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| Titel |
The lizardite phase transformation followed by in situ high-temperature Raman and FTIR spectroscopy |
| VerfasserIn |
R. Trittschack, B. Grobéty, M. Koch-Müller |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250062283
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| Zusammenfassung |
| Inertialisation of chrysotile and CO2sequestration in ultramafitesare examples of
processes in which dehydroxylation of serpentine phases play an important role.
Although research on the mechanism of this process goes back to the early 20th
century, the dehydroxylation mechanism is still not fully elucidated. This study
investigates the phase transformation of lizardite using in situ high-temperature
micro-Raman spectroscopy as well as in situ micro-FTIR spectroscopy between
room temperature and 819Ë C in order to demonstrate the temperature-dependent
behaviour in the low-frequency region (100-1200 cm-1) with bands assigned to
Si-O and Mg-(O,OH) vibrations (Raman) as well as the high-frequency region
(3400-3800 cm-1) with hydroxyl stretching vibrations (FTIR and Raman). A thin film
pressed in a diamond anvil cell was used for FTIR spectroscopy, whereas Raman
spectroscopy was carried out on a loose lizardite powder. The spectra have been
recorded at 25Ë C increments in the temperature range where dehydroxylation
occurs, i.e. between 400 and 665Ë C. The in situ recording guarantees that the
spectra are not affected by structural changes during quenching. All observed room
temperature bands can be assigned to lizardite, although the OH-bands show a remarkable
multiplication which vanishes with heating except of one extra OH band at 3665 cm-1. The
Raman active bands of lizardite show a linear shift to lower wavenumbers with
increasing temperature. All Raman bands, except of a broad band at around 670 cm,-1
diagnostic for Si-O-Si related vibration modes, disappear at a temperature between 639
and 665Ë C. Some of them exhibiting considerable frequency jumps in the last
stage. We did not observe any water molecule related modes. Such band should be
present assuming the conventional dehydroxylation model with water as only product
(Zhang et al., 2010). The only band, which shows an increase in intensity during the
dehydroxylation is the extra, non-assigned OH-band. Assuming that this increase is not a
deconvolution artefact, the extra OH-band may be related to non-structural OH-groups that
form during the transport of the dehydroxylation products to the grain surface. A
hydrogen/oxygen/hydroxyl hopping mechanism has been proposed as alternative
transport mechanism to water diffusion (Zhang et al., 2005). The frequency jumps and
the appearance of a new band at 183 cm-1 are evidence for a hydroxyl bearing
intermediate, partially ordered phase just before the formation of forsterite. The
frequency of the new band is close to the diagnostic 189 cm-1 mode of the 10Å phase
(Fumagalli et al., 2010). The formation of a possible dehydroxylation intermediate
with a T-O-T arrangement was already proposed by MacKenzie and Meinhold
(1994). Remaining Si-O-Si bonds are also pointing to the presence of a continuous
T-sheet.
Fumagalli, P. et al. (2001): Earth Planet Sc. Lett., 186, 125-141.
MacKenzie, K.J.D. & Meinhold, R.H. (1994): Am. Mineral., 79, 43-50.
Zhang, M. et al. (2010): Am. Mineral., 95, 1686-169.
Zhang, M. et al. (2005): Am. Mineral., 90, 173-180. |
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