title: The α-β phase transition in volcanic cristobalite.
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 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 simultaneousin situheating 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 Al3+and Na+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.
authors.affiliation: Department of Earth Sciences, Durham University, South Road, Durham DH1 3LE, UK ; Institute of Hazard, Risk and Resilience, Durham University, South Road, Durham DH1 3LE, UK ; Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, Munich 80333, Germany.
authors.forename: David E
authors.initials: DE
authors.lastname: Damby
authors.affiliation: Department of Earth Sciences, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Edward W
authors.initials: EW
authors.lastname: Llewellin
authors.affiliation: Department of Earth Sciences, Durham University, South Road, Durham DH1 3LE, UK ; Institute of Hazard, Risk and Resilience, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Claire J
authors.initials: CJ
authors.lastname: Horwell
authors.affiliation: Camborne School of Mines, College of Engineering, Mathematics and Physical Sciences, University of Exeter, Penryn TR10 9EZ, UK.
authors.forename: Ben J
authors.initials: BJ
authors.lastname: Williamson
authors.affiliation: Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK.
authors.forename: Jens
authors.initials: J
authors.lastname: Najorka
authors.affiliation: Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK.
authors.forename: Gordon
authors.initials: G
authors.lastname: Cressey
authors.affiliation: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK.
authors.forename: Michael
authors.initials: M
authors.lastname: Carpenter
keywords: cristobalite; phase transitions