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Heat capacity and phase equilibria of wadeite-type K2Si4O9 |
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| Authors: | Wenjun Yong E Dachs A C Withers E J Essene |
| Institution: | 1. Department of Geological Sciences, University of Michigan, Ann Arbor, MI, 48109-1005, USA 2. Fachbereich Materialwissenschaften, Universit?t Salzburg, Hellbrunnerstr. 34, 5020, Salzburg, Austria 3. Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN, 55455, USA |
| Abstract: | The low-temperature heat capacity (C p) of Si-wadeite (K2Si4O9) synthesized with a piston cylinder device was measured over the range of 5–303 K using the heat capacity option of a physical properties measurement system. The entropy of Si-wadeite at standard temperature and pressure calculated from the measured heat capacity data is 253.8 ± 0.6 J mol−1 K−1, which is considerably larger than some of the previous estimated values. The calculated phase transition boundaries in the system K2O–Al2O3–SiO2 are generally consistent with previous experimental results. Together with our calculated phase boundaries, seven multi-anvil experiments at 1,400 K and 6.0–7.7 GPa suggest that no equilibrium stability field of kalsilite + coesite intervenes between the stability field of sanidine and that of coesite + kyanite + Si-wadeite, in contrast to previous predictions. First-order approximations were undertaken to calculate the phase diagram in the system K2Si4O9 at lower pressure and temperature. Large discrepancies were shown between the calculated diagram compared with previously published versions, suggesting that further experimental or/and calorimetric work is needed to better constrain the low-pressure phase relations of the K2Si4O9 polymorphs. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. |
| Keywords: | Si-wadeite K2Si4O9 Heat capacity Entropy Phase equilibria Thermodynamic calculation UHP |
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