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| Titel |
Incipient amorphization of metastable α-quartz induced by ultrahigh pressure (>15 GPa?): first report in an eclogite |
| VerfasserIn |
Rosaria Palmeri, Maria Luce Frezzotti, Gaston Godard |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040367
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| Zusammenfassung |
| Pressure-induced amorphization (PIA) in the solid state (i.e., without melting) is a
well-known process that has been experimentally observed for several materials and
minerals1-2, including α-quartz1-4. Apart from meteorite impacts5, it has been predicted as
irrelevant under natural P-T conditions because it can develop only under high pressures
(e.g., 15–40 GPa for α-quartz1-4,6), in metastable materials, either at very low temperature
or during a shock. Such pressures can only be found at extreme depths within the Earth, at
which crystal-crystal transitions are expected rather than amorphization1-2, because of the
high temperatures.
The present study reports the first discovery of incipient PIA of α-quartz in an eclogite
from the Lanterman Range (Antarctica)7. It was found in 50-μm-sized monocrystalline
quartz enclosed in omphacite, which shows a clear-to-opalescent lustre. In-situ Raman
spectroscopy (Siena) and X-ray micro-diffraction using a synchrotron radiation source
(ESRF, ID13; Grenoble) allowed to evidence some anomalies in this quartz. The
“anomalous” quartz is characterized by weak α-quartz modes, the broadening of the main
α-quartz peak at 465 cm-1, and additional vibrations at 480–485, 520–523, and 608 cm-1.
X-ray diffraction circular patterns display irregular and broad α-quartz spots, some of
which show an anomalous d-spacing tightening of about 2%. They also show some
very weak, hazy clouds that have d-spacing compatible with coesite but not with
α-quartz.
These results are evidence of (a) disordering and incipient amorphization, (b) slight
densification of the lattice, together with (c) the development of cryptic coesite. Disorder and
incipient amorphization of α-quartz are revealed by the decrease of the main A1 Raman mode
at 465 cm-1 and the appearance of the 480–485 (D1) and 608–cm-1 (D2) Raman defect
bands8. These peculiarities are known to occur in quartz during experimental PIA1,3,9.
Actually, the Raman spectra are very similar to those obtained for quartz partly amorphized
during shock experiment9. The d-space tightening revealed by X-ray is the evidence of the
slight lattice densification. Development of cryptic coesite is also suggested by a
weak Raman peak at about 521 cm-1 and by broad, weak spots in X-ray diffraction
patterns8.
This is interpreted as the first record of incipient pressure-induced amorphization of a
mineral formed in the Earth. The geological mechanism that has produced such incipient
amorphization still remains unexplained. A shock (due to a seism?) is preferred to a local
overpressure at the inclusion scale (due to expansion mismatch between quartz and its host
mineral10). This finding throws new light on the modality of the quartz-coesite transition and
on the pressure regimes (non-lithostatic vs. lithostatic) during ultrahigh-pressure
metamorphism.
References
1. Sharma, S.K., Sikka, S.K. (1996). Progress in Materials Science, 40, 1–77.
2. Richet, P., Gillet, P. (1997). Eur. J. Mineral., 9, 907–933.
3. Hemley, R.J. et al. (1988). Nature, 334, 52–54.
4. Kingma, K.J. et al. (1993). Science, 259, 666–669.
5. Goltrant, O. et al. (1992). Phys. Earth Planet. Int., 74, 219–240.
6. Hazen, R.M. et al. (1989). Solid State Communications, 72, 507–511.
7. Ghiribelli, B., Frezzotti, M.L., Palmeri, R. (2002). Eur. J. Mineral., 14, 355–360.
8. Palmeri, R., Frezzotti, M.L., Godard, G., Davies, R.J. (2009). J. Metamorphic Geol.,
27, 685-705.
9. De Resseguier T. et al. (2003). J. Appl. Phys., 94, 2123-2129.
10. Guiraud, M., Powell, R. (2006). Earth Planet. Sci. Lett., 244, 683–694. |
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