OXIDATION/REDUCTION OF IRON IN IRON-RICH
TOURMALINE FROM PEGMATITES AS A RESPONSE TO
THE HIGH-TEMPERATURE TREATMENT
Jan Filip1
Milan Novák2
3
Ferdinando Bosi & Henrik Skogby4
1
Centre for Nanomaterial Research, Palacký University, Olomouc, Czech Republic
Department of Geological Sciences, Masaryk University Brno, Czech Republic
3
Department of Earth Sciences, Sapienza University of Rome, Italy
4
Department of Mineralogy, Swedish Museum of Natural History, Stockholm, Sweden
2
Keywords: tourmaline, pegmatites, high-temperature treatment
INTRODUCTION
Tourmaline is a typical accessory
to minor and rather exceptionally also
major mineral in rocks of highly variable
origin and chemical composition, including
granitic pegmatites. Although tourmalines
serve as a very suitable indicator of
compositional evolution of the host rocks
chiefly due to its ability to incorporate
large number of elements (Selway et al.
1999), chemical composition of tourmaline
is significantly controlled also by crystalstructural constraints. The general
formula of tourmaline can be expressed as
XY3Z6(T6O18)(BO3)3V3W (X = Na, Ca, □; Y
= Mg, Fe2+, Mn2+, Al, Fe3+, Mn3+, Li; Z =
Al, Fe3+, Cr3+, Mg, Fe2+; T = Si, Al, B; V
= OH, O and W = OH, F, O). Tourmaline
of schorl composition is by far the most
common tourmaline found in pegmatites.
Nevertheless, Bosi & Lucchesi (2007)
recently predicted that ordered schorl is
structurally unstable and natural schorl
always shows Fe and Al disorder over Y-site
and Z-site. In order to better understand
such constraints, tourmaline samples of
rather simple chemical composition were
heat-treated under various conditions to
reveal role of Fe2+/Fe3+ and their ordering
in crystal structure of Mg-poor, Li-free and
Estudos Geológicos v. 19 (2), 2009
F-poor samples (oxy-schorl, foitite-olenite).
Up to now there is a lack of information
dealing with high-temperature behavior of
tourmaline – the limited number of studies
include those of Kraczka et al. (2000), Fuchs
et al. (2002), Pieczka & Kraczka (2004), Ertl
& Rossman (2007), Castaneda et al. (2006)
and McKeown (2008). However, all those
published experiments were performed
exclusively in oxidizing conditions (i.e.,
in air) on restricted range of chemical
compositions of tourmaline samples.
Experiments performed under reducing
and inert atmospheres could bring results
describing more comprehensively the role
of iron in tourmaline structure in relation
to occupancy of V- and W-sites and also to
better assess the temperature paths of both
tourmaline post-crystallization history
and host pegmatite evolution. Moreover,
high-temperature experiments performed
on well defined tourmaline crystals could
provide essential knowledge applicable
not only in geosciences (mineralogy,
petrology), but also for tourmaline
applications in material science and
chemistry. In the present paper we report
on the thermally-induced oxidation/
reduction of iron in iron-rich tourmalines
under air and hydrogen atmospheres at
700, 800 and 900 °C.
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OXIDATION/REDUCTION OF IRON IN IRON-RICH TOURMALINE FROM PEGMATITES AS A
RESPONSE TO THE HIGH-TEMPERATURE TREATMENT
MATERIAL AND METHODS
Two iron-rich tourmaline samples,
differing in their compositions and origin,
were used in this study:
1) Black, fine-grained aggregates
and individual grains of tourmaline, up
to 5 mm in size, from irregular pegmatite
patches enclosed in biotite-nephelineplagioclase gneisses at the Cancrinite
Hill, east of Bancroft, southern Ontario.
Pegmatite patches consist of albite,
dark blue sodalite and yellow cancrinite
as hosts of tourmaline, and accessory
magnetite, zircon and apatite. Tourmaline
is homogeneous, Na-rich (0.91-1.00 apfu),
Al-poor (5.55-5.68 apfu), Fe-rich (2.913.01 apfu Fe2+tot), Mg-poor (0.28-0.35
apfu) and F-poor (≤ 0.03 apfu). It indicates
along with the results of Mössbauer
spectroscopy (Figure 1) oxy-schorl.
2) Tourmaline from simply zoned
abyssal pegmatite, Miskovice (Czech
Republic) forms black crystals, up to 3
cm in size (Cempirek & Novák 2003).
Tourmaline is homogeneous, Na-poor
(0.54-0.57 apfu), Al-rich (7.48-8.19 apfu
Z
Al + YAl + TAl ), Fe-moderate (1.041.17 apfu Fe2+tot), Mg-poor (0.17-0.28)
and F-free. Low Na and Fe, high X-site
vacancy (0.34-0.40 apfu) and very high Al
indicate foitite-olenite.
In order to study reversibility of
gradual oxidation/reduction of iron within
the tourmaline structure, we have chosen
the annealing temperatures well below
the temperature where the structure
decomposition occurs (typically around
880 to 920 °C; Pieczka & Kraczka 2004),
but high enough to initiate the oxidation/
reduction of iron at reasonable period
of time (above 500-550 °C; Pieczka &
Kraczka 2004). Therefore, the tourmaline
crystals (mm-sized fragments) were
thermally treated in air and hydrogen
atmosphere at 700, 800 and 900 °C for
sufficient period of time to fully oxidize
122
and reduce iron within the tourmaline
structure. In air, the heating was performed
in conventional muffle furnace; whereas
for hydrogen the horizontal glass-tube
furnace was used. Crystals were placed
onto a sample holder and inserted into the
pre-heated furnace in order to eliminate
the heating-up time. After annealing,
all samples were quenched and both
ground into powder and also kept in the
form of thin single-crystal absorbers
for subsequent analyses. The process of
oxidation/reduction of iron was monitored
using Mössbauer and FTIR spectroscopy.
Parts of the crystals were used for the
single-crystal X-ray diffraction study.
57
Fe transmission Mössbauer spectra
of a powdered samples (grinded under
acetone) were accumulated in a constant
acceleration mode using a 57Co in Rh
source and 1024 channel detector at room
temperature and 25 K. The isomer shift
was calibrated with regards to an α-Fe
foil at room temperature. Spectra were
fitted by Lorentz functions using the
computer program CONFIT2000. For a
single-crystal Mössbauer spectroscopy,
the crystals were mounted onto a lead
plate with an aperture of appropriate size
and measured using of 57Co point source.
Unpolarized FTIR absorption spectra
in the OH stretching frequency region of
tourmaline samples were recorded from
4000-3000 cm-1 using a Bruker Equinox
55S spectrometer equipped with NIR
source, LN2 cooled InSb detector and CaF2
beam-splitter. Circular sample apertures
with 100 μm in diameter were used. A total
of 100 scans in air were performed for each
sample and the respective background
with a resolution of 4 cm–1.
Chemical composition was studied
on the electron microprobe Cameca SX
100 at accelerating voltage 15 kV, beam
current 30 nA, using well characterized
natural and synthetic minerals as standards.
Estudos Geológicos v. 19 (2), 2009
Jan Filip et al.
RESULTS AND DISCUSSION
As a result of annealing the tourmaline
fragments turned to dark brownish-red
(oxidized) or greyish-black (reduced)
when viewed through very thin edges. The
Mössbauer spectra of the tourmalines heattreated in air at 700 °C show a dramatic
increase of the Fe3+ component, up to 100
% of iron atoms when annealed for 111
hours (Figure 1). The spectral parameters
of the oxidized tourmalines are close to
those characteristic of buergerite (cf. Dyar
et al. 1998) - the spectrum can be fitted by
two strongly overlapped doublets of Fe3+
( EQ = 0.87–0.94 and 1.21–1.28 mm/s,
respectively). According to Pieczka &
Kraczka (2004) they could correspond
to [Fe3+Y/Z|O5(OH)] and [Fe3+Y/Z|O6]
Estudos Geológicos v. 19 (2), 2009
octahedra. Therefore, in the presence of
oxygen (i.e., air) the thermal oxidation
proceeds according to the reaction: Fe2+
+ OH- + ¼O2 = Fe3+ + O2- + ½H2O (cf.,
Pieczka & Kraczka 2004).
Under hydrogen atmosphere and
the same temperature and time (700 °C,
up to 111 h), there is no obvious change
in Fe3+/Fetot ratio (within the experimental
error). Moreover, the comparable Fe3+/Fetot
ratios were obtained when the tourmaline
fragment were heat-treated at 800 °C for a
period of time up to 111 hours, where the
partial melting on the surface of fragments
was observed. At 900 °C the tourmaline
totally decomposes after annealing for less
than 1 hour. The electron microprobe data
shows a release of fluorine upon initial
heating under hydrogen.
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OXIDATION/REDUCTION OF IRON IN IRON-RICH TOURMALINE FROM PEGMATITES AS A
RESPONSE TO THE HIGH-TEMPERATURE TREATMENT
Figure 1 - Mössbauer spectra of raw and heat-treated tourmaline samples from localities Bancroft
(left) and Miskovice (right). Dark gray - Fe3+, light gray - Fe2+, medium gray - Fe2.5+.
Subsequently, the fully oxidized
tourmaline samples were subjected to
repeated reduction under hydrogen at both
700 and 800 °C. However, only partial
reduction of Fe3+ was observed (Figure 1,
bottom). Again, the iron was fully reduced
just upon the tourmaline decomposition
124
(i.e., at temperature higher than 800 °C or
at 800 °C and annealing for more than 100
hours).
FTIR spectra, recorded before and
after annealing of tourmaline samples,
show an important variations within
the 3800 to 3400 cm-1 spectral region
Estudos Geológicos v. 19 (2), 2009
Jan Filip et al.
corresponding to OH-bond stretching.
Partial deprotonation – in the case of
oxidized samples – represents charge
compensation to the oxidation of Fe2+
in the Y-site. As the OH groups occupy
two structurally distinct sites, it is not
surprising that upon oxidation just one of
them vaporized (the one weaker bonded
within the structure; cf., Castaneda et al.
2000, 2006).
The possibility to fully oxidize the
iron within the tourmaline structure but
not to reduce the iron to higher extent
that was the original value could indicate
that the structure of almost pure endmember schorl would not be stable when
all the iron atoms are in Fe2+ form. This
is supported by the fact that prior full
reduction of the iron atoms the tourmaline
decomposes and immediately appears
the iron in metallic form (confirmed by
Mössbauer spectroscopy; not shown).
Therefore, the presented results imply
that tourmaline structure of end-member
schorl composition is not stable and must
be stabilized by the presence of certain
amount of ferric iron and/or disorder of
Fe and Al over the Y- and Z-sites (cf.,
Bosi & Lucchesi 2007). Moreover, with
respect to the ease of oxidation of iron in
tourmaline structure this could be used,
under some circumstances, to evaluate
the post-crystallization history of both
tourmaline and tourmaline host-rock (e.g.,
pegmatites).
Acknowledgments
This work was supported by the
research projects MSM6198959218 to JF
and MSM0021622412 to MN. Major part
of this work has been done during stay
of JF at the Department of Mineralogy,
Swedish Museum of Natural History,
Stockholm (project Synthesis, SE-TAF
4065). Samples for this study were kindly
Estudos Geológicos v. 19 (2), 2009
provided by the Moravian Museum in
Brno, Czech Republic.
REFERENCES
Bosi, F. & Lucchesi, S. 2007. Crystal
chemical
relationships
in
the
tourmaline
group:
Structural
constraints on chemical variability.
American Mineralogist, 92: 10541063.
Castaneda, C., Oliviera, E. F., Gomes,
N., Soares, A. C. P. 2000. Infrared
study of OH sites in tourmaline from
the elbaite-schorl series. American
Mineralogist, 85: 1503-1507.
Castaneda, C., Eeckhout, S.G., da Costa,
G.M., Botelho, N.F., De Grave, E.
2006. Effect of heat treatment on
tourmaline from Brazil. Physics and
Chemistry of Minerals, 33: 207-216.
Cempirek, J. & Novák M. 2003.
Mineral assemblages and chemical
composition of Al-rich tourmaline
from abyssal pegmatites of the
Bohemian Massif. Book of Abstracts
from LERM 2003, Nové Město na
Moravě, 12-13.
Dyar, M.D., Taylor, M.E., Lutz, T.M.,
Francis, C.A., Guidotti, C.V.,
Wise, M. 1998. Inclusive chemical
characterization
of
tourmaline:
Mossbauer study of Fe valence and site
occupancy. American Mineralogist,
83: 848-864.
Ertl, A. & Rossman, G.R. 2007. Box
a: Preliminary study of highertemperature heat treatment of canary
tourmaline. Gems & Gemology, 43:
322-322.
Fuchs, Y., Lagache, M., Linares, J. 2002.
Annealing in oxidising conditions
of Fe-tourmalines and correlated
deprotonation of OH groups. Comptes
Rendus Geoscience, 334: 245-249.
Kraczka, J., Pieczka, A., Wasilewski, W.,
125
OXIDATION/REDUCTION OF IRON IN IRON-RICH TOURMALINE FROM PEGMATITES AS A
RESPONSE TO THE HIGH-TEMPERATURE TREATMENT
Brzozka, K. 2000. Mössbauer study of
thermal oxidation of Fe2+ in tourmaline.
Proceedings of the All-polish Seminar
on Mössbauer Spectroscopy, 30: 8085.
McKeown,
D.A.
2008.
Raman
spectroscopy, vibrational analysis,
and heating of buergerite tourmaline.
Physics and Chemistry of Minerals,
35: 259-270.
126
Pieczka, A. & Kraczka, J. 2004. Oxidized
tourmalines - a combined chemical,
XRD and Mossbauer study. European
Journal of Mineralogy, 16: 309-321.
Selway, J. B., Novák, M., Černý, P.,
Hawthorne, F.C. 1999. Compositional
evolution of tourmaline in lepidolitesubtype pegmatites. European Journal
of Mineralogy, 11: 569-584.
Estudos Geológicos v. 19 (2), 2009