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Monoclinic-to-orthorhombic phase transition in Cu2(AsO4)(OH) olivenite at high temperature: strain and mode decomposition analyses

Published online by Cambridge University Press:  28 February 2018

Serena C. Tarantino
Affiliation:
Dipartimento di Scienze della Terra e dell'Ambiente, Università di Pavia, via Ferrata 9, I-27100 Pavia, Italy CNR-IGG, Sezione di Pavia, via Ferrata 9, I-27100 Pavia, Italy
Michele Zema*
Affiliation:
Dipartimento di Scienze della Terra e dell'Ambiente, Università di Pavia, via Ferrata 9, I-27100 Pavia, Italy CNR-IGG, Sezione di Pavia, via Ferrata 9, I-27100 Pavia, Italy
Athos M. Callegari
Affiliation:
Dipartimento di Scienze della Terra e dell'Ambiente, Università di Pavia, via Ferrata 9, I-27100 Pavia, Italy
Massimo Boiocchi
Affiliation:
Centro Grandi Strumenti, Università di Pavia, via Bassi 21, I-27100 Pavia, Italy
Michael A. Carpenter
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

Abstract

A natural olivenite single crystal was submitted to in situ high-temperature single-crystal X-ray diffraction from room temperature (RT) to 500°C. Unit-cell parameters were measured at regular intervals of 25°C, and complete datasets collected at T = 25, 50, 100, 150, 200, 250, 300, 400 and 500°C. Evolution of unit-cell parameters and structure refinements indicates that olivenite undergoes a structural phase transition from P21/n to Pnnm at ~200°C, and eventually becomes isostructural with the other members of the olivenite-mineral group. Volume expansion with temperature is larger in the monoclinic phase – where it follows a non-linear trend – than in the orthorhombic one. Axial and volume expansion coefficients of the orthorhombic olivenite phase are positive and linear and similar to those of the other Cu-bearing member of the mineral family, namely libethenite, but rather different from those of the Zn-analogue arsenate adamite.

Distortion of Cu polyhedra is quite high in the olivenite monoclinic phase at RT and goes towards a relative regularization with increasing T until the phase transition occurs. In the orthorhombic phase, no significant variation of the polyhedral distortion parameters is observed with increasing temperature, and maximum expansion is along the b direction and governed by corner-sharing. Landau potential provides a good representation of the macroscopic changes associated with the phase transition, coupling between the strains and the order parameter is responsible for the nearly tricritical character of the transition.

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Article
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

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