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Durham University

Department of Earth Sciences

Staff Profile

Publication details

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

Author(s) from Durham

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.