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Cryolite, Na 3 AlF 6 , 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), 1019.2 ± 0.4 °C (50 MPa), and 1028.7 ± 0.4 °C (100 MPa). Both pressure-temperature relationships are linear: (dT/dp) α-β  = 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: ΔV α−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 % ΔV 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 Vβ,1284 = 8.49 ± 0.17 J/(mol·bar); coeficients for the volumetric thermal expansion in the form of the third-order polynomial equation are: V 298  = 7.080 ± 0.012 J/(mol·bar), a 1  = (1.39 ± 0.20)·10 -4  K -1 , a 2  = (.2.15 ± 0.51)·10 -7  K -2 , and a 3  = (2.68 ± 0.34)·10 -10  K -3 . The total (dT/dp)m of cryolite is very similar to that of villiaumite (NaF), whereas ΔV m /V β,1284  of cryolite is smaller than for other alkali halides (NaF, NaCl).

Phase transitions and volumetric properties of cryolite, Na 3 AlF 6 : Differential thermal analysis to 100 MPa