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Thermochemical properties have been either measured or estimated for synthetic monazite, LaPO 4 , and dissakisite, CaLaMgAl 2 (SiO 4 ) 3 OH, the Mg-equivalent of allanite. A dissakisite formation enthalpy of -6,976.5 ± 10.0 kJ mol -1 was derived from high-temperature drop-solution measurements in lead borate at 975 K. A third-law entropy value of 104.9 ± 1.6 J mol -1 K -1 was retrieved from low-temperature heat capacity (C p ) measured on synthetic LaPO 4 with an adiabatic calorimeter in the 30-300 K range. The C p values of lanthanum phases were measured in the 143-723 K range by differential scanning calorimetry. In this study, La(OH) 3 appeared as suitable for drop solution in lead borate and represents an attractive alternative to La 2 O 3 . Pseudo-sections were calculated with the THERIAK-DOMINO software using the thermochemical data retrieved here for a simplified metapelitic composition (La = P REE + Y) and considering monazite and Fe-free epidotes along the dissakisite-clinozoïsite join, as the only REE-bearing minerals. Calculation shows a stability window for dissakisite-clinozoïsite epidotes (T between 250 and 550°C and P between 1 and 16 kbar), included in a wide monazite field. The P-T extension of this stability window depends on the bulk-rock Ca-content. Assuming that synthetic LaPO 4 and dissakisite-(La) are good analogues of natural monazite and allanite, these results are consistent with the REE-mineralogy sequence observed in metapelites, where (1) monazite is found to be stable below 250°C, (2) around 250-450°C, depending on the pressure, allanite forms at the expense of monazite and (3) towards amphibolite conditions, monazite reappears at the expense of allanite.

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Abstract. The lattice parameters of synthetic diaspore (A10(OH)) and synthetic and natural corundum (A120•) have been measured at simultaneously elevated pressure and temperature up to 7 GPa and 800øC (1000øC for corundum) using a MAX 80 cubic anvil high-pressure apparatus. Several experimental runs were carried out for both minerals, in which the samples were first compressed at room temperature and then heated; NaC! served as an internal standard for pressure calibration. In most runs the samples were mixed with Vaseline or NaC1 to ensure hydrostatic pressure-transmitting conditions. Some runs, mainly at pressures < 4 GPa, were performed without using a pressure medium.