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| Titel |
Anomalous elastic behaviour of hydrous sanidine megacrysts from the Eifel, Germany |
| VerfasserIn |
Jürgen Schreuer, Kathrin Demtröder, Peter Sondergeld, Sara Dehn |
| 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 |
250035716
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| Zusammenfassung |
| Sanidine megacrystals from Volkesfeld (Riedener Kessel, East Eifel volcanic field) are well
known for their fast irreversible changes of optical properties at moderately high annealing
temperatures (starting from 750-C) [1]. These unique optical effects are probably
related to fast changes of the Al/Si-order, the origin of which is not yet understood.
Polarised infrared spectra and NMR studies indicate a relatively high amount of
water (about 200 wt-ppm) in the Eifel sanidines compared to low-sanidine crystals
from Madagascar (~ 1 wt-ppm) [2,3] possessing no unusual optical properties at
high temperatures. In order to clarify the role of water for the observed anomalies,
we investigated the elastic properties of sanidine megacrystals from four different
eruptive centres (Essingen, Kerpen, Rockeskyll, Volkesfeld) of the quarternary
volcanic fields of the Eifel between room temperature and 1100-C with the aid of
resonant ultrasound spectroscopy (RUS). Low-sanidine from Madagascar served as a
reference.
The Eifel sanidine megacrystals are all very similar in respect to their chemical and
structural properties. The chemical composition NaxK1-xAlSi3O8 as obtained by EMPA
varies between x = 0.15 (east Eifel volcanic field) and 0.27 (west Eifel volcanic field) with
up to 2 at-% celsian and less than 0.02 at-% anorthite. High-resolution DSC and
thermogravimetric measurements revealed an irreversible weight loss at about 950-C which
corresponds to about 250 wt-ppm H2O. Single crystal structure analyses yielded 2t1 - 0.58
for the Al-occupation of the t1 tetrahedral position indicating nearly fully disordered
high-sanidines. For comparison, the potassium-rich low-sanidines from Madagascar are
characterised by 2t1 = 0.69.
The temperature evolution of the elastic constants of ”dry” Madagascar sanidine is
monotonic and fully reversible in successive runs. However, at about 890-C a small peak
appears in the ultrasound attenuation above a slowly rising background. In contrast, the
hydrous Eifel sanidines show an exponentially increasing ultrasound dissipation above 800-C
followed by a rapid irreversible softening of all resonance frequencies by about 8% above
970-C. The observed anomalies are most likely related to the mobility of water dissolved in
the crystal structure of the Eifel sanidines. On first heating the water slowly gets mobile
above 800-C and leaves the crystal structure at about 970-C. The high mobility of the water
molecules allow for a new, less ordered equilibrium state to be reached on the time scale of
the experiment. The increase of the configurational entropy as well as the development
and propagation of micro-cracks lead to an elastic softening of the macroscopic
crystals.
[1] C. Zeipert and W. Wondratschek: Ein ungewöhnliches Temperverhalten bei Sanidinen
von Volkesfeld/Eifel. N. Jb. Mineral., Mh. 9 (1981) 407-415.
[2] A. Beran: A model of water allocation in alkali feldspar, derived from
infrared-spectroscopic investigations. Phys. Chem. Minerals 13 (1986) 306-310.
[3] E.A. Johnson and G.R. Rossman: A survey of hydrous species and concentrations in
igneous feldspars. Am. Min. 89 (2004) 586-600. |
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