Clay Minerals (1993) 28, 285-296

I N F L U E N C E OF G E O L O G I C A L M A T E R I A L IN T H E C O M P O S I T I O N OF S U R F A C E W A T E R S OF G A L I C I A

( N W S P A I N ) . G E N E S I S OF C L A Y M I N E R A L S

R. C A L V O D E A N T A AND F. M A C I A S

Departamento de Edafologia, Universidad de Santiago de Compostela, Spain

(Received 10 December 1991; revised 14 September 1992)

A B S T R A C T: The composition of fluvial waters can be used to predict the processes of formation and evolution of secondary minerals in Galician soils. These processes are similar in areas with different rocks, from acid to basic, the only exception being strongly serpentinized materials. In general, the nature of the rock has less influence than the characteristics of the alteration system, which is always open, acid and freely draining. Mineral neoformation is always of the monosialitic type, having different degrees of evolution. The incipient phases tend to form gibbsite as a more stable mineral, and halloysite, allophanes and imogolite as metastable forms. With time, a crystalline kaolinite would be the only stable species. These results agree with the mineralogy and the andic or ferralic properties of the soils.

Soil format ion constitutes an impor tant part of the chemical, mineralogical and organizat ional changes during the weathering of geological materials . Mineral neoforma- tion is ini t iated at the beginning of meteor izat ion at the contact surfaces of the pr imary minerals in the rocks and continues in the edaphic horizons and from the solutions that leave the systems. As a result of the equil ibria that are establ ished between the soil components and hydrodynamic aspects, different fluid phases are formed whose composi t ion can be used to establish the tendencies of al terat ion and neoformat ion of the system, whenever a model of equil ibrium is applicable.

In Galicia, different studies have shown that the main chemical changes, with relative losses of more that 90% of Ca, Mg and Na and of 50% of K and Si, occur when the mater ia l still retains the original rock structure, whereas the process of format ion of B and A horizons can be defined as isochemical (Macfas et al., 1982). As the liquid phase passes through the soil, very impor tant variat ions of the active species are produced , due to the changes in the different physical/chemical parameters that affect the acid/base equil ibria, redox and surface phenomena (Calvo de An ta et al., 1987a; Alvarez et al., 1992). In the surface horizons, not knowing the thermodynamic constants of the organic components and of the complexes that form with the metals , together with the influence of the biotic and hydrodynamic processes, make it impossible to guarantee that the model of solution- mineral equil ibrium by itself can explain the global behaviour of the system. However , the tendencies es t imated from the composi t ion of the solution of the subsurface horizons and from the surface water correspond adequate ly to the dominant minerals in the clay fraction of the soils and al tered mater ia ls in Galicia, so the equil ibr ium model seems to be applicable (Calvo de An ta et al., 1987b; Fernandex & Macias, 1989).

In this article the influence of geological mater ia l in mineral neoformat ion is s tudied from

�9 1993 The Mineralogical Society

286 R. Calvo de An ta and F. Macias

the composition of streams and rivers of Galicia selected because their entire course is over the same type of rock, from acid to ultrabasic in type.

M A T E R I A L S A N D M E T H O D S

The study area is situated in the NW of Spain. The majority of the area is characterized climatically by high precipitation and mild temperatures (Fig. 1 and Table 1). The most frequent moisture regime is udic type (USDA, 1990), with a transition towards xeric to the SE of the region and in small areas with thin soils. The temperature regime is mesic (USDA, 1990). The weathering systems are characterized by an important loss of elements caused by the high renewal of the contact solutions as well as by the type of existing process (acidolysis or acid complexolysis in A horizons and hydrolysis in B and C horizons) (Macfas et al., 1982). The mobility sequence is Ca, Na>Mg>K>Si>A1 , Fe. The soils are acid (pH 4.5- 5.5), rich in organic matter in the surface horizons (>5% C), with an abundance of A1 in the exchange complex and forming complexes with the organic matter.

The dominant mineralogy in the clay fraction is an association of 1:1 phyllosilicates (kaolinite and/or halloysite) and micas degrading to hydroxyaluminium interlayered vermiculites. The presence of gels and poorly crystallized material is relatively frequent in incipient soils and altered materials (Garcfa-Rodeja et al., 1987; Romero et al., 1992a,b). Gibbsite appears as a product of the incipient alteration of acid and basic rocks in areas with

SCHISTS (SCH)

SHALES (SH)

~ GRANITES (G)

BASIC AND ~LTRABASIC ROCKS (GB, A, GR, N, S,)

SEDIMENTS

FI6. I. Locations of streams and rivers analysed.

Surface waters and clay minerals 287

o

o

<

~~

� 9 cO I ~ I ',~ I

mmmmm m~

N N N N ~ N

0 �9 �9 �9

o o o o o o o o o o

N N N N N ~ N N N N

e~

-a

co

e~ .. ~>

~C~

o

e ~

< . ~ ~

~ .~ =

M

288 R. Calvo de Anta and F. Macias

good drainage. Goethite is frequent in the B and C horizons of soils developed on ultrabasic and basic rocks and schists and gneisses rich in biotite.

The most frequent type of soil has an A umbric epipedon followed by a B cambic (Humic Cambisols, FAO, 1990). The eroded areas have soils of Leptosol and/or Umbric Regosol type, in which andic properties are frequent; true Humic Andosols are developed on basic rocks. In stable areas and on alterable materials, there are more evolved soils showing ferralic properties (Ferralic Cambisols) (Macias et al., 1982).

The serpentinized areas are very different, presenting a mollic (saturated in Mg) epipedon followed by a poorly developed B cambic horizon. The clay fraction comprises inherited minerals (chlorite, antigorite, chrysot i le . . . ) , kaolinitic minerals and vermiculites (and occasionally talc and chalcedony). In areas that are poorly drained, smectites have been recorded (Calvo de Anta et al., 1987c).

In the fall of 1991, 63 samples were taken of streams and rivers of Galicia carefully selected in such a way that their entire course was over the same type of rock: granites, schists, basic rocks (gabbros), metabasic (amphibolites, gneisses and granulites) and ultrabasic rocks (serpentinites) and shales. The samples from shale areas were taken from two regions with different moisture regime, shale-G, in an udic regime, similar to the rest of the materials, and shale-B, of xeric character (Fig. I and Table 1). The samples were taken within a period of less than 36 hours, so that the effect of climatic variations would be minimal. A time of the year following a long dry spell was selected, because this is considered favourable for solution-mineral equilibrium (Garcfa Paz et al., 1977).

Samples were filtered through a 0-45/~m Millipore filter, pH determined, and then analysed for fluorine (by ion selective electrodes, and using TISAB II decomplexing solution, Orion), sulphates, chlorides and nitrates (by ionic chromatography), phosphates and Si (colorimetry of the molybdic-blue complex), A1 (colorimetry, Dougan & Wilson, 1974), Ca and Mg (atomic absorption spectrophotometry), and K and Na (emission spectrophotometry). For calculation of the ionic activities, the Solmineq.88 package (Kharaka etal., 1989) was used and the results used in diagrams of equilibrium formed from the thermodynamic data of different authors, according to the characteristics of the analysed system.

R E S U L T S A N D D I S C U S S I O N

Composition of the surface waters. Concentration and activity of the main ions

The pH varies between 6-1 and 8.1 (Table 2) and the average values in the areas of the most precipitation follow the order: shale-G < granites < schists < basic and metabasic rocks < shale-B < serpentinites. This sequence agrees with the composition of the rocks and with the pH values of the soils and soil solutions (Table 1); in streams and rivers all the values are higher than in the soil solutions. The climatic influence is observed when the results obtained from the same type of material in two different regions are compared, e.g. shale-G (average value of pH in river water is 6-59) and shale-B (7.39).

The concentrations of the alkaline and alkaline earth elements are very low. Variations according to the abundance and availability of the elements in each type of rock are known. For instance, the highest Mg 2+ values are produced in the serpentinites, while the lowest values were registered in the granites and shales-G. The basic rocks and the shales-B (from the xeric region), have the highest concentrations of Ca 2+, and K + is higher in

Surface waters and clay minerals 289

L

?

�9

N N

e~ o

�9

c~

0

0

G

F~F~

d-

~L

+I +I +I +I +I +I +I +I +I +I +I +I ~ ~ o

+I +I +I +I +I +I +I +I +I +I

6

+I +I ~ ~ ~ ~

+i +I +i +I +I

�9 d

+I +I +I +I +I +I +I ~ ~ ~ ~

+I +I +I +I +I +I +I +I +I +I +I ~ ~o ~ ~ +i

+I +I +I +I +I +I +I +I +I +I +I +I ~ ~ ~ ~4~

+I +I +I +I +I

�9 d

+I +I +I +I +I +I +I ~ oo ~- ~ oo

+I +I +i +I +i +I +I +I +I +I

+I +I +I +I +I +I +I +I +I +I

6

+ I + I ~

~ 6 6

6

+I +I +I +I +I

6 ~6~

+I +I

6

+I +I +I

6 6

I [ I

o ~x xx

~ + + X~ r x ~ ~

T~

d

290 R. Calvo de Anta and F. Macias

granites and schists. The distribution of Na + and C1- is highly affected by proximity to the sea. Greater concentrations of Si occur in the basic rocks and especially in the ultrabasic rocks. In vaporitic surfaces and in fissure systems in altered layers in these ultrabasic rocks the concentration of SiO2 is frequently >0.83 mmol 1-1, which makes it possible to justify the presence of amorphous silica and processes of silicification in these areas (Macfas et al., 1983). The concentration of SO42 varies widely (0.009-0-28 mmol 1-1) due to different possible causes (pollution, proximity to the sea etc.) and seems to be mainly related to the irregular distribution of mineralizations of S in different rocks. For the rivers analysed, the highest values were registered in some areas of shales, serpentinites and, to a lesser degree, in amphibolites and schists.

Phosphorus and A1 are two elements that are present in very low concentrations and consequently their variations are of very little relevance (Table 2). The very low concentration of P must be related to the low contents in the majority of the rocks with the abundance of reactive forms of A1 and Fe (particularly in the basic rocks). The highest values were registered in granitic areas due to the greater content of P-containing mineral.

The relationships between Mg/Si, Ca/Si and Ca/SO4 are different among the various types of parent rocks, with some high Mg/Si values in serpentinized area and Ca/Si in shale- B, (Table 2). The charge balance calculated from the average concentration values (Fig. 2) shows the low level of ions from the alteration processes with respect to the importance of the values related to external factors, like the influence of the sea, especially for Na and C1 and to a lesser degree for SO42 , and the biogenic processes and equilibria with the atmosphere, which determine the levels of NO3- and HCO3-.

These results enhance those obtained by analysis of the solid fraction of soils and alteration layers formed as a result of rapid and intense drainage in a large part of the territory where average temperatures above 12~ favour the degree of geochemical

NEGATIVE POSITIVE CHARGE: mmol (+or-)1-1

GRANITE

SHALE -G

SHALE -B

GABBRO

A/4PHIBOLITE

GRANULITE

GNEISS

SCHIST

SERPENTINITE

Icll

c is] I

Ic ls , 1 c Ilcll

C C1

[

llNal F Ca " 1"5

C a M ~ Na ]

i i i i i | 1 1 i i i

2 . 4 1 . 6 0 . 8 0 0 . 8 1 . 6 2 . 4

Fro. 2. Water rivers charge balance (mean values). N:NO3 ; C:HCO3-; S;5042 , (H +, AI(OH)z n+ and PO4 3 contribution was negligible).

Surface waters and clay minerals 291

evolution reached. The strongly diluted nature of the drainage solutions conceals the significant differences among the different geological areas.

The calculations of activities yielded results of around 10-4-10 -5 M for Ca 2+ , Mg 2+, K + , HaSiO4 and SO~ 2 with slight differences between the different rocks (Table 3). The activity of Mg 2+ in the serpentinized areas is the only notable result. The lowest values were recorded in phosphates with activities <10 -11 and even <10 -13 M. The aA1 +3 is very low, with strong variations, from 10 9 M in shales, to values close to 10 13 M in serpentinites. The dominant species of A1 are, in all the samples, A1 hydroxylated species, and from them A1OH4- (10 6 ~), while the fluoride and sulphate complexes always have activities < 1 0 - 7 M

and <10 10 M, respectively (Table 3). These results are very different from those observed in previous studies of soil solutions in

Galicia (Alvarez et al., 1992). In these solutions, in addition to a higher concentration of A1, the species with a fluoride complex (including complexes with organic matter in the surface horizons) are dominant. This variation is undoubtedly related to the increase in pH, and the variations in pCO2 and activity of the organic acids. The data obtained in solutions from O/A horizons of forest soils of Galicia give values >3.0 mmol 1-1 of soluble organic compounds, that decrease sharply in A horizons (0.3-0.5 mmol 1 1) and B and C horizons (<0.15 mmol 1-1). Organic matter in solution was not detectable in the rivers analysed. On the other hand, it is well known that the pCO2 is higher in surface horizons (10-100 times higher than atmospheric pCO2) than in surface running water, in which there is a tendency towards equilibrium with the atmosphere.

The importance of the A1-F complexes in soil solutions must be also related to the greater content of F, particularly in A horizons in soils on granites (frequently >100 ktmol 1 1), and decreases suddenly when the water percolates through the soil. The concentration of F in the river is always <5 ktmol 1 1 (Table 2).

Mineral neoformation processes

From the activity data and using the Solmineq.88 package the indices of saturation for different minerals and the evolutionary tendencies in each area have been calculated. Considering only the possible secondary minerals with A1, those having positive saturation indices are practically the same in all the river courses, independent of the geological nature of the substratum (Table 4). Only the serpentinites have specific tendencies, with the possibility to form saponites, chlorites, talc and chalcedony.

In all the samples, the mineral with a greater saturation index is kaolinite, and this coincides with the general tendency of geochemical evolution towards monosialitization (fermonosialitization in basic rocks), and with the dominant mineral in the clay fraction of the saprolites and more mature soils. (Macias et al., 1982). On the other hand, the high number of minerals with positive saturation indices is outstanding.

A more precise analysis of the tendencies of the system must take into consideration the effect of the temperature (-15~ and the possible presence of phases with different degrees of crystallinity.

Because of the low concentration of sulphates and phosphates, the most important geochemical system for understanding the processes of mineral neoformation must be SiO2- A1203-H20. A diagram has been established at 288 K using different species and thermodynamic constants (Fig. 3a,b). The representation in this diagram of the activity data makes it possible to check that there are several minerals that can be oversaturated

292 R. Calvo de Anta and F. Macias

F.

I

o

�9

m~

.a. .a- ,A ,A .a- ,A ~ . + ~ . + , - . . ,

~. ~. ' ! . ~ ~. ~ ~ "!.

,o o~ ~. ~ ~ ~. ~ ~. "!.

< 2 s ~ X A * ~ X 2

L <

<

�9

<

d

a m • ~ ~ ~ . ~ •

A '.6 ~5 A A A A ~A ~5

~ ~ ~ ~ 2 o ~ ~ , ~ ~ ~ ~ ,

Surface waters and clay minerals

TA~I~E 4. Mineral saturations afforded by the Solmineq.88 software (Kharaka et al., 1989).

293

Phase of greater Other saturated Occasionally Rock saturation phases saturated phases

Granites K B,Ga,G,H,I,S A Shales K B,Ga,G,H,I,S A Schists K B,Ga,G,H,I,S A Gneisses K B,Ga,G,H,I,S Granulites K B,Ga,G,H,I,S Amphibolites K B,Ga,G,H,I,S Gabbros K B,Ga,G,H,I,S Serpentinites K B,Ga,G,H,I,S,Sp C1,Ch,T

K: kaolinite; B: boehmite; Ga: microcrystalline gibbsite; G: gibbsite; H" halloysite; I: illite; S: smectites; Sp: saponites; A: alunite; CI: chlorite; Ch: chalcedony; T: talc. The stability constants considered for different dissolution reactions at 298 K were: alunite: -2-79; boehmite: 6.95; gibbsite: 34-92; amorphous gibbsite: -34-42; halloysite: 8-91; kaolinite: 6-23; illite: 10-34; chalcedony: -3.73; smectites: 5.8645.14; saponites: 25-85-26-88; chlorite: 68.57.

with respect to the majority of the liquid phase. The solution seems to be saturated with respect to gibbsite and kaolinite and sometimes with respect to bayerite, halloysite and imogolite. This confirms the great abundance of phases that could be found in a metastable form.

The H4SiO 4 activity has average values around the saturation in quartz, being minimal in the granitic rocks and shales and maximum in the serpentinites. The logA13+ + 3pH parameter has values of 9-10, independent of aH4SiO4, indicating that it is not a mineral of the kaolinite type that controls the A1 activity, but microcrystalline gibbsite with a thermodynamic constant similar to that proposed by Hem & Roberson (1967). This result is similar to the one found by Bourrie et al. (1989) in temperate and tropical areas.

If the thermodynamic constants of crystalline kaolinites (KI) (Fig. 3b) are considered, the most stable mineral in all examples would be kaolinite and, therefore, all the edaphic systems should be directed towards its formation. However, for the mineral of the kaolinite type, a great variety of thermodynamic data have been shown to be responsible for properties such as particle size, crystallinity degree, crystallinity imperfections, etc. This means that if kaolinites of low crystallinity are considered (K4) (Fig. 3a), these will be less stable than gibbsite in the conditions in the Galician rivers. Intermediate data, like that corresponding to the K2 and K3 kaolinites, would give results between both situations.

The formation of well crystallized kaolinite is kinetically slow, and more so at temperatures that are relatively low. In addition to this, it can be carried out in stages (Sposito, 1985), from lower to higher crystallinity. In this way, two well defined situations could be considered in the soils of Galicia. For soils and incipient alterations, kaolinite, if it exists, would be of low crystallinity, and therefore the most stable mineral would be gibbsite, and metastable forms of low-order degree could appear (halloysite, imogolite). This situation agrees with previous studies related to poorly developed Galician soils in which the existence of properties related to the presence of aluminous forms of high reactivity was evident, such as the rapid flocculation of organic matter, high fixation capacity of phosphates, reactivity of NaF, variable charge, high absorbing capacity in relation to the pH changes, etc., (Macias et al., 1978; Garcia-Rodeja et al., 1987). The

294 R. Calvo de Anta and F. Macias

mineralogical studies showed the abundance of gibbsite (Macias, 1981) and, recently, of halloysite, alnminous gels, allophanes and imogolites (Romero et al., 1992a,b).

At a later stage the systems will evolve towards the most stable species, kaolinite (Fig. 3b), which is the major abundant mineral in the most evolved soils in Galicia. In these

e~ co

++ 03

7: o

:=

++

o

12

10

12

10

a)

A I ( O H 3 ) a .

. . . . . . . . ...... :~

~ ~i:::::: ~:i::::::i::i i::i::i i::i: i::i: i ::i:: i : i i::i:i:: ::::::::::iiii!i;:i::iiii!::!::!:@ :ii:ii~:i ~: ;: !)iii!~i )i~ii)i ii!::ii ::iii:i~!ii :::::::::::::::::::::::::::::::::::::::::::~ BAYERITE

~.~~;:::)37i!ii~ii3~3{i3:f:3:?!:33:{3:3];3~i~)!i?3!!}!?3{]i3:{d~]..:?i~3~:i! ---~..IvIICROGIBBSI E i::i::i::i::!::~i~! ! i i :?~

::::::::::::::::::::::: i ii:: :, ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ~;~:~/: ~:i~::~::!~:~ii::::::: ::iiii:.~i i :i::::':'" ' ii~ "~ ::: !:!! i!!:i i~:::;!::!i!:i::;i:i:i~iiii::i i::i::iiii!#:!!!!i!ii;i:iiiiiiii::i:: :: :: :: i i i i : " ...... i~::i::!:: i ::i::i i i i ) GIBBSITE

I I i i i i

- 5 , 0 - 4 , 0 - 3 , 0

l o g H4SiO 4

1

b)

x

A I ( O H 3 ) a .

BAYERITE

i iii!ii!i iiii ^ a i r A ' ~ e ~ I I C R O G I B B S I PE ~ - x x

GIBBSITE

o G

�9 SCft

r GB

�9 A

X S

+ Gr

a SH

�9 N

.......... :::::::::::::::::::::: STABILITY

ii::i::!::!::!::!il CONDITIONS

:::: ):::::::::) )::::: : : GIBBSITE

I i I I A I

- 5 , 0 - 4 . 0 - 3 , 0

log H4SiO 4

F1G. 3. Equilibrium diagram of the SiOz-AI203-HzO system considering stable and unstable phases of differing degrees of crystallinity (a: initial stages; b: mature stages). Gibbsite: May et al., (1979); microgibbsite: Hem & Roberson, (1967); bay�9 and amorphous AI hydroxide: Sarazin (1979); kaolinites K-l/K-4: Kinrick (1966); halloysite: Busenberg, (1978); imogolite: Aurousseau et al.

(t987), (G: granites; SCH: schists; GB: gabbros; A: amphibolites; S: serpentinites; Gr: granulites; SH:

shales; N: gneiss),

Surface waters and clay minerals 295

soils t h e r e a re n o g ibbs i t e n o r a n d i c p r o p e r t i e s , h a v i n g f r e q u e n t l y fe r ra l ic p r o p e r t i e s

(espec ia l ly in t he m a t e r i a l s t h a t a re t he r i ches t in Fe ) .

T h e r e f o r e , t he m o d e l of t he s o l u t i o n / m i n e r a l e q u i l i b r i u m ag rees wel l w i th t he ex is t ing

m i n e r a l s in t he soils a n d a l t e r e d m a t e r i a l s of Ga l ic ia . T h e r e a re n o i m p o r t a n t d i f f e r ences

d u e to the n a t u r e of t he r o c k a n d t h e type of s e c o n d a r y m i n e r a l f o r m e d d e p e n d s m a i n l y o n

t he m e a n fac tors , h igh p r e c i p i t a t i o n , r ap id d r a i n a g e , e tc . a n d t he e v o l u t i o n d e g r e e r e a c h e d .

O n l y t he s e r p e n t i n i z e d a r ea s h a v e a specif ic cha rac t e r i s t i c , b e c a u s e of t he poss ib i l i ty of

f o r m a t i o n o f m i c r o s y s t e m s wi th m i n e r a l s l ike talc, s a p o n i t e s a n d c h a l c e d o n y .

R E F E R E N C E S

AEVAREZ E., GARClA-RODEJA E. & CALVO OE ANTA R. (1992) Parent soil material and toxic species of Al in acid soils of Galicia. Fresenius Environmental Bull. 1,553-558.

AUROUSSEAU P., BOURR1E G. & CURM1 P. (1987) Organisation mindralogie et dynamique de l'aluminium dans les sols acides et podzoliques en climat temp6rde et oc6anique. Pp. 85-105 in: Podzols and Podzolization (Righi & Chauvel, editors). AFES-INRA, Rennes.

BOURRIE G., GRINALDI C. & REaEARD A. (1989) Monomeric versus mixed monomeric-polymeric models for aqueous aluminium species: Constraints from low-temperature natural waters in equilibrium with gibbsite under temperate and tropical climate. Chem. Geol. 76, 403-417.

BUSEr~BERG E. (1978) The products of the interaction of feldspars with aqueous solutions at 25~ Geochim. Cosmochim. Acta 42, 1679-1686.

CAEVO DE ANTA R., FERNANDEZ L. & VElCA A. (1987a) Composici6n de la soluci6n del suelo en medios naturales de Galicia. An. Edaf. Agrobiol. 46, 621-641.

CAEVO DE ANTA R., FERNANDEZ L. & VEIGA A. (1987b) Establidad mineral de suelos desarrollados a partir de rocas bfisicas y ultrab~isicas de Galicia. An Edaf. Agrobiol. 46, 643-665.

CALVO DE ANTA R.M., MACIAS F. & BUURMAN P. (1987c) Procesos de alteraci6n y neoformaci6n mineral en medios serpentfnicos de Galica. Cuad. Laboratorio Xeol6xico de Laxe, 11, 161-170.

DOUGAN W.K. & WILSON A.L. (1974) The absorptiometric determination of aluminium in water. A comparison of some chromogenic reagents and development of an improved method. Analyst, 99, 413-430.

F.A.O. (1990) Mapa Mundial de Suelos. Leyenda Revisada. Informes sobre Recursos Mundiales de Suelos 60, 142pp.

FERNANDEZ L. & MACIAS F. (1989) Neoformaci6n de minerales de la arcilla en la Espafia peninsular: tendencias termodin~imicas basadas en la composici6n de las aguas de los rfos espafioles. Cuad. Lab. Xeol6xico de Laxe, 14, 19-28.

GARCfA PAZ C., MAClAS F. & DIAZ-FIERROS F. (1977) Relaci6n entre la composici6n qufmica de las aguas superficiales y la mineralogia de los suelos de Galicia. Acta Cientifica Compostelana, 14, 337-363.

GARCIA-RODEJA E., SILVA B., MACIAS F. (1987) Andosols developed from non-volcanic materials in Galicia, NW Spain. J. Soil Sci. 38, 573-591.

HEM J.D. & ROBERSON C.E. (1967) Form and stability of aluminum hydroxide complexes in dilute solution. U.S. Geol. Surv., Water-Supply Pap. 1827 A, 55pp.

KHARAKA Y., GUNTER W., AGGARWAL P., PERKINS E. & DEBRAAE J. (1989) Solmineq.88: A Computer Program for Geochemical Modelling of Water-rock Interactions. U.S. Geol. Surv. Menlo Park, California, 419pp.

KITrRICK J.A. (1966) Free energy of formation of kaolinite from solubility measurements. Am. Miner. 51, 1457-1466.

MACIAS F., PUma M. & GUITIAN F. (1978) Caracteres ~indicos de suelos sobre gabros de Galicia (NW de Espafia). An. Edaf. Agrobiol. 37, 187-203.

MAOAS F. (1981) Formation of gibbsite in soils and saprolites at temperate humid zones. Clay Miner. 16, 43-52. MACIAS F., CALVO DE AN:rA R., GARCIA P., GARCIA-RODEJA E. & SILVA B. (1982) El material original: su formaci6n e

influencia en las propiedades de los suelos de Galicia. An Edaf. Agrobiol., 41, 1747-1786. MAY H.M., HELMKE P.A. & JACKSON M.L. (1979) Gibbsite solubility and thermodynamic properties of hydroxy-

aluminium ions in aqueous solution at 25~ Geochim. Cosmochim. Acta 43, 861-868. ROMERO R., ROBERT M., ELSASS F. & GARCIA C. (1992a) Evidence by transmission electron microscopy of

weathering microsystems in soils developed from crystalline rocks. Clay Miner. 27, 21-33.

296 R. Calvo de A n t a a n d F. Mac ias

ROMeRO R., ROBERT M., ELSASS F. & GARCIA C. (1992b) Abundance of halloysite neoformation in soils developed from crystalline rocks. Contribution of transmission electron microscopy. Clay Miner. 27, 35-46.

SARAZIN (1979) Gdochimie de l'aluminium au cours de l'dlt(ration des granites et des basaltes sous climat tdmpdrd. PhD thesis, Univ. Paris VII, France.

SPOSITO G. (1985) Chemical models of weathering in soils. Pp. 1-18 in: The Chemistry of Weathering (J.I. Drever, editor). Nato Advanced Study Inst., SeE C.

USDA (1990) Keys to Soil Taxonomy, fourth edition. SMSS Technical Monograph 6, Blacksburg, Virginia.