GEOLOGICA CARPATHICA, 50, 5, BRATISLAVA, OCTOBER 1999
CONVERSION OF SMECTITE TO AMMONIUM ILLITE
IN THE HYDROTHERMAL SYSTEM OF HARGHITA BÃI, ROMANIA:
SEM AND TEM INVESTIGATIONS
and LUCRETIA GHERGARI
Laboratory of Mineralogy,
Polytechnic University of Timisoara, Timisoara-1900, Romania
Department of Mineralogy, Babes-Bolyai University, Cluj Napoca-3400, Romania
(Manuscript received February 19, 1998; accepted in revised form December 9, 1998)
Abstract: Random and ordered mixed-layer smectite-ammonium illite (NH
-I/S) were identified by XRD analysis in
the fossil hydrothermal system of Harghita Bãi (Eastern Carpathians, Romania). Morphologies of NH
-I/S were stud-
ied by SEM and TEM. SEM image of random NH
-I/S displays a cellular or cornflake texture. Ordered NH
exhibits a scalloped morphology with curled edges. Ribbon crystals of illite rise from the surfaces of plate aggregates.
Two generations of NH
-I are distinguished: one developed from a smectite precursor and another from a kaolinite
precursor. Hairy illite and illite pseudomorphs after book-like aggregates of kaolinite are present. TEM images show
the evolution of morphology from flake to lath habit during the smectite to illite conversion. Particles with veil
characteristics and flake particles are observed in random mixed-layered NH
-I/S. Diffuse lath shaped illite devel-
oped on the previous morphology in the expandability range 7020 %S is observed. Euhedral lath shaped illite is
present in NH
-I/S, below 10 %S. The reaction of smectite-to-illite is continuous, leading to lath shaped illite.
Key words: Eastern Carpathians, Harghita Bãi area, XRD, SEM, TEM, random and ordered NH
The reaction of smectite into illite during the diagenetic, meta-
morphic and hydrothermal processes has been described over
more than 25 years by several authors (Perry & Hower 1970;
Hower et al. 1976; Boles & Franks 1979; Nadeau & Reynolds
1981; Inoue 1986; Inoue et al. 1987, 1988, 1990; Buatier et al.
1992; ucha et al. 1993, 1996; Christidis 1995). During the
conversion of smectite to illite, the proportion of illite layers
and K or NH
contents increase with temperature and geologi-
cal time. Typically, K is the interlayer cation fixed in illite
structure, however NH
can play an analogical role.
Detailed investigations by SEM and TEM of random and or-
dered mixed-layer illite/smectite have been made by Nadeau
et al. (1985), Inoue (1986), Inoue et al. (1987), Keller et al.
(1986), Yau et al. (1987), Christidis (1995), ucha et al.
(1996). Their conclusion concerning the mechanism of smec-
tite to illite conversion is that of dissolutioncrystallization.
Amouric & Olives (1991), studying the illite/smectite sam-
ples described by Inoue et al. (1987), have shown that both
mechanisms of dissolutioncrystallization and solid state
transformation are involved at the same time in the process
of illitization of smectite.
An experimental study of the changes in particle morpholo-
gy during illitization of smectite was made by Whitney & Vel-
de (1993). They found that this reaction followed four steps:
dissolution, epitaxial nucleation and growth of illite on smec-
tite substrata, coalescence of thin illite particles to form aggre-
gates and infillings, and syntaxial growth of aggregates.
However, the understanding of the mechanism of this reac-
tion is limited. No single model can explain the transition of
smectite to illite. On the basis of published data, Moore &
Reynolds (1989) quoted four reported genetic models to ex-
plain this reaction: the MacEwan crystallite model, the fun-
damental particle model, the segregated model and two solid
A complete transition from smectite to ammonium-illite
-I) has never been reported in the literature. However,
some ordered mixed-layer NH
formed during diagenesis, have been described by Copper &
Hydrothermal occurrences of NH
-I or/and mica are
known in Slovakia, Japan and the U.S.A. Ammonium seric-
ite was identified by Kozaè et al. (1977) in the Neogene Vol-
canism of Slovak Carpathians (Vihorlat Mts.). In the Japa-
nese volcanic arc, Higashi (1978, 1982) reported NH
-I as a
new mineral named tobelite (after Tobe mine). Also,
Kawano & Tomita (1988) described ammonium-bearing dio-
mica from Aira district. Wilson et al. (1992)
characterized hydrothermal tobelitic material, ordered
-I/S, 28% expandable layers (%S), which
occur in hydrothermally altered black shale from the Oquirrh
Mts. (Utah, U.S.A.).
In this contribution, morphological characteristics of ran-
dom and ordered mixed-layer NH
-I/S and the evolution of
smectite into illite in the Harghita Bãi hydrothermal area
(Neogene volcanism of Eastern Carpathians, Romania) were
studied by scanning electron microscopy (SEM) and trans-
mission electron microscopy (TEM).
The Neogene volcanic activity in the Eastern Carpathians
represents a subsequent stage of the magmatism associated
Present address: Department of Geosciences, University of Aveiro, Aveiro-3810, Portugal
with the Carpathian orogen. The CãlimaniGurghiuHarghi-
ta volcanic arc is the youngest unit of the Carpathians and it
developed in two stages, represented by two structural com-
partments: lower volcanoclastic and upper stratovolcanic
(Fig. 1). The volcanic edifice of Harghita Bãi situated in a
craterial join of a stratovolcano, developed during two main
stages: stratovolcanic and intrusive.
An intrusive microdiorite and andesite formation (upper
Pliocene) succeeds a variety of andesitic rocks developed
closer to the surface. The hydrothermal alteration of Harghita
Bãi is related to the evolution of a porphyry copper system
and it is characterized by the following stages: biotitic, am-
phibolic, chloritic and argillic (Stanciu 1984).
The fragment of the hydrothermal area investigated by Bob-
os (1994) in the mining works from 0 to 110 meters below
the present surface is characterized by two types of alteration:
an advanced argillic alteration (kaolinite ± dickite ± pyrophyl-
lite assemblage and sudoite + Na-rectorite ± K-illite assem-
blage), interpreted as a satellite alteration of the porphyry
copper system, and a late alteration enriched in NH
to the breccia pipe structures (BPS). NH
-I alteration is super-
imposed on the porphyry copper system. The late alteration
took place after the change of the fluid regime from lithostatic
to hydrostatic, which yielded hydraulic fracturing expressed
by BPS (Bobos 1994). The BPS are devoid of ore minerals
and are composed of irregular polygonal blocks of argillized
pyroxene andesite enriched in NH
-I. The postmagmatic event
related to the BPS is supposed to be associated with residual
hydrothermal fluids enriched in nitrogen and boron. The spa-
tial relationship between the BPS and the alteration zones en-
riched in ammonium is shown in Fig. 2.
Materials and methods
Both random and ordered mixed-layer NH
-I/S, as well as
-I, occur in the Harghita Bãi hydrothermal area. Samples
containing random mixed-layer NH
-I/S are mixtures con-
taining small amounts of other clay minerals (kaolinite and
chlorite) and occur outside the BPS at depths of 110 meters.
Ordered mixed-layer NH
-I/S (10 to 40 %S) come from the
external part of the BPS and were collected at the depth
range of 50 to 110 meters.
The < 2
m fractions were separated by sedimentation and
treated by Jacksons (1975) technique. X-ray diffraction
(XRD) patterns were recorded for oriented specimens, pre-
pared by pipetting clay suspensions of < 2
m fractions onto
glass slides (concentration of 10 mg/cm
). The saturation
with ethylene glycol for the expandability measurement was
carried out over 8 hours at 60 °C.
XRD analysis was carried out with a Philips PW-1730 auto-
mated system using a Cu, K
radiation and a graphite mono-
chromator. Samples were analysed in the range 2 to 50° 2
ing 0.5° divergence and 1°
receiving slits, 0.02° 2
counting time of 1sec/step. The expandability of NH
the ordering type (Reichweite) were determined using the meth-
ods of rodoñ (1980) and Watanabe (1988). The accuracy of
%S measurements by both methods is approximately ± 5 %S.
For SEM observations the rocks were crushed into small
pieces and then cleaned by ultrasonics. Samples were mounted
on a carbon holder. Analysis was carried out with a Jeol-35
scanning electron microscope operated at 15 kV and 25 kV.
For TEM analysis, the < 2 µm fractions were dispersed in
water, using an ultrasonic vibrator. Very dilute drops were de-
posited over copper microgrids covered with a Formvar film.
Laurylamine was intercalated in sample HB-1 (NH
R = 0) to prevent the collapse of smectite layers under high
vacuum (Murakami et al. 1993). Investigation was performed
Fig. 1. Geological sketch of central area of Harghita Mts.: 1 up-
per structural compartment; 2 lower structural compartment; 3
craterial area; 4 argillization area; 5 centre of eruptions.
Fig. 2. The location of mixed-layer NH
-I/S and NH
-I in the brec-
cia pipe structure.
SEM AND TEM INVESTIGATIONS OF SMECTITE, HARGITA BÃI 381
Fig. 3. XRD patterns (air dry and ethylene glycol) of random and ordered mixed-layer NH
using a Tesla B-500 and a Hitachi H-9000 transmission elec-
tron microscopes, both operated at 80 kV.
XRD patterns of random and ordered NH
-I/S were re-
corded in air-dry and ethylene glycol states (Fig. 3). The
measured expandability is listed in Table 1. The samples
with random NH
-I/S are mixtures with chlorite or kaolinite.
Most samples with ordered NH
-I/S do not contain other
minerals in the < 2 µm fractions. Only in the sample HB-6
was a small quantity of kaolinite detected.
-I/S enriched in ammonium (HB-7; 10 %S) shows the
d(005) reflection at 2.05 Å in XRD traces. The XRD patterns
of sample HB-8 (5 %S) show two distinct d(005) reflections,
both in air-dry and ethylene glycol states: a small shoulder at
2.03 Å corresponding to NH
-I/S and the reflection at 2.01 Å
corresponding to K-I/S. The two peaks were assigned to two
separate phases of K-I/S and NH
-I/S. This is consistent with
Fig. 4. Text on the next page.
SEM AND TEM INVESTIGATIONS OF SMECTITE, HARGITA BÃI 383
with sharp edges having the typical cornflake texture of
montmorillonite, could be seen (Fig. 4a, sample HB-1). With
the decrease of expandability, the texture changes and a large
number of flat lying flakes is observed (Fig. 4b, sample HB-2;
70 %S). The clay texture in sample HB-3 (40 %S) is domi-
nated by flakes with a scalloped morphology, apparently
platy crystals (Fig. 4c). The coalescence of thin flat or ribbon
illite is observed. This type of morphology coexists with the
digitated texture, where ribbons of illite are arranged like the
fingers of a hand. Below 30 %S, further changes in morpholo-
gy are well evidenced. Elongated ribbon illite crystals grow-
ing from flakes or scalloped crystals are shown in Fig. 4d
(sample HB-5). Ordered NH
-I/S, below 10 %S exhibits a lath
morphology. Illite with a hair texture (Fig. 4e) was identified
in sample HB-8 of K-I/S + NH
-I/S (5 %S). The length of the
hairy illite is more than 10 µm and they are very thin. The hair-
like morphology coexists with euhedral quartz. Similar
paragenesis (hairy K-I/S and euhedral quartz) was reported
from the Shinzan hydrothermal area (Japan) by Inoue (1986).
The hair illite morphology does not occur in samples of am-
monium clays. NH
-I also formed from kaolinite precursor
minerals, is well observed in an illite pseudomorph after
books of kaolinite (Fig. 4f, sample HB-7).
Transmission electron microscopy
Extremely thin particles of pure smectite are shown in Fig. 5a,
(sample HB-1). Individual aggregates have a dominant flake-
like habit and curling edges (cornflake texture). Flake-like mor-
phology is also well defined in random mixed-layer NH
with 70 %S (sample HB-2) and the first thin lath illite particles
grown of smectite aggregates were observed (Fig. 5b). Random-
ly organized laths on a core particle of smectite can be seen.
With the decrease of expandability, the flake morphology will
gradually disappear. Partially ordered NH
-I/S (40 %S to
30 %S, HB-3 and HB-4) display illite crystals with a lath-like
habit, growing on a smectite support (Figs. 5c and 5d). The lath
crystal broadens towards the smectite flake.
Morphological investigations of NH
-I/S (R = 1) evidence
laths and short fibres developed on smectite along one or more
preferential directions with a variable angle of 40° to 60° be-
tween them. With the decrease of expandability, the smectite
morphology decreases (Fig. 5e; HB-6). It disappears in NH
below 20 %S. Ordered mixed-layer NH
-I/S, having below
10 %S, display mainly a lath morphology elongated along the
a* direction without flake-like habits (Fig. 5f, sample HB-7;
10 %S). The lath illite correspond to 1M polytype (Güven
1974). In Fig. 5g (sample HB-7) two generations of illite can
be distinguished. Lath particles of illite show straight and
pseudo-hexagonal edges. These indications of crystal habit
can constitute the genetical features for illite formed from
smectite and from kaolinite precursor minerals.
The smectite to illite reaction via mixed-layer I/S clay min-
erals has been interpreted in the literature as a solid state trans-
formation or a neoformation process. Solid state transforma-
Fig. 4. SEM images: a smectite exhibits a cornflake texture
(sample HB-1, 95 %S); b random NH
-I/S (HB-2, 70 %S) show-
ing flat lying flakes; c on the edge of scalloped (S) morphology
are present thin flat or ribbon crystals of NH
-illite rising from
smectite (HB-3, 40 %S); d ribbon (R) crystal of NH
4, 30 %S); e hair-like (H) and plate morphology of illite in the
sample of K-I/S + NH
-I/S (HB-8, 5 %S); f pseudomorphose of
-illite after a kaolinite book (HB-7, 10 %S).
HB-5 -72 m
Table 1: Location of samples (depth below surface) and XRD char-
acteristics of the NH
-I/S investigated from the < 2
the chemical composition of this sample, which showed an
increase of K and a decrease of NH
content, as compared to
sample HB-8 (Bobos et al. 1995).
Scanning electron microscopy
SEM images of random mixed-layer NH
95 %S exhibit a cellular morphology. The flakes of smectite
tion is imagined as alteration of smectite crystals into illite
crystals due to Al
substitution for Si
(Hower et al. 1976). The neoformation process consists of
dissolution of smectite and crystallization and growth of il-
lite (Nadeau et al. 1985; Eberl & rodoñ 1988; Inoue et al.
The reactions progress is influenced by the chemistry of
fluids, porosity and permeability, with water playing the
main role in the dissolution of smectite and the crystalliza-
tion of illite (Whitney 1990). Thin illite particles grow and
coalesce with increasing temperature and time (Eberl & ro-
doñ 1988). According to Jiang & Peacor (1990), the smectite
to illite transition involves only metastable phases controlled
by kinetic factors.
In experimental hydrothermal conditions, Whitney (1990)
demonstrated that original smectite layers disappeared at 50
%S and then, the first thin illite layers were constituted. Nor-
mally, dissolution of smectite is considered to be very fast
(Eberl et al. 1986) and take place in randomly mixed-layer I/S.
The first lath illite crystals, coexisting with flake smectite,
were observed by Inoue et al. (1987, 1988) and Christidis
(1995) at 8070 %S. Thus, fast dissolution of smectite is re-
ported in their results. In contrast, in ordered NH
-I/S (R = 1,
> 20 %S) the morphology of original smectite (Figs. 5c, 5d,
5e) persists and it tends to disappear only in NH
less than 20 %S. Illite crystals are oriented at 120° and at
60°40° angles on smectite aggregates of NH
-I/S (70 %
20 %S), suggesting crystal growth and epitaxial coalescence.
In fact, in the expandability range 7020 %S there is a pro-
gressive evolution from flake smectite to lath illite, where
syntaxial illite grown on smectite is clearly evidenced. Ac-
cording to Whitney & Velde (1993), syntaxial lateral growth
of secondary illite layers on primary smectite could appear
during the illitization of smectite.
Two types of processes (dissolutionprecipitation) may be
either the topotactic replacement, where the original smectite
or both minerals, smectite and illite are maintained, but may
produce overgrowths and growth of new crystals (Whitney &
Northrop 1988). Lath shape NH
-I grown on smectite, can be
considered as individual particles which would become in-
creasingly inter-grown or can be interpreted, as aggregates of
inseparable units formed of smectite and illite. Layers of il-
lite and smectite are pictured as intimately interlayer (Altan-
er & Bethke 1988) and the MacEwan model may be a sup-
port in the case of random and ordered NH
Two cases are involved to explain the morphologies of
-I/S, characterized by the coexistence of flaky smectite
and lath illite:
1. The fluid flow rate in the smectitic environment was in-
sufficient for an extensive dissolution and leaching, develop-
ing the porosity around the smectite flakes. The subsequent
reaction was characterized by slow dissolution of smectite
and epitaxial growth of illite on smectite surface by topotaxy.
Pore chemistry inhibited the smectite to illite reaction;
2. The presence of Na, Ca and Mg interlayer ions are
known to inhibit the illitization process of smectite.
The dissolution of smectite is related to water in the geologi-
cal system. Water with a catalyst role is important for transport
and crystallization, although, a small amount of water can
slow the dissolution of smectite and enhance extensive substi-
tution of the smectite through illite. The reaction of smectite il-
litization was inhibited due to the deficit of water in the system
and therefore, it was controlled by the porosity. A complete il-
litization process of both smectite and kaolinite is nevertheless
attested in the BPS through NH
Taking into account the second case, the presence of Mg
ions in the solution will retard the rate of illitization. The
chemistry of ordered mixed layer NH
-I/S (40 to 20 %S),
shows values of the Mg content in the range 0.310.41/
(Bobos et al. 1995). As a matter of fact, Roberson
& Lahann (1981) demonstrated in experimental conditions
the role of Na
in the pore fluids during the
smectite to illite conversion.
Therefore both stages are involved in the illitization of
smectite from the Harghita Bãi area and can explain the long
rate of smectite dissolution. The availability of NH
enced more or less the low rate of the reaction. No other by-
product was identified in the clay fractions of the BPS. On the
contrary, kaolinite chlorite, quartz and cristobalite associated
-I/S were identified outside the BPS. The NH
ability was sufficiently in the main channel and the smectite
and kaolinite were consumed, leading to illite layers.
Fig. 5. Text on the next page.
SEM AND TEM INVESTIGATIONS OF SMECTITE, HARGITA BÃI 385
Fig. 5. TEM images: a smectite with fluffy
morphology (HB-1, 95 %S); b flaky habit
and first appearance of thin lath particles at 70
%S (HB-2); c syntaxial growth of lath illite
on smectite precursor mineral, (HB-3, 40 %S);
d syntaxial growth of NH
-illite on smectite
(HB-5, 28 %S); e NH
-illite crystals with a
lath habit dominating smectite flaky habit (HB-
6, 20 %S); f predominantly NH
lath morphology (HB-7, 10 %S); g lath NH
illite crystals with straight and pseudo-hexago-
nal edges (sample HB-7, 10 %S).
Our data obtained by SEM and TEM illustrate morpholog-
ical change during the smectite to illite reaction in a fossil
hydrothermal system of Harghita Bãi. A long rate of smectite
dissolution (around the BPS), illite grown on smectite and a
short rate of smectite illitization (in the BPS) were observed.
Kaolinite and smectite were two precursor minerals which
occurred in the external argillic alteration envelope related to
the porphyry copper system. Kaolinite underwent a rapid illi-
tization, before the illitization of smectite took place, as a re-
sult of the reaction of kaolinite illitization not being depen-
dent on temperature (rodoñ & Eberl 1984).
An attempt has been made to explain whether the type of
interlayer cation or the thermodynamic conditions were the
main factors influencing the changes of morphology during
the illitization of smectite. The type of interlayer cation does
not influence the typical morphologies described in the liter-
ature along the smectite to illite reaction and therefore, water
and pore chemistry played the main role in the illitization
process of smectite from Harghita Bãi area.
Acknowledgements: The authors are indebted to two anony-
mous reviewers for helpful suggestions, comments and for
improving the English. I.B. thanks Vlado ucha for his fruit-
ful help in clays investigation along of several years. Also,
thanks are due to T. Farcas for his technical assistance during
the TEM analysis.
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