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The α-β phase transition in volcanic cristobalite
David E Damby1, Edward W Llewellin2, Claire J Horwell3
1Department 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.
Journal of Applied Crystallography
|September 23, 2014
Summary
Volcanic cristobalite, containing aluminum and sodium, transitions between alpha and beta phases at a lower temperature than synthetic cristobalite. This finding impacts respiratory hazard assessments of volcanic ash.
Area of Science:
- Mineralogy and Materials Science
- Geochemistry
- Crystallography
Background:
- Cristobalite is prevalent in volcanic ash from dome-forming eruptions.
- Understanding volcanic cristobalite is crucial for assessing respiratory hazards.
- Al3+ and Na+ substitutions in volcanic cristobalite affect its crystal structure and phase transitions.
Purpose of the Study:
- To investigate the dynamics and energy changes of the alpha-beta phase transition in volcanic cristobalite.
- To compare the transition behavior of volcanic cristobalite with synthetic cristobalite.
Main Methods:
- X-ray powder diffraction with simultaneous in situ heating.
- Differential scanning calorimetry.
Main Results:
- Volcanic cristobalite has a larger cell volume than synthetic cristobalite at ambient temperature.
- Volcanic cristobalite exhibits lower thermal expansion and a lower alpha-beta transition temperature (~473 K) compared to synthetic cristobalite (>543 K).
- The transition displays a hysteresis loop, with coexisting alpha and beta phases, indicating varying transition temperatures among crystals.
Conclusions:
- The presence of Al3+ and Na+ defects influences the phase transition behavior of volcanic cristobalite.
- The unique properties of volcanic cristobalite, including its transition temperature and diffraction pattern, can lead to ambiguity in phase identification.
- These findings are important for accurately assessing the health risks associated with inhaling volcanic ash.
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