A DETAILED SURVEY ON THE ACTUAL AND POTENTIAL SOIL ACID ITY AT THE BANG PAKONG LAND DEVELOPMENT CENTRE, THAILAND

N. van Breemen Department of S o i l Sc ience and Geology, U n i v e r s i t y of Agr icu l ture Wageningen

Manop Tandatemiya & Sopon Chanchareonsook Land Development Department Bangkok, Thailand

Introduction

The Bang Pakong Land Development Centre in Amphur Bang Pakong, Chachoengsao

province, Thailand, has been established by the Soil Conservation & Management

Division of the Land Development Department, in order to obtain information about

suitable reclamation measures for the saline soils in this area.

___-

The work at the station was started in 1967 with the clearing of the original

(mangrove) vegetation. The first cropping experiments began in 1968. Apart from

the construction of ditches (partly surrounding the station) and roads, and the

building of ridges for upland crops in a certain area, no special reclamation

procedures have so far been applied.

Because brackish water can still enter the area via a forked tidal channel (con-

nected with the Bank Pakong river) bordering the station in the West and the

South-East, and fresh water is not yet sufficiently available, at present the

soils are probably as saline as before the establishment of the station. Only

where the soil was piled in ridges (90 cm high and 5 metres wide) did the salini-

ty become low enough to permit experiments with field crops (corn, sorghum, cot-

ton, sisal, and sesbania) and fruit crops (sapodilla, guava, rose apple, pomelo,

jujube, jack fruit, mango, and coconut). All field crops died however, and among

the fruit crops only sapodilla and coconut showed a more or less normal growth.

In the southernmost area of the station (not included in the survey described

here), transplanted rice (variety K K - I ) was grown during the 1970 wet season.

Two fertilizer ( 1 2 - 2 4 - 1 2 ) applications of 125 and 6 2 . 5 kg/ha resp. were given.

The rice yield was quite high: 1940 kg/ha.

Salinity may have played a part in the failure of most crops, hut the development

of severe acidity (with pH values between 2 and 3 ) as a result of pyrite oxida-

tion in the top of the ridges appeared to he a major factor in hampering the pro-

ductivity.

159

t

A study was therefore undertaken on the regional distribution of the actual and

potential acidity of the soils at different depths. The results of the study,

carried out at the beginning of 1970, are reported here.

General information

The station is situated in a flat mangrove swamp, in an area inside a convex

bend of a meander of the Bang Pakong river.

The vegetation near the station is dominated by Bruguiera gymnorrhiza, Nipa fru-

ticans, and Xylocarpus granatum, with some Excoecarcia agallocha, Ceriops decan-

dra and Rhizophora apiculata, while the undergrowth consists mainly of Derris

heterophyllia and Acrostichum aureum. The climate is tropical savanna, with an

annual precipitation of about 1360 mm. The sediments have a clayey to silty tex-

ture, are poorly ripened, and are generally reduced at depths below IO to 30 cm, showing greenish gray ( 5 GY 4 / 1 ) and gray ( 5 Y 4 / 1 ) colours. The surface horizons

of the better drained soils have a brown matrix with strong brown mottles. Occas-

ionally yellow (jarosite) mottles occur.

The salinity is invariably high: the electrical conductivity of the groundwater

varies between 1 1 and 40 mmho/cm. The pH of the reduced soil under field condi-

tions is close to 6.5 , but may drop to values around 2 and 3 if the soil is allo- wed to dry and oxidize.

Methods

At some 150 sites, located in a square grid pattern of about 10 x 10 m, soil

samples were taken with a screw auger at depth of 15-30, 45-60, and 75-90 cm

below the surface. The matrix colour was noted and the samples were brought into

100 cc plastic jars, which were closed immediately. Within 1 to 2 weeks after sampling, the pH of the soil was measured with a sturdy glass electrode, inserted

into the (soft, wet) soil, and a reference-electrode placed at the surface.

Next the samples were oxidized by repeated air-drying and rewetting. After three

months the soils were wetted to a consistence that allowed insertion of a glass

electrode, and the pH was measured again. The data thus obtained have been compi-

led into two maps:

1 ) showing the distribution of soils in different acidity classes under

field conditions, and

2) do. after repeated air-drying and rewetting.

Finally, contour lines with intervals of 5 cm are shown in Map 3 , which has been

160

derived from a contour map compiled by.the Soil and Water Conservation Section

before the establishment of the station.

Results and discussion

Under field conditions all samples from depths below 45 cm have a pH between

6 and 7 . Much lower pH values however, occur in the topsoil of the ridges (pH

invariably between 2 and 4 1 , and in the surface layer of one area of undisturbed

soil in the Northern part of the station (pH between 3 and 5 , see Map I). At all other surveyed points, the topsoil appeared to be at most slightly acid (pH

between 5 and 6 . 5 ) .

After repeating drying and rewetting, all subsoils (75 -90 cm) became very acid:

pH 1 . 9 - 3 . 8 , average 2 .9 .

Except for the soils in the N.W. fringe of the surveyed area, the samples of the

second layer (45-60 cm) showed an even sharper pH drop upon drying: pH 1 .9 -3 .8 ,

average 2 .6 .

By contrast, the potential acidity of the topsoil shows enormous local variati-

ons: hardly any pH drop was observed in the N.W. part of the station, but the potential acidity increased dramatically in S.E.direction. About half of the

napped area has soils with a surface layer that gives a pH drop to well below 4

after drying (see Map 2).

TABLE I . AVERAGE VALUES AND STANDARD DEVIATIONS OF THE pH AFTER AIR DRYING FOR THE UNITS DISTINGUISHED IN MAP 2 .

MAPPING UNIT pH AT DIFFERENT DEPTHS

15-30 cm 45-60 cm 75-90 cm

5 . 4 2 0 . 6 4.6 2 0 . 4 2.8 + 0 . 3

5 . 4 2 0 . 6 2.7 2 0 . 5 2 . 9 + 0 . 4

4 .6 2 0 . 4 2 . 5 f_ 0 . 3 2 . 8 + 0 . 2

I V 3 .6 2 0 . 5 2 .5 f_ 0 . 3 2 .9 0 .2

- I

I1

I11 -

-

V 2.7 f_ 0 . 4 2 .6 5 0 . 4 3 . 0 5 0 . 3

Comparison of Map 2 with Map 3 shows a clear correlation between the potential

acidity of the topsoil and the elevation: the potential acidity increases accor-

ding as the soils have a lower elevation. The same follows from Fig.], which

shows the relationship between the pH of the topsoil after air-drying, and the

different elevation classes given in the contour map.

161

Because quickly buffering substances (lime) occur only very locally at depths

below 60 cm, the potential acidity of most samples undoubtedly reflects the

amount of pyrite in the soil. This was confirmed by the results of pyrite deter-

minations in two profiles (see Fig.2).

The available data permit the following conclusions:

a) The pyrite content increases with depth down to between 50 and 90 cm and decreases further down in the profile.

b) The maximum pyrite content occurs higher in the profile according as the

elevation of the soil surface is lower.

c) The higher elevated soils contain less pyrite throughout the profile than

the lower lying soils.

The stronger accumulation of pyrite in the lower areas is undoubtedly the result

of a more favorable environment for the reduction of sulphate (regular supply

of sulphate during high tides, continuously reduced conditions, high production

of organic matter).

The observations about the matrix colours indicate that the soil profiles are best

developed in the N.W. part of the station (topsoil colour has chroma's 3 - 4 ) ,

whereas in the Southern half the soils are completely reduced at depths greater

than 15 cm below the soil surface (greenish gray and gray colours, chroma 1 ) .

In the zone between these two areas, the topsoil shows some evidence of oxidation

(grayish brown matrix colours, chroma 2). Also here a correlation with the topo-

graphy is apparent: as a result of better drainage conditions in the higher ele-

vated areas, oxidized conditions prevail in the topsoil and the influence of oxi-

dation diminishes as drainage conditions become poorer in the lower areas. Exclu-

ding the ridged areas, it appears that the soils with a low pH at the surface are

found in that part of the station where considerable quantities of pyrite occur

in the topsoil and where drainage conditions allowed oxidation of topsoil under

natural conditions.

To the N.W. of this area, the soils are even better drained, but the pyrite con-

tents at the surface were too small to bring about appreciable acidification. By

contrast, the soils in the S.E. part have higher pyrite contents, but here the

poorer drainage conditions prevent oxidation.

The results of this study permit the following recommendations for the management

of the soils of the station and for similar areas in the neighbourhood:

a) Desalinization by leaching with fresh water can, theoretically, be car-

162

ried out without danger of serious acidification, but only in the higher elevated

areas and provided the groundwater level is kept within 40 to 60 cm below the surf ace.

b) A more practical approach for the desalinization of the higher elevated

areas for upland crops is to build ridges according to the 3-step scheme shown

in Fig. 3a. By the 2-step system (Fig. 3b), which has been applied at the sta-

tion, potentially acid soil is brought to the surface, resulting in very low pH

values in the rootzone.

c) Because liming is probably prohibitively expensive, the soils in the

low lying areas should never be allowed to dry out. If fresh water is sufficient-

ly available, rice can be grown there, probably with good results. However, in

general, mangrove forestry, shrimp breeding etc. may well be more economical

types of land use in such areas.

d) In order to get an impression of the suitability for agriculture in si-

milar areas in this region, information on the topography (contour-maps) and the

potential acidity of the soil at different depths at a number of selected sites

seems indispensable.

Finally it may be useful to point out that experience with many potentially

acid soils has shown that drying and rewetting of potentially acid soil samples

in the laboratory invariably yields a pH that is some 1 to 2 units lower than

the pH that would be reached if the same soil were oxidized under natural condi-

tions.

Even if the area of the station were to be drained intensively by drainage ca-

nals, it is highly unlikely that oxidation of the soil would result in pH values

as shown in Map 2. Such extremely acid conditions do develop, however, if poten-

tially acid soil is piled up in ridges, where very rapid oxidation can take place.

Acknowledgement

This work was supported by funds from the Netherlands Foundation for the Advan-

cement of Tropical Research (WOTRO). The results were published with the permis-

sion of the Director General of the Department of Land Development, Thailand.

163

t

hkp l : pH of the topsoi l (15-30 cm) under Map 2: pH of the topsoi l a f t e r drying and re-

wetting i n the laboratory.At depths

below 60 cm ( I ) or 30 cm (11-v) the pH i s t o between 2 and 3 .

f i e l d conditions; a l l deeper layers have a pH between 6 and 7 . The ridged area (topsoil pH 2-4) is shown by hatching.

Symbol Depth (cm) PH I 15 - 60 5-6.5

I1 15 - 30 5-6.5 I11 15 - 30 4-5

I V 15 - 30 3-4

v 15 - 30 2-3

165

- E v

c u a a, a

166

Fig.3. Two different ways in which

the earth was piled up in

ridges; for explanation see

text.

167

Summar2

A de ta i l ed survey based on pH measurements of f r e s h and dried soil samples from a recen t l y reclaimed mangrove swamp shows t h a t i a ) p o t e n t i a l l y acid topsoils are

found only i n the r e l a t i v e l y low ly ing areas and ibl subso i l s are p o t e n t i a l l y

acid regardless of topography. The f i nd ings are discussed i n r e l a t i o n t o the condi t ions for p y r i t e accumulation. A few recommendations are given for t he mana- gement of t he soils i n quest ion.

Résumé

La d i s t r i b u t i o n de l ' a c i d i t é p o t e n t i e l l e dans un so l de mangrove récentement récuperé a é t é determinée p a r un prélèvement en d e t a i l d ' échan t i l l ons e t mesurage

de leur pH en é t a t f ra i che ainsi que séché. Les données montrent que l ' a c i d i t é p o t e n t i e l l e dans l e s couches s u p e r f i c i e l l e s e s t l i é e exclusivement a m s i t e s bas,

pendant dans l e s sous-sol 1 ' a c i d i t é p o t e n t i e l l e e s t présent paiqtout e t indépendent de l a t o p o g r q h i e . Les données sont d i scu tées en rapport avec l e s condi t ions f a - vorables à l 'accumulation de l a p y r i t e . Quelques mesures pour I'amAnagement des

s o l s en quest ion sont recommendées.

Besumen

Se determina l a d i s t r ibuc ión de l a acidez p o t e n t i a l en un suelo de manglares re-

c i e n t m e n t e recuperado por medio de un mostreo detal lado, mediendo e l pH de l a s m e s t r a s en e l estado fresco y después del secarlos. Las resultas indican que pura

la s capas superiores , l a acidez po ten t ia l e x i s t e e x c l u s i v m e n t e en los s i t i o s

bajos . Mientras para la s capas i n f e r i o r e s se encuentra l a a c i d e z p o t e n t i a l en

cualquier parte independiente de l a topografia. Se discute l o s datos en relucidn

con l a s condiciones ambientales fomentando l a acmulacidn de l a p i r i t a . FinuZmente se recomienda unas medidas para e l mejorar de lo s suelos consabidos.

Zusamenfasswig

Im neu l i ch urbar gemachten n k q p " e b o d e n wurde die p o t e n t i e l l e Az id i t t i t k a r t i e r t

au f Grand ausf i ihrl icher Probeentnahmen, sowie pH-.^ilessungen b e i Bodenproben i r r

frischen und auch im getrockneten Zustond. Auf Grund der Data ze i zen d i e oberen

Horizonte e ine ausschl iess l iche Lokal is ier ing der p o t e n t i e l Len P z i d i t t i t i n den Geliinde-DepTessionen, ijährenll d i e i n den tiefeeren Horizcnten 5beruZ.Z vorkommende p o t e n t i e l l e A z i d i t t i t zeigt sick mabhtingig :]rim ?iProre l ie-r des SeZri'ndes. Die ir-

gebnisse verden im Lich te dei. ZUT h ~ r i t a J C P u ~ ~ ht iovr yihrenl/en :;-nztBnde er

Ein ige Eodenmelicrat ionc-iassnah~e~ werden emrifchlen.

168

MICROPEDOLOGICAL OBSERVATIONS ON PYRITE AND ITS PEDOLOGICAL REACTION PRODUCTS

D.van D u n i and L.J.Pons U n i v e r s i t y of Agr icu l ture , Department of S o i l :&enoe avid Geology Wageningen

I. Introduction

Apart from a micromorphological description of a cat clay profile from Malaysia

by ESWARAN (1967) and of some incidental microscopical observations on pyrite in

reduced muds no micromorphological studies of well developed acid sulphate soils

have been published up to now.

At the Laboratory of Soil Science of the University of Agriculture in Wageningen

micromorphological studies are being carried out for several years and many of

these pertain to acid sulphate soils, one of the main research subjects of this

laboratory. Several students included micromorphological research of acid sul-

phate soils from several countries in their graduate pedological studies. Some

results of these studies are presented in this article. The basis of the present

article is a graduate study by the first author (VAN DAM 1971) on a number of

well developed acid sulphate and related soils from Thailand, sampled by the

second author in 1968 (PONS and VAN DER KEVIE 1969). Also other samples of acid sulphate soils were used, e.g. those already described by KOOPMANS (1967) from

the Niger delta (Nigeria) and from Mijdrecht (The Netherlands). Also soils sampled

for other purposes were included e.g. from Purmerend and from Elst (The Nether-

lands).

The aim of this article is to review the micromorphological characteristics of

various forms of pyrite present in reduced muds and of the reaction products

related to the oxidation of pyrite in acid sulphate soils.

2. The origin of pyrite in marine muds and of its oxidation products in acid sulphate soils

As described by many authors (a.o. SLAGER and VAN SCHUYLENBORGH 1970; LOVE and

AMSTUTZ 1966) pyrite is formed in a reduced environment with a continuous supply

of sulphates in the presence of easily decomposable organic matter, sulphate

reducing bacteria being an important interagent.

The main reaction involved can be written according to VAN BKEEYEN (1968) as

SO?- + Z(CHz0) + H 2 S + 2HCO; ( 1 )

169

(a microbiologically mediated reaction conditioned by Eh 0 . 0 - 0 . 2 V and pH 5.5 and 9)

3 H L S + Fe(OH)3 + 2FeS + So + 6 H 2 0

FeS + So -f Fesp

( 2 )

(3)

(both pure chemical reactions)

The boundary conditions of its formation make pyrite a common element of marine

sediments.

Three forms of pyrite are distinguished by PONS (1963): primary, secondary and

tertiary pyrite. Primary pyrite is of geogenetical origin and consists of the

synsedimentary allochtonic part of the pyrite and the autochtonic pyrite associ-

ated with the synsedimentary part of the organic matter. =secondary and terti-

ary pyrite are formed during and after the last phase of sedimentation when auto-

chtonic organic matter originating from the vegetation already settled on the se-

diment, respectively on the soil, is used for sulphate reduction. The secondary

pyrite can be considered as a component of mixed geogenetical and pedogenetical

origin, and is formed before and during complete ripening of the soil. The ter-

tiary pyrite is formed when the sedimentation and ripening of the soil has termi-

nated completely and the soil has been subject to renewed inundations with saline

or brackish water. Typical for the primary pyrite is the rather homogeneous distri-

bution of relatively small pyrite aggregates. Secondary and tertiary pyrite are

always heterogeneously distributed and form larger aggregates always closely

associated with organic tissues. Yicroscopically the secondary and tertiary forms

are hardly distinguishable.

With drainage under natural or artificial influence (respectively tidal actions

or reclamations) air will penetrate the originally totally reduced mud. At this

moment the pedogenesis starts and the oxidation of pyrite gives rise to several

oxidation products.

Pyrite oxidizes to form ferric hydroxide and sulphuric acid according to

Microorganisms of the Thiobacilli group are mainly responsible for the oxidation

of ferrous ions to ferric ions. The process is slow at high pH and rapid under

acid conditions when dissolved ferric ions may act as an oxidant (cf.VAN RREEMEN

1973) .

I70

Depending on the composition of the sediment, especially on the presence and

quantity of neutralizing components a s CaC03 and bases adsorbed to the soil

complex, the reaction may develop along one of the following pathways:

a) In the presence of enough Ca-carbonates (shells, shell fragments and

limestone-rock fragments) or adsorbed bases, the drop in pH is small and the

following reactions are taking place:

b) If only small amounts of Ca carbonates and/or adsorbed bases are present

the buffering system of reaction (6) fails and the pH will drop. In this case

reaction ( 4 ) is promoted, many H+ ions are produced and a basic iron sulphate

(jarosite) is being formed, buffering the system at a pH of 3.8. The reaction

writes as follows:

Excess of oxygen and lack of Fe(OH)3 causes a further drop in pH to very low

values. Dissolved iron and sulphate ions which both are mobile, may give rise to

insoluble, visible reaction products at some distance from the pyrite source.

At low pH values some clay minerals as montmorillonite and other silicate minerals

as glauconite and chamosite become unstable (PONS and VAN OER KEVIE 1969) and start

to hydrolyze. During this weathering process HbSi04 is being formed (8) and is

precipitated as cryptocrystalline silica.

In older acid sulphate soils in which H+ and SO:- ions are leached and/or neutra-

lized, the jarosite tends to hydrolyze according to

KFe3(S0,+)2(0H)6 + 3H20 3Fe(OH)3 + K+ + 2S0:- + 3H+ (9)

At the same time, especially along pores and channels ferric hydroxide will

easily be dehydrated giving rise to goethite and even haematite. In thin sections

of the different horizons of acid sulphate soils developed from marine sediments

various products of the above mentioned reactions are visible. Yost important in

this respect are:

1 . Pyrite in various forms, mainly in reduced subsoils (C, horizons), but also

in aerated calcareous soils, a s relicts that as yet have n o t been oxidized.

171

2 . Ferrous iron and sulphate ions, although not visible when formed in an

early stage of oxidation probably are indicated by "empty" places, significant

for interpretation of the processes involved (see also Section 5 ) .

3 . Amorphous ferric hydroxides (and sometimes dehydrated and crystallized

forms of ferric hydroxides like limonite and goethite) especially in calcareous

or deacidified soil horizons with respectively a high and a rather high pH level.

4 . Gypsum, in horizons that were originally calcareous, or to which Ca was

added subsequently by inundation water or liming. Gypsum may also be formed when

the soil complex does contain Ca2+-ions.

5. Jarosite, in more or less oxidized acid C and A horizons.

6. Cryptocrystalline silica in acid oxidized C and A horizons. g g

g g

3 . Identification of pyrite and its reaction products

The micromorphological investigations were carried out with a set up allowing

microscopic observation of thin sections with transmitted normal and polarized

light and with incident mercury light. Together with incident mercury light a blue

filter was used.

Pyrite and the above mentioned reaction products show characteristic colour varia-

tions in the above mentioned three kinds of light. This enables a relatively

simple microscopic recognition, which, in nearly all cases is satisfactory for the

purpose. The roughly described characteristics are listed in Table I .

Pyrite, strongly humified organic matter and some pieces of amorphous silica are

all opaque in transmitted light. Incident mercury light however immediately allows

to identify any pyrite crystals by their intensive light green reflection as well

as cryptocrystallized silica by its soft bluish green reflection.

The identification of specific minerals may become difficult, when they occur as

mixtures. This is often the case with amorphous ferrit hydroxide and jarosite. In

most of these cases however, successful recognition is possible after some ex-

perience, particularly with discriminate application of incident mercury light.

172

TABLE I . ROUGH DESCRIPTIVE MICROSCOPICAL CHARACTERISATION OF COMPONENTS OF ACID SULPHATE SOILS

INCIDENT LIGHT MERCURY TRANSMITTED LIGHT COMPONENT NORMAL POLARIZED

Pyrite

Organic matter

Amorphous hydroxide

Goethite

Haema t i t e

Jarosite

Crypto- crystalline silica

opaque opaque

brownish brownish black black

redbrown isotropic redbrown

orange yellow; orange yellow; mixed with mixed with matrix : matrix: olive green olive brown

bright red bright red, anisotropic

very light light reddish reddish brown brown to to yellowish yellow, fine grey anisotropic

light grey to light grey to dark grey to dark grey to black black, aniso-

tropic

bright light green

no reflection, sometimes greenish

if pure: no reflection; if mixed with soil matrix: green to black depending on concentration

if pure: no reflection; if mixed with matrix: bright green

stone red reflection

very bright green to marine green

blueish green

4 . The micromorphology of pyrite

ai CZassi f icat ions

Mainly with a view to quantitative determination of pyrite content using micro-

scopic enumeration, microscopically visible pyrite-elements have been classified

according to their size, shape and distribution pattern. In an earlier attempt

the second author (PONS 1 9 6 4 ) counted crystals and crystal clusters (crystallaria)

in suspensions of crushed sediments using six size classes. This method allows

quick and for practical purposes fairly reliable semi-quantitative determination

of pyrites.

The first author developed this method for use in thin sections refining the

classification system to suit micromorphological description and interpretation.

A full report of the improved method and its results will be published elsewhere.

At present the refined classification will be presented and its genetical impli-

cations briefly discussed.

Pyrite commonly occurs in the shape of microcrystals in reduced marine sediments

and in the reduced subsoils (G-horizons) of acid sulphate s o i l s and other soils

173

developed on marine sediments. According to the size and reflection of the

individual crystals under the microscope the first author distinguished the fol-

lowing three crystal size classes (VAN DAM 1971):

I. The pyrite crystals are very small ( < l p ) , their reflection with mercury

light is medium and they cannot be recognized separately.

11. The microcrystals are small (l-2p), their reflection is bright and they

are separately visible, but no individual shapes can be recognized.

111. The microcrystals are bigger (2-611) and they sharply reflect the

incident mercury light. The shape of individual crystals is well visible.

Pyrite microcrystals of Class I11 are clearly angular (probably cubic)

as easily can be seen in Photographs 1 and 2. It is very probable that

also the microcrystals of Class I and I1 are angular.

Pyrite microcrystals of different size, as described above, are nearly always

concentrated in composite structures, often in the shape of spherical aggregates

(Pyrite Crystallaria, see Photograph 2). These were originally described by

LOVE (1962) from carboniferous shales as well as from recent sediments. He named

the spherical aggregates "framboids" and explained the origin of the bags, sur-

rounded by a clearly visible skin and densely filled up with pyrite microcrystals,

as filled up micro-organisms. RICKARD (1969, 1970) states that "sulphate-reducing

bacteria do not exert any direct influence on the textures of pyrite. Slow pro-

duction of pyrite from the initial mackinawite under the influence of surface-

active agents from the autolyzed bacteria and their waste products, may result

in the formation of framboidal forms of pyrite." According to RICKARD the pyrite

framboids may be pseudomorphic after organic spherules o r they may have formed

by the infilling of gaseous vacuoles in rocks and sediments by pyrite.

The pyrite aggregates (crystallaria) were classified by the first author (VAN DAM

1971) according to their size in the following aggregate size classes:

1 . < 2p; 2: 2-6p; 3: 6-2511; 4 : 25-10011; and 5: > 10011

According to the distribution pattern of pyrite in the soil and its association

with other soil components four classes are distinguished:

A: Pyrite is scattered throughout the matrix.

B: Pyrite is associated with organic matter. c: Pyrite i s present in soil pores.

D: Pyrite is filling the inside of diatoms or foraminifers.

I 7 4

This classification of the distribution pattern of pyrite is closely related to

the division of the pyrite into primary, secondary and tertiary pyrite (PONS 1963) .

b ) The r e l a t i o n s o f d i s t r i b u t i o n pa t t e rn c las ses t o primary, secondary and t e r t i a r y p y r i t e

The most typical distribution pattern of primary pyrite is Class A : pyrite scat-

tered in the matrix (Photograph 3). Sometimes, especially when the original

sediment is rich in synsedimentary organic matter, the primary pyrite may occur

according to Class B, but in these cases the organic matter particles are always small and show horizontal orientations (Photograph 4 ) .

During the process of sedimentation concentrations of primary pyrite may be

formed by the filling in of cracks and animal pores (Class C; Photograph 3). These concentrations never show any traces of organic matter.

The pyrite of Class D, occurring in the inside of diatoms and foraminifers also

belongs to the primary pyrite (Photograph 5). Especially in sandy and/or silty

sediments the diatoms and foraminifers will have been added to the sediment by

transport from shallow seabottoms. The walls of these diatoms and foraminifers

may be destroyed, setting free the microcrystals. A l s o destruction of the pyrite

framboids is occurring (Photograph 6), during as well as after the sedimentation. In these cases the individual microcrystals of pyrite are present in a scattered

distribution pattern in the matrix according to Class A (Photograph 3 ) .

Secondary as well as tertiary pyrite is nearly always associated with organic

matter (Class B). Sometimes the organic matter has vanished and consequently the

pyrite distribution has to be classified as Class C (Photographs 7 and 8 ) . In

these cases however, nearly always the holes, in which the former organic matter

was present, can he distinguished as being the site of former plant remains.

Secondary and tertiary pyrite distributed according to Class B occurs nearly

always in big clusters of pyrite crystallaria in or against coarse to rather

coarse, more or less decomposed plant tissues. The pyrite may be present as pyrite

crystallaria tubes in pores of decomposed roots (Photograph 7 ) , as pyrite crystal-

laria strands and clusters in the inside of rather well intact roots (Photograph

9 ) or other kinds of plant tissues (Photograph IO), or even as pyrite crystallaria

tumors, originating under the epidermis of roots, destroying it by its growth

(Photograph 11).

The distinction between secondary and tertiary pyrite may he relatively easy

visible or when the plant remains show a vertical orientation. Most of the times

175

however it is very difficult to distinguish between secondary and tertiary pyri'

by means of micromorphological investigations only. Secondary and tertiary pyrit

may be present as scattered individual pyrite microcrystals when original pyrite

crystallaria have been destroyed (see Photograph 7, where the destruction of the pyrite crystallaria was probably due to the preparation of the thin sections).

e i Relat ions between s i z e of microcrystals , s i z e o f p y r i t e c r y s t a l l a r i a and d i s t r i b u t i o n pat terns in d i f f e r e n t sediments

The size of the pyrite microcrystals and of the crystallaria (framboids) that

are scattered in the matrix (Class A) shows some correlation with the texture of

the sediment. In sediments rich in clay (coastal clay in Surinam and marine Beem-

ster clay in The Netherlands) the framboids are small ( < l o l l , mainly crystal

aggregate size class 1 and 2) and are composed of small microcrystals (Class I).

In sediments rich in silt the size of the pyrite crystallaria is mainly of

aggregate size Class 3 with individual microcrystals of Class I and 11. Such

a correlation and also the occurrence of sedimentary concentrations of pyrite

(Photograph 3 ) point to a sedimentary origin of the bulk of the primary pyrite.

A post sedimentary origin from synsedimentary organic matter is improbable for

the majority of Class A pyrite. Sometimes also Class B pyrite must be considered

as primary pyrite (Photograph 4 ) .

The size of the pyrite crystallaria of distribution Class B (nearly always

secondary pyrite) lies in most cases between 20 and 501~. (Aggregate Size Class 3

and 4) and their microcrystals belong to Class I1 (Photographs 6 , 7 , 9 , 1 0 and 1 1 )

and sometimes to Crystal Size Class 111 (Photograph 2). Pyrite crystallaria of

Class B inside of more or less intact cells (e.g. Photograph 4) are mostly smaller than 2011 and belong in majority to Aggregate Size Class 2 or 3. Their

microcrystals are also smaller and belong to Crystal Size Class I.

The pyrite microcrystals occurring in the inside of foraminifers and diatoms

(Class D ) are in most cases coarse and belong to Crystal Size Class 111 (Photo-

graph 5). In this case the framboids are absent or not well developed.

5. The micromorphology of oxidation products of pyrite

The pedogenesis of acid sulphate soils and other soils developing on pyritic ma-

rine sediments starts as soon as air is penetrating the soil along cracks and

rootholes. The first oxidation of the soil mass and of the pyrite takes place

176

along these structural features and as a result the oxygen pressure and the state

of reduction may vary from place to place, particularly near the oxidation-

reduction boundary. This transition horizon with an irregular oxidation front,

in which totally reduced and rather impermeable soil plasma occurs next to

oxidized walls of cracks and rootholes, may vary in thickness from 20 to 60 cm. Within this horizon occur steep gradients in redox potentials, in moisture

content and after some time also in pH. This causes intensive mass movements

and precipitations in selective sites.

Amorphous ferric hydroxides (Reaction 41

The beginning of oxidation of pyrite is evident as a diffuse ring of ferric

hydroxides at the periphery of pyrite crystals or crystallaria (Photograph 1 2 ) .

Pyrite, present in diatoms and in the central parts of pyrite concentrations,

may stay unoxidized for a very long time, even in a totally oxidized environment.

With rapid oxidation of pyrite the resulting ferric hydroxides are found always

close to the site of their origin and associated with remaining pyrites. In

weakly oxidizing environments the slow pyrite oxidation first produces ferrous

iron, which is soluble. This, together with H+ and SO;- ions will move to the

walls of rootholes and cracks where subsequently the ferrous iron is oxidized

to ferric iron. Ferric hydroxides may precipitate on the walls of the structural

elements in the form of neoferrans and quasiferrans, sometimes forming thick

skins (Photograph 13). The ferric hydroxide skins around the cracks and root-

holes, particularly i n the upper s o i l horizons may also include dehydrated forms

of ferric hydroxide e.g. crystallized goethite and/or even haematite. This is

frequently the case in the intensely red coloured cambic-B horizons of the well

developed acid sulphate soils of Thailand (Photograph 1 4 ) as a result of dehydra-

tion in the long dry season.

Gypswfl

Gypsum crystallizes wherever sufficient sulphate and Ca ions are combining

(Reactions 5 and 6). Concentrations of gypsum are mainly present as crystal tubes

and are formed in rootholes and cracks as a result of evaporation and saturation

of the soil solution (Photograph 15). They also may occur as neo- and quasicutans

in the soil matrix around cracks and rootholes. I n some profiles high quantities

of gypsum occur throughout the s o i l e.g. when the original s o i l material was

calcareous. In other soils the gypsum is concentrated in horizons where inundation

water containing CaCo-,, is meeting the soil solution with SO:- and H+ ions

(VAN DER KEVIE 1972). In these cases concentrations of gypsum are found directly

above or in the upper part of jarosite horizons (Photograph 1 6 ) . I77

Jarosite

The highly insoluble jarosite (basic ferric potassium sulphate) is precipitated

in places where SO:- and ferric and potassium ions are meeting. If oxygen can

penetrate the soil and reach the pyrite crystals easily, the jarosite is found

in contact with the pyrite. With slow oxidation of pyrites the soluble oxidation

products will move to the cracks and channels with higher oxygen pressure and

there, along the walls and in the holes, very fine crystalline jarosite is formed,

mostly as jarositans and as neojarositans (Photograph 17).

Sometimes the holes are totally filled up with pure jarosite (Photograph 1 7 ) . In

many cases skins, composed of a mixture of jarosite and ferric hydroxide are

formed, the composition changing gradually from the inside to the outside of

the skins. Older jarosite precipitations may hydrolize along their outer bounda-

ries under the influence of leaching SO:- and H

crystalline goethite is often present (Photograph 17).

+ . ions and at these places fine

CryptocrystaZZine s i l i c a

Cryptocrystalline precipitations of silica, recognizable by grey colours

in transmitted plain light and a very typical bluish

dent mercury light, are commonly present in the matrix at some distance from

pyrite remnants. Sometimes the silica occurs only homogeneously mixed with the

matrix, in other cases it is present as clearly visible concentrations (Photo-

graph 19) , mostly with shapes related to holes and cracks (neo- and quasicutans)

and to accumulations of jarosite and ferric hydroxide (cutans: Photograph 17). When the silica is homogeneously mixed with the soil matrix, the silicic acid

is most probably derived from clay minerals. In cases with more concentrated

cryptocrystalline silica precipitations greater minerals e.g. glauconite may

be the source of the silicic acid.

green reflection in inci-

Zonation

The micromorphology of thin sections often expresses the movements of soluble

oxidation components and the subsequent precipitation as insoluble products in

relation to structural soil features.

In the deeply developed, rather old acid sulphate soils of Thailand the transition

horizon between the totally reduced subsoil ( G horizon) and the oxidized upper

horizons (Cg horizons) is rather broad. In this transition horizon (Cg-G horizon)

already some structure has been developing, but as yet totally reduced inner

parts of prisms occur next to oxidized walls of cracks. In vertical sections

of this transition horizon a typical zonation can be observed from the inside to

the outside of the peds (see Figure I ) .

Some of these zones have distinct boundaries, but also gradual transitions occur,

e.g. between zones c and d and e in Figure 1 .

For the explanation of this zonation one has to consider the physico-chemical

gradients that occur in the Cg-G horizon between the inner and outer parts of

the prisms.

Especially during the dry season, gradients of the oxigen pressure, the redox

potential, the pH, and the moisture conditions are causing a mass transport of

oxidation products of pyrite towards the cracks.

According to Reactions 5 and 9, soluble Fe2+, Sof- and H+ ions will be produced

in Zone a and transported through Zone b. In Zone c, a part of the H+ ions reacts

with the clay minerals, giving cryptocrystalline silica (Reaction 8 ) . In Zone d,

a part

SO:- ions together with other FeZ+ ions is precipitated as jarosite (Reaction 8) in Zone e. An other part of these SO?- ions together with Ca2+ ions originating

from some CaC03 (Reaction 5) and from the adsorbed Ca of the complex is preci- pitated as gypsum (Reaction 6).

Deeply developed acid sulphate soils of Thailand show the same at shallower depths,

immediately above the transition horizon (Cg-G) in a Bg-horizon in which the con-

centration of jarosite reaches its maximum (PONS and VAN DER KEVIE 1969 and

VLEK 1971). In the upper part of this jarositic B-horizon yellowish mottles are

present together with brown ones and in some places even red ones. On the peds

of this horizon cutans are present composed of three zones (Figure 2).

The genesis of this zonation can be explained with an interference of the earlier

mentioned dry season moisture gradient promoting jarosite accumulation at the

periphery of the prisms (Zone a in Figure 2) and a reverse gradient during the

wet season promoting the hydrolysis of the jarosite (Reaction 9) and the leaching of SO$- and H+ with the water percolating through the cracks. After the hydrolysis and leaching amorphous ferric hydroxide remains (Zone b in Fig.2), the outer part

of which changing into crystalline goethite due to dehydration during the dry season

(Zone c in Fig.2).

In some cases crystalline ferric oxides of brown colours and of red colours (respectively goethite and haematite) are present next to each other (see Photo-

graph 14). only red coloured haematite can be found.

of the Fe2+ is oxidized to amorphous ferric hydroxide. A part of the

In the upper horizons of the typical red mottled acid sulphate soils

179

Acknowledgement

For the microscopical determination of pyrite and of the different oxidation

products of pyrite, and for the micromorphological investigations, carried out

at the Laboratory for Micromorphology of the Department of Soils and Geology

of the University of Agriculture, much help was met with from Mr.S.Slager and

Mr.A.G.Jongmans. For the determination of the oxidation products Mr.J.van Schuy-

lenborgh and Mr.N.van Breemen carried out some chemical investigations and Mr.

R.Schoor1 some röntgenographical ones on small samples from "Anschliff" pieces.

W e also thank Mr.S.Slager, Mr.L.van der Plas, Mr.N.van Breemen, Mr.R.Miedema and

many others for the discussions in relation with this paper. A special word of

thank also for the colleagues who placed additional samples at our disposal.

180

4

F i g u r e 1 ped channe l

I A

I-

a -

b -

c -

d -

e -

f -

The interior of the peds with unoxidized pyrite (e.g.Photograph 3 )

A zone without any visible pyrite oxidation products (Photographs 17, 20)

A rather broad zone with cryptocrystalline silica in the soil matrix (Photographs 17, 19, 20)

A zone with ferric hydroxide, mostly present as neo-ferrans or quasiferrans (Photographs 18, 20) A zone with jarosite, partly as neo-jarositans, partly as cutans or as crystal tubes of jarosite (Photographs 17, 18, 20) The inner part of the crack or channel is filled with gypsum as a crystal tube (Photograph 7 )

Figure 2 ped channel

r-----l-

a - An outer zone of goethite along the cracks or channels

b - A zone with amorphous ferric hydroxide c - An inner zone with jarosite

181

LIST OF PHOTOGRAPHS

Numbers en ter parenthesis, e .g . (2), r e f e r t o indicatory s igns on photographs

Photograph 1 . A pyrite crystallarium of about 50 p diameter (Class 4 ) composed of

relatively large pyrite micro-crystals ( 1 ) . The microcrystals are about 5 )i in diameter (Class 111) and their angular form is clearly visible.

Combined transmitted p l a i n l i g h t and incident mercury l i g h t , 1 3 8 ~

Photograph 2 . Two pyrite crystallaria ( 2 , 3 ) of tertiary pyrite associated with

organic matter from sphagnum peat ( 1 ) that was inundated with seawater. The pyrite

crystallaria are spherical (framboïds) and the individual microcrystals are

present inside of a pseudo skin (2). The crystallaria belong to the Size Class 4

and the clearly angular microcrystals to Class 111 respectively Class 1/11. The density of the infilling of both crystallaria is very different. Acid sulphate

peat soil from Groot Mijdrecht (The Netherlands), depth 55-70 cm.

Combined transmitted p la in l i g h t and incident mercury l i g h t , 312x

Photograph 3. Reduced subsoil of a heavy textured calcareous marine clay soil,

Purmerend, The Netherlands, depth 308-323 cm, with homogeneously scattered pyrite crystallaria and microcrystals (Class A) in the matrix ( 1 ) and a pyrite crystal

sheet ( 2 ) (Class B ) . Both distribution classes of pyrite must be considered as

primary pyrite. The pyrite crystal sheet represents a crack or animal pore filled up with pyrite during the sedimentation.

Trunsmitted p la in Zight, 81x

Photograph 4 . Fine (IO microns) primary pyrite crystals (1 ) occurring inside syn-

sedimentary bands of organic matter ( 2 ) . The organic matter is horizontally orien- ted, parallel with the sedimentation stratigraphy. From CG-horizon at 140 depth

of profile from Thailand.

Combined transmitted p l a i n l i g h t and inc ident mercury l i g h t , 81x

Photograph 5. Pyrite microcrystals present in the inside of foraminifers and dia-

tomae ( 1 ) . The pyrite microcrystals are of Size Class 111. Calcareous marine clay soil from The Netherlands, Purmerend, depth 163-178 cm.

Combined transmitted p la in l i g h t and inc ident mercury l i g h t , 1 3 8 x

Photograph 6. A number of tertiary pyrite crystallaria (Class 4 ) (framboïds) in

different stages of decomposition. Some are intact ( 1 ) and show the original sphe- rical shape, surrounded by a pseudoskin, others ( 2 ) are decomposed and the indivi-

dual microcrystals start to scatter. Acid sulphate peat soil from Groot Mijdrecht, The Netherlands, same profile as Photograph 2.

Combined transmitted p la in l i g h t and inc ident mercury l i g h t , 273x

183

Photograph 7. A cluster of rather large pyrite crystallaria (Class 4 ) forming

an irregular pyrite crystallaria tube ( I ) , partly filling up a pore of a former

root. The root is totally decomposed and the distribution class is transitionary

between Class B and C. Totally reduced subsoil of calcareous marine clay soil

from Purmerend, The Netherlands, depth 223-238 cm.

Combined transmitted p l a i n l i g h t and inc ident mercury Zight, 1 3 8 ~

Photograph 8. Secundary pyrite present as irregular distributed pyrite crystalla-

ria (1 ) between the sandgrains (2). The crystallaria show very different sizes

(Class 3 and 4 ) and are filled with microcrystals of Class 1/11. Sandy gley soil

(Cat Sand Soil) from Reutem, The Netherlands, depth 60-75 cm.

Combined transmitted p la in l i g h t and incident mercury l i g h t , 138x

Photograph 9. Tertiary pyrite occurring as irregular strands of rather small py-

rite crystallaria ( 1 ) (Class 3) in rather well intact pieces of root of Sphagnum

(2). Acid sulphate peat soil from Groot Mijdrecht, The Netherlands, depth 55-70 cm.

Same profile as Photograph 2.

Combined transmitted p la in Zight and incident mercury l i g h t , 138x

Photograph 10. Secundary or tertiary pyrite present as clusters of pyrite cry-

stallaria ( 1 ) in partly decomposed tissues (Class B ) . Acid sulphate soil from

Groot Mijdrecht, The Netherlands, depth 115-130 cm.

Combined transmitted p l a i n l i g h t and inc ident mercury l i g h t , 81x

Photograph 1 1 . Secundary or tertiary pyrite occurring as a kind of pyrite crystal-

laria tumor ( I ) (Distribution Class B ) . The medium sized pyrite crystallaria

(Size Class 3 and 4 ) developed under the epidermis of a Phragmites root pressing

it upwards. Other pyrite crystallaria have been developed in the inside of the

root. Peaty marine clay soil from Groot Mijdrecht, The Netherlands, depth 80-95 cm.

Combined transmitted p la in l i g h t and inc ident mercury Zight, 1 3 8 ~

Photograph 12. Pyrite microcrystals filling the inside of a diatom shell ( 1 ) present

in the matrix of a marine sandy clay (2). The microcrystals are ShnTTing the first

signs of oxidation. Between the black unoxidized pyrite crystals a brownish vague

zone is visible where the pyrites are already oxidized. Marine sandy clay soil from

Purmerend, The Netherlands.

Combined transmitted p l a i n l i g h t and incident mercury l i g h t , 138x

Photograph 13. Ferric hydroxide cutans ( 1 ) around grains of calcium-carbonate and

quartz. The pyrite grains are oxidized and the ferric hydroxide is partly present

at the original sites of the pyrite (2), partly as cutans around other grains. The

ferric hydroxide in this horizon is very immobile due to a high amount of calcium-

184

carbonate, prohibiting the development of a low pH by pyrite oxidation. Strongly

stratified sandy and clayey layers of marine sediment from the Haarlemmermeer,

The Netherlands, depth 95-100 cm.

Transmitted p l a i n l i g h t , 200~

Photograph 14. Well crystallized microcrystals of haematite ( 1 ) (droplets)

(HAMILTON, 1964), partly scattered in the soil matrix, partly present in deformed

cutans. Red-brown coloured with transmitted normal light, very dark red in pola-

rized light, bright orange-red coloured in nature. Upper part of the cambic B-

horizon of well developed acid sulphate soils of Thailand.

Transmitted p l a i n l i g h t , 2OOx

Photograph 15. A concentration of well developed gypsum crystals forming together

a crystal tube ( 1 ) developed in a channel of an acid sulphate soil. The soil

matrix of the heavy clay soil shows on spots weak plasma reorientation ( Z ) , as

a result of the crystallization pressure of the gypsum. Thin section from 75-90 cm,

directly under the red coloured cambic-B-horizon. Profile from Nakhon Pathon,

Thailand.

Transmitted l i g h t , crossed po lar i ze r s , 3 1 x

Photograph 16. Concentrations of gypsum ( 1 ) occurring in horizons rich in jarosite

(2). When jarosite concentrations are formed in well developed acid sulphate soils,

by subsequently added calcium-carbonate (e.g. by irrigation water rich in calca-

reous silt), the jarosite may hydrolize and gypsum is formed in its place. Acid

sulphate soil from Thailand, depth 57-72 cm.

Combined transmit ted p l a i n l i g h t and inc iden t mercury l i g h t , 138x

Photograph 17. Along a wall of a roothole (1 ) neocutans of microcrystalline jaro-

site (2) are present. The transition of the (partly broken) neojarositan to the

matrix is very diffuse. At the outer border of the neojarositan mixtures of amor-

phous ferric hydroxide with jarosite are present in different concentrations, giv-

ing red brown colours (3). In the roothole a high birefringent cutan of goethite

is visible (4). Acid sulphate soil of Groot ?lijdrecht, The Netherlands, depth

55-70 cm.

Transmitted l i g h t , crossed po lar i ze r s , 81x

Photograph 18. Nearly pure, cryptocrystalline jarosite ( I ) , filling up a roothole of the jarositic horizon of a well developed acid sulphate soil from Rangsit (Thai-

land) CG-horizon, 130-135 cm. The wall of the roothole shows neo- and quasiferrans

(2). The black spots in the jarosite are ferric hydroxides (3). Transmitted p l a i n Zight and i nc iden t mercury l i g h t , 4 8 x

185

Photograph 19. Concentrations of cryptocrystalline silica, present as neocutan ( 1 )

in a acid sulphate soil of Thailand, depth 80-95 cm. Transmitted pZain l i g h t , 126x

Photograph 20. Ped surface in the jarositic horizon of an acid sulphate soil from

Rangsit (Thailand) CG-horizon, 155-170 cm. From inside to outside the ped a zone

without any pyrite or visible pyrite oxidation products ( I ) , a zone with concen-

trations and quasicutans of silica (2), a quasicutan to cutan of ferric hydroxi-

de ( 3 ) and a cutan of jarosite ( 4 ) are visible.

Transmitted l i g h t , crossed po lar i ze r s , 1 2 6 ~

186

O N

I92

REFERENCES

BREEMEN, N.van. 1968. Formation and conversion of sulphides and sulphates in

marine sediments and cat clay soils. Translated title of a not published

literature

DAM, D.van. 197

produkten.

products.)

1971.

ESWARAN, H. 196

14:259-265.

study in Dutch. Univ.of Agric., Wageningen.

. Mikromorfologisch onderzoek over pyriet en pirietoxidatie-

(Micromorphological investigations on pyrite and pyrite oxidation

Report of a graduate study in Regional Soil Science. Wageningen.

. Micromorphological study of a "cat-clay'' soil. Pedologie (Gent)

HAMILTON, R. 1 9 6 4 . A short note on droplet formation in iron crusts. In:

Jongerius, A. (Ed.): Soil Micromorphology, pp.277-278.

KEVIE, W. van der. 1972. Physiography, classification and mapping of acid sul-

phate soils. Paper 1nt.Symp.Acid Sulphate Soils. Wageningen. 1973.

KOOPMANS, R.K. 1967. Geogenesis and pedogenesis in different sedimentation en-

vironments from the Niger delta and some aspects of pyrite research. Transl.

title (Dutch) of a report of a graduate study in Regional Soil Science.

Wageningen, 1967.

LOVE, L.G. 1962. Biogenic primary sulphide of the Permian Kupferschiefer and

Marl Slate. Econ.Geol.57, 3:350-366.

LOVE, L.G., and AMSTUTZ, 1966. Review of microscopic pyrite. Fortschr.Miner. 4 3 : 273-309.

PONS, L.J. 1963. Pyrites as a factor controlling chemical "ripening" and forma-

tion of "cat-clay" with special reference to the coastal plain of Surinam.

Proc.Congr.Agr.Res.Guianas. Paramaribo. 1 9 6 3 . Bu11.82:141-162. Agr.Exp.Stat.

Paramaribo.

PONS, L.J. 1964. A quantitative microscopical method of pyrite determination in soils. In: Jongerius, A . (Ed.): Soil Micromorphology, pp.401-409.

PONS, L . J . , and KEVIE, W.van der. 1969. Acid Sulphate Soils in Thailand. Studies

on the morphology, genesis and agricultural potential of soils with catclay.

Soil Survey Report No.81. Dept.of Land Dev. Bangkok.

RICKARD, D.T. 1969. The microbiological formation of iron sulphides. Stockholm.

Contributions in Geology, Vol.XX. 3:49-66.

RICKARD, D.T. 1970. The origin of framboids. Lithos 3269-293. 193

SLAGER, S . , JONGMANS, A.G., and PONS, L.J. Micromorphology of some tropical

alluvial clay soils. J . S o i l Sci.21:233-241.

SLAGER, S . , SCHUYLENBORGH, J.van. 1970. Morphology and geochemistry of three

clay soils of a tropical coastal plain (Surinam). Agr.Res.Rep.734. PUDOC,

Wageningen.

VLEK, P. 1971. Some morphological, physical and chemical aspects of Acid Sulphate

Soils in Thailand. Soil Survey Report No.84. Dept.of Land Dev. Bangkok.

194

Summary

The micromorphological charac te r i s t i c s of pyrite and i t s oxidat ion products i n acid sulphate s o i l s are described, c l a s s i f i e d and i l l u s t r a t e d w i th photographs.

The genet ical impZications of shapes, configurations and d i s t r i b u t i o n pa t t e rns

o f the relevant micromorphological elements are discussed.

Résume'

Les caractéris t iques micromorphologiques de l a pyrite e t ses produi ts d'oxidation dans les s o l s sulfate's acides sont d é c r i t e s , c l a s s i f i é e s e t i l l u s t r é e s par des

photographies. L ' in t e rpré ta t ion génétique des formes, configurations et répar t i -

t ions p a r zones e s t discute'e.

Res umen

Se describe y c l a s i f i c a las cualidades micromorfológicas de l a p i r i t a y los minerales proviniendo de SU oxidación en suelos de s u l f a t o s ácidos i l u s t rando las con fotografias. S e d i scu ta l a i n t e rpre tae ión genét ica de las formas, coloca-

ciones y e l repartimento en zonas de los minerales concernientes.

195

D I S C U S S I O N

RICKARD: Methods i n v o l v i n g m i c r o s c o p i c s e i z i n g o f f r a m b o i d s and t h e n c a l c u -

l a t i o n f r o m t h a t t o t h e amount o f p y r i t e canno t be a c c u r a t e because f r a m b o i d s

v a r y w i d e l y i n d e n s i t y . Framboids g e n e r a l l y a r e n o t s o l i d . I n t h e l a b o r a t o r y i t

i s p o s s i b l e t o c r e a t e f rambo ids w i t h a d e n s i t y l e s s t h a n u n i t y , wh ich f l o a t .

PONS: R e s u l t s o f t h i s method were i n d e e d b e t t e r where f r a m b o i d s were c rushed

o r r e l a t i v e l y few as compared t o s i n g l e p y r i t e c r y s t a l s . C r u s h i n g b r o k e i n many

cases t h e s k i n around t h e f r a m b o i d s a f t e r wh ich t h e m i c r o c r y s t a l s s l i p p e d o u t .

R ICKARD: The s k i n s have been i d e n t i f i e d t o be o f o r g a n i c m a t t e r . The fram-

b o i d s a r e n o r m a l l y i m p e r f e c t and n e a r l y a lways o c c u r t o g e t h e r w i t h s i n g l e m i c r o -

c r y s t a l s . The mechan ica l s t r e n g t h o f t h e f r a m b o i d s i s due t o t h e i r unusua l con-

j u g a t i o n r a t h e r t h a n t o t h e s k i n h o l d i n g t h e m i c r o c r y s t a l t o g e t h e r . A c c o r d i n g t o

LOVE, who used c r u s h i n g o f s o i l t o s e p a r a t e f r a m b o i d s , t h e s e l a t t e r t h e r e b y a r e

r a r e l y c o m p l e t e l y b roken down.

RICKARD: C l a s t i c p y r i t e from r o c k s ( C o r n w a l l ) , d e p o s i t e d a f t e r e r o s i o n i n

c o a s t a l sed imen ts a r e v e r y s t a b l e i n o x i d i z e d c o n d i t i o n s because t h e c r y s t a l s a r e

p r o t e c t e d b y a f i l m o f M a c k i n a w i t e formed t o p o t a c t i l l y on t h e c r y s t a l s s u r f a c e .

The same m i g h t h o l d f o r p y r i t e p r e s e r v e d i n a e r a t e d s o i l s . M a c k i n a w i t e f i l m i s

n o t v i s i b l e by normal m ic roscopy , o n l y b y e l e c t r o n - m i c r o s c o p y .

I96