The α–β phase transition in volcanic cristobalite

Damby, David E. and Llewellin, Edward W. and Horwell, Claire J. and Williamson, Ben J. and Najorka, Jens and Cressey, Gordon and Carpenter, Michael (2014) The α–β phase transition in volcanic cristobalite. Journal of Applied Crystallography, 47 (4). pp. 1205-1215. ISSN 0021-8898 essn 1600-5767 DOI 10.1107/S160057671401070X

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Abstract

Cristobalite is a common mineral in volcanic ash produced from dome-forming eruptions. Assessment of the respiratory hazard posed by volcanic ash requires understanding the nature of the cristobalite it contains. Volcanic cristobalite contains coupled substitutions of Al3+ and Na+ for Si4+; similar co-substitutions in synthetic cristobalite are known to modify the crystal structure, affecting the stability of the [alpha] and [beta] forms and the observed transition between them. Here, for the first time, the dynamics and energy changes associated with the [alpha]-[beta] phase transition in volcanic cristobalite are investigated using X-ray powder diffraction with simultaneous in situ heating and differential scanning calorimetry. At ambient temperature, volcanic cristobalite exists in the [alpha] form and has a larger cell volume than synthetic [alpha]-cristobalite; as a result, its diffraction pattern sits between ICDD [alpha]- and [beta]-cristobalite library patterns, which could cause ambiguity in phase identification. On heating from ambient temperature, volcanic cristobalite exhibits a lower degree of thermal expansion than synthetic cristobalite, and it also has a lower [alpha]-[beta] transition temperature (~473 K) compared with synthetic cristobalite (upwards of 543 K); these observations are discussed in relation to the presence of Al3+ and Na+ defects. The transition shows a stable and reproducible hysteresis loop with [alpha] and [beta] phases coexisting through the transition, suggesting that discrete crystals in the sample have different transition temperatures.

Item Type:	Article
Additional Information:	© David E. Damby et al. 2014
Uncontrolled Keywords:	2014AREP; IA67;
Subjects:	03 - Mineral Sciences
Divisions:	03 - Mineral Sciences 07 - Gold Open Access
Journal or Publication Title:	Journal of Applied Crystallography
Volume:	47
Page Range:	pp. 1205-1215
Identification Number:	10.1107/S160057671401070X
Depositing User:	Sarah Humbert
Date Deposited:	18 Aug 2015 16:00
Last Modified:	18 Aug 2015 16:00
URI:	http://eprints.esc.cam.ac.uk/id/eprint/3454

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