The α–β phase transition in volcanic cristobalite. (1st August 2014)

• Record Type: Journal Article
• Authors: Damby, David E.; Llewellin, Edward W.; Horwell, Claire J.; Williamson, Ben J.; Najorka, Jens; Cressey, Gordon; Carpenter, Michael
• Is Part Of: Journal of applied crystallography. Volume 47:Part 4(2014:Aug.)
• Page Start: 1205
• Page End: 1215
• Publication Date: 2014-08-01
• Subjects: Crystallography -- Periodicals; 548.05
• DOI: 10.1107/S160057671401070X
• Languages: English
• ISSNs: 0021-8898

Abstract

<abstract abstract-type="main" xml:lang="en"> <title> <x xml:space="preserve">Abstract</x> </title> <p>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 Al<sup>3+</sup> and Na<sup>+</sup> for Si<sup>4+</sup>; similar co‐substitutions in synthetic cristobalite are known to modify the crystal structure, affecting the stability of the α and β forms and the observed transition between them. Here, for the first time, the dynamics and energy changes associated with the α–β phase transition in volcanic cristobalite are investigated using X‐ray powder diffraction with simultaneous <italic>in situ</italic> heating and differential scanning calorimetry. At ambient temperature, volcanic cristobalite exists in the α form and has a larger cell volume than synthetic α‐cristobalite; as a result, its diffraction pattern sits between ICDD α‐ and β‐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 α–β transition temperature (∼473 K) compared with synthetic cristobalite (upwards of 543 K); these observations are discussed in relation to the presence of Al<sup>3+</sup> and Na<sup>+</sup> defects. The transition shows a stable and reproducible hysteresis loop with α and β phases coexisting through the transition, suggesting that discrete crystals in the sample have different transition temperatures.</p> </abstract>