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Order parameter variation through the C2/m-P2₁/m phase transition in cummingtonite

¹ Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany
² Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K.
³ Dipartimento di Scienze della Terra, via Ferrata 1, 27100 Pavia, Italy

Correspondence: ^* E-mail: tiziana.boffa-ballaran{at}uni-bayreuth.de

The C2/m-P2₁/m phase transition in natural cummingtonites, with different compositions, has been investigated by single-crystal X-ray diffraction and powder absorption IR spectroscopy. Variations in the intensity of type b (superlattice) reflections above room temperature are consistent with the transition being thermodynamically continuous and conform to the solution to a 2–4–6 Landau potential. Parameters extracted from IR spectra collected both above and below room temperature suggest that the local structural evolution differs slightly from the average macroscopic behavior observed by X-ray diffraction. Changes in wavenumber of an isolated absorption band at ~1130 cm^–1 and changes in the integrated intensity of a band at ~760 cm^–1 can be described by the solution to a 2–4 Landau potential with saturation below room temperature. Absorption bands due to O-H vibration and libration, however, appear to show discontinuities in the variation of their wavenumbers at the transition point. A local mechanism for the transition might involve a framework distortion providing the main driving force, with coupling to an order-disorder process for the protons.

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