Smectite formation produced by weathering in a coarse granite saprolite in Galicia (NW Spain)

Ž .Catena 35 1999 281–290

Smectite formation produced by weathering in až /coarse granite saprolite in Galicia NW Spain

Teresa Taboada ), Carlota GarciaDepartamento de Edafologıa y Q. Agrıcola, Facultad de Biologıa, UniÕersidad de Santiago de Compostela,´ ´ ´

15706 Santiago de Compostela, Spain

Received 20 May 1997; accepted 10 November 1997

Abstract

Ž .Weathering of a calcalkaline granite was studied in the south of Galicia NW Spain where theaverage annual precipitation is 1400 mm and the average annual temperature is 128C. The originalrock contains perthitic K-feldspars, plagioclases with inclusions of muscovite and opaque miner-als, quartz and chloritized biotite, with apatite, zircon, sphene and opaques as accessories. In thesaprolite the structure of the rock is preserved, the plagioclases show up to grade 4 weathering andthe biotites, between 2 and 3. Weathering in quartz and potassium feldspar crystals is manifestedonly by fracturing. Biotite changes following the parallel linear model and its weathering productsare interstratified biotite–vermiculite and iron oxyhydroxides. The plagioclases change to amicrogranular material by pseudomorphic transformation. This material, which substitutes theplagioclase, includes small clearly delimited units which retain zones with the optic characteristicsof muscovite. X-ray diffraction analyses of microsamples show that they are formed by 1:1diocthaedral phyllosilicate, smectite and a small quantity of mica. From these facts we concludedthat smectite is formed inside the plagioclase crystals, and probably originates from the inclusionsof muscovite contained in these crystals, as it is suggested by the microscopic study which showsthe increase in volume which occurs when the crystals of muscovite are transformed. q 1999Elsevier Science B.V. All rights reserved.

Keywords: Granite; Weathering; Micromorphology; Biotite; Plagioclase; Muscovite; Smectite

1. Introduction

Ž .At present, Galicia NW Spain has an average annual rainfall of 1400 mm and anaverage annual temperature of 128C. Under these conditions, the weathering products of

) Corresponding author. Fax: q34-81-596904; E-mail:[email protected]

0341-8162r99r$20.00 q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0341-8162 98 00107-6

( )T. Taboada, C. GarciarCatena 35 1999 281–290282

the granite rocks in open systems are 1:1 dioctahedral phyllosilicates, vermiculitesŽ . Žusually Al hydroxide interlayered , Fe oxyhydroxides goethite and non-crystalline

. Ž . Žcompounds and gibbsite under strong leaching conditions Macıas, 1981; Macıas et´ ´al., 1982; Taboada et al., 1990, 1992; Taboada, 1992; Romero et al., 1992; Taboada and

. ŽGarcıa, 1997a,b . Smectites were only found in cases of paleoalterations Gey et al.,´. Ž .1992 , hydrothermal alteration Calvo de Anta et al., 1983 or, exceptionally, in poorly

Ž .drained environments Macıas et al., 1982 .´The study of a granitic weathering profile in southern Galicia has shown the presence

of smectites, which cannot be explained by poor drainage or by hydrothermal alteration.As the soil is polycyclic, we cannot rule out the possibility that the smectite has beenformed under different environmental conditions than the prevailing ones, and the aim ofthis study was to ascertain the mineral from which it is derived.

2. Area descriptions, methods and material studied

The weathering profile studied was developed from a calcalkaline granite ofmedium-coarse grain, found on a class 5 slope, with eastern orientation and at analtitude of 770 m. The surrounding terrain is mountainous and there are abundant rockyoutcrops.

Ž .An Umbric Regosol F.A.O., 1989 , in which two cycles can be differentiated by thepresence of a ‘stone-line’, was found developed on the granite. From the older cycle

Ž .only the saprolite is preserved 2C horizon , while the youngest is represented by an ACŽ . Ž .horizon 45–80 cm and an Ah horizon 0–45 cm .

Ž .The soil temperature regime is mesic Soil Conservation Service, 1975 and theŽmoisture regime is a transition between udic and xeric Martınez Cortizas, 1988;´

.Martınez Cortizas and Carballeira, 1988 .´ŽIn order to characterize the soil, granulometric analysis Soil Conservation Service,

.1975 was carried out and determinations were made of pH in H O and in KCl, and of2Ž .the cation exchange capacity Peech et al., 1947 .

For the study of microstructural and mineralogical transformations, samples wereŽ .taken in bags and ‘Kubiena Boxes’. The terminology of Stoops et al. 1979 and Bullock

Ž .et al. 1985 was used to describe the thin sections. For mineralogical characterisationŽ .XRD analysis using Phillips PW 1279 equipment on bulk samples as well as on sand

silt and clay fractions was carried out. Diffraction patterns of powder and orientedmagnesium and potassium-saturated samples were obtained, solvated in ethylene glycoland subjected to different heat treatments. For identification of the smectite type, a

Ž .modification of the test of Greene-Kelly 1953 , as proposed by Lim and JacksonŽ .1986 , was carried out.

In order to concentrate the ferromagnesium minerals, a Franz magnetic separator wasused.

Free iron oxides of the fine earth samples were determined using the dithionite–Ž . Ž .citrate–bicarbonate DCB method Mehra and Jackson, 1960 and non-crystalline iron

Ž .compounds using ammonium oxalate Blakemore, 1981 .

( )T. Taboada, C. GarciarCatena 35 1999 281–290 283

3. Results

As can be seen in Table 1, the saprolite and the soil horizons have coarse textures, pHvalues in H O of less than 5 and in KCl close to 4. It is, therefore the soil is acidic,2

unsaturated in basic cations and with cation exchange capacity dominated by aluminium.

3.1. Petrography of fresh rock

Ž .The original rock Fig. 1 is an inequigranular calcalkaline granite with porphyrictendency. Its main constituents are potassium feldspar, plagioclases, quartz and biotite

Ž .and it contains apatite, zircon, sphene and opaques pyrite and iron oxyhydroxides asaccessories.

The potassium feldspar is basically perthitic microcline and they constitute the largestŽ .crystals in the rock. The plagioclases An are subdiomorphic, occasionally zoned10 – 30

and almost all of them have small inner crystals of muscovite. Idiomorphic crystals ofopaque minerals are also seen in the plagioclases. Quartz is alotriomorphic with manymyrmeketic intergrowths. Partial cloritization is seen in the biotite crystals.

3.2. Micromorphology of saprolite

Ž .The initiation of weathering of the rock saprock is shown mainly by the presence oftransmineral as well as inter and intra-mineral cracks whose sizes range between 0.1 and2 mm.

In the saprolite the structure of the rock is preserved, the plagioclases show up toŽ .grade 4 weathering and the biotites, between 2 and 3 Stoops et al., 1979 . Weathering

in quartz and potassium feldspar crystals is manifested only by fracturing.Ž .Biotites tend to open along cleavage lines linear parallel alteration , although in

Ž .some cases the opening is not uniform and gives rise to ‘microboudinage’ Fig. 2 . Atthe edges of the biotite crystals, between the exfoliation planes and in the small cracksŽ .with diameters less than 0.1 mm which surround these crystals, Fe-oxyhydroxidecoatings can be seen. In the most extreme cases, degradation of biotite forms pseudo-

Ž .morphic nodules of weathered material similar to that described in Bullock et al. 1985 .Plagioclases are pseudomorphically altered to a microgranular material which is

usually diffusely impregnated by yellow, orange and even reddish colours. Inside the

Table 1Ž .Granulometric analyses USDA and some chemical properties of saprolita and soil

Hor. Sand Silt Clay pH H O pH KCl CEC Bs Fe O Fe O2 2 3DCB 2 3OXŽ . Ž . Ž . Ž . Ž . Ž .% % % % % %

Ah 79.0 15.2 5.8 4.3 4.0 5.05 14.1 0.69 0.58AC 80.7 14.0 5.3 4.5 4.3 2.74 11.7 0.48 0.232C 65.3 15.8 8.9 4.8 4.2 0.58 0.06

Ž . y1CEC in cmol q kg , Bssbase saturation in %, Fe O sdithionite–citrate–bicarbonate extractable2 3DCB

iron, Fe O sacid ammonium oxalate extractable iron.2 3OX


Fig. 1. Fresh rock: quartz, K-feldspar, biotite and plagioclase crystal with muscovite inclusions. XPL=140.

Fig. 2. Biotite crystal with ‘microboudinage’. PPL=140.


Fig. 3. A large part of the micrograph shows the weathering products of plagioclase crystals stained by ironoxyhydroxides. Within these granules, small randomly distributed opaque minerals can be seen, and in thelower right hand corner, vermiform crystals which are weathering products of the original muscoviteinclusions. On the upper right hand side, fractured K-feldspar crystals can be observed. PPL=58.

Žpseudomorphic nodules inclusions of tiny crystals of opaque minerals probably iron.oxides, as they appear red with incident light can still be seen. They have clearly

defined edges and do not have coloured halos, a fact which is even more apparent whenŽ .all of the plagioclase nodule is yellowish brown in colour Fig. 3 .

As can be seen in the aforementioned micrograph, there are small clearly delimitedunits within the microgranular material which substitutes the plagioclase. These have thefollowing characteristics:

-In plane-polarized light they are transparent and have some cleavage lines.-With crossed polars, parts of the units which retain the polarization colours of

Ž .muscovite and straight extinction can be observed Fig. 4 .-They are vermiform in shape and expand, thus pushing the surrounding materialoutwards. Figs. 5 and 6 clearly show the displacement of the grains of the opaqueminerals positioned around these units.

3.3. Mineralogical eÕolution

XRD analyses of the rock and the saprolite show that during weathering the amountsof feldspars and micas in the bulk sample decrease.

In saprolite, the sand fraction contains quartz, feldspars, micas and traces of mixed-layer minerals of the mica-vermiculite type. XRD analyses of the clay fraction show


Fig. 4. Detail from Fig. 3 in which, in the centre, remains of muscovite crystals can still be seen. XPL=140.

Fig. 5. The weathering of muscovite in expansible 2:1 minerals results in the displacement of the primitivehematite inclusions in the plagioclase. PPL=140.


Fig. 6. The weathering of muscovite in expansible 2:1 minerals results in the displacement of the primitivehematite inclusions in the plagioclase. XPL=140.

effects corresponding to disordered kaolinite together with the presence of an swellingmineral which shows the typical behaviour of a smectite. Thus the oriented samples,when saturated with magnesium show, amongst others, a peak at 1.5 nm which isdisplaced to 1.78 nm on solvating with ethylene glycol and which collapses at 1.0 nm onheating to 5508C; when saturated with K collapses at 1.0 nm on heating the sample to

Ž .3308C indicating the presence of an Al-hydroxide interlayer and when saturated withŽ .lithium, heated and solvated with glycerol show a peak at 1.8 nm Fig. 7C .

The silt fraction has a composition intermediate between the sand and clay fractions.With the aim of determining the primary mineral from which each of the transforma-

tion products was formed, grains of the alterable minerals of the rocks were analysedseparately.

Biotites: the concentration of biotites was made by magnetic separation of saprolitesample; when saturated with magnesium these samples show XRD patterns with peaksat 2.4, 1.4, 1.2 and 1.0 nm which do not sweel on solvating with ethylene glycol andwhich collapse at 1.0 nm on heating to 5508C; these correspond to biotite and an

Ž .interstratified biotite–vermiculite Fig. 7a , thus the swelling mineral is not formed frombiotite. Microstructural studies have shown that there are iron oxy-hydroxides formedfrom biotite too. Identification of these compounds is not easy as in the XRD patternsthe peaks coincide with those of the most abundant minerals in the samples studied,although small peaks were seen at 0.269 and 0.418 nm which may correspond to

Ž .hematite or goethite. Furthermore, results from selective extraction techniques Table 1reveal that most of the iron is present in a crystalline form.


Ž . Ž .Fig. 7. A X-ray diffraction patterns of magnetic fraction. 1: oriented sample Mg-saturated , 2: ethyleneŽ .glycol solvated, 3: heated at 5508C. B X-ray diffraction patterns of a microsample of weathered plagioclase

Ž .grain. 1: orientated sample, 2: ethylene glycol solvated. C X-ray diffraction patterns of the heatedLi-saturated clay at 2508C with subsequent glycerol solvation.

Plagioclases and inclusions: as the saprolite preserves the structure of the rock, andthe micromorphological study shows pseudomorphic alteration of the plagioclases whilethe K-feldspars remain unaltered, microsamples were taken from the white colouredalteration zones in order to identify the degradation products of the plagioclases. The


XRD patterns of the oriented samples show peaks at 1.5, 1.0, 0.7, 0.44, 0.5 and 0.356nm; the first of these is displaced at 1.8 nm on solvating the sample with ethylene glycol

Ž .and collapses at 1.0 nm on heated at 5508C Fig. 7b . The minerals found weretherefore, smectite and 1:1 dioctahedral phyllosilicate, as well as a small quantity ofmica, which in this case is probably muscovite, as the peak at 1.0 nm is similar inintensity to that at 0.5 nm.

4. Discussion

The results show that the weathering products of biotites are interstratified biotite–vermiculite and iron oxyhydroxides.

Smectite is formed inside the plagioclase crystals and probably originates from themuscovites contained in these crystals, this being supported by the microscopic study aswell as by the increase in volume which is seen when muscovite is transformed into thisswelling mineral displacing the iron oxide inclusions which the host mineral originally

Ž .contained Figs. 5 and 6 .The possible evolution of the sericites included in the plagioclases, to smectites, has

Ž .previously been mentioned by Dejou et al. 1977 .The different evolution of biotite and muscovite which is seen in the weathering of

Ž .this granite is to be expected, because as Robert 1973 pointed out, smectite is formedmore easily from dioctahedral micas, where the tetrahedral charge is lower, than fromtrioctahedral micas. Transformation of dioctahedral micas does not implies modifica-tions in the tetrahedral layer, and may take place after a small decrease in the overallcharge of the mineral. The same author proposed the term ‘transformation smectite’ forsmectites derived from micas.

The type of smectite found in the profile from the analyses seems to be a behaves asa beideillite. Since the potassium-saturated sample has to be heated to 3308C to producethe collapse at 1.0 nm, a layer of Al-hydroxide must be contained in this beidellite.

As it was mentioned, it is not common to find smectites amongst the weatheringproducts of granites in Galicia, because of the high rainfall and strong leachingenvironments where if takes place. The fact that smectites are formed in this instance,

Žmust be attributed to the particular microsystem in which they appear inside the.weathered plagioclase graines where the fine matrix which substitutes the original

crystal slows down leaching.

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Smectite formation produced by weathering in a coarse granite saprolite in Galicia (NW Spain)

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