EPR Spectroscopic and Calorimetric Study of Minerals of the Amblygonite–Montebrasite Series from Rare-Metal Pegmatite of the Voron’i Tundras (Kola Peninsula) and Sangilen Highlands (Tuva)

The paper reports the first comprehensive physicochemical study of minerals of the amblygonite LiAlPO₄F–montebrasite LiAlPO₄(OH) series. An EPR spectroscopic and calorimetric study of montebrasite LiAlPO₄(OH)_0.9F_0.1 from the Shuk-Byul rare-metal granite pegmatites (Sangilen Highlands, Tuva) and amblygonite LiAlPO₄F_0.5(OH)_0.5 from pegmatites of the Voron’i Tundras (Kola Peninsula) was carried out. Using the EPR method, the radiation-sensitive paramagnetic O^– centers were discovered. They were formed without participation of impurity elements in the regular sites of crystal lattice. The possibility use of minerals of the amblygonite–montebrasite series for EPR geochronometry has been demonstrated for the first time. Using the method of high-temperature melt dissolution calorimetry on a Tian-Calvét microcalorimeter, the enthalpies of formation from elements \({{\Delta }_{f}}H_{{{\text{el}}}}^{0}\) (298.15 K) = –2326.3 ± 2.2 kJ/mol for montebrasite with the composition LiAl(PO₄)(OH)_0.9F_0.1 and for amblygonite with the composition LiAl(PO₄)F_0.5(OH)_0.5 (‒2347.9 ± 3.1 kJ/mol) are obtained; and the values of this parameter are calculated for the end members with an 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 \({{\Delta }_{f}}G_{{{\text{el}}}}^{0}\) (298.15 K) for intermediate and end members of the amblygonite–montebrasite series are estimated.