M A RDI Res. Bul l . ) . 2 : ( .50 - 57 )

PYRITE AND ITS OXIDATION PRODUCTS

IN AN ACID SULPHATE SOIL

Eswnnnx, H. ,qNo Joslpt t , K.T.

A<'c'elt tet l f ()r l )uhl i&ti t tn t tn l9 Nttverrrher. 197'tr

RINGKASAN

Ka i i ben tuk py r i t e dan has i l pengoksa i c l annya t e l ah d i ka l i dengan i r i s n i p i s dan t e rop ( ) r l g

ha lus e l ec t r o r r s cann ing . Py r i t e pe r rngka t kedua se la l unya d i dapa t i sebaga i f r an rbo i c l . Hab lo r -

hab lo r vang t e rd i r i da r i pada f r an rbo ids ada lah py r i t ohed ra l a t aup t t n oc tahec l r a l .

Pe rubahan ben tuk -pa l su f r an rbo ids kepada i a ros i t e t i dak d i dapa t i c l a l am pe ro fa i l i n i .

Ja ros i t e t e rhab lo r danpada l a ru tan t anah dan d i dapa t i da l an r r uan -e kosong . Hab lo r -hab lo r

t a ros i t c ada lah be rben tuk k i ub -pa l su a tau t abu la r .

Ha ide roksa id bes i an ro rphous dan goe th i t e ada lah ke iad ian -ba ru i , ang l a i n r i i da l an r

t anah . Kesemuanya t e rhad kepada l ap i san pe rn r , t kae rn . da lan r bahag i : r n be rL rda ra pe ro fa i l .

INTRODUCTION

Ac ic l su lpha te s r t i l s and po ten t i a l l v ac id su lpha le : , ( ) i l s wh i ch ( ) ccL t r qu i t c ex tens i vc l \

in Malavsia - - est imates sug-sest at least 1 nr i l l ion acres. are chafactcr ised general l \ ' .

thoLr-eh not inevi labl_r- . b1- rhe presence ( ) f jar ( )s i te in the or id ised state and pvr i tc (nra in l1")

in thc u l lox id ised state. Current s tudies bc ins carr ied out b\ Vl ,ARDl are a in led at charac '

rer is i l lg the vast array of ac id and potent ia l l l ' ac id sulphate s() i l 1) pes throu-uh chenl ica l

character isat ion, f loodin-g and soi l c( ) lumn leaching expcr i lnents in addi t ion l ( ) svstet l la t lc

mrt rpht tk tg ica l s tudies. Pyr i te and i ts ox idal ive be hal i t rur in f l r - rences the I ta ture and pr( ) -

per t ies of ac id sulphate soi ls which invar iabl l dcvelop throu,s l r aerat ion and c l ra inagc.

The mineralo-chen.r is1r1, , o f ac id sulphate soi ls is thus maler ia l l \ in f lLrcnced hry p)r i te and

i t s behav iou r i n so i l env i r ( )nn ren l s . R l c rann (1973 ) i nves t i ga t cd ex tens i ! c l \ t he gencs i s and

oxidat ion products of pvr i te in -ueological sedinrents but no indepth s ludics of th is sor t

has been at tenlpted in vary ing pedolo-u ical envi ronnrents. An in i t ia l nr icronrorphokreical

s tudr- was at lempted by EswrnaN (1967) on the d is t r ibut ion of prr i te in one tvpe of ac id

su lp i ra t c so i l f r on r Ma la . vs ia . V . cN I ) rN , r , e t u l . ( 1973 ) and M l t , n t , v t . qN , e t u l . ( 1973 )exan r i ned

the ur icromorpholoey ( ) f sonre ac id sulphate s() i ls of the Nether lands us ing scannins e lect r ( )nmicroscopic (SEM) technic lues in thei r s tuCies of pvr i te and i ts or idat ion producls.

The present contr ibut ion a ims at examinin-s pvr i te and i ts or idat ion products in a prof i le

f ronr Malaysia. the deta i ls of which are as fo l lows:-

Ah 0 - l 0 c n r

l0 - 60 cnr

Very dark brown ( l0YR2 l2) . nro is t ) : orsanic c la l - : moist . f r iab le : f ine

granular : abundant f ine roots: c lear wavv boundrv.

Brown ( l0YR5i3, moist ) wi th much organic mat ter s ta in ing. c la1 ,ntoist. fr iable: weak c()Ar{e prisnratic breakin-e down to nroderate me-

dium subangular b lock-n ' p lent i fu l medium and f ine r ( )ots : c( )nt inuot lsclayskins: clear wavy boundary'.

50

B g 6 0 - 8 0 c m

BCg 80 100 cnt

Brorvn ( l0YR-5/3, moist ) : c lay: wet . s l i -eht ly s t icky, weak mediunr su-

bangular b locky: few f ine roots. d iscont inuous c la l ,sk ins main ly a long

ro() t channels: many nredium dis t inct s t rong brown (7. -5Y5i 8) . r 'e l low(5Y8 t6 ) and o l i ve 1 ' e l l ow (2 .5Y6 / [ J ) mo t t l es : - s radua l i r r e -eu la r boun -

dary.

B rowr r ( l 0YR 5 i 3 . mo is t ) and l i gh t g re1 , (2 .5Y7 i 0 , n ro i s t ) ; c l ay : we i

st icky: s t ructure less, nrassive: c( )mmon ntediunr l ,e l low (5YS i6) anr l

o l ive yel low (2.5\ '6 i 8) mot t les; d i f fuse i r re-sular boundary.

CG 100 cm * Crc1. , lnd b lue _9rev colours: c lay: wet . s l icky. s t ructure less, nrassive.

' l -uhle I s ives sonre chent ica l proper l ies of th is soi l .

TABLE I . CHF-MICAL PROPERTIES OF SOIL

P H i nHo r i zo l t d i s t i l l ed pH t n

\ \ ater 0.2 KC I

A h

B

Bg

BCg

C'G

( onduc - Wa te rt i v i t y s o l u b l e

n rhos x Su lpha te1 06 (i(

6NHC I' f . r 1 3 1

s t r l . 6 N H C IS ' / r S ' / c Fq0 r

3 . 3 2 9

?.9 2.7

2.9 2.6

2 .3 2 .6

2.8 2.1

Organ i cm a t t e r

( ' r

J J

l 3

I I

t l

t 2

550

980

l , 5 t i 0

l . 5 l i 0

1.9(x)

0.09

0 . l 8

0 .74

o.14

l . 4 E

0.69

1.06

.1 .1 I

4 .3 7

.1.S7

0.48 0 061

1.86 0 .089

) . 1 4 0 . 1 0 7

1 . 9 2 0 . 1 0 7

2.90 0 .098

MATERIALS AND METHODS

T'he prof i le is sanrpled in the s late of Selansor , W.

faquept . For the present s tudv. four large bkrcks (30

sanrDles were taken at f5 cnr in tcrvals fnrnt which a

centre of the conta incr was cut r lu t and intpregnated

sect ions were cut for the nr icr ( )nr( ) rDholos ical s tudr ' .

Ma lavs ia . l t i s c l ass i f r ed as a Su l -

X l0 X l0 cnr) of undist r - r r -becl

b lock (15 X l 0 X 5 cn r ) f r on r t he

u' i th p last ic . Af ter hardening th in

The renra in ing uninrpregnated sanrple was a i r -dr ied and emploved for the. SEM. Af terthc SEM studv. the nrater ia l ( )n the stub was subject to X-rav d i f f ract ion anal l ,s is fornr inera l idcnt i f icat ion and conf i rnrat ion.

RESULTS

a ) Pvrite

The antount and d is t r ibut ion of pvr i te var ies wi th depth. The 50 to 75 cnt zone has

the most pvr i te . T 'he pvr i te ( )ccurs as s ingle crvsta ls or as aggregates for which the ternrf rantboidal is mosl apt . The f ramboids thenrselves ntay be d is t r ibuted s ingl l , or ( )ccur asclusters. These palterns are sunrmarised in

' l 'uble 2.

5 l

The significance of Table 2 is that the -50 - 75 cm zone is one where perhaps tornration

of pyrite is most active. In the overlaving material the general process is one whcre there

is a homo_senisation of the soil material and a more random distribution of the p1'rite

crystali. The framboids do not break up during this process.

T A B L E 2 . F O R M A N D D I S T R I B U T I O N O F P Y R I T E

0 -25Depth (cm)

2 5 - 5 0 5 0 * 7 5 75 - r00

R

VC

VC

t .

2 .

J .

4.

Single crystals, usuallv cubic, scattered

in matr ix

Single framboids scattered in matrix

Clusters of framboids scattered in

matrix

Clusters of framboids associated with

organic matter and in channels

R

VC

VC

C

C

C

C

C : common; VC : verv common; R : rare

In the last horizon, a significant number of the p1'rite comprises of sin-ule cubic crystals.

These may be pr imary pyr i te (PoNs. 1964\ f l rmed dur ing sedimentat ion. The f ramboids.

being associated with organic matter are post-sedimentar.r. ' .

Plate /. (,4 ) shows clusters of framboids and few isolated framboids which are incor-

porated in the matrix of the soil (type 3). Or-sanic matter associated with them can be

discerned only with diff iculty. The micrograph is frtxr the 25 -50 crn depth. Micrograph

B of plate I shows the typical clusters present in channels of the 50 - 75 cnt zone (t1"pe ' l).

Remnants of organic matter can be seen lining the walls of the channcls. These fram-

boids are lar-eer than 100 microns and are exceptional. On the average, they are silt-sized.

Micrograph C shows a single framboid in the 0 - 2-5 cm depth, in the matrix of the stl i l

mater ia l ( type l ) . The indiv idual pvr i te crysta ls are octahedral .

Micrographs A and B of plate ? show the SEM photo-eraphs of the pyrite. Most of

the f ramboids however, are pentagonal dodecahedral or pyritohedral. Micrographs of

these are not included here as similar forms are i l lustrated by MtEut'nal, et al. (1913

Framboids composed of cubic crvstals are absent.

C

C

CVC

b) J arosite

Jarosite is confined to the first two

(7.5Y8/4) ; in para l le l l ight i t is br i -eht

Mtrnrnan, et al. (1973) have il lustrated

ln these framboids, the core is black

soil studied.

horizons. In hand specimens it is whitish yellow'

yellow and under crossed nicols, l ight oran-ee-yellow.

the epitactic alteration of pyrite framboids to jarosite.

while the rim is yellow. This is not present in the

s')

Plate I

A. Frantboids of pyr i te , as c lusters and indiv iduals" incorporatcd in ntat r ix of so i l mate '

r ia l . Polar ised l ight . Ma-un. 80.

B. Clusters of pyr i te f ramboids in channels

Inc ident halogen l ight wi th b lue f i l ter . Ma-en. 300.

C. Single framboid of pyrite composed of bipyramidal crystals.

Incident halogen light without f i l ter. Magn. 300.

l ) . Jarosi te and anrorphous ferr ic hydroxide.

Polar ised l ight . Magn. 80.

E. Jarosi te wi th local goeth i te cry-sta l l i tes as b lack dots.

Inc ic lcnt halogen l ight . Magn. 300.

F. Goethite crystal aggregates

Parallel I ight. Ma_en. 80.

Plate 2

A. Single f ramboid of pyr i te adher ing to soi l mater iat

SF.M. Magn. 2.500.

B. Indiv idual crysta ls of f ramboids (A) are octahedral .

SEM. Magn. 25,000.

C. Jarosite crystals on void wall.

SEM. Magn. 5,000.

D. Goethite crystal aggregates in soil.

SEM. Magn. 6,000.

5 3

Plate I.

54

Plute 2

55

All the jarosite is present as yellow masses fi l l ing up large irregular voids (plate I ,micrographs D and E) and mostly occurring in the 25 - 50 cm depth. The release of

oxidation products through the slow oxidation of pyrite is translocated in voids and when

potassium ions are also present, crystall isation of jarosite results.

Plate 2 micrograph C shows the jarosite crystals under the SEM. Higher magnifications

indicate at least two crystal habits - pseudocubic and tabular. There is no preferential

arr rngement pat tern in the neojaros i te.

c) Amorphous ferric hydroxides

Precipitates of amorphous ferric hydroxides are frequent in the top 50 cm of the profile.

Frequently they are associated with decomposed organic matter making it diff icult to

differentiate one from the other in thin sections. They are also present as diffuse sub-cutanic features and as fine diffuse nodules. Sometimes they occur together with jartrsite.

Plate 1, micrograph D shows a thick mass of reCdish amorphous ferric hydroxides.

d ) Goet hit e

This is only present in the 0 - 25 cm layer and in this profi le is typically associated with

the jarosite. Plate l, micrograph E shows the slow alteration from the void wall into the

mass of the jarosite. The alteration proceeds through hydrolysis of the jarosite and in

micrograph E, f ine crystall i tes of goethite which has a metall ic lustre under incident l ight

can be vaguely discerned.

Micrograph f of plate / shows relatively large crystals of goethite. These have a bright

red colour and can be easily handpicked. Plate 2, micrograph D is a SEM photo of such

a specimen, the composition of which is confirmed as -eoethite from X-ray dilTractionanalysis. Higher ma-enifications indicate that each of the -elobular forms in micrograph D

is composed of a number of lenticular crystals of goethite. The morphology of the crys-tals is similar to those shown bv Esw,qnaN. et al. (1973\.

DrscusstoN

Several other oxidation products are reported in l i terature reference. These were ncltdetected in this sludy. When the -eround waters are rich in calcium, one of the reactionproducts is gypsum. SEM micrograph of suhedral gypsum is shorvn by Mrcorrt n et ul .(1973). Due to the relatively rapid recharge of water in the Malaysian soils, gypsum nlaybe only locally present.

Though only four samples from the profi le are studied, a distinct zonation with respectto relative amounts of pyrite or its oxidation products is present. This zonation is afunction of the ground watertable. In the 0 - 25 cm layer, amorphous ferric hydroxideand goethite with minor amounts of jarosite and pyrite are present. The 25 - 50 cm layeris characterised by a dominance of iarosite with minor amounts of the others. The nextlayer is made up essentially of framboids of pyrite: no jarosite is present. The last layerhas both framboids and the single crystals of pyrite.

This differential mineralogical zonation needs to be defined more accurately by detailedfield studies.

The question of the pyrite framboids has yet to be resolved. According to Rtcxann (1973),

the pyrite framboids ntay be pseudomorphic after organic spherules. However, these fram-

boids can be produced in the lab in the absence of organic matter or bacterial ctl lonies.

As amorphous gels frequently attain a globular form, a metastable -eel phase can be

postulated for the genesis of the pyrites.

SUMMARY

The morphology of pyri te and i ts oxidation products q ere studied in thin sections.,r ' i th the scanning electron microscope. Secondary pyri te is frequently present as framboids.The crystals comprising the framboids are p.vri tohedral or octahedral.

Pseudomorphic alterat ion of the framboids to jarosite is not present in this prof i le. Thejarosite crystal l izes from the soi l solut ion and is present in voids. The jarosite crystalsare pseudocubic or tabular.

Amorphous iron hydroxides and goethite are the other neoformations in the soi l . Theseare confined to the surfa:e horizon, in the aerated part of the profi le

REFERENCES

Eswena.N, H. (1967\ Micromorphological studv of a cat-clay soil.

Pedofogie (Gent 14 :259 -265\ .

EswenaN, H. eNn N.G. RncuuM.x-IAN (19'73\ The microfabric of petroplinthite.

.Sr.'r l Scl. Soc. Arn. Proc. 37, 79 -82.

CHow, W.T. (1968) A pre l iminary study on ac id sulphate soi ls in W. Malaysia.

Prtrc. 3rd Malays. Soil Conf . Kuching, 5l - 58.

MrrornR, R., A.C. .loNcnalNs AND S. SLaNcen (1973) Micron.rorphological observations on

pyrite and its oxidation products in four Holocene alluvial soils in the Netherlands.

4th Int. Working Meeting, Kingston Can:.da.

In Press.

PrrNs, L.J. (1964) A quantitative microscopi:al method of pyrite determination in soils.In : Joneer ius (Ed). : Sci l Micromorpholo_ey, 4.01 - 409.

Rlcxanl ; , D.T. (1973) Sedimentary i ron sulp ' , . i ie format ion.ln : Dost (Ed) . Acid sulphate soi ls .

Publ. 18, I, of IILRI, Wageningen, 28-62.

VeN Dnu, D. aNn L."/. Poxs (1973) Microoedological observations on pyrite and its pedo-logical reaction products.

In: Dost (Ed) . Acid sulphate soi ls .Puhl. l lLRI, Wageninge,?, lE, II 169 -194.

) l