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Phase transitions and volumetric properties of cryolite, Na₃AlF₆: Differential thermal analysis to 100 MPa

David Dolej^* and Don R. Baker

Department of Earth and Planetary Sciences, McGill University, 3450 rue University, Montreal, Quebec H3A 2A7, Canada

Cryolite, Na₃AlF₆, is the most abundant aluminofluoride mineral in highly evolved felsic suites and their pegmatites, but its phase transitions and thermodynamic properties at elevated pressures are unknown. We used a simple modification of the TZM pressure vessel to perform differential thermal analysis of cryolite at high pressures. Temperatures of the -ß transition are as follows: 559.30 ± 0.23 °C (1 atm), 562.10 ± 0.28 °C (47 MPa), and 567.33 ± 0.23 °C (101 MPa). Cryolite melting temperatures increase as follows: 1011.4 ± 0.2 °C (1 atm), 1019.2 ± 0.4 °C (50 MPa), and 1028.7 ± 0.4 °C (100 MPa). Both pressure-temperature relationships are linear: (dT/dp)_-ß = 78.4 ± 8.4 °C/GPa and (dT/dp)_m = 174 ± 12 °C/GPa. Application of the Clapeyron relationship leads to the following volumetric changes: V_-ß = 0.089 ± 0.019 J/(mol·bar) and V_m = 1.49 ± 0.12 J/(mol·bar). Despite the significant self-dissociation in the cryolite liquid, melting sensu stricto (without dissociation) dominates the heat and volumetric changes during melting in comparable amounts: 83.3 ± 6.7 % H_m and 68 ± 15 % V_m and suggests that the degree of dissociation has no significant effect on the (dT/dp)_m. Evaluation of previous and current volumetric data for cryolite polymorphs leads to V_ß,1284 = 8.49 ± 0.17 J/(mol·bar); coefficients for the volumetric thermal expansion in the form of the third-order polynomial equation are: V₂₉₈ = 7.080 ± 0.012 J/(mol·bar), a₁ = (1.39 ± 0.20)·10^–4 K^–1, a₂ = (–2.15 ± 0.51)·10^–7 K^–2, and a₃ = (2.68 ± 0.34)·10^–10 K^–3. The total (dT/dp)_m of cryolite is very similar to that of villiaumite (NaF), whereas V_m/V_ß,1284 of cryolite is smaller than for other alkali halides (NaF, NaCl).

Key Words: DTA • TGA • cryolite • phase transition • thermodynamics

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