Abstract:
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 (α-β 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), 10 19.2 ± 0.4 °C (50 MPa), and 1028.7 ± 0.4 °C (100 MPa). Both pressure-temperature relationships are linear: (dT/dp)αβ= 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: ΔVα-β = 0.089 ± 0.019 J/(mol·bar) and ΔVm = 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% ΔHm and 68 ± 15% ΔVm 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 Vβ1284 = 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 ΔVm/Vβ.1284 of cryolite is smaller than for other alkali halides (NaF, NaCl).
