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High-pressure behaviour of lawsonite; a phase transition at 8.6 GPa

Isabelle Daniel, Guillaume Fiquet, Philippe Gillet, Max W. Schmidt and Michael Hanfland
High-pressure behaviour of lawsonite; a phase transition at 8.6 GPa
European Journal of Mineralogy (August 2000) 12 (4): 721-733


The structural behaviour of lawsonite CaAl (sub 2) Si (sub 2) O (sub 7) (OH) (sub 2) .H (sub 2) O has been studied under quasi-hydrostatic conditions in a diamond-anvil cell to 18 GPa at room temperature, using angle-dispersive X-ray powder diffraction and Raman spectroscopy. With increasing pressure, we observe a phase transition at P = 8.6(3) GPa, characterized by (1) the splitting of diffraction lines, (2) the emergence of new Raman bands, and (3) significant changes in the frequency shifts of the hydroxyl O-H stretching modes. The transition is displacive and fully reversible, without any detectable hysteresis. The high-pressure phase, referred to here as lawsonite III can be indexed into a monoclinic unit-cell, with a = 5.6833(3), b = 8.5944(4), c = 12.8773(5) Aa, gamma = 91.42(4) degrees and V = 628.80(4) Aa (super 3) at P = 10.6 GPa. The space-group of lawsonite III is likely to be C112 (sub 1) /m, which is the unconventional representation of P2 (sub 1) /m related to the low-pressure Cmcm symmetry. Assuming the change in space-group from Cmcm to C112 (sub 1) /m, the resulting components of the spontaneous strain tensor are analyzed in terms of the change in point group from mmm to 2/m. The pressure-dependence of the fourth power of the symmetry-breaking component e (sub 6) (super 4) is linear, indicating a tricritical character for the transition. Both X-ray diffraction and Raman spectroscopy indicate that the overall aluminosilicate framework of lawsonite is retained through the transition. However, monoclinic lawsonite III is approximately 40% less compressible than the low-pressure orthorhombic polymorph. The Raman spectroscopic results, in good agreement with recent infrared ones reveal that this decrease in compressibility is likely to be related to the increase in the hydrogen bond strength involving the hydroxyl groups in the structure, whereas the hydrogen bond system around the water molecule does not appear to be modified at the transition.

ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 12
Serial Issue: 4
Title: High-pressure behaviour of lawsonite; a phase transition at 8.6 GPa
Affiliation: Universite Claude Bernard-Lyon I, Laboratoire de Sciences de la Terre, Villeurbanne, France
Pages: 721-733
Published: 200008
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 31
Accession Number: 2001-025985
Categories: Geophysics of minerals and rocks
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
Secondary Affiliation: CNRS, FRA, FranceEuropean Synchrotron Radiation Facility, FRA, France
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200109
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