a SpringerOpen Journal
Kaufhold et al. SpringerPlus 2014, 3:598
http://www.springerplus.com/content/3/1/598RESEARCH Open AccessPorosity and distribution of water in perlite from
the island of Milos, Greece
Stephan Kaufhold1*, Anke Reese2, Werner Schwiebacher3, Reiner Dohrmann1,4, Georg H Grathoff5,
Laurence N Warr5, Matthias Halisch6, Cornelia Müller6, Ulrich Schwarz-Schampera1 and Kristian Ufer1Abstract
A perlite sample representative of an operating mine in Milos was investigated with respect to the type and spatial
distribution of water. A set of different methods was used which finally provided a consistent view on the water at
least in this perlite. Infrared spectroscopy showed the presence of different water species (molecular water and
hydroxyl groups / strongly bound water). The presence of more than 0.5 mass% smectite, however, could be
excluded considering the cation exchange capacity results. The dehydration measured by thermal analysis occurred
over a wide range of temperatures hence confirming the infrared spectroscopical results. Both methods point to
the existence of a continuous spectrum of water binding energies. The spatial distribution of water and/or pores
was investigated using different methods (CT: computer tomography, FIB: scanning electron microscopy including
focused ion beam technology, IRM: infrared microscopy). Computer tomography (CT) showed large macropores
(20 – 100 μm) and additionally revealed a mottled microstructure of the silicate matrix with low density areas up
to a few μm in diameter. Scanning electron microscopy (FIB) confirmed the presence of μm sized pores and IRM
showed the filling of these pores with water. In summary, two types of pores were found. Airfilled 20 – 100 μm
pores and μm-sized pores disseminated in the glass matrix containing at least some water. Porosity measurements
indicate a total porosity of 26 Vol%, 11 Vol% corresponding to the μm-sized pores. It remains unsolved wether the
water in the μm-sized pores entered after or throughout perlite formation. However, the pores are sealed and no
indications of cracks were found which indicated a primary source of the water, i.e. water was probably entrapped
by quenching of the lava. The water in these pores may be the main reason for the thermal expandability which
results in the extraordinarily porous expanded perlite building materials.
Keywords: Perlite; Porosity; Water distribution; Volcanic glass; FIB; CT-analysis1 Introduction
Perlite is a hydrous volcanic material dominated by alu-
mosilicate glass. As an industrial material, perlite is
mostly used in its expanded form, i.e. after heat treat-
ment resulting in a light-weight macroporous product.
Most of the expanded perlite is used for building
construction in plasters, mortars, and tiles. Minor com-
ponents of perlite are phenocrysts or microlites which
formed before eruption of the magma (e.g. feldspar or
biotite). Technically, the term perlite is used for glassy vol-
canic rock which can be thermally expanded to about 20
times of its volume (Koukouzas et al. 2000). Scientifically* Correspondence: s.kaufhold@bgr.de
1BGR, Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2,
D-30655 Hannover, Germany
Full list of author information is available at the end of the article
© 2014 Kaufhold et al.; licensee Springer. This i
Attribution License (http://creativecommons.or
in any medium, provided the original work is pthe term perlite is used for hydrated volcanic glass. The
most common volcanic glass is obsidian. The water
content of typical obsidian is about 0.1 mass%. Larger
water contents of obsidians mostly result from post-
emplacement secondary hydration. Perlites, in contrast,
contain up to 5 mass% water. They are believed to form
upon hydration of volcanic glass (Ross and Smith 1955).
Classical perlites (round particles with an onion like ap-
pearance) (Lorenz and Gwosdz 2000) are distinguished
from banded perlites (Allen 1988). The water required for
perlite formation (glass hydration) is supposed to enter
through small cracks present in the volcanic glass.
Diffusion of water into the glass may cause the cracks
or crack formation facilitates water diffusion (Marshall
1961; Friedmann et al. 1966; Denton et al. 2009). At
the wall of the cracks the glass dissolves and smectites an Open Access article distributed under the terms of the Creative Commons
g/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction
roperly credited.
Kaufhold et al. SpringerPlus 2014, 3:598 Page 2 of 10
http://www.springerplus.com/content/3/1/598crystallizes as alteration proceeds (Denton et al. 2009).
The typical hydration shells around the primary parti-
cles are about 20 μm in size (Friedmann et al. 1966).
The smectite at the walls would explain the presence
of both, hydroxyls and molecular water in the perlite.
Keller & Picket (1954) detected hydroxyls as well as
hydrogen bound water using infrared (IR) spectros-
copy. According to Friedmann et al. (1966) both
primary magmatic and meteoric water causing the
post-formational hydration of the glass can be found in
perlites. This water, in contrast to the hydration shell, was
supposed to be located within the Al-Si-framework. The
difficulty of distinguishing different types of perlite water
also results from the fact that the primary water of the
volcanic glass varies in both amount and composition,
with variable mixtures of hydroxyls and molecular water
(Stolper 1982; Eckert et al. 1988; Dobson et al. 1989; Silver
et al. 1990; Pandya et al. 1992).
The water in perlites can be measured by IR spectros-
copy. Dobson et al. (1989) found different binding ener-
gies which could be correlated with IR stretching
vibration. The amount of water is measured e.g. consid-
ering the 3550 cm−1 vibration (e.g. (Nichols et al. 2002)).
Also NIR spectroscopy can be used (Stolper 1982).
Differential thermal analysis (DTA) is commonly used
for distinguishing hydroxyls and molecular water in clay
minerals. However, few studies use DTA for hydrated
volcanic glasses. One DTA curve was published by
Tazaki et al. (1992) but the almost continuous mass de-
crease was difficult to interpret.
Few studies about the spatial distribution of water in
perlites were published. Tazaki et al. (1992) studied freshly
hydrated volcanic glasses with TEM and described spher-
ical structures containing hydroxyls and concluded that
these spherical structures could be precursors to the for-
mation of clay minerals. Wysoczanski and Tani (2006)
published IR image analysis based on the 3550 cm−1 vibra-
tion. They found that the glass particles all contained
some water whereas low water domains corresponded to
phenocrysts. However, they could not reveal the water dis-
tribution within the glass.
The present study, therefore, was conducted to im-
prove the understanding of the spatial distribution
of water in perlites. Knowledge about the spatial
distribution would improve the understanding of the
thermal expansion of technically produced expanded
perlite.
Both, bulk methods to characterize the binding of the
water (infrared spectroscopy, IR, and differential thermal
analysis combined with a mass spectrometer, DTA-MS)
were used along with three different 2D or 3D methods
(computer tomography, CT, infrared microscopy, IRM,
and focused ion beam technology in combination with a
scanning electron microscope, FIB).The perlite investigated in the present study was from
the most important European perlite mines located in
Milos, Greece. Most of the European perlite is derived
from the Greek islands of Milos, Kimolos, and Kos
(Koukouzas et al. 2000). From the middle part of the
Upper Pliocene to the late Quarternary, Milos was more
or less continuously affected by extensive volcanism.
The perlites are supposed to result from the youngest
volcanic activity in Milos which in contrast to the
older ones is supposed to result from relatively shallow
magma chambers (Fytikas et al. 1986). This perlite is
characterized as calc-alkaline rhyolite with commonly
more than 85 mass% glass and some phenocrysts.
According to Koukouzas and Dunham (1994) different
textural types can be distinguished: pumiceous perlite
(light-weight and frothy features, only present in mines),
hard perlite (dense and reddish and only appearing in
the Kerdari Cap), and classical perlite (dense and
mostly in contact with the hard and the pumiceous
perlite in the mines). Much is known about the geo-
logical history of Milos and some information about
compositional variation of Milos perlites and their ap-
plicability is available (Koukouzas et al. 2000) but none
about the spatial distribution of the water within the
perlite.
2 Materials and methods
The perlite investigated in the present study was col-
lected in a running perlite mine in Milos (Greece). Three
different specimens representing three different regions
in the outcrop were collected. They differed with respect
to their color (reddish, cream, white). However, no dif-
ference in water content or type of water (measured with
DTA-MS and IR) was detected. Therefore, each of them
would have been suitable for studying the spatial distri-
bution of water of perlite of this deposit. One of the
cream white particles (about 3 cm3) was selected for
producing a polished thin section. This particle (the part
not used for the production of the thin section) was fur-
ther used for the investigation of the spatial distribution
of water. For the computer tomography (CT) investiga-
tion a small piece (ca. 1 mm3) was cut off. In addition
an expanded 1 mm3 sized perlite specimen (again about
1 × 1 × 1 mm = 1 mm3) that was taken from an industri-
ally produced product (thermally expanded perlite) was
investigated with CT.
For the characterization of the bulk material, three
particles with the different colors were ground together
to represent the entire outcrop.
The methods used were selected to i) characterize the
bulk material and ii) to gather information about the spatial
distribution of pores and or water. The latter was thought
to be difficult because of expectedly small pores and/or
water domains. Therefore, apart from light microscopy,
Kaufhold et al. SpringerPlus 2014, 3:598 Page 3 of 10
http://www.springerplus.com/content/3/1/598methods with higher resolution and different strengths
were used.
Bulk material characterisation
X-ray diffraction (XRD) patterns were recorded using a
PANalytical X’Pert PRO MPD Θ-Θ diffractometer (Cu-
Kα radiation generated at 40 kV and 30 mA), equipped
with a variable divergence slit (20 mm irradiated length),
primary and secondary soller slits, a Scientific X’Celera-
tor detector (active length 0.59°) and a sample changer
(sample diameter 28 mm). The samples were investigated
from 2° to 85° 2Θ with a step size of 0.0167° 2Θ and a
measuring time of 10 sec per step. For specimen prepar-
ation, the top loading technique was used and quantifica-
tion performed based on Kaufhold et al. (2010).
The chemical composition of powdered samples was de-
termined using a PANalytical Axios. Samples were pre-
pared by mixing with a flux material (Lithiummetaborate
Spectroflux, Flux No. 100A, Alfa Aesar) and melting into
glass beads. The beads were analyzed by wavelength
dispersive X-ray fluorescence spectrometry (WD-XRF). To
determine loss on ignition (LOI) 1000 mg of sample ma-
terial was heated to 1030°C for 10 min.
For measuring mid (MIR) infrared spectra the KBr
pellet technique (1 mg sample/200 mg KBr) was applied.
Spectra were collected on a Thermo Nicolet Nexus FTIR
spectrometer (MIR beam splitter: KBr, detector DTGS
TEC). The resolution was adjusted to 2 cm−1.
Thermoanalytical investigations were performed using
a Netzsch 449 F3 Jupiter thermobalance equipped with a
DSC/TG sample holder linked to a Netzsch QMS 403 C
Aeolus mass spectrometer (MS). 100 mg of powdered
material previously equilibrated at 53% relative humidity
(RH) was heated from 25–1100°C with a heating rate of
10 K/min.
The cation exchange capacity (CEC) was measured
using the Cu-Triethylenetetramine method (Meier and
Kahr 1999; Kaufhold and Dohrmann 2003). A sample
mass of 0.3 and 0.4 g was used to increase the smectite
detection limit up to about 0.5 mass%.
The porosity was determined based on measuring the
particle or specific density (AccuPyc 1330 of micromeri-
tics using He) and bulk or envelope density (micromeri-
tics GeoPyc 1360 using a free-flowing, finely divided, dry
powder (DryFlow®) as the fluid medium instead of a li-
quid with a lower diameter limit of 50 μm). Different
types of samples were investigated: powder, 1–2 mm
particles, >2 mm, and after melting the sample (1150°C
for 6 hours).
Methods to gather spatial information
Water can be detected by its characteristic infrared vi-
brations and the spatial distribution can be probed by
IR-microscopy. In the present study a Thermo NicoletContinuum FT-IR microscope was used. A freshly
broken even surface of the perlite was fixed on the x-y
stage. The following experimental conditions were se-
lected: beam splitter CaF2, detector MCT/A, aperture 5,
spectroscopic range of each spectrum 1000 – 4000 cm−1
with a resolution of 4 cm−1 and 16 scans each, and the
step width was 3 μm. Because of the typically low inten-
sity at large wave length the water deformation band at
about 1635 cm−1 was selected to produce a 2-D plot.
The micro-computed tomography (μ-CT) imaging was
performed with an “nanotom s 180” device, developed
by GE Sensing & Inspection Technologies and using the
product line of phoenix x-ray. This CT has a special
high power nanofocus tube (180 kV/15 W) with an ad-
justable focal spot size down to 1 μm in diameter, which
enables very sharp imaging data sets. After the scanning
process, the 3D data sets have been evaluated with VG
Studio Max 2.0. Phase segmentation has been performed
by using quantification tools, such as edge detection and
phase contrast filter operations, to ensure high accuracy
phase thresholding and volume determination. After-
wards, 2D as well as 3D visualization of regions of interest
within the samples took place. The scanning parameters
for the investigated samples were voltage 55 kV, current
80 μA, projections 1500, average 7, skip 3, timing 1000 ms
and voxel size 1.06 μm.
For the high-resolution SEM focused ion beam (FIB-
SEM) investigation a Zeiss Auriga field equipped with a
field emission cathode and extra-large charge coupled
device (CCD) 80 mm2 CCD detectors for energy disper-
sive x-ray (EDX) analyses was used. Before starting the
milling process the sample was sputtered with Pd. A
“slice-and –view” procedure was run by milling 25 nm
thick cuts in the form of a cross-section 20 × 20 μm in
size. A full description of the methodology is described
in Warr and Grathoff (2011).
3 Results
3.1 Basic characterization of the perlite
The representative perlite sample consisted mainly of
silicate glass (“amorphous” in Figure 1). Minor compo-
nents were biotite, quartz, and feldspar. The material
can be characterized as peraluminous rhyolite glass with
biotite, quartz, and feldspar phenocrysts. The most
abundant element was Si (75 mass% SiO2), followed by
Al2O3 and minor amounts of the common earth and al-
kaline metal oxides. The chemical composition along
with the LOI were determined after drying of the sample
at 105°C. Hence, externally bound water is not included
in the LOI value given in Figure 1. The mineralogical
composition was quantified using the XRD Rietveld
technique, which confirmed the presence of more than
90 mass% of an amorphous phase and about 5 mass%
minor crystalline components (2.6 mass% quartz, 2.6
Figure 1 Mineralogical and chemical composition of the perlite sample.
Kaufhold et al. SpringerPlus 2014, 3:598 Page 4 of 10
http://www.springerplus.com/content/3/1/598mass% feldspar, 0.6 mass% biotite). SEM-EDX investiga-
tions and light microscopy of the polished section revealed
some additional minor components, namely apatite,
zircon, hornblende, and ilmenite, which were below
the XRD detection limit.
The cation exchange capacity was practically 0 meq/
100 g. In none of the 6 separate measurements any
systematic reduction of the extinction at 578 nm was
detected. The smectite content, therefore, is less than
0.5 mass%.
The microstructure of the perlite was investigated by
SEM and CT (Figure 2). The low density of the perlite
results from the extensive network of pores between 20
and 100 μm. The significantly higher porosity of the
industrially expanded perlite was imaged. The pore
walls of this material are comparably thin. The section
through the expanded perlite shows a hierarchy of differ-
ent silicate skeletons (Figure 3, right). Mostly clusters of
smaller pores are arranged in a silicate skeleton with
slightly thicker walls (circle in Figure 3).
3.2 Characterization of the water binding
To investigate the binding energies of the water, ther-
mal analysis and IR measurements were conducted.
Thermal analysis showed a dehydration peak rangingFigure 2 Microstructure of raw perlite visualized by CT (left) and SEMfrom approximately 100°C up to 400°C (Figure 4) with
a peak slightly below 300°C. This peak was observed in
the DSC curve - with increasing temperature with the
negative values reflecting the endothermal dehydration
reaction and the upward peak indicating an exothermal
reaction such as recrystallization occurring at ca. 930°C -
as well as in the water mass spectrometer (MS) curve
(mass 18). Other natural water containing materials
such as swelling clays or even zeolites typically show
much sharper dehydration peaks i.e. a water desorption
within a smaller temperature range. The small DSC-
peak at 573°C resulted from the quartz inversion. The
amount of water lost from 100 to about 600°C was 2.7
mass% (read from the TG curves, not shown here)
which is in reasonable agreement with the LOI data
shown before.
The MIR spectrum after drying for 24 h at 150°C in
vacuum still showed the presence of molecular water
(1630 cm−1 + intensity around 3400 cm−1) along with
the typical Si-O and Al-O-Si vibrations resulting from
the crystalline and the amorphous silicates (Figure 5). In
the OH stretching region no distinct band could be found.
Instead a broad band ranging from the typical OH stretch-
ing region of Al-rich clay minerals (±3630 cm−1) to the
region being typical of water adsorbed in micropores(right).
Figure 3 Microstructural investigation of expanded perlite by CT (left pseudo 3D image, right 2D cross-section). The circle shows that
small pores with thinner walls are included in spherical structures with somewhat larger walls.
Kaufhold et al. SpringerPlus 2014, 3:598 Page 5 of 10
http://www.springerplus.com/content/3/1/598(e.g. 3410 cm−1 in the case of ferrihydrite) was observed.
The DTA curve could be interpreted in a way that the
water inside the silicate glass could not leave at lower tem-
peratures, which would shift the dehydration towards larger
temperatures. However, such kinetic effects do not affect
the IR spectrum. Therefore, both, the IR and DSC results,
indicate the presence of a continuous range of differently
bound water molecules ranging from structural OH groups
similar to those present in clay mineral structures to free
molecular water. The bands at larger wavenumbers (e.g.
3620 cm−1) can also result from water bound directly to
cations (strongly bound to a surface as inner sphere com-
plex (Bishop et al. 1994)).3.3 Spatial information
Light microscopy
Light microscopy was performed to find fluid inclusions
which cannot be detected by SEM or CT methods. Both
types of perlite texture, the classical texture (Figure 6,
left) and the banded type of perlite (Figure 6, right), were
observed. Light microscopy also revealed an extensive
network of cracks which could have been the pathway
for water which hydrated the volcanic glass.Figure 4 DSC curve (left) and MS-H2O curve (right) of the perlite samComputer tomography (CT)
CT allows investigation of the 3D structure of a sample and
provides 2D sections. The grey values of the image shown
in Figure 7 (1 μm thick 2D section) correspond to the dens-
ity of the investigated particles. A mottled structure was
observed within the glass particles. Hence even the interior
of the volcanic glass is not as homogeneous as expected.
Disseminated dark spots were observed which indicated a
possibly existing porosity. The diameter of these spots was
1–4 μm. The spots were dark because they were transpar-
ent in the 1 μm section due to water or gas filling.
In addition an automatic grey scale analysis was per-
formed. Most of the detected grey values were black
(corresponding to interval 1 in the top right insert in
Figure 7). The second most grey value was attributed to
the volcanic glass (interval 3). However, a closer look at
the interval 3 peak reveals a shoulder towards slightly
lower density grey values. These could be the hydrated
walls of the glass surface or otherwise represent an
artifact. This grey value interval 2 was selected and the
corresponding areas were marked in blue (Figure 7).
These particles or areas are almost entirely located at
the particle edges indicating the beginning of hydration
of the outer surfaces of the glass particles.ple.
Figure 5 IR spectrum of the perlite sample (pellet dried for 24 h at 150°C in a vacuum oven).
Kaufhold et al. SpringerPlus 2014, 3:598 Page 6 of 10
http://www.springerplus.com/content/3/1/598FT-IR-microscopy
The investigated area of the even surface (unpolished)
was approximately 150 μm × 100 μm (Figure 8). Altogether
more than 2000 spectra were collected across this area. The
individual spectra were dominated by the Si-O-stretching
bands. Because of the reflectance mode the quality of
the spectra was poor but differences in the water
deformation region around 1635 cm−1 were observed.
After normalization of all spectra the extinction of this
band was plotted as 2-D image (Figure 8). This image
proved the presence of water in several small areas with
a diameter of about 5 μm. These domains appear circu-
lar, which is an artifact caused by the resolution of the
microscope which is not sufficient to detect the shape
of the water domains. Nevertheless, the IR-microscopy
showed the presence of water in these small domains.
FIB-SEM
Crossbeam FIB-SEM investigations carried out in the high-
vacuum mode provided much clearer SEM images of the
glass and of possible hydration products. In the left image
of Figure 9 small sub-micron sized flakes were observed.Figure 6 Light microscopical analysis of the thin section of the perliteThese could be dust but they were evenly distributed over
the surface and showed a smectite like morphology (the
typical rose like aggregates). In the right image of Figure 9
a coating of some parts of the surface was observed which
again considering the morphology could be clay minerals.
These phases could be smectites or other hydration prod-
ucts of the volcanic glass. However, the content of these
clay minerals is much below the detection limit of the
XRD and CEC method as discussed before. They can only
be found on a few surfaces and hence they are not import-
ant from a statistical point of view.
The 2D polished slices and “slice-and-view reconstruc-
tions” (Figure 10) show the size and shape of two of the
pores and potential alteration rims around the pore as well
as minor heterogeneities in the glass (Figure 10). The back
scatter electron (BSE) images revealed pores coated by
higher density glass, which may be partly an artifact, based
on that some of the rims contain Gallium derived from
the milling processes. Using the 3D-visualisation soft-
ware AVIZO Fire (www.vsg3D.com) one of the pores was
investigated in more detail (Figure 10, right). Some colum-
nar structures were observed in the pore.sample.
Figure 7 Grey value analysis of a CT section and magnified detail of the image. The top right insert shows the grey value distribution over
the entire sample.
Kaufhold et al. SpringerPlus 2014, 3:598 Page 7 of 10
http://www.springerplus.com/content/3/1/598Porosimetry
The porosity was determined by He- and Dryflow®-pyc-
nometry. The density of the glass without any pores
(after melting) was about 2.6 g/cm3 a typical value of
silicate glasses. Without extensive heat treatment and
grinding the μm-sized matrix porosity was thought to be
intact but He could of course - only - enter the larger
macropores. The specific density of this material was
about 2.3 g/cm3. This difference, therefore, can be ex-
plained by the existence of the μm-sized matrix-porosity
accounting for 11 Vol%. The Dryflow®-density of the
larger grain fraction was 1.8 g/cm3 which results in a
total porosity of about 26 Vol%. This value is supposed
to include the macropores. The porosity of the silicate
matrix is about 10%.
Discussion
Infrared spectroscopy showed extinction over a wide
range of wavenumbers covering both the OH-stretching
region of clay minerals as well as the spectral region typ-
ical of free water. This indicated the presence of a range
of water binding forms which is in accordance with
Dobson et al. (1989). As a further tool to study water
binding, differential thermal analysis was applied which
confirmed the IR results. The MS-H2O curve (evolved
gas analysis measured by mass spectrometry) showed
dehydration over an unusually broad temperature range
(from 100 to 400°C) which was already observed by Tazaki
et al. (1992). Desorption of free water is commonly ob-
served between 100 and 200°C well separated from dehy-
droxylation (500–750°C). However, no separate secondpeak which could be assigned to dehydroxylation was ob-
served. Instead one broad peak was found which may cor-
respond to both dehydration and dehydroxylation. This
conclusion is supported by the IR results which also indi-
cated the presence of a range of different water bindings,
ranging from the typical hydroxyl stretching vibrations to
adsorbed water. The hydroxyls do not result from smectite
because the smectite content as determined by the cation
exchange capacity (CEC) method was ≤0.5 mass%. This
content was too low to conclude that the hydroxyls were
associated to smectite. One explanation would be that the
walls of an extensive pore system are covered with hy-
droxyls or at least strongly bound water which in terms of
binding energy are similar to clay mineral hydroxlys but in
this case only cover the very surface.
Using the thermal gravimetry curve the water liberated
from the perlite could be quantified (about 2.7 mass%).
This value was slightly larger than the loss on ignition
(LOI = 2.5 mass%) which can be explained by the fact
that the sample was stored at 53% r.H. prior to thermal
analysis but dried at 60°C prior to LOI determination.
Both values, however, are in good agreement.
The aim of the present study was to investigate the
spatial distribution of the water. Different methods were
used, each with specific strengths and weaknesses. Com-
puter tomography provided a 3D image of density distri-
butions. Using image analysis a grey value distribution
was calculated which showed slightly darker surfaces of
the particles which indicated areas of lower density,
probably a beginning hydration. As explained above, the
smectite content was too low to explain this observation.
Figure 8 Infrared microscopy of the perlite sample. Top: 2-D plot of the extinction of the 1635 cm−1 vibration of water, bottom: photograph
of the perlite surface and investigated area.
Figure 9 SEM (high-vacuum) image of some pores of the perlite showing indications for hydration products.
Kaufhold et al. SpringerPlus 2014, 3:598 Page 8 of 10
http://www.springerplus.com/content/3/1/598
Figure 10 SEM (high vacuum) image of FIB slice 107 (left) and AVIZO Fire-visualization (3D slice and view reconstruction) of one of the
pores based on the FIB measurements (right, image width 10 μm).
Kaufhold et al. SpringerPlus 2014, 3:598 Page 9 of 10
http://www.springerplus.com/content/3/1/598It could be explained by a small hydration layer but fur-
ther interpretations would be speculative. The most
interesting CT-results were found by considering 1 μm
slices (2D assessment of the 3D data set). Of course the
large pores which were observed with light microscopy
were well resolved. In addition, however, the massive
glass particles appeared uneven. Dark spots close to the
resolution of the method were observed all over the glass
particles. Analytical artifacts were excluded and the hy-
pothesis was that these areas could be pores. To further
investigate this assumption high resolution scanning elec-
tron microscopy in combination with the focused ion
beam technology allowing 3D analysis was used. This
method confirmed the presence of pores in the μm range
and hence supported the view that the density heterogen-
eity measured by CT corresponded to a system of pores.
FIB-SEM proved that the pores are closed and isolated.
Neither CT nor FIB were suitable to determine eventual
pore filling. Therefore, IR microscopy was used. The ex-
tinction at 1635 cm−1 over a 150 × 110 μm area showed
spots indicating water. The shape of these spots was an
artifact because of the maximum resolution of method.
However, IR microscopy proved the presence of water in
small domains. At first sight these domains were consid-
ered to be analytical artifacts. However, the size and distri-
bution of these domains fits well with the pores found byFigure 11 Concluding figure showing the mottled like microstructure
presence of small pores detected by the SEM (FIB).CT and FIB. Therefore, these domains are supposed to
correspond to the pores observed by CT and FIB and,
moreover, identified at least a part of the filling of the
pores. To show that the three methods probably identified
the same pores all results were plotted in one figure (to be
able to compare the scale; Figure 11).
The total porosity was 26 Vol% and the μm-sized
matrix-porosity accounted for 11 Vol%. IR microscopy
showed that the pores are at least partially filled with
water. A 3D-FIB visualization of one pore showed col-
umnar structures in the pore. These could be precipita-
tions or newly formed phases. Further discussion about
them would be highly speculative.
Most of the perlites are supposed to have formed upon
post-emplacement hydration. The μm-sized finely dis-
tributed pores identified in the present study, however,
are ubipresent in the volcanic glass and the pores are
not supposed to be connected. The pores observed are
all closed. One option to explain the abundance of this
water is that it was trapped when the magma was
quickly cooled. This had to be investigated further, e.g.
using isotope methods.
Summary and conclusions
Perlites are believed to form upon post-formational hydra-
tion often leading to smectite formation. The presence ofrevealed by CT, the filling of pores with water (IR), and the
Kaufhold et al. SpringerPlus 2014, 3:598 Page 10 of 10
http://www.springerplus.com/content/3/1/598smectite in the investigated sample, however, can be ex-
cluded. CEC measurements with high sample masses are
accurate enough to prove that less than 0.5 mass% smect-
ite was present. Infrared spectroscopy and thermal analysis
showed the presence of a continuous range of water bind-
ing, ranging from hydroxyls/strongly bound water to mo-
lecular water. Using CT and FIB an extensive pore system
of closed pores was found with pore diameters in the
range of 1–5 μm. In the same range, infrared microscopy
revealed domains with significant extinction in the water
deformation region. Because of similar size and distribu-
tion these signals were believed to represent the filling of
the pores. According to the consistent picture gained from
applying a set of different methods, the glass particles of at
least the investigated material contain appreciable small
water filled pores. It remains unsolved wether the water in
these pores entered after or throughout the emplacement.
However, the pores are sealed and no indications of cracks
were found which indicates a primary source of the water,
i.e. water was probably entrapped by quenching of the
lava. The water in these pores may be important for the
possible formation of clay minerals out of perlites and
may have implications for the formation of bentonites in
Milos. The water in the μm-sized pores may be the main
reason for the thermal expandability of this perlite.Competing interests
The authors declare that they have no competing interests.Authors’ contributions
SK was the supervisor of the study and wrote most of the text, AR – as
bachelor student – did a lot of analytical work in the lab and the literature
study, WS provided samples, access to the pit and the scientific question,
RD took care about the smectite content determination and revised the
manuscript, GG and LW performed the FIB measurements and the 3-D
segmentation of the SEM data, CM performed the μ-CT measurements
which were further processed by MH, US-S did the light-microscopical
analysis, and KU calculated the quantitativemineralogical analysis based
on XRD Rietveld. All authors read and approved the final manuscript.
Author details
1BGR, Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2,
D-30655 Hannover, Germany. 2Georg-August-University Göttingen,
Geowissenschaften und Geographie, Goldschmidtstr. 3, 37077 Göttingen,
Germany. 3Knauf, Knauf Aquapanel GmbH, Kipperstraße 19, 44147 Dortmund,
Germany. 4LBEG, Landesamt für Bergbau, Energie und Geologie, Stilleweg 2,
D-30655 Hannover, Germany. 5Ernst-Moritz-Arndt Universität Greifswald,
Institute für Geographie and Geologyie, Friedrich-Ludwig-Jahn-Str. 17a,
D-17487 Greifswald, Germany. 6LIAG, Leibniz-Institut für Angewandte
Geophysik, Stilleweg 2, D-30655 Hannover, Germany.
Received: 1 October 2014 Accepted: 6 October 2014
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doi:10.1186/2193-1801-3-598
Cite this article as: Kaufhold et al.: Porosity and distribution of water in
perlite from the island of Milos, Greece. SpringerPlus 2014 3:598.
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