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Mineralogical variability in weathering microsystems of A granitic outcrop of Galicia (Spain)
CATENA
Vol. 10, 225-236
Braunschweig 1983
MINERALOGICAL VARIABILITY IN WEATHERING MICROSYSTEMS OF A GRANITIC OUTCROP OF GALICIA (SPAIN)
R.M. Calvo, E. Garcia-Rodeja & F. Macias, Santiago de Compostela SUMMARY Several products of weathering have been studied in a granitic outcrop in Galicia (N.W. Spain). Alteration took place under a humid temperate climate. The various materials and the microsystems in which they occur are: (1) fissures containing gibbsite and/or 1:1 sheet silicates of varying degrees of crystallinity,(2) fissures containing 2:1 sheet silicates ofbeidellite type, (3) encrustations of siliceous material showing varying degrees of structural order, with or without organic matter, and (4) encrustations of non-crystalline matter composed chiefly ofAl, P and Si. The results demonstrate the importance of considering the different microsystems which contribute to the formation of secondary products in a given weathering system. 1..INTRODUCTION In recent years the study of mineralogical transformations taking place during weathering has made significant progress due fundamentallyto the introduction of thermodynamic and kinetic concepts and the attention paid to the heterogeneous nature of weathering systems. The earlier global, zonal approach to weathering processes has thus given way to a series of refinements in which the need to know the specific conditions of microsystems has become ever more apparent. MILLOT (1964), PEDRO (1978), TARDY (1969), HENIN et al. (1968), PEDRO et al. (1975), CHESWORTH (1977), MEUNIER & VELDE (1976), NALOVIC & PEDRO (1978), ESWARAN (1979) ... This paper seeks to characterize the great variety of weathering products found in a granitic area of Galicia (NW Spain) and to relate them to the specific situations in which each of them occurs. The general tendency of the weathering undergone has been described in earlier papers reporting for a single granitic outcrop the chemical, mineralogical and structural analysis of various profiles which illustrate different stages of the process: more or less weathered rock, saprolites, granitic sands and soils developed over these materials (CALVO et al. 1981). 2. MATERIALS AND METHODS The granitic outcrop studied here lies to west of the town of Padr6n (La Corufia). It forms part of a series of shelves carved out during the Eocene and bounded to the east by the great fault which crosses Galicia from north to south. The parent rock is a leucogranite composed of microcline, albitic plagioclase, quartz (muscovite and biotite). The present humid mesothermic climate yields an irregularly distri-
226
CALVO, GARCIA-RODEJA, & MACIAS
buted annual rainfall of over 1700 mm and a mean temperature of 11°C, with about 10°C between the summer and winter average. Right from the first stages of weathering, gibbsite and a kaolinite phyllosilicate form, chiefly at the expense ofplagioclase. The quantity of clay formed ranges from 10 to 20% of the < 2 mm fraction of the material that has already lost its original structure. Variation with respect to depth within weathering profiles is slight, though in the finest fractions of the organicrich surface horizons a reduction in gibbsite content is accompanied by the increased presence of weathering products of mica trending towards aluminous vermiculites (CALVO et al. 1981). The material examined in the present study occurs in a wide variety oflocalised weathering environments including: fissures filled with fine matter, coatings upon the surface of other consolidated or unconsolidated material and precipitates in small cavities. The nature of the material has been determined by X-ray diffraction using samples untreated except for saturation with K or Mg and samples pretreated with ethyleneglycol or heat; by DTA; by IR spectroscopy; by optical microscopy. Standard techniques of chemical analysis were used: extraction with ammonium oxalate at pH 3 (McKEAGUE & DAY 1966); with hot and cold 50/0Na2CO3 (JORGENSEN et al. 1970); and with boiling 0,5 M NaOH (HASHIMOTO & JACKSON 1958). The analysis of both the extracts and of dissolved whole samples was carried out by atomic absorption spectrophotometry. In some cases the P and Si contents were also determined by colorimetry after complexation with molibdenum blue (GUITIAN & CARBALLAS 1975). Specific surfaces were measured by adsortion of EGME (HEILMAN et al. 1965), phosphate fixation by Kawai's method (KAWAI 1981), and reactivity to NaF following FIELDES & PERROT (1966) and BRACEWELL et al. (1970).
3. 3.1.
RESULTS FISSURE SYSTEMS WITH GIBBSITE AND/OR 1:1 PHYLLOSILICATES
One of the most common features of these weathering profiles is the presence of fissures of a width not usually exceeding 0,5 cm filled with fine material. They run vertically or subvertically through the entire observable depth of the profiles, sometimes displaying numerous ramifications (Photo 1). The filling is most often a mixture of gibbsite and 1:1 phyllosilicates in very varied proportion (Fie,. la) ranging from an almost complete absence of phyllosilicates to their clear predominance (Fig. lb). Small quantities of quartz, mica in varying stages of degradation, and non-crystalline material rich in aluminium and silicon, are found together with the gibbsite and phyllosilicates. The non-crystalline material is identified by its higher reactivity in chemical tests (Table 1), by the presence in DTA traces of small endothermic effects between 350 and 400"C and by the 2 to 10% loss of weight suffered between 100 and 200"C. In general it is the samples richest in gibbsite that also contain the greatest quantity of this amorphous material. Characteristically the gibbsite too presents a formless, non-idiomorphic, both in fissures and in saprolites, though in the latter tiny well-crystallized specimens have also been commonly observed (GUITIAN et al. 1981). The asymmetry index derived from DTA diagrams for 1:1 phyllosilicates ranges from 1.3 to 3.0. This clearly reflects a variability in the 1:1 minerals similar to that in gibbsite. The origin of these fissure-filling materials is obscure. In some cases they would seem to be associated with in situ weathering of vein minerals previously affected by tectonic pro-
WEATHERING MICROSYSTEMSIN GRANITE
227
Tab. 1: ANALYTICAL DATA FROM SAMPLES OF GIBBSITE/I:I PHYLLOSILICATE FISSURE MATERIAL (A11 and A3) AND OF NON-CRYSTALLINE COATINGS (A4 and A7).
-
~
1:1 phyllosilicates gibbsite
~ ~
micas quartz
.~
AI203 t %
~
Samples with crystalline material A-11 A-3 60% 10% 30% 50%
Samples rich in non-crystalline material " A--4 A-7 Dominant minerals 5% 3% % by DTA
x x
x x
x x
x x
21,3 46,0 -
22,5 44,0 -
13,0 28,2 17,2
13,0 30,3 15,0
SiO2 t/Al203 t A1203 t/P205 t
0,8 -
0,9 -
0,8 2,3
0,7 2,8
. -~8 oo ~
A1203 F (4) % SiO2 F (4) % A1203 C (3) % SiO2 C (3) %
n.d. n.d. n.d. n.d.
2,3 1,7 23,8 6,0
25,3 6,8 1,9 6,0
27,0 6,0 1,9 3,9
~ o= G"
SIO2/A1203F SIO2/A1203C
n.d. n.d.
1,3 0,4
0,5 5,4
0,4 3,5
AI203 F/A1203 t n.d. ~ AI203F-{--C/A1203t n.d. X SiO2 F/SiO2 t n.d. SiO2 C + F/SiO2 t n.d.
5,0 59,3 7,5 34,2
89,7 96,4 52,3 98,4
89,1 95,4 46,1 76,2
SiO:t % .~ .~
P2Os t %
~
.~ 0 ~
AI203 O %
Impurities x present
Molar ratios in total analysis
Molar ratios in extracts % extracted with respect total
SiO2 o %
1,5 1,6
1,9 0,8
24,8 6,0
22,4 4,5
SiO2 o/A1203 o
1,8
0,7
0,4
0,3
Molar ratios in extracts
-~ ~ ~ ~ A1203 o/A1203 t ~a ~ ~ × SiO2 o/SiO2 t
3,3 7,5
4,3 3,5
88,6 46,1
73,9 34,7
% extracted with respect total
SiO2 e %
33,1 4,3
31,2 5,4
27,0 6,8
28,4 6,8
~ SIO2/A1203 e
0,2
0,3
0,4
0,4
Molar ratios in extracts
o Z ~ A1203 e/A1203 t × SiO2 e/SiO2 t
72,0 20,2
70,9 24,0
95,7 52,3
93,7 52,3
% extracted with respect total
~_u.
pH FNa 2 min pH FNa 30 min
9,4 9,7
9,2 9,5
10,1 11,4
10,8 11,6
FIELDES & PERROT (1966) lg : 50 ml satured NaF
Z o
meq OH-/100 gr meq OH-/m 2
~. "~ .*~-- O 3: .E= _
.~ '~: Z
AI203 e %
2 ~~ .~,
o ~. o
o ~ ~
89 136 1,02×10 -2 1,46)
603 293 meq. OH- after 25 mins 1,60X10 -2 0,8X10 -2 BRACEWELLetal. 1970
Specific surf. m2/g 87
93
376
353
EGME (HEILMAN et al. 1965)
pH 7 mgrs P/100gr 1050 pH 4,5 1155
1425 1480
3375 3630
3000 3630
KAWAI (1980) adding 6.45 mm P/g of material
228
CALVO, GARCIA-RODEJA & MACIAS
985
D.TA
m
300
566
X. RD a~
u~
A
a)
b) Fig. 1: XRD and DTA of fissure materials; a) gibbsite dominant; b) phyllosilicate 1:1 dominant.
cesses (in a number of profiles fissures occur that have been fractured and displaced several centimetres). In other cases they seem to be inffllings derived from more highly weathered areas, and transported into cracks opened in the rock during weathering. We are thus tentatively led to the conclusion that the majority of the secondary material has been formed by surface weathering of unstable minerals (plagioclases, felspars, etc.), with tectonic processes and associated hydrothermal weathering effects in some cases opening the way. 3.2.
FISSURES WITH 2:1 PHYLLOSILICATES
Only one of the profiles studied is of this type. It consists of an approximately 50 cm wide band of 1 to 2 cm veins whose soft shiny surfaces are reminiscent of fault planes (Photo 2). The results of XRD and DTA diagrams (Fig. 2) to indicate the presence ofa 2:1 phyllosilicate, possibly a low charge vermiculite or a high charge smectite, very difficult to tell apart by standard techniques. However, the presence of a 060 reflection below 1.53/~ and the fact that ethyleneglycol caused re-expansion of a sample treated with Li and heated to 250°C
WEATHERING
MICROSYSTEMS
IN GRANITE
229
Photo 1: Fissures in a weathering profile.
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Photo 2: Weathering figure with smectite material.
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.7:
,,;
Ill
230
CALVO,GARCIA-RODEJA&MACIAS
931"
521"
oc ~
(f
9i9 2 / ( e
XR.D
b~
~ (d ~7,3
~
(060)
(b
(a
Fig. 2: DTA and XRD diagrams of smectitic material, a) Mg saturated; b) (060) diffraction; c) treatment with ethyleneglycol; d) heated at 500°C; e) K saturated; f) Li saturated, heated at 250°C and treated with ethyleneglycol (GREENE-KELLY test).
(GREENE-KELLY 1955) support the latter alternative, dioctahedral mineral of beidellite type. In some samples small quantities of gibbsite have also been identified by DTA. The coexistence of beidellite with gibbsite is nevertheless thermodynamically puzzling (KITrRICK 1969), only plausible at all in surface media and under uncommon conditions (CHESWORTH 1980). In the present case we regard the most likely explanation to be that the two minerals formed under different conditions and at different times. This part of Galicia contains many hydrothermal in origin and unstable in the present weathering system. The accompanying gibbsite developed more or less as in the gibbsite/1:1 phyllosilicates fissures. Another possibility that cannot be ruled out is that we are dealing here with a mixed system in which gibbsite is an intruder originating in the principal mineral of the fine fraction.
3.3.
SILICEOUS COATINGS
In certain situations, particularly on the upper and lower surface of the large, siliceous material is found to form a hard coating several millimetres thick from which black knobby protuberances project out like microstalagmites for several millimetres, giving a botryoidal
WEATHERINGMICROSYSTEMSIN GRANITE
231
438
1~5
°C
Fig. 3: DTA of siliceous coatings with different range of order. appearance (Photo 3). DTA diagrams (Fig. 3a) show a low temperature endotherm which at higuer temperatures merges into a pronounced exotherm peak between 400 and 500°C. This is attributable to the oxidation of oganic matter trapped in the silica and responsible for the original dark colouring. The behaviour of the coatings under X-ray is not like that of typical non-crystalline material but, follows the pattern of other opaline materials described in the literature (JONES et al. 1963) with a diffuse band between 8.8 and 10/~ along with effects characteristic of polymorphs of crystalline silica, specially cristobalite (4.04 and 3.18 ~) and quartz (3.32 and 4.26 ~). These coatings seem to have originated as precipitates from solutions locally supersaturated with silica due to the rapid evaporation undergone by water running over the surface of the granite blocks. The biological activity may be supposed to play an important role in the formation of these features. KUZNETSOV (1975) and SAAVEDRA et al. (1981) have suggested that in such cases silica is precipitated when evaporation takes simultaneously with the bacterial decomposition of organo-siliceous complexes. Somewhat similar material to that described above is also found in small cavities in which water accumulates for short periods. In general organic matter is now absent and the colouring whitish. Although morphologically similar to the surface coatings (Photo 4), the degree of crystaliinity is higher and in DTA diagrams the inversion endotherm characteristic of quartz is recognisable (Fig. 3).
232
CALVO, GARCIA-RODEJA& MACIAS
51~.m.~mI
Photo 3: Siliceous coating forming microstalactites
Photo 4: Scanning electron micrograph of white precipitates of siliceous material without organic matter (Photo: F. GUITIAN).
Photo 5: Scanning electron micrograph of formations of biogenic silica (Photo: F. GUITIAN)
WEATHERING MICROSYSTEMS IN GRANITE
Photo 6: Non-cristalline coatings rich in phosphorus. General aspect.
~'~"~W"
.
I
Photo 7: Non-cristalline coatings rich in phosphorus. Detail in transversal cut.
Photo 8: Thin section of non-cristalline coatings rich in phosphorus.
"., ~ a , ~
.I ....................... . I . . 4 .
.......................... . _
. . . . . . . . .
:~" . . . .
:,'
233
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,.I ....
: ~ . . : ~ . , ~ -
.~.~._~i
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( i
,"~1
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!
:::::::::::::::::::::::::::::: , i~ ~ii.~:.::., !.;IE.7..:.~&._~!,!!
,
234
CALVO, GARCIA-RODEJA & MACIAS
1052 D.TA
"C~0. 32O
X.R.D
i~
~ Fig. 4: XRD and DTA plots of noncrystalline coatings; a) without other crystallinecomponents; b) with gibbsite.
Finally we may mention the presence of small siliceous deposits which under the electron microscope appear like a tissue of living cells, so that a biological origin is to be presumed (Photo 5). 3.4.
NON-CRYSTALLINE COATINGS RICH IN PHOSPHORUS
On the walls of fissures near the surface, it is common to find whitish coUophorm-like encrustations with conchoid fracture, composed principally of non-crystalline combinations of Si, A1 and P (Photos 6 and 7). The thickness of these coatings is variable, though usually less than 1 cm. The coatings are commoner and thicker beneath the subhorizontal surfaces of rocks washed by run-off from the higher parts of the massif. Under the petrographic microscope an isotropic matrix of low refractive index is seen to contain two general types of differentiated microstructure. The first is massive, irregularly shaped and bordered by a brownish fringe composed of small accumulations of Fe, and the second is typical of desicated geles (Photo 8). Under the SEM, a fractured skin with noncrystalline appearance is observed. X-ray spectra confirm the low degree of crystallinity of this material. Two bands are distinguishable, a weak one between 7 and 11 /~, with a maximum at 10 A, and another, even flatter, between 2.8 and 4.0 A. The second does not show up in all eases. Features corresponding to impurities ofgibbsite and forms of silica are
WEATHERINGMICROSYSTEMSIN GRANITE
235
occasionally observed (Fig. 4). The DTA diagrams have a pronounced endotherm with a maximum at 169°C and an equally intense exotherm at 1052°C, together with the 300°C endotherm corresponding to gibbsite in the samples in which it is present (Fig. 4). The composition of two samples and their behaviour under various chemical tests is shown in table 1 together with similar data for two samples from type 1 (gibbsite/l:l phyllosilicate) fissures. The high P205 content falls between the values quoted for allophanes (which rarely approach 100/o, WEAVER & POLLARD 1975), and pure aluminium phosphates. But the possibility of these figures reflecting a mixture ofaluminium phosphate with siliceous material is ruled out by the molar A1/P ratio, which instead of being approximately 1 is greater than 2. Thus the aluminium present must be bound to both silicon and phosphorous. Further information concerning these materials is provided by their high reactivity in NaF test. It should be pointed out, however, that the high reactivity with NaF is to be regarded less as a chemical effect than as the result of the high specific surface area (above 300 m2/g in the crystalline samples). When the reactivity data for NaF are expressed in meq. OH-/m 2 the crystalline and non-crystalline samples are indistinguishable from one another. Solubility tests likewise reflect the difference between the two types of deposit. The crystalline (gibbsite-bearing) samples are barely soluble in oxalate or cold sodium carbonate, but highly soluble in hot carbonate or boiling NaOH. On the other hand about 90% of the'A1 of the easily soluble noncrystalline coatings is extracted by cold carbonate or oxalate, with molar SIO2/A1203 ratios below unity for the solutes. To find out whether this non-crystalline material was still capable of fixing phosphate, it was"subjected to KAWAI's test (1980), saturation being effected at pH 4.5 and pH 7.0. The results (Table 1) show that in spite of already having a high phosphate content this amorphous material can indeed fix more phosphate than the crystalline samples. This agrees with findings from other parts of Galicia, where phosphate uptake per 100 g of crystalline material ranges from 1000 to 1400 mg, whereas for non-crystalline material the figures lie between 3000 and 4500 mg, more phosphorus being adsorbed the lower the initial P/AI ratio (GARCIA-RODEJA 1981). All these results seem to indicate that the most likely hypothesis as regards the formation of the material is that in an initial crystalline or non-crystalline alumino-silicate capable of reacting with phosphate groups, silicon has been progressively replaced by phosphorus. Recent papers by other authors mention similar mechanisms. VEITH & SPOSITO (1977) for example, have shown that aluminium phosphate of formula Ai(OH)2H2 PO4 forms and silica is released when non-crystalline alumino-silicates react with phosphates at neutral to acid pH. However, another possibility is suggested by the work of ALVAREZ et al. (1980), who show that gibbsite may fix phosphate. To sum up, though their mechanism of formation is imperfectly known, there can be little doubt that very diverse combinations of Al, Si and P do occur. They originate as weathering products of granitic rocks in leaching environments, and evolve into mixtures of aluminium phosphate, gibbsite, allophanes of varying SiO2/Al203 ratio and 1:1 phyllosilicates of low crystaUinity. The direction of this evolution depends upon the relative proportions of the elements present and the specific conditions of each microsystem. REFERENCES
ALVAREZ, R. et al. (1980): Silicate and phosphate adsortion on gibbsitc studied by X-ray photoelectron spectroscopyangular distributions. Soil Sci. Soc. Am. J. 44(2), 422-425.
236
CALVO,GARCIA-RODEJA& MACIAS
BRACEWELL, J.M. et al. (1970): An assessment of some thermal and chemical techniques used in the study of the poorly ordered aluminosilicates in soils. Clay Min. 8, 325-337. CALVO, 1LM. et al. (1981): Alteraci6n del material granitico del Monte Meda (Galicia): datos preliminares. Cuad. Lab. Xeol. Laxe, 2, 245-251. CHESWORTH, W. (1977): Weathering stages of the common igneous rocks index minerals and mineral assemblages at the surface of the earth. J. Soil Sci., 28, 490-497. CHESWORTH, W. (1980): The haplosoil system. Am. J. Sci. 280, 969-985. ESWARAN, H. et al. (1977): The micromorphology ofgibbsite forms in soils. J. Soil Sci. 28, 136-143. FIELDES, M. & PERROT, K.W. (1966): The nature of allophane in soils. III. Rapid field and laboratory test for allophane. N.Z.J. Soil Sci. 9, 623-629. GARCIA-RODEJA, E. (1981): Thesis. In preparation. Santiago. GREENE-KELLY, 1L (1955): Dehydradation of the montmorillonite minerals. Mineral Mag. 30, 604-615. GUITIAN RIVERA, F. & CALVO, ILM. (1981): Evoluci6n de los minerales primarios de las rocas grardticas de Galicia. Aplicaci6n de las T6cnicas de microscopia electronica (SEM) y an~.lisis de microsonda. An. Edaf. Agrobiol. GUITIAN OJEA, F. & CARBALLAS, T. (1976): T~cnicas de anhlisis de suelos. Pico Sacro. Santiago de Compostela. HASHIMOTO, I. & JACKSON, M.L. (1958): Rapid dissolution ofallophane and kaolinite-halloysite after dehydratation. Clays Clay Min. 7, 102-113. HEILMAN, M.D. et al. (1965): The EGME technique for determining soil surface area. Soil Sci. 100, 409-413. HEN1N, S. et al. (1968): Considerations sur les notions de stabilit~ et d'instabilit6 des mineraux en fonction des conditions du milieu: essai de classification des syst~mes d'agresion. Trans. 9th Int. Cong. Soil Sci. Vol. III, 79-90. JONES, R.L. et al. (1963): Microfossils in Wisconsin loess and till from western Illinois and eastern Iowa. Science 140, 1222-1224. JI3RGENSEN, S.S. et al. (1970): Assessment ofgibbsitic materials in soil clays by differential thermal analysis and alkali dissolution methods. J. Thermal Analysis 2, 467-477. KAWAI, IC (1980): The relationship of phosphorous adsorption to amorphous aluminum for characterizing Andosols. Soil Sci. 119, 81-88. KITrRICK, J.A. (1969): Soil minerals in the AI203-SiO2-H20 system and a theory of their formation. Clays Clay Min. 17, 157-167. KUZNETSOV, S.I. (1975): The role of microorganisms in the formation of lake bottom deposits and their diagenesis. Soil Sci. 119, 81-88. MCKEAGUE, D. (1966): Dithionite and oxalate extractable Fe and AI as aids in differentiating various classes of soils. Can. J. Soil Sci. 46, 13-22. MEUNIER, A. & VELDE, B. (1976): Mineral reactions at grain contacts in early etapes of granitic weathering. Clay Min. 11,235-240. MILLOT, G. (1964): Geologie des argiles. Masson. Paris. NALOVIC, L. & PEDRO, G. (1978): New concepts on the modalities of rock alteration and the evolution of soils. (The role of concentrated microsystems and the importance of fluctuations of the medium), llth Cong. ISSS. Canada. PEDRO, G. (1968): Distribution des principaux types d'alterations chimiques h la surface du globe. Presentation d'une esquisse geographique. Rev. Geog. Phys. Geol. Dinam. 10, 457-470. PEDRO, G. et al. (1975): Sur la necessit6 et l'importance d'une distinction foundamentale entre type et degree d'alteration. Application au probl~me de la definition de la ferrallitisation. C.R. Acad. Sci. Paris, 280, ser D, 825-828. SAAVEDRA, J. & SANCHEZ CAMAZANO, M. (1981): Origen de niveles continentales silicificados con alunita en el preluteciense de Salamanca, Espafia. Clay Min. 16, (in press). TARDY, Y. (1969): Geochimie des alterations Etude des ar~nes et des eaux de quelques massis cristallins d'Europe et d'Asie. Mem. Serv. Geol. Als-Lor., Strasbourg, N 31,199 p. VEITH, J.A. & SPOSITO, G. (1977): Reactions of aluminosilicates, aluminium hydrous oxides and aluminium oxide with o-phosphate, the formation of X-ray analogue of variscite and Montebrasite. Soil Sci. Soc. Am. J. 41,870-876. WEAVER, C.F. & POLLARD, L.D. (1975): The chemistry of clay minerals. Developments in Sedirnentology 15. Elsevier. Amsterdam. Address of authors: R.M. Calvo, E. Garcia-Rodeja, F. Macias, Edafologla y Geologla. Fac. Biologia. Univ. Santiago de Compostela. Espafia.
Vol. 10, 225-236
Braunschweig 1983
MINERALOGICAL VARIABILITY IN WEATHERING MICROSYSTEMS OF A GRANITIC OUTCROP OF GALICIA (SPAIN)
R.M. Calvo, E. Garcia-Rodeja & F. Macias, Santiago de Compostela SUMMARY Several products of weathering have been studied in a granitic outcrop in Galicia (N.W. Spain). Alteration took place under a humid temperate climate. The various materials and the microsystems in which they occur are: (1) fissures containing gibbsite and/or 1:1 sheet silicates of varying degrees of crystallinity,(2) fissures containing 2:1 sheet silicates ofbeidellite type, (3) encrustations of siliceous material showing varying degrees of structural order, with or without organic matter, and (4) encrustations of non-crystalline matter composed chiefly ofAl, P and Si. The results demonstrate the importance of considering the different microsystems which contribute to the formation of secondary products in a given weathering system. 1..INTRODUCTION In recent years the study of mineralogical transformations taking place during weathering has made significant progress due fundamentallyto the introduction of thermodynamic and kinetic concepts and the attention paid to the heterogeneous nature of weathering systems. The earlier global, zonal approach to weathering processes has thus given way to a series of refinements in which the need to know the specific conditions of microsystems has become ever more apparent. MILLOT (1964), PEDRO (1978), TARDY (1969), HENIN et al. (1968), PEDRO et al. (1975), CHESWORTH (1977), MEUNIER & VELDE (1976), NALOVIC & PEDRO (1978), ESWARAN (1979) ... This paper seeks to characterize the great variety of weathering products found in a granitic area of Galicia (NW Spain) and to relate them to the specific situations in which each of them occurs. The general tendency of the weathering undergone has been described in earlier papers reporting for a single granitic outcrop the chemical, mineralogical and structural analysis of various profiles which illustrate different stages of the process: more or less weathered rock, saprolites, granitic sands and soils developed over these materials (CALVO et al. 1981). 2. MATERIALS AND METHODS The granitic outcrop studied here lies to west of the town of Padr6n (La Corufia). It forms part of a series of shelves carved out during the Eocene and bounded to the east by the great fault which crosses Galicia from north to south. The parent rock is a leucogranite composed of microcline, albitic plagioclase, quartz (muscovite and biotite). The present humid mesothermic climate yields an irregularly distri-
226
CALVO, GARCIA-RODEJA, & MACIAS
buted annual rainfall of over 1700 mm and a mean temperature of 11°C, with about 10°C between the summer and winter average. Right from the first stages of weathering, gibbsite and a kaolinite phyllosilicate form, chiefly at the expense ofplagioclase. The quantity of clay formed ranges from 10 to 20% of the < 2 mm fraction of the material that has already lost its original structure. Variation with respect to depth within weathering profiles is slight, though in the finest fractions of the organicrich surface horizons a reduction in gibbsite content is accompanied by the increased presence of weathering products of mica trending towards aluminous vermiculites (CALVO et al. 1981). The material examined in the present study occurs in a wide variety oflocalised weathering environments including: fissures filled with fine matter, coatings upon the surface of other consolidated or unconsolidated material and precipitates in small cavities. The nature of the material has been determined by X-ray diffraction using samples untreated except for saturation with K or Mg and samples pretreated with ethyleneglycol or heat; by DTA; by IR spectroscopy; by optical microscopy. Standard techniques of chemical analysis were used: extraction with ammonium oxalate at pH 3 (McKEAGUE & DAY 1966); with hot and cold 50/0Na2CO3 (JORGENSEN et al. 1970); and with boiling 0,5 M NaOH (HASHIMOTO & JACKSON 1958). The analysis of both the extracts and of dissolved whole samples was carried out by atomic absorption spectrophotometry. In some cases the P and Si contents were also determined by colorimetry after complexation with molibdenum blue (GUITIAN & CARBALLAS 1975). Specific surfaces were measured by adsortion of EGME (HEILMAN et al. 1965), phosphate fixation by Kawai's method (KAWAI 1981), and reactivity to NaF following FIELDES & PERROT (1966) and BRACEWELL et al. (1970).
3. 3.1.
RESULTS FISSURE SYSTEMS WITH GIBBSITE AND/OR 1:1 PHYLLOSILICATES
One of the most common features of these weathering profiles is the presence of fissures of a width not usually exceeding 0,5 cm filled with fine material. They run vertically or subvertically through the entire observable depth of the profiles, sometimes displaying numerous ramifications (Photo 1). The filling is most often a mixture of gibbsite and 1:1 phyllosilicates in very varied proportion (Fie,. la) ranging from an almost complete absence of phyllosilicates to their clear predominance (Fig. lb). Small quantities of quartz, mica in varying stages of degradation, and non-crystalline material rich in aluminium and silicon, are found together with the gibbsite and phyllosilicates. The non-crystalline material is identified by its higher reactivity in chemical tests (Table 1), by the presence in DTA traces of small endothermic effects between 350 and 400"C and by the 2 to 10% loss of weight suffered between 100 and 200"C. In general it is the samples richest in gibbsite that also contain the greatest quantity of this amorphous material. Characteristically the gibbsite too presents a formless, non-idiomorphic, both in fissures and in saprolites, though in the latter tiny well-crystallized specimens have also been commonly observed (GUITIAN et al. 1981). The asymmetry index derived from DTA diagrams for 1:1 phyllosilicates ranges from 1.3 to 3.0. This clearly reflects a variability in the 1:1 minerals similar to that in gibbsite. The origin of these fissure-filling materials is obscure. In some cases they would seem to be associated with in situ weathering of vein minerals previously affected by tectonic pro-
WEATHERING MICROSYSTEMSIN GRANITE
227
Tab. 1: ANALYTICAL DATA FROM SAMPLES OF GIBBSITE/I:I PHYLLOSILICATE FISSURE MATERIAL (A11 and A3) AND OF NON-CRYSTALLINE COATINGS (A4 and A7).
-
~
1:1 phyllosilicates gibbsite
~ ~
micas quartz
.~
AI203 t %
~
Samples with crystalline material A-11 A-3 60% 10% 30% 50%
Samples rich in non-crystalline material " A--4 A-7 Dominant minerals 5% 3% % by DTA
x x
x x
x x
x x
21,3 46,0 -
22,5 44,0 -
13,0 28,2 17,2
13,0 30,3 15,0
SiO2 t/Al203 t A1203 t/P205 t
0,8 -
0,9 -
0,8 2,3
0,7 2,8
. -~8 oo ~
A1203 F (4) % SiO2 F (4) % A1203 C (3) % SiO2 C (3) %
n.d. n.d. n.d. n.d.
2,3 1,7 23,8 6,0
25,3 6,8 1,9 6,0
27,0 6,0 1,9 3,9
~ o= G"
SIO2/A1203F SIO2/A1203C
n.d. n.d.
1,3 0,4
0,5 5,4
0,4 3,5
AI203 F/A1203 t n.d. ~ AI203F-{--C/A1203t n.d. X SiO2 F/SiO2 t n.d. SiO2 C + F/SiO2 t n.d.
5,0 59,3 7,5 34,2
89,7 96,4 52,3 98,4
89,1 95,4 46,1 76,2
SiO:t % .~ .~
P2Os t %
~
.~ 0 ~
AI203 O %
Impurities x present
Molar ratios in total analysis
Molar ratios in extracts % extracted with respect total
SiO2 o %
1,5 1,6
1,9 0,8
24,8 6,0
22,4 4,5
SiO2 o/A1203 o
1,8
0,7
0,4
0,3
Molar ratios in extracts
-~ ~ ~ ~ A1203 o/A1203 t ~a ~ ~ × SiO2 o/SiO2 t
3,3 7,5
4,3 3,5
88,6 46,1
73,9 34,7
% extracted with respect total
SiO2 e %
33,1 4,3
31,2 5,4
27,0 6,8
28,4 6,8
~ SIO2/A1203 e
0,2
0,3
0,4
0,4
Molar ratios in extracts
o Z ~ A1203 e/A1203 t × SiO2 e/SiO2 t
72,0 20,2
70,9 24,0
95,7 52,3
93,7 52,3
% extracted with respect total
~_u.
pH FNa 2 min pH FNa 30 min
9,4 9,7
9,2 9,5
10,1 11,4
10,8 11,6
FIELDES & PERROT (1966) lg : 50 ml satured NaF
Z o
meq OH-/100 gr meq OH-/m 2
~. "~ .*~-- O 3: .E= _
.~ '~: Z
AI203 e %
2 ~~ .~,
o ~. o
o ~ ~
89 136 1,02×10 -2 1,46)
603 293 meq. OH- after 25 mins 1,60X10 -2 0,8X10 -2 BRACEWELLetal. 1970
Specific surf. m2/g 87
93
376
353
EGME (HEILMAN et al. 1965)
pH 7 mgrs P/100gr 1050 pH 4,5 1155
1425 1480
3375 3630
3000 3630
KAWAI (1980) adding 6.45 mm P/g of material
228
CALVO, GARCIA-RODEJA & MACIAS
985
D.TA
m
300
566
X. RD a~
u~
A
a)
b) Fig. 1: XRD and DTA of fissure materials; a) gibbsite dominant; b) phyllosilicate 1:1 dominant.
cesses (in a number of profiles fissures occur that have been fractured and displaced several centimetres). In other cases they seem to be inffllings derived from more highly weathered areas, and transported into cracks opened in the rock during weathering. We are thus tentatively led to the conclusion that the majority of the secondary material has been formed by surface weathering of unstable minerals (plagioclases, felspars, etc.), with tectonic processes and associated hydrothermal weathering effects in some cases opening the way. 3.2.
FISSURES WITH 2:1 PHYLLOSILICATES
Only one of the profiles studied is of this type. It consists of an approximately 50 cm wide band of 1 to 2 cm veins whose soft shiny surfaces are reminiscent of fault planes (Photo 2). The results of XRD and DTA diagrams (Fig. 2) to indicate the presence ofa 2:1 phyllosilicate, possibly a low charge vermiculite or a high charge smectite, very difficult to tell apart by standard techniques. However, the presence of a 060 reflection below 1.53/~ and the fact that ethyleneglycol caused re-expansion of a sample treated with Li and heated to 250°C
WEATHERING
MICROSYSTEMS
IN GRANITE
229
Photo 1: Fissures in a weathering profile.
""
'
"
I
*
'o5
t
.
..
i
,.
". ~
,
i~
.. '~
5
:,
.:~','" ",
~
'i;
,..',
.
-4.
-
[;,
~¢
,,..<:. /
,~
/
.,,,.
Photo 2: Weathering figure with smectite material.
• ..
.7:
,,;
Ill
230
CALVO,GARCIA-RODEJA&MACIAS
931"
521"
oc ~
(f
9i9 2 / ( e
XR.D
b~
~ (d ~7,3
~
(060)
(b
(a
Fig. 2: DTA and XRD diagrams of smectitic material, a) Mg saturated; b) (060) diffraction; c) treatment with ethyleneglycol; d) heated at 500°C; e) K saturated; f) Li saturated, heated at 250°C and treated with ethyleneglycol (GREENE-KELLY test).
(GREENE-KELLY 1955) support the latter alternative, dioctahedral mineral of beidellite type. In some samples small quantities of gibbsite have also been identified by DTA. The coexistence of beidellite with gibbsite is nevertheless thermodynamically puzzling (KITrRICK 1969), only plausible at all in surface media and under uncommon conditions (CHESWORTH 1980). In the present case we regard the most likely explanation to be that the two minerals formed under different conditions and at different times. This part of Galicia contains many hydrothermal in origin and unstable in the present weathering system. The accompanying gibbsite developed more or less as in the gibbsite/1:1 phyllosilicates fissures. Another possibility that cannot be ruled out is that we are dealing here with a mixed system in which gibbsite is an intruder originating in the principal mineral of the fine fraction.
3.3.
SILICEOUS COATINGS
In certain situations, particularly on the upper and lower surface of the large, siliceous material is found to form a hard coating several millimetres thick from which black knobby protuberances project out like microstalagmites for several millimetres, giving a botryoidal
WEATHERINGMICROSYSTEMSIN GRANITE
231
438
1~5
°C
Fig. 3: DTA of siliceous coatings with different range of order. appearance (Photo 3). DTA diagrams (Fig. 3a) show a low temperature endotherm which at higuer temperatures merges into a pronounced exotherm peak between 400 and 500°C. This is attributable to the oxidation of oganic matter trapped in the silica and responsible for the original dark colouring. The behaviour of the coatings under X-ray is not like that of typical non-crystalline material but, follows the pattern of other opaline materials described in the literature (JONES et al. 1963) with a diffuse band between 8.8 and 10/~ along with effects characteristic of polymorphs of crystalline silica, specially cristobalite (4.04 and 3.18 ~) and quartz (3.32 and 4.26 ~). These coatings seem to have originated as precipitates from solutions locally supersaturated with silica due to the rapid evaporation undergone by water running over the surface of the granite blocks. The biological activity may be supposed to play an important role in the formation of these features. KUZNETSOV (1975) and SAAVEDRA et al. (1981) have suggested that in such cases silica is precipitated when evaporation takes simultaneously with the bacterial decomposition of organo-siliceous complexes. Somewhat similar material to that described above is also found in small cavities in which water accumulates for short periods. In general organic matter is now absent and the colouring whitish. Although morphologically similar to the surface coatings (Photo 4), the degree of crystaliinity is higher and in DTA diagrams the inversion endotherm characteristic of quartz is recognisable (Fig. 3).
232
CALVO, GARCIA-RODEJA& MACIAS
51~.m.~mI
Photo 3: Siliceous coating forming microstalactites
Photo 4: Scanning electron micrograph of white precipitates of siliceous material without organic matter (Photo: F. GUITIAN).
Photo 5: Scanning electron micrograph of formations of biogenic silica (Photo: F. GUITIAN)
WEATHERING MICROSYSTEMS IN GRANITE
Photo 6: Non-cristalline coatings rich in phosphorus. General aspect.
~'~"~W"
.
I
Photo 7: Non-cristalline coatings rich in phosphorus. Detail in transversal cut.
Photo 8: Thin section of non-cristalline coatings rich in phosphorus.
"., ~ a , ~
.I ....................... . I . . 4 .
.......................... . _
. . . . . . . . .
:~" . . . .
:,'
233
'
,.I ....
: ~ . . : ~ . , ~ -
.~.~._~i
~ :~i::
I~
"
~: . . . "
~..J .......
( i
,"~1
,
!
:::::::::::::::::::::::::::::: , i~ ~ii.~:.::., !.;IE.7..:.~&._~!,!!
,
234
CALVO, GARCIA-RODEJA & MACIAS
1052 D.TA
"C~0. 32O
X.R.D
i~
~ Fig. 4: XRD and DTA plots of noncrystalline coatings; a) without other crystallinecomponents; b) with gibbsite.
Finally we may mention the presence of small siliceous deposits which under the electron microscope appear like a tissue of living cells, so that a biological origin is to be presumed (Photo 5). 3.4.
NON-CRYSTALLINE COATINGS RICH IN PHOSPHORUS
On the walls of fissures near the surface, it is common to find whitish coUophorm-like encrustations with conchoid fracture, composed principally of non-crystalline combinations of Si, A1 and P (Photos 6 and 7). The thickness of these coatings is variable, though usually less than 1 cm. The coatings are commoner and thicker beneath the subhorizontal surfaces of rocks washed by run-off from the higher parts of the massif. Under the petrographic microscope an isotropic matrix of low refractive index is seen to contain two general types of differentiated microstructure. The first is massive, irregularly shaped and bordered by a brownish fringe composed of small accumulations of Fe, and the second is typical of desicated geles (Photo 8). Under the SEM, a fractured skin with noncrystalline appearance is observed. X-ray spectra confirm the low degree of crystallinity of this material. Two bands are distinguishable, a weak one between 7 and 11 /~, with a maximum at 10 A, and another, even flatter, between 2.8 and 4.0 A. The second does not show up in all eases. Features corresponding to impurities ofgibbsite and forms of silica are
WEATHERINGMICROSYSTEMSIN GRANITE
235
occasionally observed (Fig. 4). The DTA diagrams have a pronounced endotherm with a maximum at 169°C and an equally intense exotherm at 1052°C, together with the 300°C endotherm corresponding to gibbsite in the samples in which it is present (Fig. 4). The composition of two samples and their behaviour under various chemical tests is shown in table 1 together with similar data for two samples from type 1 (gibbsite/l:l phyllosilicate) fissures. The high P205 content falls between the values quoted for allophanes (which rarely approach 100/o, WEAVER & POLLARD 1975), and pure aluminium phosphates. But the possibility of these figures reflecting a mixture ofaluminium phosphate with siliceous material is ruled out by the molar A1/P ratio, which instead of being approximately 1 is greater than 2. Thus the aluminium present must be bound to both silicon and phosphorous. Further information concerning these materials is provided by their high reactivity in NaF test. It should be pointed out, however, that the high reactivity with NaF is to be regarded less as a chemical effect than as the result of the high specific surface area (above 300 m2/g in the crystalline samples). When the reactivity data for NaF are expressed in meq. OH-/m 2 the crystalline and non-crystalline samples are indistinguishable from one another. Solubility tests likewise reflect the difference between the two types of deposit. The crystalline (gibbsite-bearing) samples are barely soluble in oxalate or cold sodium carbonate, but highly soluble in hot carbonate or boiling NaOH. On the other hand about 90% of the'A1 of the easily soluble noncrystalline coatings is extracted by cold carbonate or oxalate, with molar SIO2/A1203 ratios below unity for the solutes. To find out whether this non-crystalline material was still capable of fixing phosphate, it was"subjected to KAWAI's test (1980), saturation being effected at pH 4.5 and pH 7.0. The results (Table 1) show that in spite of already having a high phosphate content this amorphous material can indeed fix more phosphate than the crystalline samples. This agrees with findings from other parts of Galicia, where phosphate uptake per 100 g of crystalline material ranges from 1000 to 1400 mg, whereas for non-crystalline material the figures lie between 3000 and 4500 mg, more phosphorus being adsorbed the lower the initial P/AI ratio (GARCIA-RODEJA 1981). All these results seem to indicate that the most likely hypothesis as regards the formation of the material is that in an initial crystalline or non-crystalline alumino-silicate capable of reacting with phosphate groups, silicon has been progressively replaced by phosphorus. Recent papers by other authors mention similar mechanisms. VEITH & SPOSITO (1977) for example, have shown that aluminium phosphate of formula Ai(OH)2H2 PO4 forms and silica is released when non-crystalline alumino-silicates react with phosphates at neutral to acid pH. However, another possibility is suggested by the work of ALVAREZ et al. (1980), who show that gibbsite may fix phosphate. To sum up, though their mechanism of formation is imperfectly known, there can be little doubt that very diverse combinations of Al, Si and P do occur. They originate as weathering products of granitic rocks in leaching environments, and evolve into mixtures of aluminium phosphate, gibbsite, allophanes of varying SiO2/Al203 ratio and 1:1 phyllosilicates of low crystaUinity. The direction of this evolution depends upon the relative proportions of the elements present and the specific conditions of each microsystem. REFERENCES
ALVAREZ, R. et al. (1980): Silicate and phosphate adsortion on gibbsitc studied by X-ray photoelectron spectroscopyangular distributions. Soil Sci. Soc. Am. J. 44(2), 422-425.
236
CALVO,GARCIA-RODEJA& MACIAS
BRACEWELL, J.M. et al. (1970): An assessment of some thermal and chemical techniques used in the study of the poorly ordered aluminosilicates in soils. Clay Min. 8, 325-337. CALVO, 1LM. et al. (1981): Alteraci6n del material granitico del Monte Meda (Galicia): datos preliminares. Cuad. Lab. Xeol. Laxe, 2, 245-251. CHESWORTH, W. (1977): Weathering stages of the common igneous rocks index minerals and mineral assemblages at the surface of the earth. J. Soil Sci., 28, 490-497. CHESWORTH, W. (1980): The haplosoil system. Am. J. Sci. 280, 969-985. ESWARAN, H. et al. (1977): The micromorphology ofgibbsite forms in soils. J. Soil Sci. 28, 136-143. FIELDES, M. & PERROT, K.W. (1966): The nature of allophane in soils. III. Rapid field and laboratory test for allophane. N.Z.J. Soil Sci. 9, 623-629. GARCIA-RODEJA, E. (1981): Thesis. In preparation. Santiago. GREENE-KELLY, 1L (1955): Dehydradation of the montmorillonite minerals. Mineral Mag. 30, 604-615. GUITIAN RIVERA, F. & CALVO, ILM. (1981): Evoluci6n de los minerales primarios de las rocas grardticas de Galicia. Aplicaci6n de las T6cnicas de microscopia electronica (SEM) y an~.lisis de microsonda. An. Edaf. Agrobiol. GUITIAN OJEA, F. & CARBALLAS, T. (1976): T~cnicas de anhlisis de suelos. Pico Sacro. Santiago de Compostela. HASHIMOTO, I. & JACKSON, M.L. (1958): Rapid dissolution ofallophane and kaolinite-halloysite after dehydratation. Clays Clay Min. 7, 102-113. HEILMAN, M.D. et al. (1965): The EGME technique for determining soil surface area. Soil Sci. 100, 409-413. HEN1N, S. et al. (1968): Considerations sur les notions de stabilit~ et d'instabilit6 des mineraux en fonction des conditions du milieu: essai de classification des syst~mes d'agresion. Trans. 9th Int. Cong. Soil Sci. Vol. III, 79-90. JONES, R.L. et al. (1963): Microfossils in Wisconsin loess and till from western Illinois and eastern Iowa. Science 140, 1222-1224. JI3RGENSEN, S.S. et al. (1970): Assessment ofgibbsitic materials in soil clays by differential thermal analysis and alkali dissolution methods. J. Thermal Analysis 2, 467-477. KAWAI, IC (1980): The relationship of phosphorous adsorption to amorphous aluminum for characterizing Andosols. Soil Sci. 119, 81-88. KITrRICK, J.A. (1969): Soil minerals in the AI203-SiO2-H20 system and a theory of their formation. Clays Clay Min. 17, 157-167. KUZNETSOV, S.I. (1975): The role of microorganisms in the formation of lake bottom deposits and their diagenesis. Soil Sci. 119, 81-88. MCKEAGUE, D. (1966): Dithionite and oxalate extractable Fe and AI as aids in differentiating various classes of soils. Can. J. Soil Sci. 46, 13-22. MEUNIER, A. & VELDE, B. (1976): Mineral reactions at grain contacts in early etapes of granitic weathering. Clay Min. 11,235-240. MILLOT, G. (1964): Geologie des argiles. Masson. Paris. NALOVIC, L. & PEDRO, G. (1978): New concepts on the modalities of rock alteration and the evolution of soils. (The role of concentrated microsystems and the importance of fluctuations of the medium), llth Cong. ISSS. Canada. PEDRO, G. (1968): Distribution des principaux types d'alterations chimiques h la surface du globe. Presentation d'une esquisse geographique. Rev. Geog. Phys. Geol. Dinam. 10, 457-470. PEDRO, G. et al. (1975): Sur la necessit6 et l'importance d'une distinction foundamentale entre type et degree d'alteration. Application au probl~me de la definition de la ferrallitisation. C.R. Acad. Sci. Paris, 280, ser D, 825-828. SAAVEDRA, J. & SANCHEZ CAMAZANO, M. (1981): Origen de niveles continentales silicificados con alunita en el preluteciense de Salamanca, Espafia. Clay Min. 16, (in press). TARDY, Y. (1969): Geochimie des alterations Etude des ar~nes et des eaux de quelques massis cristallins d'Europe et d'Asie. Mem. Serv. Geol. Als-Lor., Strasbourg, N 31,199 p. VEITH, J.A. & SPOSITO, G. (1977): Reactions of aluminosilicates, aluminium hydrous oxides and aluminium oxide with o-phosphate, the formation of X-ray analogue of variscite and Montebrasite. Soil Sci. Soc. Am. J. 41,870-876. WEAVER, C.F. & POLLARD, L.D. (1975): The chemistry of clay minerals. Developments in Sedirnentology 15. Elsevier. Amsterdam. Address of authors: R.M. Calvo, E. Garcia-Rodeja, F. Macias, Edafologla y Geologla. Fac. Biologia. Univ. Santiago de Compostela. Espafia.