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Thermal effect in opal below room temperature.

M J Buerger1, G L Shoemaker

  • 1Institute of Materials Sciences, University of Connecticut, Storrs, Conn. 06268.

Proceedings of the National Academy of Sciences of the United States of America
|November 1, 1972
PubMed
Summary

Opal exhibits unique thermal properties due to the behavior of its bound water. This water, trapped in nanopores between cristobalite particles, melts at sub-zero temperatures, influencing opal's thermal effects.

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Area of Science:

  • Mineralogy
  • Solid-state Chemistry
  • Materials Science

Background:

  • Opal was previously thought to be amorphous silica.
  • Levin and Ott (1932) identified opal's X-ray pattern as high-temperature cristobalite.
  • The anomalous presence of cristobalite below its transition temperature required explanation.

Purpose of the Study:

  • To investigate the anomalous thermal behavior of opal.
  • To determine the cause of observed heat effects below ambient temperatures.
  • To understand the role of water in opal's thermal properties.

Main Methods:

  • X-ray powder diffraction to analyze cristobalite structure.
  • Differential Thermal Analysis (DTA) using a du Pont 900 Thermoanalyzer.
  • Controlled heating and cooling experiments to observe thermal effects and water loss.

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Main Results:

  • Opal displayed exothermic and endothermic heat effects below ambient temperatures.
  • These thermal effects were linked to the presence of water, not the cristobalite phase transition.
  • Water loss occurred nearly linearly up to 422°C, with complete loss at that temperature.
  • Rehydrated opal exhibited the thermal effects, while water-free opal did not.

Conclusions:

  • The anomalous thermal behavior of opal is attributed to its bound water.
  • Water is confined within nanoscale voids (hundreds of Angstroms) between cristobalite particles.
  • This confined water exhibits unusual properties, melting around -50°C, distinct from bulk water.