Ž .Geoderma 89 1999 287–305

Precipitation and dissolution of gibbsite duringweathering of andesitic boulders in humid tropical

West Java, Indonesia 1

B. Mulyanto a, G. Stoops b,), E. Van Ranst b

a Department of Soil Sciences, Faculty of Agriculture, Bogor Agriculture UniÕersity,Jl. Raya Pajajaran, Bogor, Indonesia

b Department of Geology and Soil Science, UniÕersiteit Gent, Krijgslaan 281, S8, B-9000 Gent,Belgium

Received 24 February 1998; accepted 24 August 1998

Abstract

Several weathered andesite boulders, sampled in Typic Humitropepts from the Cigudeg area,Ž . ŽWest Java Indonesia , were studied using micromorphological, mineralogical XRD, TG, DTA,

.SEM, EDXRA and chemical techniques. Because all data of the boulders are similar, only theresults of the largest boulder are presented in this paper. Three zones are distinguished: the rim ofthe boulder core, a transition layer, and outer layers. In the rim of the core weathering attacks theglass matrix and the anorthite rich parts of the plagioclases. Even before sufficient iron is liberatedfrom pyroxenes goethite is deposited in their cracks, as a first step to the formation of a boxwork.In the transition layer mono- and divalent cations have been lost, but Al shows an absoluteaccumulation as gibbsite in plagioclase pseudomorphs and pores. It is presumed that the Aloriginates from the more internal parts of the core. Towards the outer layer gibbsite getscompletely removed, supposedly as the result of a lowering of pH, and the plagioclase pseudo-morphs become filled with illuvial kaolinitic clay. Illuviation of clay, and the import of iron, fromthe surrounding soil material, put limits to the use of isovolumetric calculations. The goethiteboxworks after pyroxenes reflect the original pattern of fractures, rather than cleavage patterns.q 1999 Elsevier Science B.V. All rights reserved.

Keywords: micromorphology; gibbsite; weathering; andesite; Indonesia

) Corresponding author. Laboratorium voor Mineralogie, Petrologie en Micropedologie, Univer-siteit Gent, Krijgslaan 281, S8 B-9000 Gent, Belgium. Tel.: q32-9-264-4561; Fax: q32-9-264-4984; E-mail: georges.stoops@rug.ac.be

1 Publication no. 96r001 of the International Training Centre for Post-graduate Soil Scientists-Gent.

0016-7061r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0016-7061 98 00088-3

( )B. Mulyanto et al.rGeoderma 89 1999 287–305288

1. Introduction

As part of a more general pedogenic survey of a small watershed in WestŽ .Java Mulyanto, 1995 , a detailed study on weathering under tropical perudic

conditions has been made of a series of andesitic boulders present in the soilprofiles. The study focuses on mineral transformations using chemical, miner-alogical, optical and in situ submicroscopic techniques. Because micromorpho-logical and in situ submicroscopical studies deal with the mineral scale ofobservation they provide insight in the mechanisms of weathering of primaryminerals and the formation of secondary minerals.

Although weathering of andesitic rocks has been described in many regionsŽ .e.g., Hendricks and Whittig, 1968; Glasmann, 1982; Jongmans et al., 1994such studies are scarce with respect to the extended perudic regions of Indone-sia. Especially the study of boulders and pebbles can yield important informa-

Žtion on physical and chemical conditions in the soil e.g., Veldkamp et al., 1990;

Fig. 1. Location of the study area.

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.Jongmans et al., 1993 which may be quite different from the conditions in theŽ .underlying parent rock Eswaran et al., 1973 . The study revealed some trends

about gibbsite occurrences which were hitherto less documented, especially thedisappearance of gibbsite upon ongoing weathering.

2. Materials and methods

2.1. Materials

The studied site is located in the rolling upland on a crest slope of a smallŽ X X .watershed in the southern part of the Cigudeg area 6833 S, 106830 E , Bogor

Ž .district, West Java, along the way from Bogor to Pandegelang Fig. 1 . Thegeological substrate is indicated on the geological map as ‘undifferentiated

Ž .volcanic rocks’ Effendi, 1986 . The major soil types in the study area are TypicHumitropepts on the back slope, with Oxic Humitropepts on the crest slope and

Ž .Typic Tropaquents on the foot slope Mulyanto, 1995 . The soils are welldrained, the temperature regime is isohyperthermic, the moisture regime perudic,

Ž .and all profiles belong to a clayey, kaolinitic family Soil Survey Staff, 1992 .The original forest was first replaced by rubber plantations during the colonialtime, and recently transformed to cocoa and coconut plantations.

Samples were taken from exfoliating boulders exposed in a road cut in theTypic Humitropept on the back slope. Three boulders were studied in detail, the

ŽFig. 2. Location of samples within the exfoliated andesite core-stone. C: core matrix 5 Y 5r2,. Žplagioclase 7.5 YR 8r1, pyroxene 7.5 Y 3r2, powder 10 YR 8r1 ; T: transition layer matrix 2.5

.Y 8r4, plagioclase 10 YR 8r8, pyroxene 5 YR 4r8–5r8, powder 10 YR 7r6 ; O : outer layer 11Ž . Ž . Ž .powder 10 YR 7r4 ; O : outer layer 2 powder 5 YR 6r4 ; O : outer layer 3 5 YR 8r3 .2 3

()

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Table 1

Element Core Transition Outer Layers

bC T O O O1 2 3

Ž . Ž . Ž . Ž . Ž . Ž .a Chemical composition wt.% , pH and bulk density BD of core C , transition T and outer O layers of a weathered andesitic boulderSiO 61.1 42.8 46.7 45.7 56.62

Al O 16.8 31.6 23.4 23.9 27.52 3aFe O 6.2 14.5 14.8 18.6 4.12 3

TiO 1.0 1.3 1.2 1.3 1.42

MnO 0.1 0.0 0.0 0.0 0.0P O 0.2 0.1 0.0 0.0 0.02 5

CaO 5.3 0.1 0.0 0.0 0.0MgO 2.7 0.1 0.1 0.1 0.1Na O 2.8 0.1 0.1 0.1 0.12

K O 2.4 0.1 0.1 0.0 0.02

H O 1.6 10.2 13.7 10.3 10.12

Total 100.2 100.9 100.1 100.0 99.9

SiO rAl O 6.1 2.2 3.4 3.2 3.52 2 3

pH 7.1 5.1 4.6 4.6 4.6y3Ž .BD Mgrm 2.4 1.5 1.4 1.3 1.3

3Ž . Ž . Ž . Ž . Ž .b Chemical composition expressed in cgrcm of core C , transition T and outer O layers of a weathered andesitic boulder isovolumetric method

SiO 146.6 63.1 65.3 59.4 73.72

Al O 40.3 47.3 32.7 31.1 35.82 3

Fe Oa 15.1 21.7 20.7 24.2 5.32 3

TiO 2.4 1.9 1.7 1.7 1.82

MnO 0.2 0.0 0.0 0.0 0.0P O 0.5 0.1 0.0 0.0 0.02 5

CaO 12.7 0.1 0.0 0.0 0.0MgO 6.5 0.1 0.1 0.1 0.1Na O 6.7 0.1 0.1 0.1 0.12

K O 5.8 0.1 0.1 0.1 0.02

H O 3.8 15.2 19.2 13.4 13.12

aFeOq Fe 0 .2 3bNo longer isovolumetric.

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largest found at the soil surface, the others, respectively, in the AC and BCŽhorizons. As they were yielding similar results only data on the largest, 90=70

.cm are presented in this study.

Ž . Ž .Fig. 3. XRD of bulk samples oriented from weathered andesite from the core C up to the outerlayers without any treatment.

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2.2. Methods

Ž .Large thin sections 90=120 mm were prepared after impregnation withŽ .polystyrene as described by Benyarku and Stoops 1999 and studied using a

polarising microscope. For thin section description the concepts and termsŽ .proposed by Bullock et al. 1985 were used. In order to avoid confusion, the

use of the term groundmass was restricted to soil material, whereas the termmatrix was used for the fine material within the rock. The original mineralogical

Ž .composition modus of the andesite was assessed by counting 1000 points on athin section from a fresh sample of the centre of the core. Small subsamples

Ž .were ground and analysed by X-ray diffraction XRD after saturation withMg2q, glycol solvation and saturation with Kq followed by heating at 110, 350and 5508C, respectively. The same was done for clay fractions separated fromthe transition and outer layers. Red and white spots considered as weatheredpyroxene and plagioclase grains, respectively, were sampled with a needle andXRD was performed using a Gandolfi camera.

Total elemental analysis of bulk samples, using AAS, was done according toŽ .Van Hende 1976 . Bulk density of the clods was measured according to the

Ž .method of Blake and Hartge 1986 . Uncovered thin sections were studied withŽ .a Scanning Electron Microscope SEM Jeol 6400 equipped with an Energy

Ž .Dispersive X-Ray Analyser EDXRA JXA-6400 in order to determine thechemical composition of the constituents.

Table 2XRD-analysis of ground samples of the core, transition zone and outer layers of the andesiteboulder

Minerals Core Tranition Outer layers

C T O O O1 2 3

Rock mineralsPlagioclase qqq – – – –Cristobalite qq qqq qqq qq qTridymite q qq qq qqq qqqAugite q – – – –Hypersthene q – – – –

Weathering productsMixed layers – q q q qq0.7 nm halloysite – q qq qqq qqqGoethite – q q q qGibbsite – qqq qq q –

Note: q: low, qq medium, qqq high, –: not detected.

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3. Results

3.1. Macroscopic description

The weathered rim of the andesitic boulder is about 10 to 15 cm thick andexfoliated. One undisturbed sample was taken from the core, one from the

Ž .transition layer and three from the outer layers Fig. 2 .

3.2. Chemical analysis

The chemical composition and bulk density of the core and weathered layersof the andesitic rock, determined from bulk samples, are presented in Table 1a.During weathering, pH and bulk density gradually decrease towards the outer

Fig. 4. XRD of untreated oriented clay fraction separated from bulk samples of weathered andesiteŽ . Ž .core-stone, from the least weathered T up to the ultimate weathered material O .3

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Ž . 2qFig. 5. XRD pattern of oriented clay samples of the outer layers O , saturated with Mg , and3

Kq, glycol solvated and heated.

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Ž .layer. Using the isovolumetric method Millot and Bonifas, 1955 , the chemicalŽ . Ždata were recalculated Table 1b . A drastic decrease of the basic cations Ca,

.Mg, K and Na in the weathering layers and a steady decrease of Si indicate a

Fig. 6. DTA and TG patterns of ground samples of the different layers of an exfoliated andesitecore stone.

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pronounced leaching of these elements. The Al content increases in the transi-tion layer and gradually decreases towards the outer layers. Iron shows a relativeaccumulation, except in the outer shell.

3.3. XRD and DTA analyses

The XRD patterns of the ground samples are presented in Fig. 3 and theirinterpretation is summarised in Table 2.

Ž .The core contains plagioclases 0.63, 0.404, 0.322 and 0.317 nm , cristo-Ž . Žbalite, 0.401, 0.313, 0.282 and 0.248 nm , tridymite 0.427, 0.408, 0.378,

. Ž . Ž .0.360, 0.343 nm , augite 0.294 nm and hypersthene 0.318, 0.298, 0.255 nm .A weak, broad peak at more than 0.7 nm indicates already the presence of someweathering products.

In the transitional layer, the reflections of plagioclases and pyroxenes havedisappeared, while the intensity of the tridymite, and especially the cristobalite

Ž .peaks strongly increases. In addition, gibbsite 0.485, 0.436 nm , goethiteŽ . Ž0.418, 0.245 and 0.268 nm , and probably halloysiterkaolinite 0.72, 0.445 and

.0.353 nm are observed. The 0.98 nm reflection probably belongs to mixedlayers with halloysite.

In the outer layers, the tridymite reflections become stronger than those ofcristobalite. The gibbsite reflections gradually disappear, whereas the reflectionsof goethite, halloysiterkaolinite, and mixed layers become more pronouncedcompared to the transition layer.

The XRD patterns of the separated clay are presented in Fig. 4. They showstrong reflections of cristobalite, tridymite, and very weak reflections of plagio-clase, halloysite, and probably chlorite and mixed layers. The clay fraction of

Ž . Ž . Ž .the outer O layer Fig. 5 also contains, besides cristobalite 0.404 nm ,3Ž . Ž .tridymite 0.410 nm and plagioclase 0.63 and 0.32 nm , newly-formed phyl-

losilicates. Kaolinite is shown by the 0.72 and 0.358 nm reflections, andhalloysite by the broadened 0.7 nm peak to about 0.9 nm. Heating the sample at1108C increases the intensity of the 0.72 nm reflections; whereas the broadeningdisappears, indicating the presence of hydrated 1:1 phyllosilicate layers. Heatingat 5508C destroys the 0.72 nm reflection.

The broadened peak, situated between 1.0 and 2.4 nm in the Kq saturatedsample indicates irregular 1:1r2:1 mixed layers. Reflections at 1.0 and 0.5 nm,present after all treatments, indicate mica. The shift of the 1.5 nm peak in the

Ž .Plate I. Petrography of the fresh core and initial weathering of plagioclases. a GlomerophyricŽ . Ž .fabric with phenocrystals of pyroxene p and plagioclase f in feldspar and glass matrix of fresh

inner core. Note the glass inclusions in the feldspars and the opaque inclusions in the pyroxenes.Ž . Ž .PPL. b Idem, XPL. c Initial weathering of plagioclase phenocrystals, microlites and matrix in

Ž .outer part of core. PPL. d Idem, XPL.

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Mg2q saturated sample, towards 1.7 nm after glycol solvation indicates swellinglayers.

Ž .DTA and TG curves Fig. 6 show that the endothermic reaction at 3008C,due to the dehydroxylation of gibbsite, is very significant in the transition layer,and gradually disappears towards the outer layer, where the endothermic peak ofkaolinite is dominant.

3.4. Microscopic study

The fresh rock in the central part of the core has a medium grainedŽ . Ž .glomerophyric fabric Plate Ia and b . The matrix 52.5% consists of colourlessŽ . Ž .glass and microlites "40 mm of plagioclase; phenocrysts "1 mm comprise

Ž Ž . Ž .. Ž .plagioclase bytownite 4.2% and anorthite 27.0% , augite 9.4% and hypers-Ž .thene 3.4% . Tridymite, cristobalite, biotite and opaques are observed only in

Ž .small amounts 3.5% and therefore their individual weathering pattern will notbe discussed in this paper. Plagioclases display a normal zonation, polysynthetictwinning, cleavages and inclusions of grey glass, pyroxenes and gas bubblesŽ .Plate Ia and b . Pyroxene have mainly opaque inclusions, with metallic lustre;occasionally also inclusions of gas or fluid bubbles occur. The fresh grains showno deuteric alteration products. As weathering of hypersthene and augite seemsto be very similar, no distinction between both minerals is made in this study.

The prismatic microlites of plagioclase in the matrix of the outer zone of thecore are replaced by dotted clay sized or colloidal materials with grey interfer-

Ž .ence colour Plate Ic and d , probably cryptocrystalline gibbsite or halloysite;some yellow staining of the matrix is locally observed. In the transition layer thematrix as a whole becomes isotropic in yellow stained parts, whereas unstainedzones have a stipple speckled b-fabric with a grey interference colour. In theouter layers, part of the matrix is transparent and yellow, the other part iscompletely opaque in transmitted light and red in incident light. In the latter, the

Ž . Ž .pores are filled with either goethite yellow or hematite red .In the outer zone of the core, weathering of the zoned plagioclase phenocrysts

starts from their anorthite-rich centre; grains become speckled and trouble inŽ .transmitted light Plate Ie and f . The anorthite rich centre is more reactive

Ž .because of a higher proportion of ionic bonds Nahon, 1991 . The turbiditygradually increases and isotropic zones appear. Pseudomorphs after plagioclases,without specific internal fabric, are formed containing the following secondary

Ž . Ž .materials: i isotropic material, ii randomly organised gibbsiterhalloysite, and

Ž .Plate II. Advanced weathering of plagioclases in transition layer. a Gibbsite pseudomorphs afterplagioclase phenocrystals in transition layer. Note that first a gibbsite boxwork was formed which

Ž . Ž .became afterwards filled with microcrystalline gibbsite. PPL. b Idem, XPL. c Detail of gibbsitepseudomorph after plagioclase phenocrystal. Note the relicts of original glass inclusions, not

Ž .replaced by gibbsite. PPL. d Idem, XPL.

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Ž .iii clay minerals with low grey interference colour oriented parallel to the wallŽ .of the cracks and in contact voids Plate II . The latter accumulation is probably

due to a micro-transport in the boxwork. The optical data are confirmed byGandolfi XRD of a hand-picked pseudomorph indicating at this stage thepresence of plagioclase, partly dehydrated halloysite and gibbsite.

In the transition layers the secondary materials in the plagioclase pseudo-morphs show signs of disorganisation due to dissolution, resulting in theformation of large voids. Although the homogeneous grey 1st order interferencecolour of the secondary minerals seems to indicate one homogeneous phase of

Ž .1:1 phyllosilicates, the back scattered electron image BSEI at an enlargementof 200 times shows at least 2 different grey levels. Point analyses in the lighterpart indicates a SiO rAl O molar ratio 1.6 suggesting a 1:1 mineral, most2 2 3

probably halloysite, whereas the darker part, directly in contact with the contactŽ .voids, contains almost 100% aluminium expressed on total of cations which

indicates gibbsite. The 750 times enlargement shows the crystal habit and thepores in between the grains of gibbsite, while the fabric of halloysite remainsunclear. Weathering of the pseudomorphs after plagioclase is continued in theouter layer by congruent dissolution of gibbsite, leaving relatively large voidsand halloysite. Yellowish infillings with microlaminated, parallel oriented claywith low interference colours are formed by illuviation of fine material fromoutside the boxwork.

Weathering of the pyroxenes starts in the outer zone of the core from cracksw x Ž . Ž .perpendicular to the 001 -axis or related to inclusions. The 100 and 010

cleavage planes play only an accidental role. Coatings of goethite are depositedon the walls of the cracks, forming a boxwork. Congruent dissolution of thepyroxene results in fragments with denticulate basal planes isolated in the cells

Ž .of the boxwork, separated from the goethite septa by contact voids Plate IIIa .In the transition layer, goethite boxwork pseudomorphs with pyroxene habit canstill be recognised, rarely containing cores of the original minerals. The contactvoids are partially filled by red rounded aggregates, 10–20 mm in diameter.Considering the colour, the chemical composition determined by EDXRA andGandolfi XRD, the material is determined as goethite. Towards the contact void,the percentage of iron decreases, while the content of aluminium and silicaincreases, pointing to the presence of phyllosilicates. In voids corresponding tothe original cracks thin coatings of limpid, yellowish clay occur. In some cases,

Ž .Plate III. Weathering of pyroxene phenocrystals and general aspects. a Weathering of pyroxenephenocrystals in outer core, as a first step to the formation of a goethite boxwork. Note the

Ž .congruent dissolution of the pyroxene. PPL. s: Fe septa, p: pyroxene fragments, v: contact void.Ž .b Goethite boxwork after pyroxene surrounded by layered infillings of fine clay, oriented

Ž . Ž . Ž . Ž .alternatively parallel i and perpendicular n to the walls of the voids. PPL. c Idem, XPL. dIlluvial clay forming coatings and infillings in transmineral fissures in the outer layer. Goethiteboxworks after pyroxene phenocrystals are still visible. PPL.

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clay particles are oriented parallel to the walls, in others perpendicular to theŽ .walls. Combinations of both were also observed Plate IIIb and c . Perpendicular

orientation points to clay neoformation, probably combined with local transloca-tions considering the parallel orientation patterns. In the outer layer, the goethitepseudomorphs after pyroxenes are preserved.

Ž . Ž .Structural micropores like cracks 200–500 mm and cavities 500–800 mmare rare in the centre of the core; in their outer margin, cracks are common andsometimes filled by gibbsite microcrystals oriented perpendicular to the wall andsurrounded by hematite hypocoatings. Coatings of yellowish clay, with low

Ž .interference colours occur in planar voids in the transition layer Plate IIId . Inplanar voids in the outer layer, coatings of clay stained by goethite or of gellicmaterials, or a combination of both are locally found. Others are coated byradiating fibrous reddish yellow goethite microcrystals.

4. Discussion

As weathering progresses in the boulder from outside to the centre, it can beassumed that the outer layers have passed through the same processes which actnow on the more central part of the core. Micromorphology shows that the

Ž .isovolumetric calculations Table 1b are not valid for the most outward layerŽ .O as the influence of illuviated clay becomes important, although the rock3

structure is still preserved during the isovolumetric weathering stage. Clay fromexternal origin invades plagioclase and pyroxene boxworks, even as largerplanar voids.

Ž .Isovolumetric calculations Table 1b show that the mono- and divalentcations drastically decline in the transition layer; also the Si content drops about50%. This is in agreement with the XRD data and the micromorphologicalobservations, showing the disappearance of plagioclases and pyroxenes. Thesame trend is continued towards the outer layers. The few mono- and divalentcations left over are probably present on the exchange complex of the clays; the

ŽSi is partly present as free SiO relative accumulation of tridymite and2.cristobalite observed in XRD , and partly as components of the clay. On the

Ž .contrary, the Al content shows an absolute increase about 15% in the transitionlayer, corresponding to high amounts of gibbsite in the XRD, DTA and TG,followed by a gradual decrease of Al towards the outer layers, where gibbsite isabsent. The micromorphological observations also point to an accumulation ofgibbsite in the plagioclase boxworks and transmineral fissures, starting in theexternal part of the core, and reaching a maximum in the transition layer,followed by the disappearance of gibbsite towards the outer layers. The relativeposition voids–gibbsite–halloysite–plagioclase in the plagioclase boxwork inthe transition layer suggests that the gibbsite is due to either the removal of

Ž .silica desilicification of halloysite , or to a direct gibbsite precipitation fromsolutions.

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According to micromorphological observations, weathering of plagioclases isŽ .already in a rather advanced stage in the outer core no chemical data available ,

as shown by the presence of pseudomorphs of an isotropic substance. It ispresumed that, due to the high abrasion pH of the weathering environment alsoAl is mobilised and moved towards the more outward transition layer. Thegibbsite dissolution in the outer layers is in agreement with observations of

Ž . Ž .Furian 1994 in soils of Brazil, and of Dedecker and Stoops 1999 in saprolitesŽ .of Malaysia. Holdren and Speyer 1985 suggest that in the range of pH 6–8

dissolution of feldspar is stoichiometric and equal amounts of Si and Al areliberated. They partly combine to form kandites, as revealed by micropedologyand Gandolfi XRD, and partly move to the transition layer where, due to a dropof the pH, Al becomes less soluble and is precipitated as gibbsite, overlaying thekandites already present, whereas the Si is gradually leached out. According tomicromorphological, chemical, XRD and DTA data the gibbsite is gradually

Ž . Ž .dissolved in the outer layers. Nahon 1991 and Velde 1992 cited that gibbsiteŽ . Ž .is dissolved at pH below 5. According to Kittrick 1969 and Tan et al. 1973

the dependency of the gibbsite on the pH value is shown by the equation:3 q 3qAl OH q3H q3H 0mAl P6H OŽ . 2 2

This equation indicates that a decrease in pH shifts the reaction to the right,leading to the dissolution of gibbsite.

Ž .An inward movement of Al from the outer layers to the transition layerseems difficult to explain.

Ž .The absolute accumulation of Fe O up to about 10% according to the2 3

isovolumetric calculations is visible in thin sections as a yellowish and reddishimpregnation of the weathered matrix and as goethite septa in the pyroxeneboxworks. As the formation of the goethite coatings in the pyroxenes starts evenbefore sufficient iron can be liberated by these minerals, it is clear that animportant input from outside is necessary. The goethite boxworks are thus notnecessarily the result of an in situ accumulation of pyroxene weatheringproducts, which anyway would be insufficient to fill the volume. They may

Ž .have, however, contributed to it, as suggested by the presence of Si 8–22%Ž .and Al 14–20% in the boxwork. Morphologically, the weathering of the

pyroxenes can be considered as a congruent dissolution. Goethite accumulationin the pyroxene boxwork takes place in two steps: first formation of septa ofmicrocrystalline goethite, and later deposition of cryptocrystalline materials inthe contact voids and empty cells. Similar observations have been reported on

Ž .garnet weathering by Embrechts and Stoops 1982 . It is striking that septa formaccording to fractures only, and cleavage directions seem unimportant.

Table 1b clearly shows that Fe is the only element not showing a decrease,Ž .except for the outer layer O where it is a result of local reducing conditions.3

In the outer layers, the tridymite XRD-reflections appear stronger than thoseof cristobalite, probably because tridymite, which formed at lower temperatures

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Ž .Mitchell, 1975 is less sensitive to weathering than cristobalite resulting in arelative accumulation.

5. Conclusions

Within 10–15 cm from the margin of the core the weathered andesitic rockshowed drastic change in fabrics and mineralogical composition. Most feldsparsand pyroxenes as well as the glass of the matrix are eliminated. Mono- anddivalent cations are immediately leached even as an important part of the silica.Altering pH conditions control the behaviour of Al, which in an advancedweathering stage first precipitates and accumulates in feldspar pseudomorphs asgibbsite, and upon ongoing weathering redissolves and is removed. We ex-plained the gibbsite coatings covering the kandite coatings in the transition layeras a result of Al precipitation, rather than desilicification as described by other

Ž .authors e.g., Jongmans et al., 1994 .Goethite boxworks after pyroxenes, already starting to form in the outer rim

of the core, are more the result of an absolute accumulation of iron than arelative one caused by in situ weathering. The septa of the boxwork reflect oldpatterns of fractures rather than cleavages.

Coatings of clay, oriented perpendicular to the walls of voids in the outerlayer point to neoformation of clay minerals.

Although isovolumetric calculations seemed valuable as a first approxima-tion, micromorphology combined with submicroscopic analyses show that strictlyspoken the method cannot be applied in the outer layer, since interlayer transportand precipitation of iron and clay illuviation took place.

Acknowledgements

This research was supported by the Algemeen Bestuur van de Ontwikkel-Ž .ingssamenwerking ABOS scholarship no: DVZ 3.953.704 of Belgium and a

research grant no: FKFO 2.0017.93. from the National Science Foundation.

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