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A high-pressure structural study of lawsonite using angle-dispersive powder-diffraction methods with synchrotron radiation

Published online by Cambridge University Press:  05 July 2018

A. R. Pawley  and
D. R. Allan
A. R. Pawley*
Affiliation:
Department of Earth Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK
D. R. Allan
Affiliation:
Department of Physics and Astronomy, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, UK
*
*E-mail: alison.pawley@man.ac.uk
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Abstract

Structural refinements of lawsonite have been obtained at pressures up to 16.5 GPa using angle-dispersive powder diffraction with synchrotron radiation on a natural sample contained in a diamond anvil cell. Lawsonite compresses smoothly and relatively isotropically up to 10 GPa. Its bulk modulus is 126.1(6) GPa (for K’ = 4), consistent with previous results. A trend of decreasing Si–O–Si angle indicates that compression is accommodated partly through the narrowing of the cavities containing Ca and H2O in the [001]ortho direction. At 10–11 GPa there is a phase transition from Cmcm to P21/m symmetry. The occurrence of a mixed-phase region, spanning >1 GPa, indicates that the transition is first order in character. The phase transition occurs through a shearing of (010)ortho sheets containing AlO6 octahedral chains in the [100]ortho direction, which causes an increase in βmono. Across the transition, the number of oxygens coordinated to Ca increases from 8 to 9, causing an increase in the average Ca–O bond length. The compressibility of P21/m lawsonite could not be determined due to solidification of the methanol/ethanol pressure-transmitting medium. On the basis of an experiment in which the P21/m lawsonite structure was heated to 200°C at 12.0 GPa, we predict a shallow positive P-T slope for the phase transition, and therefore no stability field for P21/m lawsonite in the Earth.

Keywords

lawsonitehigh-pressurephase transitionsynchrotron radiationpowder diffraction
Type
Research Article
Information
Mineralogical Magazine , Volume 65 , Issue 1 , February 2001 , pp. 41 - 58
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2001

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