GEOLOGICA CARPATHICA, 51, 1, BRATISLAVA, FEBRUARY 2000
DISSOLUTION OF K-FELDSPAR INTO Si-Al GEL
AND CRYSTALLIZATION OF HALLOYSITE IDENTIFIED
IN THE KAOLIN DEPOSIT OF SÃO VICENTE DE PEREIRA (PORTUGAL)
IULIU BOBOS and CELSO GOMES
Department of Geosciences, University of Aveiro, Aveiro-3810, Portugal
(Manuscript received January 18, 1999; accepted in revised form September 28, 1999)
Abstract: Dissolution of K-feldspar into Si-Al non-crystalline phase was identified in the kaolin deposit of São
Vicente de Pereira (Portugal). The gel having a SiO
ratio approximately equal to the unity, crystallized into
halloysite-0.7 nm. SEM investigations carried out on K-feldspar grains show a vacuolar texture characterized by etch-
pits of various dimensions. The pits exhibit various shapes, either polygonal or circular. The gel was identified by
SEM on the K-feldspar surfaces and by TEM in the < 2 µm clay fractions. TEM images display tubes of halloysite-0.7
nm surrounding flakes of Si-Al gel. Relicts of K-feldspar pseudomorphs submitted to amorphization are identified by
TEM. SAED analyses confirmed the non-crystalline state of the gel. Electron microprobe analyses attested a very fast
destabilization of the chemical structure of K-feldspar grains.
Key words: Portugal, kaolin deposit, scanning and transmission electron microscopy, X-ray diffraction, K-feldspar
dissolution, Si-Al gel, halloysite-0.7 nm.
Alteration of feldspars into kaolinite is due to weathering or
hydrothermal processes (e.g. Keller 1978; Wilke et al. 1978;
Meunier & Velde 1979; Anand et al. 1985, among others).
The nature of feldspar alteration and formation of secondary
minerals is strictly related to geochemical environments.
Several cases of feldspar weathering have been well docu-
mented by high resolution-transmission electron microscopy
investigations (Eggleton & Buseck 1980; Banfield & Eggle-
ton 1990; Robertson & Eggleton 1991). These contributions
concluded that feldspar alteration produce secondary phases
either illite or dehydrated smectite via non-crystalline inter-
mediate phases and then, kaolinite or halloysite is formed.
Feldspar alteration can produce kaolinite directly from solu-
tion and halloysite via a non-crystalline phase (Eswaran &
Bin 1978), whereas epitactic/topotactic alteration of K-feld-
spar into secondary minerals is not possible (Gilkes et al.
1986). Romero et al. (1992) have shown the transformation
of feldspar into proto-crystalline material similar to allo-
phane either in composition or morphology. Therefore, it is
assumed that the feldspar alteration involves two distinctive
processes: i) dissolution of feldspars into solution and ii)
subsequent precipitation from solution of kaolinite or other
Non-crystalline Fe-Si-Al- oxy-hydroxides and amorphous
aluminium hydroxide have been identified as being formed
from feldspars at the earliest weathering stage (Eggleton
1987). Other papers reported that Fe and Si rich high defect
fibrous materials are transformed into halloysite (Tazaki &
Fyfe 1987). Sequences of gibbsite are always associated
with weathering of feldspar. However, other authors (Berner
& Holdren 1979) did not find surficial layers of secondary
material on the weathered feldspars surfaces.
Feldspar dissolution kinetics is a subject thoroughly stud-
ied (e.g. Murphy & Helgeson 1987; Drever 1994; Blum &
Lasaga 1991, etc.) and the majority of the publications sug-
gest that feldspar dissolution rates are pH independent in
the range ~ 5 to 8.
It is well known from literature that hydrated halloysite
(i.e. halloysite-1 nm), is formed from glass rich volcanic ma-
terials and displays spherulitical or spheroidal morpholo-
gy (Kirkman 1975, 1977, 1981; Gomes & Massa 1991) and
halloysite-0.7 nm showing tubular morphology is formed
from feldspars or micas (Estoule-Choux & Blanchet 1987).
Synthesis experiments accomplished by Parham (1969) in
the laboratory supported the idea that halloysite-0.7 nm rep-
resented an early stage of structural organization in the pro-
cess of kaolin formation.
In the present paper K-feldspar grains collected in the ka-
olin deposit of São Vicente de Pereira (Portugal), were
chemically and morphologically studied by electron micros-
copy techniques, in order to identify a relationship between
K-feldspar and halloysite-0.7 nm, the ultimate product of al-
The kaolin deposit of São Vicente de Pereira is situated at
the north-western border of the Ossa Morena zone (Fig. 1)
and it is widely extended along an alignment NNW-SSE,
parallel to the fault which separates the Lourosa Unit from
the Arada Unit. The estimated kaolin deposit extension is
50 BOBOS and GOMES
approximately 11 km
. The Lourosa Unit, is characterized by
HP metamorphism (sillimanite, garnet, K-feldspar) and in-
cludes several injection dykes of a two mica granitoids. The
kaolin deposit of São Vicente de Pereira is related solely to
migmatite rocks exhibiting argillic alteration. The textural
integrity of migmatite rocks is well preserved in several plac-
es of São Vicente de Pereira kaolin deposit, being recognized
several facies, such as: equigranular, fine-grained, granophyric
and pegmatite. Hydrothermal activity in the kaolin deposit of
São Vicente de Pereira and to the north of the deposit, is asso-
ciated with a deformational system which includes faults, frac-
tures and dilatation zone ductile shear zone (Bobos &
Gomes 1998). Hydrothermal fluids were genetically and spa-
tially related to fluids of metamorphic origin and felsic intru-
sive rocks (Bobos & Gomes, unpublished data).
Two types of alteration stages were described: hydrothermal
and supergene (Bobos & Gomes 1998). Hydrothermal alter-
ation is characterized by a relict greisen type alteration, repre-
sented by quartz + muscovite bearing F, Cl and quartz + tour-
maline assemblages, and by an argillic alteration represented
by well ordered kaolinite. Silicification and tourmalinization
are two late hydrothermal processes identified in the São Vice-
nte de Pereira area, as well as in the north part of this area. The
quartz ± tourmaline bearing assemblage occurs as veins of 10
to 15 cm wide and veinlets of disseminated tourmaline in the
Weathering is expressed by poorly ordered kaolinite ± illite
assemblage, as well as by kaolinite into halloysite-0.7 nm
transformation (Tari et al. 1999; Bobos & Gomes, unpublished
data). Two generations of halloysite-0.7 nm were distin-
guished by TEM: one formed from kaolinite and the other
from K-feldspar; the last being the aim of the present paper.
In the kaolin deposit of São Vicente de Pereira, fenocrys-
tals of K-feldspar are completely altered into halloysite-0.7
nm, sometimes associated with poorly ordered kaolinite.
Small K-feldspar grains silicified and undigested by hydro-
thermal solutions, were found inside of the deposit; they
were submitted to morphological investigations. Usually,
they occur in a relict pegmatite facies. For comparison appar-
ently fresh K-feldspar grains occurring in the unaltered mig-
matite complex, were morphologically investigated as well.
Materials and methods
Separation of the < 2 µm fractions in the alteration prod-
ucts of K-feldspar grains and kaolin rock were carried out
by the sedimentation method using Stokes law. Dispersion
of clay minerals has been achieved without any chemical
treatment, except the use of ammonia buffer in the suspen-
sion (three drops of ammonia for one litre of distilled wa-
ter). All the separated clay fractions were washed up and
concentrated by centrifugation. The < 2 µm clay fractions
were air-dried at 40 °C overnight to avoid a possible conver-
sion of halloysite-1 nm into halloysite-0.7 nm. The amor-
phous phase was extracted from K-feldspar grains collected
inside the kaolin deposit, using Jacksons (1974) technique
(chapter II, pp. 72) and then, XRD analysed.
Samples of K-feldspars, as well as specimens containing
the < 2 µm clay fractions of their alteration products, pre-
pared as non-oriented mounts were examined by X-ray dif-
fraction (XRD) with a Rigaku Geigerflex D/max. C se-
ries machine, using CuK
radiation and a scanning speed of
, per min.
The sample of halloysite-0.7 nm have been previously de-
scribed from the structural and morphological points of view
(Bobos et al. 1996), being investigated again in order to
check a relationship between gel and halloysite-0.7 nm.
Morphological investigation was carried out using a scan-
ning electron microscope (Jeol JSM-35C), equipped with
EDAX facilities and operated at an accelerating voltage of
15 kV. A few partially altered K-feldspar grains were
crushed into several small grains and then cleaned ultrasoni-
cally. Samples of these remaining grains were mounted on a
carbon holder and then, gold coated.
Microscopic observation was carried out with a Hitachi H-
9000 transmission electron microscope (TEM), working at
Fig. 1. Geological sketch of the northwestern sector of Ossa Morena
zone (after Chaminé et al. 1995). 1: São João de Ver Unit (metavol-
canite; metaporphyry; micaceous schist; metagraywacke); 2: Arada
Unit (green schist; amphibolite schist; amphibolite; black quartzite);
Espinho Unit (staurolite + micaceous schist; quartzite with garnet)
Lourosa Unit (micaceous schist; ortho-gneiss; migmatite; amohibo-
lite); 5: Hercynic Granitoids (syn D3); 6: Ante-Hercynic Granitoids;
7: Post-tectonic (Granitoid of Madalena); 8: Deposit of Kaolin (K),
argillic alteration zone.
K-FELDSPAR DISSOLUTION INTO Si-Al GEL AND CRYSTALLIZATION OF HALLOYSITE 51
80 kV, equipped with an X-ray energy dispersive spectrome-
ter (EDAX) and selected area electron diffraction (SAED).
SAED was used to determine the crystalline or amorphous
character of the secondary phases, using apertures of 550
m, in order to select 11.5
areas of the mineral. For
calibration of the SAED patterns an internal standard of gold
was used. The < 2 µm fractions were dispersed using an ul-
trasonic bath and from the dispersion, after a convenient di-
lution, a few drops of the suspension were spread over cop-
per microgrids. All microgrids were covered with a Formvar
film, used as a specimen holder.
K-feldspar grains and the < 2 µm clay fractions were in-
vestigated by XRD, in order to appreciate both structural
characteristics and degree of alteration.
XRD traces of K-feldspar grains, show well resolved d
reflection lines (Fig. 2a and b). The two XRD patterns of K-
feldspar show differences in intensity of both the 131 and
131 reflection lines. Also, the intensity decreases for both the
241 and 241 reflection lines. This means, that some order
disorder structural transformation took place in the K-feld-
The height of the background of XRD patterns of K-feldspar
grains increases in the range of 2040 (° 2
), which means that
an amorphous phase is its associated. Quartz and small
amounts of illite (< 3 %) were found associated with K-feld-
Aluminosilicate gel extracted from K-feldspar grains was
XRD recorded. The XRD pattern obtained, exhibits a gener-
al linear background, except for a broad diffusion peak in the
range of 2035, (°2
The XRD pattern of halloysite-0.7 nm (Fig. 2c) is charac-
terized by the increase of the (020) reflection line intensity,
whereas the (110) and (111) reflection lines appear just as
weak modulations of the (02,11) diffraction bands. Besides,
the (001) reflection line has an asymmetrical shape. The hal-
loysite-0.7 nm sample is not admixed with kaolinite as re-
vealed after hydrazine treatment (Range et al. 1969).
The sample shown in Fig. 2c was selected from a well or-
dered kaolinite to halloysite-0.7 nm transition series identi-
fied in the same geological profile of the kaolin deposit (Bo-
bos et al. 1996).
K-feldspar grains were collected from two distinct places:
outside and inside the kaolin deposit. The surfaces of the K-
feldspar grains were examined. The typical cleavage planes
are still preserved (Fig. 3a) in K-feldspar megacrystals, that
had been collected in the unaltered migmatite complex.
Weathering products, having irregular forms, are identified
in the right side of the same picture (Fig. 3a). Electron micro-
probe analyses indicated for those weathering products high
contents of Al and small contents of Si, Fe and K.
Altered K-feldspar grains, collected inside the kaolin de-
posit, exhibit porous surfaces. On the other hand, at the
surfaces of altered K-feldspar grains, belonging to the peg-
matite facies, both structural and morphological changes
could be observed; the last is characterized by vacuolar
structure exhibiting etch-pits of variable dimensions
(Fig. 3b). As a rule, the pits are well aligned along the K-
feldspar crystal cleavage planes. However, an ordered align-
ment of the pits couldnt be identified. In fact, a disordered
network of pits was identified (Fig. 3c and 3d). The pits ex-
hibit various shapes from rhombic to large polygonal holes.
They indicate, that the K-feldspar has been rapidly dissolved
or corroded by solutions with an acid pH, whereas no sec-
ondary mineral phases were produced. Also, no intermediate
mineral phases were recognized on K-feldspar surfaces by
SEM. Small amorphous flakes, 23
m in size, with a Si-Al
composition on the K-feldspar surfaces could be identified.
The flakes exhibited a light white colour and are concentrat-
ed in a small nucleus (Fig. 3c and 3d). The flakes of Si-Al
gel growth on K-feldspar grains were identified, even after
thoroughly cleaning grains with ultrasound. Probably, they
correspond to epitaxial growths on feldspar grain surfaces.
Various zones showing either agglomerated or isolated flakes
of Si-Al composition were also identified (Fig. 3c and 3d).
From these flakes, small tubular crystals of halloysite-0.7 nm
were nucleated and then, grown in sheaf-like agglomerates.
Several sheaves of halloysite-0.7 nm tubes were also identi-
fied in large holes on K-feldspar surfaces. Detailed images of
halloysite-0.7 nm tubes grown in sheaf-like agglomerate are
shown in Fig. 3e and 3f. However, isolated tubes of hal-
loysite-0.7 nm and very small pseudohexagonal kaolinite
plates (< 0.5 µm) can be clearly observed in the pictures (Fig.
3b and 3f).
The < 2 µm clay fractions extracted from the altered K-
feldspars and the < 2 µm halloysite-0.7 nm fractions (Fig. 2c)
were investigated by TEM. The sample whose XRD pattern
is shown in Fig. 2c, contains both long and short tubes of hal-
loysite-0.7 nm, Si-Al gel and partially rolled kaolinite plates.
Rolled kaolinite plates do not constitute a subject of the re-
search being carried out.
Non-crystalline irregular plates with SiO
sulting from analytic electron microscopy) were identified
in the < 2 µm fractions extracted from the K-feldspar. The
Si-Al gel was derived from the dissolution of K-feldspar.
Relict pseudomorphs or amorphized feldspar crystals, rec-
ognized by their habit, are observed in the < 2 µm clay frac-
tions (Fig. 4a). An incipient rolling-up at flake edges of Si-
Al gel or a crinkle film was recognized. Small halloysite-
0.7 nm tubes surrounding the irregular flakes of Si-Al gel
(Fig. 4b and 4c) could be identified. Large and small folding
could be identified seen. In Fig. 4c is shown the gel and the
tubes of halloysite-0.7 nm, whose XRD pattern is shown in
52 BOBOS and GOMES
Fig. 2. XRD patterns (random specimens) of K-feldspars (a and b) and oriented specimens of halloysite-0.7 nm (c).
K-FELDSPAR DISSOLUTION INTO Si-Al GEL AND CRYSTALLIZATION OF HALLOYSITE 53
Fig. 2c. SAED carried out in the central region of one of the
Si-Al gel flakes show only two very diffuse reflections rings
(Fig. 4d). Close to the edges of the Si-Al gel flakes, crystal-
linity becomes slightly better. It appears that the amorphous
phase corresponds to a precursor stage in the structural orga-
nization of 1:1 layers.
Electron microprobe analyses and analytic
Electron microprobe analyses carried out on feldspar grains
show particular features of their chemical compositions (Table
1). Destabilization of feldspar structure is found to be very fast
Fig. 3. SEM pictures of K-feldspar crystals and of agglomerates of weathering product characterized by high Al content and low Fe, Si
and K contents (a). Etch-pits having polygonal or circular shapes are observed on K-feldspar (c, d, e); some rhombic shape are individu-
alized and interpreted as being developed on the intersection of cleavage planes (b and d). Various flakes of Si/Al gel are well observed
on K-feldspar surface (c and d). Tubes of halloysite-0.7 nm grown and associated in sheaves (e and f).
54 BOBOS and GOMES
on a short scale (approximately equal to 510 µm), expressed
by a loss of silica and alkaline elements and a gain of alumini-
N = 3
N = 4
N = 4
N = 3
N = 8
N = 8
N = 4
(N = average chemical composition)
1, 2, 3: K-feldspar; 4, 5, 6: adjacent alteration product of K-feldspar; 7: Si/Al gel.
Table 1: Electron microprobe analyses of K-feldspar and adjacent
alteration product, and analytic electron microscopy of gel.
Fig. 4. TEM pictures of < 2 µm clay fractions showing K-feldspar pseudomorph submitted to amorphization and flakes of Si/Al gel (a).
Tubes of halloysite-0.7 nm surround Si/Al gel flakes (b and d). SAED of Si/Al gel show an amorphous state (d).
um. Chemical changes took place from the edge of pits to the
unaltered zone of K-feldspar crystal.
The presence of phosphorus and chlorine in the feldspar
structure is assumed to be related to the chemistry of hydro-
thermal fluids (Bobos & Gomes, unpublished data).
In the ternary diagram (Fig. 5), the field corresponding to
unaltered K-feldspar is noted with A. Chemical analysis relat-
ed to altered K-feldspar are plotted in the field B, showing a
loss of K and Si. Finally, the dissolution of K-feldspar passed
to a gel having a SiO
ratio close to unity, whereas K is
completely depleted. The silica resulting from the K-feldspar
structure was removed in solution and then precipitated. Fe is
not identified in the gel composition, being removed and then,
incorporated probably into free iron oxide.
The flakes of gel recognized on the K-feldspar surfaces
were found in the < 2 µm fractions and analysed by analytic
electron microscopy. Their chemistry is simple, being com-
posed solely of Si and Al.
K-FELDSPAR DISSOLUTION INTO Si-Al GEL AND CRYSTALLIZATION OF HALLOYSITE 55
The incongruent chemical reaction of the feldspar alter-
ation (Hemley 1959) to common clay minerals, can be sim-
O = Al
Helgeson (1969, 1992) assumed that from the dissolution
of feldspar, all its dissolved chemical constituents pass into
the hydrolysing solution:
O = 3 SiO
Theoretically, two different and independent chemical
processes are involved in feldspar alteration. In particular,
all the constituent elements of the K-feldspar are dissolved
activity and both reactions (2) and (3) can consti-
tute a chemical path involved in the rapid dissolution of K-
feldspar from São Vicente de Pereira kaolin deposit.
SEM investigations carried out on altered K-feldspar min-
eral grains show a vacuolar structure characterized by pits
(polygonal holes) of variable dimensions. Partially altered K-
feldspar grains exhibit porous surfaces, characterized by a
dense and irregular network of pits of various sizes. This
means that, K-feldspar has been dissolved or corroded by
acidic solutions and then, transformed into Si-Al gel. No in-
termediate mineral phases were recognized by SEM on the
K-feldspar surfaces, as we found on the K-feldspar surfaces
collected outside the kaolin deposit. Various dislocations as-
sociated with strains in K-feldspar grains may contribute to
the formation of reaction sites and to enhance the develop-
ment of the etch pits. In an initial stage of the etch-pits,
cross-section areas exhibiting rhombic shapes were devel-
oped apparently on the intersection of cleavage planes (Fig.
3a, 3b and 3c). The growth of hollow dislocations to form
etch-pits and then enlargement of these pits, both associated
with dissolution of K-feldspar, requires specific thermody-
namic conditions and chemical affinity to the hydrothermal
environment. Rapid dissolution of K-feldspar was the effect
of the high circulation rate of ascending metamorphic waters
and acid solutions along the north-western border of Ossa
Morena zone, characterized by intense fracturing and shear-
ing (Bobos & Gomes 1998).
In an acid environment the development of Al and Si-rich
layers on the feldspar surface after dissolution, may provide
favourable conditions for the crystallization of secondary
phases such as clay minerals (Petrovic et al. 1976; Aagard
& Helgeson 1983). It is also recognized that in acidic solu-
tions, K-feldspar undergoes rapid and extensive exchange
forming a surface layer of hydrogen-feldspar sever-
al unit cells thick (Blum & Stillings 1995). In kinetic terms,
several models of feldspar dissolution are described in the
literature, where different parameters (e.g. pH, temperature,
chemistry, stoichiometry, surface layer and charge density,
etc.) are considered in the theoretical approaches and re-
quired by the feldspar dissolution process (Blum & Stillings
1995). Amorphous phases formed by precipitation from so-
lution, are always thermodynamically unstable. However, the
rapid dissolution of feldspar is poorly understood so far.
Two questions resulted from the material we investigated:
what is the crystallization process of halloysite-0.7 nm from
Si-Al gel? and why did form halloysite-0.7 nm from Si-Al
gel and not kaolinite?
In natural waters, Al-aqueous species, depending on pH
and temperature, both under temperate and tropical climate,
were studied by Bourrie et al. (1989). The authors have
shown a reaction scheme for Al precipitation related to pH,
into monomeric species (e.g. Al
) at pH < 5
and then, into polymeric species at pH > 5, when gibbsite
saturation is reached. Gibbsite and bohemite were not iden-
tified in our samples and it appears that halloysite-0.7 nm
was formed without passing through intermediate mineral
phases as described before.
According to Hem & Roberson (1967), dissolution of
feldspar means a high tendency for hydrated Al to be tetra-
hedral co-ordinated by four hydroxyls. Merino et al. (1989)
explained that Al in aqueous solutions is co-ordinated by
six water dipoles and whenever pH increases some of the di-
poles loose a hydrogen ion. Hydroxyl ions being produced,
are attracted by the inner Al ion and co-ordinate hydroxyl
ions around the Al, reducing the co-ordination number from
6 to 4. The pH of its environment is directly related to the
negative charge of Si-Al gel. High pH values will favour Al
tetra-co-ordination and low pH will favour Al hexa-co-ordi-
Nevertheless, the process seems to be more complicated.
Several experimental studies of kaolinite synthesis from
aluminosilicate gel are reported in the literature. Rodrigue et
al. (1973) suggested that before kaolinite formation, the sili-
ca-alumina gel will form a structure composed of SiOAl
OH chains. De Kimpe et al. (1981) obtained kaolinite from
aluminosilicate gels and confirmed the SiOAlOH chains,
Fig. 5. Ternary diagram of SiO
O. The quantitative
chemical data of K-feldspar (Field A), alteration products (Field B),
Si-Al gel phase (Field C) and halloysite-0.7 nm were plotted.
56 BOBOS and GOMES
as an intermediate stage in the formation of kaolinite struc-
ture. Besides, beyond the pH conditions, the authors have
shown that the degree of hydrolysis of both gel and SiO
AlOH chains played a subsequent role in the process of ka-
Thus, the incorporation of aluminium in tetrahedral sites of
the structure can promote a strong tendency to curling (Bates
1959) and Al for Si substitution can cause bending of the 1:1
layers (Drever 1982). According to Merino et al. (1989) hal-
loysite may crystallize when
Al is dominant
subordinate. However, the chemistry of the solution may
control the morphology of crystals and therefore, the low or
high supersaturated solution of Si and Al will favour the
crystallization of kaolinite or halloysite-0.7 nm.
Analytical electron microscopy carried out on short and
long tubes of halloysite-0.7 nm (Fig. 2c) shows that the Al:Si
atomic ratio is greater than the unity (Tari et al. 1999), imply-
ing the presence of some tetrahedral co-ordinated Al. Neu-
tron magnetic resonance spectroscopy (
study carried out on halloysite-0.7 nm (Fig. 2c) identified a
weak resonance at 70 ppm, ascribed to tetrahedral Al (Bob-
os & Gomes, unpublished data). Nevertheless, the analytical
data is not yet available for the separated gel.
TEM shows thin and irregularly outlined of Si-Al gel
plates. An increase of crystallinity towards the edges of these
extremely thin plates is also clearly obvious by SAED as
well as the presence of tubes of halloysite-0.7 nm developed
on the edges. SEM observations carried out on K-feldspar
surfaces identified halloysite-0.7 nm grown-up in sheaves,
flakes of Si-Al gel, as well as separated tubes of halloysite-0.7
nm. Two nucleation processes of halloysite-0.7 nm are thought
to have been developed: i) heterogeneous (individual hal-
loysite-0.7 nm tubes in contact with K-feldspar surfaces) and
ii) homogeneous (sheaves of halloysite-0.7 nm tubes formed
of the bulk solution via Si-Al gel).
In the last process of halloysite-0.7 nm formation we as-
sumed that the gel or quasi-gel is an intermediary or a proto-
kaolinite structure composed of SiOAlOH chains. The
pH, temperature and chemistry of the solution played a
main role in the organization of 1:1 immature layers. It ap-
peared that a hydration process of both gel and 1:1 immature
structure, is plausible for the explanation of halloysite- 0.7
nm formation from the Si-Al gel.
Weathering of feldspars should produce first smectite or il-
lite and then, kaolinite. In other cases, early weathering of
feldspar should produce some aluminium hydroxide, which
is transformed into gibbsite, bayerite, nordstrandite, etc., de-
pending on pH, temperature and electrolite concentration
(Violante & Huang 1993).
In the kaolin deposit of São Vicente de Pereira the forma-
tion of halloysite-0.7 nm took place through a rapid dissolu-
tion of K-feldspar into Si-Al gel. No other minerals were
identified. The structural organization of Si-Al gel would
have passed through an intermediate stage, where the first
SiOAlOH chains were constituted. It seems that the
greatest hydration took place in gel, before the formation of
the first 1:1 immature layers.
Acknowledgements: I.B. thanks to Fundação para Ciência e
Technologia Lisboa for the grant provided him (Praxis XXI
BCC-4815). The authors are grateful to the three referees for
the comments on the manuscript.
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