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The α-β phase transition in volcanic cristobalite.

Damby, D.E. and Llewellin, E.W. and Horwell, C.J. and Williamson, B.J. and Najorka, J. and Cressey, G. and Carpenter, M.A. (2014) 'The α-β phase transition in volcanic cristobalite.', Journal of applied crystallography., 47 (4). pp. 1205-1215.

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 and forms and the observed transition between them. Here, for the first time, the dynamics and energy changes associated with the – 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 form and has a larger cell volume than synthetic -cristobalite; as a result, its diffraction pattern sits between ICDD - and -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 – 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 and phases coexisting through the transition, suggesting that discrete crystals in the sample have different transition temperatures.

Item Type:Article
Keywords:Cristobalite, Phase transitions.
Full text:(VoR) Version of Record
Available under License - Creative Commons Attribution.
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Full text:(VoR) Version of Record
Available under License - Creative Commons Attribution.
Download PDF (Final published version)
(520Kb)
Status:Peer-reviewed
Publisher Web site:http://dx.doi.org/10.1107/S160057671401070X
Publisher statement:This is an open-access article distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.
Date accepted:09 May 2014
Date deposited:17 June 2014
Date of first online publication:14 June 2014
Date first made open access:No date available

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