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Heat capacity and entropy of low structural state plagioclases

Abstract

The heat capacity of 11 different compositions of the low structural state plagioclase binary was measured between 5 and 773 K. The results are compared to those obtained from synthetic high structural state samples investigated in a previous study and obtained from a heat-treated natural sample of this study. The heat capacity of anorthite samples with slightly different order parameters shows large differences at ~500 K, where the Ibar1-Pbar1 phase transition occurs, which affects petrological calculations. At T = 298.15 K, the vibrational entropy versus composition behaviour of the low structural state plagioclases is almost ideal, in contrast to the high structural state plagioclases with positive excess vibrational entropies. At higher temperatures, the low structural state plagioclases show a negative deviation from ideal vibrational entropy composition behaviour in the Na-rich region.

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Acknowledgments

This work was supported by the grants from the Austrian Science Fund (FWF), project numbers P23056-N21 and P21370-N21, which is gratefully acknowledged. We also acknowledge the carful reviews of M. Tribaudino and an anonymous reviewer.

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Correspondence to A. Benisek.

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Benisek, A., Dachs, E. & Carpenter, M.A. Heat capacity and entropy of low structural state plagioclases. Phys Chem Minerals 40, 167–173 (2013). https://doi.org/10.1007/s00269-012-0556-2

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  • DOI: https://doi.org/10.1007/s00269-012-0556-2

Keywords

  • Plagioclase
  • Specific heat
  • Entropy
  • Anorthite
  • Relaxation calorimetry
  • Differential scanning calorimetry