EPR spectroscopic and calorimetric study of minerals of the amblygonite–montebrasite series from rare-metal pegmatite of the Voroniye tundry (Kola peninsula) and Sangilen highlands (Tuva)

For the first time, a comprehensive physicochemical study of minerals of the amblygonite LiAlPO4F — montebrasite LiAlPO4(OH) series was carried out. An EPR spectroscopic and calorimetric study of montebrasite LiAlPO4(OH)0.9F0.1 from rare-metal granite pegmatites Shuk-Byul (Sangilen Highlands, Tuva) and amblygonite LiAlPO4F0.5(OH)0.5 from pegmatites of the Voroniye tundras (Kola Peninsula) was carried out. Using the EPR method, radiation-sensitive paramagnetic O– centers were discovered, they were formed without the participation of impurity elements in regular sites of the crystal lattice. For the first time, the possibility of carrying out EPR geochronometry on minerals of the amblygonite–montebrasite series has been demonstrated. Using the method of high-temperature melt dissolution calorimetry on a Tiana-Calvé microcalorimeter, the enthalpies of formation from the elements ΔfH0el(298.15 K) = –2326.3 ± 2.2 kJ/mol of montebrasite with the composition LiAl(PO4)(OH)0.9F0.1 and of amblygonite with the composition LiAl(PO4) F0.5(OH)0.5 (–2347.9 ± 3.1 kJ/mol) were obtained; and the values of this parameter were calculated for the end members with ideal composition of the series: for montebrasite (–2315.5 ± 2.2 kJ/mol) and for amblygonite (–2401.6 ± 3.1 kJ/mol). The values of the standard entropy S0(298.15 K) and the Gibbs energy of formation ΔfG0el(298.15 K) for intermediate and end members of the amblygonite–montebrasite series are estimated.