On the mechanism of exfoliation of
‘Vermiculite’
S. HILLIER
1,2,
*, E. M. M. MARWA
3,4
AND C. M. RICE
3
1
The James Hutton Institute, Craigiebuckler, Aberdeen, AB15 8QH, UK,
2
Department of Soil and Environment,
Swedish University of Agricultural Sciences (SLU), P.O. Box 7014, SE-750 07 Uppsala, Sweden,
3
Department of
Geology and Petroleum Geology, University of Aberdeen, AB24 3UE, UK, and
4
Sokoine University of Agriculture,
Department of Soil Science, P. O. Box 3008, Morogoro, Tanzania
(Received 20 May 2013; revised 27 July 2013; Editor: George Christidis)
ABSTRACT: Six samples of ‘Vermiculite’ have been studied to investigate the mechanism of its
well known but poorly understood property to exfoliate. The samples were analysed quantitatively by
XRD to determine their precise mineralogical composition. Electron microprobe methods, including
elemental mapping of native potassium and of caesium (introduced by cation exchange) were used to
examine variation in the chemical composition of the particles. Most of the samples examined show
heterogeneous mineralogical compositions which occur as distinct zones within the volume of
individual particles, presenting a mosaic texture. Exfoliation is related to this mosaic distribution of
the different mineral phases within the particles. Lateral phase boundaries between vermiculite and
mica layers, or vermiculite and chlorite layers are postulated to prevent or impede the escape of gas
from a particle, resulting in exfoliation when the pressure exceeds the interlayer bonding forces that
hold the layers together. This mechanism provides a common explanation for the exfoliation of
‘Vermiculite’ by thermal methods or by treatment with H
2
O
2
. Paradoxically, one sample which
consists of pure vermiculite, in the mineralogical sense of the term, demonstrates that pure
vermiculite does not and should not exhibit the property of exfoliation. Our explanation of the
mechanism of exfoliation explains the commonly observed particle size dependence of exfoliation
and the tendency for obviously poly-phase ‘Vermiculite’ samples to show the largest coefficients of
expansion.
KEYWORDS: vermiculite, hydrobiotite, phlogopite, exfoliation, hydrogen peroxide.
As a traded commodity, the term ‘Vermiculite’ is
used to de scribe commerc ially exploited deposits of
micaceous minerals which can be exfoliated when
heated rapidly to high tempera tures (Hindman,
2006). Exfoliation involves a volume expansion
with individual platy particles expanding perpendi-
cular to the cleavage planes, bloating in an
accordion- or concertina-like fashion t o up to
2030 times their o riginal volume (Walker,
1951). In their exfoliated form, the accordions of
vermiculite are often curved, which together with
their segmented appearance evokes the vermiform
resemblance to worms, and explains the origin of
the name ‘vermiculite’. Historically, any mineral
that showed the property of exfoliation when flash-
heated was identified as ‘vermiculite’ ( Walker,
1951). Even in the earliest literature it was,
however, known th at other minerals, in particular
hydrobiotites, a lso exhibited the property of
exfoliation when heated rapidly. The term hydro-
biotite was reintrod uced by Gruner (1934) in his
classic study of vermiculites to denote minerals that
he correctly identified as inti mate interstratifications
of mica and vermiculite layers . In fact, it later
became established that hydrobiotites often showed
* E-mail: stephen.hillier@hutton.ac.uk
DOI: 10.1180/claymin.2013.048.4.01
Clay Minerals, (2013) 48, 563–582
# 2013 The Mineralogical Society
a grea ter propensity to exfoliate compared with true
vermiculites. Thus, Mi dgley & Midgley (1960)
examined sixteen commercial vermiculites, finding
that none were pure true vermiculites. They
observed that the largest degree of exfolia tion was
associated with the occurrence of hydrobiotite and
the lowest degree of exfoliation with the occurrence
of tru e vermi culite. Indeed of the eleven specimens
of vermiculite examined originally by Gruner
(1934) only three (Nos. 8, 9 and 10) were listed
as ‘easily’ or ‘readily’ exfoliating and all three were
designated as hydrobiotites, all containing appreci-
able a lkalis, mainly po tassium, in obvious contrast
to the specimens designated as true vermiculite in
which alkalis were essentially ab sent. It should be
noted, howeve r, that Gruner (1 934) did not
comment at the time on any relationship of
mineralogy to exfoliation. Subsequently, in a
classic French study Couderc & Douillet (1973)
examined a fur ther thirty one specimens of
commercial vermiculites, with samples from thir-
teen localities spread across nine different co untries.
Only two specimens, both from Brazil, appeared to
be pure true vermiculite and these showed by far
the lowest coefficients of expansion. The remainin g
specimens were identified as various mixtures of
vermiculite, interstratified mica/vermiculite types
and mica. Despite an alysing 31 specimens, Couderc
& Douillet (1973) failed to establish a detailed
relationship between exfoliation and th e miner-
alogical composition of their specim ens, noting
only that samples showing regular interstratification
of mica and vermiculite layers (hydrobiotite), as the
dominant phase showed the highest coefficients of
expansion. More recently, Justo et al. (1989)
confirmed these observations, finding much higher
coefficient s of expansion for two hydrobiotite-
dominated samples, compared to four samples of
pure vermiculite.
In the m ineralogical, as opposed to the
commercial commodity sense, vermiculite is a
precisely defined name for a group of 2:1
phyllosilicates with a layer charge of between 0.6
and 0.9 per O
10
(OH)
2
formula unit (Bailey, 1980;
Guggenheim et al., 2006). On this basis vermi cu-
lites are distinguished from smectites with lower
layer charge and micas which usually have higher
values. X-ray diffraction (XRD) patterns of
vermiculites have first-order basal spacings of
between about 14 and 15 A
˚
and if homo-ionic
and pure (in the sense of having no interstratifica-
tion with other phases such as mica) they show a
series of higher order basal peaks that are rational
on the primary basal spacing (Brindley & Brown,
1984; de la Calle & Suquet, 1988). Vermiculites
may be dioctahedral or trioctahedral although it
appears that all known macroscopic occurrences
(Foster, 1961; Douglas, 1989) and certainly a ll
commercially exploited deposits (Hindman, 2006)
are trioctahedral.
By fa r the most well known me thod of
exfoliating vermiculite is by short lived rapid so
called ‘shock’ or ‘flash’ heating. Commercially, this
is usually achieved by heating to temperatures of
around 900ºC for a few minutes or more in a
vertical or rotary furnace (Hindman, 2006). With
some vermiculites in the laboratory the onset of
exfoliation c an be observed at temperatures as low
as 300ºC, although rapid heating at 100ºC, a
temperature at which around half of the interlayer
water in ve rmiculite is lost, does not result in
exfoliation (Walker, 1951). Interestingly, other
authors have indicated that the temperature of the
onset of thermal exfoliation can also be lowered by
cation exchange treatments with Na
+
,orNH
4
+
(Muiambo e t al., 2010; Huo et al., 2012). The
experiments of Muiambo et al. (2010) were
conducted on specimens of the well known
Palabora vermiculite from Phalaborwa, South
Africa, whilst the specimen examined by Huo et
al. (2012) from China was described as an
industrial (‘‘ interstratified’’ ) vermiculite.
Microwaves have also been employed successfully
by several investigators as an alte rnative thermal
method to exfoliate vermiculite (e.g. Obut et al.,
2003; Marcos & Rodrı´guez, 2011).
As well as thermal methods, vermiculite may
also be exfoliated chemically. By far the most
effective chemica l method known is that of using
hydrogen pero xide (H
2
O
2
). The exfoliating action
of H
2
O
2
on par ticles of weathered mica was first
noted independently by Drosdoff & Miles (1938)
and Groves (1939) dur ing soil particle size analyses
procedures where H
2
O
2
was used to oxidize organic
matter. Drosdoff & Miles (1938) tested 20 museum
specimens of vermiculite and whilst most showed
no reaction at all, two samples of ‘philadephite’ (a
defunct varietal name for hydrobiotite) exfoliated
readily. Groves (1939) provided no details of the
vermiculite specimens he examined but speculated
that the exfoliation effect observed upon the action
of H
2
O
2
was possibly due to the evoluti on of
oxygen between the flakes, although he noted tha t
this did not provide explanation for the observed
564
S. Hillier et al.
‘asymmetric expansion and the zones of different
colour’. More re cently, U
¨
c¸gu
¨
l & Girgin (2002) and
Obut & Girgin (2002) investigated the effects of
H
2
O
2
on a phlogopite from Turkey. The potassium
content of this material is in fact far too low for a
pure phlogopite and XRD evidence presented in
U
¨
c¸gu
¨
l & Girgin (2002) is suggestive of some
interstratification with vermiculite. Another
chemical method that produces exfoliation is
treatment with sulphuric acid. Ruthruff (1941)
noted that hydrobiotite exfoliated greatly when
soaked in concentrated sulfuric acid for 48 h and
then spread out and allowed to dry in air, whereas
true vermiculite simi larly treated showed no
exfoliation. Walker (1951) later commented tha t
this might be used as a test to distinguish between
the two minerals. Exfoliation by sulfuric acid was
explained by Ruthruff (1941) as due to displacive
growth of sulfates between the layers. U
¨
c¸gu
¨
l&
Girgin (2002) also tested exfoliation of the Turkish
phlogopite with H
2
SO
4
, HCl, HNO
3
and H
3
PO
4
but
in contrast to H
2
O
2
no response was not ed.
In general, the mechanism of thermal exfoliation
of vermiculite is ascribed by most workers to the
rapid production of steam during flash-heating
which fo rces open the layers of vermiculite as the
steam escapes from the structure. Nonetheless,
commenting on exfoliation in h er review of
thermally modified clay minerals, Heller-Kallai
(2006) concluded that ‘‘ no completely satisfactory
explanation for the unique be haviour of vermiculite
has yet been offered’’ . It is a lso unclear if the
phenomenon of thermal ex foliation has mechanistic
factors in common with chemical exfoliation. In
terms of the basic mechanism of exfoliation there
are three main pertinent observations, and it is
useful to review these before we progress. Firstly,
many authors have pointed out that thermal
exfoliation only occurs when the particles of
vermiculite are flash-heated; in contrast slow
heating does not result in exfoliation. Secondly,
exfoliation is often noted to be a function of
particle size. Smalle r parti cles sizes typically show
less expansion compared to larger particle sizes.
And thirdly, interstratified mica/vermiculite invari-
able shows a greater propensity to exfoliate
compared to samples of pure vermiculite. As
such, most commercial vermiculi tes typically
contain large proportions of hydrobiotite rather
than true vermiculite. Any proposed mechanism
for exfoliation should be able to explain all three of
these observations.
Walker (1951) also pointed out the early
confusion that occurred in nomenclature becau se
of the variability of the mineralogy of ‘vermiculite’
over small distances. On the smallest scale this is
exemplified by single particles that may consist of
zones of mica, zones of true vermiculite and zones
of interstratification of the two. On larger scales,
heterogeneity is represented by samples of ‘vermi-
culite’ that are variable mixtures of different
minerals (e.g. vermiculite, hydrobiotite and phlogo-
pite), resulting in the potential for substantial
differences in mineralogical compos ition and
behaviour between specimens from the same
locality. No doubt if insufficient attention is given
to the characterisation of vermiculites the results
reported may serve to cloud our understanding of
the mechanism of exfoliation. Indeed, Frank &
Edmond (2001), a lthough mainly concerned with
identification of asbestos in vermiculite, are at pains
to point out that it is difficult to compare much of
the data in the literature because ve rmiculites and
hydrobioti tes have so often been grouped together
as ‘vermiculites’ in many studies, especially those
that address commercial aspects. In fact, returning
to the classic w ork of Walker (1951) it can be noted
that he saw fit to comment that of the eleven
specimens that he originally examined eight tu rned
out to be hydrobiotite.
Even though there have been many studies of
vermiculite it is clear that a completely satisfactory
explanation for the mechanism of exfoliation has
not yet been offered. There is also a growing and
active interest in exfoliation not only of clay
minerals but of a wide variety of layered materials,
by various means, for appli cations in a wide array
of tech nological and nano-technological fields (e.g.
Coleman et al., 2011). The present study was
undertake n to attempt to eluci date the mechanism
of exfoliation of ‘Vermiculite’, which probably can
be considered as the very first layered material to
be exploited for this property. Our approach has
been to combine precise mineralogical and
chemical characterisation of a range of specimens
in relation to their capacity to be exfoliated by both
thermal and chemical means.
MATERIALS AND METHODS
Samples studied
Six samples were studied, one of pure phlogopite
mica as a control and five composed mainly of
Exfoliation of verm iculite 565
vermiculites and hydrobiotites in varying amounts.
TwosampleslabelledMK-1andKL-2were
sampled in Tanzani a at the M ikese area in
Morogoro region and Kalalani area i n Tanga
region, respectively. A further sample of the
classic Palabora vermiculite (PB) from
Phalaborwa, South Africa, was supplied by
Palabora Europe Ltd. Three additional samples
were taken from The James Hutton Institute
mineral coll ection; Ver-2 a specimen of ‘Poole’
vermiculite, believed to be from the Enoree district
in S. Carolina, Ver-18 a sample from Glen
Urquh art, Invernesshire, Scotland, and Phl-3, a
large hand s pecimen of phlogopite originally
obtained from R.F.D. Parkinson and Co. and used
as the control true mica specimen.
Mineralogical ana lysis
All samples were initially crushed in a coffee
grinder for a few minutes only to avoid structural
degradation and sieved to obtain particle size
fractions of 12 mm. For XRD analysis, 3 g of
the sieved samples wer e mixed with water at a ratio
of 4:1 (water: solid) and micronized by grinding in
a McCrone mil l for 12 min, with the exception of
the phlogopite sample Phl-3 which was notably
more resistant to particle size reduction and was
therefore milled for 24 min. The resu lting slurries
were spray-d ried directly from the mill at 130ºC to
obtain random powder specimens (Hillier, 1999).
Identification of the mineral phases in the samples
was carried out after reco rding the XRD patterns of
the random powders using Co-Ka radiation selected
by a diffracted beam graphite monochromator on a
Siemens D5000 y/y diffractometer. The scanning
was carried out ove r a range of 275º on the 2y
scale with 0.02º steps and counting for 2 s/step.
Quantitative analyses of random powder XRD
patterns were carried out by a full pattern fitting
method as described in Omotoso et al. (2006).
Purified reference specimens were used as mineral
standards. The objective of this analysis was to
determine the proportion of individual minerals in
the samples.
Chemical analysis
The chemical composition of the samples was
determined by electr on probe microa nalysis
(EPMA) using a Microscan MK7 equip ped with
an energy dispersive analy ser (LINK Analytical
AN10/25S). The analysis was carried out using an
acceler ating voltage of 15 kV; pro be current of
3.0 nA; take off angle of 75º; 30 mm electron beam
diameter; and counting time of 150 s. Standa rd
minerals were used for calibr ation . Data were
acquired and processed using the LINKS ZAF4/
FLS software at the University of Aberdeen,
Department of Geology and Petroleum Geology.
Distribution of potassium in the samples
(12 mm sized material) was studied using a
scanning electron microscope (SEM) fitted with
an X-ray energy dispersive system (EDS). The
particles were mounted on glass slides and coated
with a thin conducting layer of carbon to prevent
the surfa ce from charging (Hall & Lloyd, 1981).
The SEM instrument used was an ISI-ABT55 with
EDS-LINK AN10/55S. The images were acquired
with an ISS-I-SCAN 2000 Digital Image
Acquisition System. The mapping was done with
a 15 ms dwell ti me and a 512 pix el6512 pixel
frame at the University of Aberdeen in the UK.
Additionally, selected samples were caesium
(Cs
+
) exchanged by soaking overnight in solutions
of 1
M CsCl (Hillier & Clayton, 1992) followed by
thorough rinsing in deionised water and the
subseque nt particle distribution of Cs
+
mapped by
techniques similar to those employed for potassium
mapping.
Thermal exfoliation p rocedure
The extent of exfoliation was mea sured as the
coefficient of ex pansion (K) defined by Couderc &
Douillet (1973) as the ratio of bulk densities
measured prior to and post flash-heating. Each
sample was measured in triplicate using the
precisely sized 12 mm materials separated by
sieving. No attempt was made to remove impurities
but XRD analysis (see below) showed that all
samples contained phyllosilicate contents of greater
than 92% by weight. For each sample prec isely 2 g
was weighed on an electronic balance. Sample
volume was determined by tipping the loose
fragments into a 5 ml measuring cylinder, lifting
2 cm above the table and dropping gent ly 10 times
to obtain a homogenous packing of the loose
particles in the cylinder. Thereafter, separate
specimens were flash-heated in a muffle furnace
at 400 and at 900º C for three minutes. One minute
was set for the temperature to equilibrate and two
minutes as the holding time. The weight of the
samples and their volumes were again determined
566
S. Hillier et al.
immediately after heating. Heating at 400ºC was
aimed at assessin g the effect of interlayer water loss
only, compared to 900ºC at which loss of both
interlay er water and water derived from dehydrox-
ylation of structural hydroxyl was expected.
Additionally, ten large (~1 cm
2
sized) flakes of
sample KL-2 were also tested by flash-heating at
900ºC under identical conditions to the 12mm
fraction samples.
Chemical exfoliation procedure
Following the results of thermal exfoliation tests,
all original samples were also tested for their abili ty
to be chemically exfoliated. This was conducted by
immersion/contact of small aliquots of the 12mm
fractions consisting of several tens of particles with
30% H
2
O
2
overnight.
RESULTS
Mineralogical composition
The XRD patterns of the spray-dried random
powders of the samples are presented in
Fig. 1a and b, and quantitative analyses of the
samples by the full pattern fitting method are
given in Table 1. XRD results show that two of the
samples (KL-2, Ver-18) are dominated by true
vermiculite. Indeed, sample KL-2, shows no
evidence of any other miner als. This s ample
appears to be 100% pure true vermiculite, with
integral basal reflectionsbasedonaspacingof
~14.4 A
˚
and additional non-basal peaks which
conform precisely to the typical powder pattern of
a trioctahedral vermiculite (Brindley & Brown,
1984). Sample Ver-18 has an almost identical XRD
pattern to that of KL-2, the only minor difference
being the presence of some peaks which indicate
that a minor amount of chlorite is also present in
this sample. At low angles the chlorite basal pea ks
(001, 002) a re coincident with basal peaks from the
vermiculite but greater resolution occurs at higher
angles (Fig. 1a). The control phlogopite also shown
in Fig. 1a is confirmed as pure phlogopite mica
with no evidence of any vermiculite either as a
discrete phase or as an interstratification. Figure 1b
shows XRD patterns of the three samples PB, MK-
1 and Ver-2. Two of these samples, Ver -2 and
MK-1 contain substantial true vermiculite, but both
samples also contain substantial hydrobiotite
(Table 1). The presence of hydrobiotite is indicated
most obviously by strong but broad peaks at
around 12 an d 24 A
˚
. The largest spacing peak is
sometimes referred to as a superlattice peak since
it arises from an ordered interstratification of 1 0 A
˚
(mica) and 14 A
˚
(vermiculite) layers. The term
hydrobiotite has been precisely defined ( Brindl ey
et al., 1983) but we u se it here in its most general
sense to de scribe mica/vermiculite mixed-layer
minerals, with sub-equal proportions of layer
types, although it is likely that the specimens we
have examined would conform to the strict
definition. The similar name hydrophlogopite,
although widely used by analogy, is discredited
as a mineral name. Hydrobiotite is the dominant
phase in s ample PB accounting for about 95% of
the sample, only a trace of true vermiculite being
present in this sample. Both sample PB and Ver -2
also contain minor amounts of discrete phlogopite
(Ta ble 1). The presence of accessory minerals in
vermiculites and hydrobiotites is common (Frank
& Edmond, 2001; Hindman, 2006) and other
phases identified by XRD in the various samples
include minor to trace amounts of quartz,
amphibole and talc.
TABLE 1. Quantitative mineral ogical composition of the six samples (<2>1mm sized) by full pattern fitting.
Analytical uncertaint ies at 95% confidence are given following the method described in Hillier (2003).
Vermiculite Hydrobiotite Phlogopite Chlorite Talc Quartz Amphibole
KL2 100.0 5.0
VER-18 92.0 4.9 7.4 2.0 0.6 0.8
VER-2 46.7 3.8 34.6 3.5 12.1 2.4 0.5 0.8 2.1 1.3 2.1 1.3 1.9 1.3
MK1 25.6 3.1 62.1 4.2 5.1 1.8 7.2 2.0
PB 94.6 4.9 5.1 1.8 0.3 0.7
PHL-3 98.5 5.0 1.4 1.1
Exfoliation of verm iculite
567
