 . 6

1 8 . 3 1 7 . 6 2 7 . 4 2 4 . 0 3 5 . 2 3 7 . 7

5 . 8 8 .0 1 3 . 7 1 3 . 0 1 7 . 9 21 .o 2 . 0 3 . 7 7 . 5 7 . 0 1 0 . 0 1 0 . 7

1 1 . 2 4 . 2 1 . 4 0 .6 0.1 0.0

19.0 1 5 . 9 2 2 . 6 2 0 . 6 28.0 3 1 . 7

2 5 . 6 20 .8 4 5 . 0 4 2 . 2 3 5 . 6 2 9 . 6 36.4

+)

++)

sample 12 is Miocene glauconitic material, sample 13 is Oligocene clay

This pH refers to a compound sample oÏ 12 and 13, measured immediately

after collection. After slow air-drying Ïor 6 months the pH o Ï No. 12

was 2 . 9 and the pH of No. 13 was 4 . 1 ; possible reasons Ïor the differ-

ences between the pH values of samples oxidized under Ïield conditions

and in the laboratory are discussed by van Bremen ( 4 ) .

The data are for a compound sample of 12 and 1 3 , which was dry frozen

immediately after sampling in order to prevent acidification.

+++)

UI UI

distinctly lower in 111-2. Jarosite is present both in the groundmass (jarosite

nodules) and as cutans around voids (neojarositans). In 111-1 jarosite is close-

ly associated with commonly occurring ferric nodules (which are up to several cm

in diameter) but generally not connected with the many neoferrans present here.

By contrast, in 111-2 neoferrans are surrounded almost exclusively by jarasite.

Especially in the upper part of the Miocene deposit many channel ferri-argillans

(clay cutans) present evidence of clay illuviation. These ferri-argillans show a

continuous orientation, are between 30 and 200 pm thick, and are distributed at

random throughout the groundmass. They are never covered by jarosite but are so-

metimes coated with ferric oxide.

The phosphorite horizon is rich in glauconite, hut does not contain jarosite.

Iron oxide segregations occur only sporadically.

The samples from the Oligocene clay have irregular shaped, angular inclusions

of apparently unweathered glauconitic material. V-l shows many jarositic nodules

and neojarositans, mainly in the glauconite-rich pockets. The upper part of this

horizon contains ferric nodules and neoferrans, both often associated with jaro-

site. Locally 20 to 30 jJm thick cutans of goethite occur in jarositic nodules.

Adjacent to voids, individual jarosite grains (up to 5 Ilm in diameter) are clear-

ly visible (Plate I),and the jarosite in this horizon has a distinctly coarser tex-

ture than in Horizon I1I.Horizon V-2 contains much pyrite as framboidal aggregates

( I O to 50 Um in diameter) or as cubic particles (10 to 20 l im thick) either dis-

tributed throughout the groundmass, in glauconitic grains, or as fillings (up to

1 cm in diameter) in ortho matric aggrotubules. A few star-shaped efflorescences

of gypsum (probably formed after sampling and drying) occur. Voids with shapes

resembling the outlines of large, well developed gypsum crystals are present in

Horizons IV and V-I.

PHYSICAL, CHEMICAL AND MINERALOGICAL ANALYSES

Table 1 gives data on the pH, the organic C content, the "free" Fez03 content,

the grain size distribution, and the exchange characteristics of samples from va-

rious depths.

The coversand has a large amount of the 100-250 lJm fraction. All deeper layers

show two peaks in the grain size distribution: one in the 100-500 um fraction

and one in the clay- and silt fraction. The samples from the horizons TI, IV and

+) For the analytical methods see Appendix 2

+)

56

V show distinct differences in clay (<2 pm) contents.

The Tertiary deposits have low organic C contents, except in the reduced clay at

depths below 370 cm. The free FezO, concentration increases sharply with depth

in the Miocene deposit, drops abruptly in the phosphorite horizon and remains

fairly constant in the oxidized part of the Oligocene clay. The acidity is high

throughout the profile, but the lowest pH values are found in the Yiocene stratum.

The phosphorite horizon, and especially the Oligocene deposits are distinctly

less acid.

Under field conditions the pH of the reduced subsoil is about 6, but upon slow

air-drying the pH drops to 2.9 in the glauconitic material, and to 4 . 1 in the

gray Oligocene clay. The acid character of the samples 5 , 6 , and 7 is illustrated also by the high exchangeable A l content. Exchangeable Ca and “Ig increase strong-

ly with depth. Exchangeable Na and K could not be detected in any of the samples.

The buffered CEC values of the Yiocene material are distinctly lower than those

of the Oligocene samples. This is due both to the higher clay content in V and

to the higher CEC of the clay separates in question. Except in the phosnhorite

horizons, the difference between the effective CEC (the CEC measured at the ac-

tual pH) of the acid samples and the CEC at pH 8.2 is generally small, indica-

ting that the pH-dependant part of the exchange capacity is quantitatively un-

important, probably because of the low contents or organic matter.

TABLE 2. MINERAL COMPOSITION OF THE CLAY SEPARATES, AS INDICATED BY X-RAY DIFFRACTION

The relative amounts present are indicated roughly:

+ small, ++ moderate, +++ high, -not detected

Sample No. 4 5 6 7 8 9 10 1 1 12 13

smec t i te ++ +++ +++ ++ ++ +++ +++ +++ +++ +++

chlorite/verm. ++ - - - - - - - - -

illite ++ ++ ++ + + ++ ++ ++ ++ ++

kaolinite ++ ++ +++ ++ + ++ ++ ++ ++ ++

quartz + + + + + + + + + +

The results of the X-ray diffraction of the clay separates (see Tab.2) show that

smectite(s) (especially below 300 cm), illite, and kaolinite are dominant.

57

TABLE 3 . ELEMENTAL ANALYSIS

I : clay fraction

sample No. 4 5 6 7 8 9 I 0 I1 12 13 14

5 0 . 6 50.1 4 3 . 3 4 3 . 0 5 0 . 5

15 .7 1 6 . 6 1 6 . 5 1 6 . 0 1 9 . 9

17 .3 1 6 . 8 1 9 . 3 17 .1 1 0 . 9

.3 . 2 . 3 . 2 . 3

. I . I . I . I . o

. O 2 . 5 2 . 0 1 . 6 1 . 1

. o . o . o . o .o

. 2 . I .I . 2 .2

4 .0 4 . 0 3 .8 4 . 4 3 . 8

. 6 . 6 . 7 . 7 . 8

. 7 . 4 .4 .7 1 . 2

8.1 7 . 8 1 2 . 2 1 4 . 2 9 . 9

I1 : non-clay fraction

9 2 . 3

2.5

I . 7

.I

. 2

. I

.3

1 . o . 2

.o 1.5

. 3

. o

6 4 . 7

5 .9

1 8 . 6

.2

2 . 4

. 2

.3

6.7

. 5

.o 3 . 3

. 8

. o

54 . I

20 .2

10.0

.2

.I

2 . 4

. o

. 2

3 . 8

. 7

.4

8 .0

51 . 7

1 9 . 4

1 0 . 7

.2

.I

2 . 2

.o . 2

4 . 1

. 7

.3

8 . 9

5 4 . 3

2 0 . 7

9 . 2

. 2

. o 2 . 2

. o

. 2

3 . 8

.7

. I

7 .5

(glauconite)

5 1 . 9 5 5 . 8 -

2 0 . 4 2 2 . 8 -

1 0 . 9 6 . 8 -

. 3 . 3 -

. I . I -

2 . 3 2 . 3 -

. o . I -

.2 . 2 - 3 . 8 3 . 4 -

. 8 .8 -

. 3 . I - 8 . 9 7 . 9 -

7 2 . 5

4.6

10.6

. 3

2 . 6

. 2

. 2

5 . 3

. 3

. o 4 . 2

3 . 2

.o

5 9 . 3 5 4 . 1 6 3 . 2 6 0 . 9 6 9 . 4 6 9 . 5 6 6 . 3 4 9 . 2

6 . 0 6 . 9 5 . 1 4 . 9 12.0 .8 12.3 7 . 9 5

19 .1 1 9 . 1 1 8 . 4 1 8 . 5 8 . 3 14 .7 7 . 9 2 3 . 9

. 3 .8 .6 1 . 1 . 6 2 . 9 1 .5 . 7 - - - - - - - -

2 . 7 3 . 0 3 . 2 2 . 9 1 . 7 2 .6 1 . 9 3 . 1

. 2 . 9 .9 .8 1.0 1 .0 1 . 6 . 3

. 2 .2 . 2 . 2 .2 . 2 .3 . I

8 . 2 6 . 6 6 . 8 7 . 6 3 .4 6 . 2 4 .4 7 . 7

. 2 . 2 . 5 .4 . 9 . I . 9 .I

. 2 2 . 2 . 3 .3 .1 . I . I . 2

4 . 9 5 . 3 5 . 7 6 . 0 6 . 6 3 . 5 7 . 4 5 . 9

3 . 3 . 3 1.0 2 . 3 1.3 1 . 0 0 . 4 . o .o . o . O . o .o 1 . 8 1 . 4 . o

58

TABLE 4 . NORMATIVE MINERALOGICAL COMPOSITION (IN EQUIVALENT PERCENTAGES) OF THE CLAY FRACTION (GOETHITE NORM)

HORIZON I1 111 IV V

SAMPLE NO. 4 5 6 7 8 9 10 11 1 2 13 - -

quartz 31 .4

kaolinite 5 . 5

muscovite 3 9 . 0

montmorillonite 9 . 0

goethite 1 3 . 5

rutile . 5

strengite I .2

MnO . I

excess H20 5 . 5

2 8 . 4

6 . 5

3 9 . 4

1 1 . 3

13.4

. 5

.6

.o

7 . 1

21 .a

9 . 0

3 9 . 0

11.0

1 6 . 4

.6

.a

.o

26.2

2 3 . 5

2 . 7

4 6 . 3

9 . 9

14 .6

.6

I .4

.o

3 1 . 8

2 0 . 6

11.1

3 5 . 0

22 .6

7 . 9

.7

2 . 2

.o

1 2 . 5

1 4 . 6

.a

3 5 . 7

3 8 . 6

7 . 6

.5

.6

.o

7 . 7

1 8 . 7

1 . 7

3 9 . 1

3 0 . 6

8 . 4

. 5

. 6

.o

1 3 . 5

1 3 . 8 10.0

. 4 .2

3 5 . 7 3 6 . 0

42.3 4 5 . 3

7 . 2 8.4

. 6 .6

. 2 .4

.o .o

6 . 7 4 . 3

1 1 . 4

6 .4

3 0 . 8

4 5 . 1

5 . 3

.6

. 2

.o

6 . 5

TABLE 5. NORMATIVE MINERALOGICAL COMPOSITION (IN EQUIVALENT PERCENTAGES] OF THE NON-CLAY FRACTION (EPINOFX)

In case two values are given, the first one refers to a variant with the glauconite of sample 14, the second one to a variant with Al-free glauconite

sample No. 4 5 6 7 8 9 10 I 1 12 13

quartz

kaolinite

muscovite

glauconite

feldspars

goechi te

j arosite

pyrite

gypsum rutile

phosphates+)

rest++)

excess H2O

87.4-86.7

0-1 .o 3.7-3.5 3.0-3.3 3.0

O 1.5

O O 0.2 O

1.3-0.6 3.2-2.8

27.4-34.0 O

1.4-9.8 57.5-31.9 7.7-12.8

0-6.6

4.2 O O

0.4

O 1.4-5.2

0.6-(-3.6)

60.3-60.5 O

10.7-9.3 3.7-4.3

1.9 O

16.4 O O

0.2

O 6.9-2.3 2.4-2. I

32.3-29.7 15.3-19.5 25.4-28.6 0-5 .4 O O

7.3-8.4 4.9-17.8 0-13.1

32.3-35.8 60.0-44.7 53.7-44.7 6.2-2.0 8.0-5.5 7.8-2.2 0-0.3 0-4.7 0-3.4 17.1 1.7 5.2

O O O

O O O 0. 1 0.2 0 .5

0.2 4.0 0.4 4.6-0.8 6.0-2.0 7.1-2.0

-5.7-(-0.9) 4.8-(-0.4) 8.1-4.2

28.3-27.9 0-2.3

0.8-7.0 40.2-44.1 10.5-2. O

0-0.7

12.0 O

O 0.3 0.5

7.5-3.3 7.4-5.1

49.7-43.8 7.9-19.8

5.9-1.3 14.2-18.4

2.0 O

6.8 O O

0.7 0.2

12.6-7.0 9.5-5.0 -

53.1-40.6 O O

I I .3-37.9 0-4.7

7.6-3.4

1.8

5.1

I .9

O. 1

0.2 17.8-4.4 -2.8-29.2

37.7-35.5 0.9-7.8 17.1-19.2 27.6-23.4

3.0 0-1.7

O 3.9 1.2 0.6

0.2 7. 8-3.9

14.4-1 0.9

+)

++)

Calciumphosphate in samples 1 1 , 12 and 13; strengite in the other samples Includes ottrelite, amesite, antigorite, zoisite, tremolite, xonotlite, Na-silicate and K-silicate

Quartz is present in all samples.

The yellow mottles from all appropriate horizons were found to contain jarosite

with d-spacings within 0.01 2 similar to those of pure (K)-jarosite according to

the Index to the Powder Diffraction File 1970, No. 10-443. (Published by the

Joint Committee on Powder Diffraction Standards, 1845 Walnut Street, Philadel-

phia, Pennsylvania 19103, USA).

Brown mottles from Sample 6 showed weak, broad diffraction peaks of goethite.

The elemental composition of the clay fraction and of the non-clay fraction

(calculated from the composition of the clay fraction, the composition of the

fine earth, and the clay content) i s given in Table 3. Sample 14 is unweathered

glauconite isolated by magnetic separation from freshly exposed Miocene material

sampled in the same clay pit.

+)

NORMATIVE MINERALOGICAL COMPOSITION

The normative mineralogical composition of the samples was calculated from the

elemental analyses according to the procedures outlined by van der Plas and van

Schuylenborgh (10).

The normative composition of the clay fractions is given in Table 4 . The ratio

of smectite to kaolinite is not in accordance with the X-ray diffraction data.

This ratio depends on the choice of the smectite mineral: when montmorillonite

is chosen, high amounts of smectite and low amounts of kaolinite are found. With

saponite as the smectite mineral, the reverse is true, so the smectite/kaolinite

ratio can be balanced by assuming certain m o n t m o r i l l o n i t e / , s a p o n i t e ratios.

It appears that the norm can be made to fit the mode reasonably well if part of

the smectite in the samples 9, 10, 1 1 , 12 and 13 is introduced as saponite in the

calculations.

In calculating the normative composition of the non-clay fraction, the epinorm,

which is normally used for this purpose (IO), has been adjusted to account for

the sulphur minerals jarosite, gypsum, and pyrite, and for glauconite. These mi-

nerals were introduced in the basis scheme as follows:

- If SOs and gypsum were present, SO3 was combined with the Ca0 left after sub-

stracting the amount of Ca0 necessary for Ca-phosphate. The remaining SO3 was

combined with FenO3, K z O and HzO to form jarosite ( ] / I 2 {K20.3Fe*03.4S03.8H2n}).

+) A l l S (from the total S determinations) was assumed to be present in the non- clay fraction. This may be incorrect.

61

- S was combined with ferrous iron to form pyrite.

- Next the glauconite content was calculated.

Because glauconites are very variable in composition, the glauconite of Sample

14 was used for the calculation. Based on the general formula 111 I1 1 2-y (%,Fe ) Si3+xA1 ,-xO, o ( O H ) 2, K,-x+y(A1,Fe Y

the mineral appeared to have the following composition:

I11 I1 KO. 74(*'0. ZO'Fe I . 42) (MgO. 3hYFe0. 04)"3. 64A10. 36°10(nH)2

the norm composition of sample 14 is: (in eq. X )

calcium phosphate 0.5%

quartz 0.5%

goethite 0.5%

rutile 0.1%

glauconite 9 8 . 4 %

excess H20 -8.6%

The glauconite-rich subsoil material (sample 12) h a s a very low A1,03 content in

the non-clay fraction, compared with the glauconite shown above.

Therefore a second set of calculations was carried out with Al-free glauconite

with the composition:

I11 I1 KO. sFe . 2 (Mg, Fe ) 0.8 s i40 0 (OH) 2

The results of the two calculations are given in Table 5.

The erratic occurrences of kaolinite, mica, and goethite, and the occasionally

large contribution of the rest group indicate that the result is rather speculati-

ve. This may be partly due to the assumptions that all S (and thus all jarosite)

occurs in the non-clay fraction.

DISCUSSION

The presence of an acid soil associated with jarosite mottles over a total depth

of about 3 m in both the Miocene and the Oligocene deposits is an indication of

the former presence of appreciable quantities of pyrite.

Undoubtedly the pyrite was formed under conditions very similar to those actually

prevailing in many marine near-shore environments. Since its formation, the py-

rite has been subject to appreciable redistribution, as is illustrated by the

relatively coarse pyrite particles (compared with pyrite in recent sediments)

62

found in the thin sections, and even more so, by the large pyrite concretions

(with dimensions up to several dm) commonly found in reduced strata of these

Tertiary deposits. The oxidation o f the pyrite and the formation of the acid sul-

phate "soil" probably started long ago, and not recently as a result of excava-

tion activities. This is suggested by (a) the presence of large and well develo-

ped jarosite mottles and the absence of oxidizable sulphur over a considerable

depth, (b) the uniform depth of the reduced pyritic horizon over the whole length

of the exposed wall, and (c) micromorphological evidence of clay illuviation in

horizons containing jarosite. The last factor even points to a pre-Holocene age

o f the jarosite. Van Schuylenborgh et al. ( 1 1 ) postulated that the formation of

ferriargillans under conditions in The Netherlands dates from the Pleistocene.

Whether the illuviated clay came from the present t o p 80 cms or from other over-

lying material that has been eroded in the meantime remains an open question.

The occurrence of jarosite is typical for oxidized pyritic deposits with pH va-

lues of 4 or lower (8, 6, 4 ) . At higher pH values jarosite is unstable with res-

pect to poorly crystallized ("limonitic") goethite, and will be hydrolized re-

leasing K+ and SO$- into solution. This process appears to be slow under most con-

ditions, and jarosite can occur metastably at pH values higher than 4 for a con-

siderable time (6).

The close association of jarosite with ferric nodules and with neoferrans in the

upper part of the Miocene and the Oligocene deposits indicates that part of the

jarosite is hydrolized. The presence of neoferrans surrounded by jarosite in

Horizon 111-2 and the absence of jarosite around the neoferrans of the overlying

horizon suggests that the hydrolysis takes place preferably along pores.

The occurrence of jarosite in Samples 10 and 1 1 , where the pH is well above 5, is puzzling. Metastable persistence after jarosite formation took place under

more acid conditions seems unlikely, because the high contents of exchangeable Ca

(which were probably caused mainly by the dissolution of well buffering substan-

ces such as francolite and the CaCO3 from septaria locally present i n the Oligo-

cene clay) indicate that the pH has never been much lower than it is at present.

However, it is possible that local domains with lower pH developed temporarily

upon pyrite oxidation in the pockets of Miocene glauconitic material.

This is suggested by (a) the strong acidification of the reduced glauconitic ma-

terial after air-drying (see second footnote, Table l ) and its high pyrite content

when compared to the Oligocene clay and (b) the fact that in Horizon V-1 jarosite

is mainly concentrated in the glauconitic pockets. No separate samples of natural-

6 3

ly oxidized Miocene and Oligocene material of Horizon V are available to check

whether these show differences in pH.

The sulphuric acid produced during pyrite oxidation is an aggressive weathering

agent. TO what extent has pyrite oxidation influenced the mineral composition of

the deposits?

Francolite and calcium carbonate dissolve under slightly acid or acid conditions

and at the same time exert a considerable pH buffering. Buffer intensity curves

(which show the amounts of strong acid or strong base necessary to produce a pll

change of 1 unit as a function of the pH) were calculated (cf. 12) for aqueous

systems containing CO2 and calcite or hydroxyl apatite as a representant for fran-

colite (see Fig.4).

The CaCOq-COL system tends to keep the pH between 7 and 8.5; the buffering intensity is very high and the pH can drop to values below 7 only after large

amounts of CaC03 have been dissolved. Hydroxyl apatite is a less effective buffer-

ing agent in the neutral pH range, but inactivates appreciable quantities of acid

at pH values between 4 and 5 according to:

CasOH(P0+)3 + 7H+ = 5 Ca2+ + 3H2POu + H,O

However, if the pH is below 6, hydroxyl apatite is generally unstable with respect to aluminium- and ferric iron phosphates, and even more acid can be consumed by

reactions such as:

CaSOH(POb), + 3Fe(OH)3 + l O R + = 5Ca2+ + 3FePO4.2HLO + 4H2O

Samples from an unweathered phosphorite horizon may contain more than 10% P z O s ,

so the much lower phosphate content of Sample 8 (viz. 2.5% P 2 0 5 , assumed to be

present as strengite) shows that processes of both kind have taken place.

The extent of silicate weathering is difficult to assess, because (a) the chemi-

cal and mineralogical composition of the original material is not known and (b)

the results of the norm calculations for the non-clay fraction are unsatisfactory.

There are some indications for a considerable inherited compositional variation

within the Miocene glauconitic material and within the Oligocene clay, especially

in the non-clay fraction (e.g. see the total TiOL contents in Table 3.11). So the

parent material was probably n o t homogeneous with depth, and it is speculative to

consider present dissimilarities among the different samples within a stratigra-

phic unit mainly as the effects of soil forming processes. However the tremendous

difference in the composition of the clay fractions of the unoxidized glauco-

64

nitic material (Sample 12) compared with the oxidized material (Samples 5, 6,

and 7 ) as revealed by the norm calculations, must be ascribed in a large part to

weathering of smectite to kaolinite and free silica. On the other hand, in spite

of the rather old age of profile development, the deposits still contain large

quantities of minerals such as smectites, muscovite, and glauconite, which are

susceptible to weathering under acid conditions.

Both the poor internal drainage (low porosity) at depths below 140 cm, which

probably prevents appreciable leaching, and the local presence of more easily

decomposable minerals (calcite and francolite), which inactivated much of the

sulphuric acid, may be responsible for the relatively "unweathered" character of

these deposits.

Acknowledgements

Thanks are due to L.Th.Begheijn, A.T.J.Jonker, J.D.J.van Doesburg and R.Schoor1

for the chemical and X-ray analyses.

We are grateful to Miss M.van de Bospoort of the Laboratory of Soils and Ferti-

lizers, University of Agriculture, Wageningen, who carried out the CEC de- terminations.

Finally we would like to thank R.Niedema for the micromorphological interpreta-

tion.

65

66

68

REFERENCES

BASCOMB, C.L. 1964. Rapid method for the determination of CEC of calcare-

ous and noncalcareous soils. J.Sci.Food Agr.2, 821-823.

BEGHEIJN, L.Th., SCHUYLENBORGH, J.van. 1971. Methods for the analysis of

soils. Dept. of Soil Sci. and Geol. State Agric.Univ.Wageningen. 156 p p .

BOSCH, H.C.J. 1971. Het Tertiair van de Kuiperberg bij Ootmarsum. Yeded.

Werkgr. Tert. & Kwart. Geologie - 8, 25-45 (Dutch with Engl. summ.)

BREEMEN, N. van, 1972. Pyrite oxidation and the development of acid sul-

phate soils. Proc. Int. Symp. on Acid Sulphate Soils. Wageningen 1972.

BREWER, R. 1964. Soil fabric and mineral analysis. Wiley, London. 470 p p .

BROWN, J.B. 1971. Jarosite-geothite stabilities at 25OC, 1 atm. Mineral

deposita 6, 242-252. JONGERIUS, A., HEINTZBERGER, C. 1963. The preparation of mammoth-sized

thin sections. Soil Survey Papers (Soil Survey Inst. Wageningen) - 1 , 37 p p .

KUBISZ, J. 1964. Minerals of the alunite-jarosite group. Polska Akad. Nauk.

Prace Geol. 22, 1-85 (in Pol.); C.A.65: 16688.

POELMAN, J.N.B. 1972. Soil material rich in pyrite in non-coastal areas.

Proc. Int. Symp. on Acid Sulphate Soils. Wageningen 1972.

PLAS, L. van der, SCHUYLENBORGH, J. van, 1970. Petrochemical calculations

applied to soils, with special reference to soil formation. Geoderma A, 357-385.

SCHUYLENBORGH, J. van, SLAGER, S . , JONGVANS, A.G. 1970. On soil genesis

in temperate humid climate. VIII. The formation of a "Udalfic" Eutrochrept.

Neth. J. Agric. Sci. 18, 207-214.

STUMM, W., MORGAN, J.J. 1970. Aquatic Chemistry. Wiley-Interscience. 583 p p .

-

-

69

APPENDIX 1

Detailed description of the Uelsen profile

Described by:

Date:

Location:

Climate :

Geology:

Phy s i o grap hy :

Relief:

Slope:

Altitude :

Hydrology:

Biological activity:

N. van Breemen and P. Buurman

14.10.1971

Topographical map 1:50.000 No.22;

N 500.950, E 259.200

Precipitation 775 "/y; effective precipitation 330 m / y ;

av. yearly temperature 9'~.

coversand on Miocene glauconitic sediment, Yiocene phos-

phorite and Oligocene clay

wall of excavation, exposed to the east

normal

compound slope, 2-3%

25 m + NAP Solum well drained; the Tertiary layers apparently have

a poor internal drainage. The watertable is lowered by

pumping in order to permit excavation

Common fine roots in the upper 5 cm of the profile. Qoot activity decreases gradually with depth; very few, very

fine roots present at 40 cm below the surface

Soil horizons

O - l O / 1 5 cm: very dark (10YR3/1) loamy sand (moist); weak, fine A l 1 crumb; loose; few fine pores; gravelly; clear and

wavy to

A l p l0/15 - 18/20 cm: very dark grayish brown (lOYR3.5/2, broken) or dark

brown (10YR3.5/3, rubbed) loamy sand (moist); few

whitish (leached) grains; weak, coarse, angular

blocky; very friable; few fine pores; gravelly to

cobbly (15-20%); abrupt and broken to

18/20 - 30/40 cm: dark yellowish brown (10YR4/4) loamy sand (moist); *I 3 structureless, single grain; l o o s e ; common fine po-

res; gravelly to cobbly (10%); clear and wavy to

70

IIC

111-1

111-2

30/40 - 80 cm:

80 - 140 cm:

I40 - 130/210 cm:

(Miocene)

111-3 170/210 - 250 cm:

(Miocene)

IV 250 - 300 cm:

light olive brown (2.5Y5/5) loamy sand (moist);

few greenish and brownish mottles; structureless,

massive; firm; abundant fine biopores; stone line

at the base; clear and wavy to

olive (5Y4.5/5, rubbed) clay loam (moist); many

(40%), distinct, coarse, clear, strong brown

(7.5YR5/7) mottles; abundant very fine pores; diffu-

se and wavy to

olive (7.5Y5/2) clay loam (moist); many (40%), dis-

tinct, coarse, clear, strong brown (7.5YR5/7) mot-

tles, decreasing with depth; common ( I O % ) , distinct,

medium, clear yellow (2.5Y8/5) jarosite mottles,

increasing with depth; the jarosite mottles are lo-

cally elongated in stress- or sedimental plains;

vertical to subvertical joints with almost unmott-

led matrix, and bordered by 2 cm thick hrown mot-

tled zones, occur at appr. every 50 cm; structure-

less, massive; slightly sticky, plastic; few biopo-

res; diffuse and tonguing into

light olive brown (2.5Y5/5) and very dark grayish

brown (2.5Y3/2) sandy clay loam (moist); abundant

(40%) prominent, coarse, sharp to clear, yellow

(2.5Y8/5) jarosite, banded in different directions;

glauconite grains clearly visible in jarosite mot-

tles; structureless, massive; friable; very few po-

res; diffuse and wavy to

dark gravish brown (2.5Y4/2) sandy loam; weathered

phosphorite; brown mottles on the exposed surface;

(Miocene; phosphorite) few jarosite mottles (only at the top of the layer)

along voids; diffuse and wavy to

v- 1 300 - 370 cm: gray (5Y5/1) and grayish brown (2.5Y4.5/2) clay

( S O % ) , and irregular pockets of greenish glauconitic

material; mainly in the glauconitic material: many

(Oligocene, mixed (20%) distinct, medium to coarse clear, yellow

with some Miocene) (2.5Y8/6) hard, cilindrical jarosite concretions,

71

with dark yellowish brown (lOYR4/4) mottles inside;

in the upper and lower parts of the horizon: common,

distinct, coarse, clear, dark brown (7.5YR3/2) and

strong brown (7.5YR4/6) mottles; very few mottles

in the lower IO cms; structureless to very weak an- gular blocky, massive; sticky and plastic; slicken-

sides; few medium pores; clear and wavy to

v-2 370 - 400 + cm: dark greenish gray (5GY4/1) clay (60-70%) with irre-

gular patches of very dark greenish gray (5C,Y3/1)

and dark grayish green (5G2.5/1) glauconitic mate-

rial; unmottled; structureless, massive; slightly

sticky, very plastic; some hard pyrite concretions

often elongated and several cm long.

72

APPENDIX 2

Laboratory methods

Chemical and X-ray analyses were carried out as described by Begheijn and van

Schuylenborgh (2). For the silicate analyses, all elements except Na and Yg

were determined by X-ray fluorescence. SO3 and S were determined as sulphate

hydrolysable in a hot 1 % Na2C03 solution, and as S reduceble by Sn-6N HC1 resn.

The sum SO3+S was checked by a total S determination using X-ray fluorescence.

The CEC of the fine earth was determined by saturation with I N BaC12, replace-

ment of Ba by Mg after addition of YgSOt,, and the determination of the decrease

in Mg concentration by complexometric titration ( I ) . Cations exchanged by Ra were

analysed in the BaCln-extract, using atomic absorption spectrophotometry (Ca , Y g ) ,

flame photometry (Na, K) and colorimetry ( A l ) .

The CEC at pH 8.2 was determined using RaCl2 - TEA.

7 3

Summary

The profile o f a fossil acid sulphate s o i l developed i n Ter t ia ru marine sediments

is described. SpeciaZ emphasis is given t o presentat ion and discussion 0.f geolo-

g i c a l , micro-morphological, mineralogical and chemical data per t inen t t o s o i l genesis.

Re'sume'

L ' a r t i c l e con t i en t une descr ip t ion d 'un so2 sulfate' acide f o s s i l , développe' clans

un sédiment marin d'âgge t e r t i a i r . L'accent e s t apnuyé sur l a pre'sentation e t l a

discussion des données géologiques, micromorphologinues, minéralogiques e t chimi-

ques d'importance géne'tique.

Resumen

Se describe un suelo cicido s u l f h t i c o f ó s i l desarrollado en sedimentos marinos

t e r t i a r i o s . Se hace hincapié en l a presentación y l a discusidn de datos geoló-

gicos, micro-morfo lógicos, minera ldgicos y quimicos de i n t e r é s gene'tico.

ZusammenJassung

E s wird e i n fossi Zer schwefe Zsauerer, aus marinen Y'ertiarsedimenten entstandener

Boden beschrieben. Vomehmlich werden d i e DarsteZlung und Diskussion geologischer,

mikromorphologischer, mineralogischer und chemischer Daten bodengenetischer Re-

deutung hervorgehoben.

7 4

D I S C U S S I O N

BRINKMAN: In Guyana and Surinam no c l a y m i n e r a l o g i c a l d i f f e r e n c e s c o u l d be d e t e c t e d i n p o t e n t i a l a c i d s u l p h a t e s o i l m a t e r i a l b e f o r e and a f t e r o x i d a t i o n i n the l a b o r a t o r y , n o r i n Holocene d e p o s i t s between v a r i o u s p h a s e s o f a c i d s u l p h a t e s o i l f o r m a t i o n . However, o x i d i z e d h o r i z o n s o f a P l e i s t o c e n e a c i d s u l p h a t e s o i l showed marked d e p l e t i o n o f m o n t m o r i l l o n i t e compared t o p a r t s o f t he p r o f i l e t h a t were n e v e r o x i d i z e d . How f a s t a r e c l a y m i n e r a l t r a n s f o r m a t i o n s i n a c i d s u l p h a t e s o i l s p r e c e e d i n g s ?

VAN BREEMEN: X - r a y - d e t e c t a b l e changes i n t h e r e l a t i v e amounts o f 2 : l and 1:l c l a y m i n e r a l s a r e r e p o r t e d f o r r e c e n t (Holocene) a c i d s u l p h a t e s o i l s (see m.y i n t r o d u c t o r y p a p e r ) .

75