ИСТИНА

Naturally-occurred murataite is a rare cubic structure phase with space group F͞43m, lattice parameter a = 14.9 Å, Z=4, and formula with 22 cations and 43 anions [8]R6[6]М112[5]М24[4]ТХ43, where R = Y, Na, Са, Мn; М1 = Ti, Nb, Na; М2 = Zn, Fe, Ti, Na; T = Zn; X = O, F, OH [1]. In synthetic samples the varieties with three- (like in natural murataite – M3 or 3C; C means cubic) five- (M5 or 5C), seven- (M7 or 7C), and eight-fold (M8 or 8C) elementary fluorite unit cell have been also found. Pyrochlore R2M12X7 and murataite are suggested to form a polysomatic series [2], i.e. the structure of 5C, 7C and 8C phases consists of pyrochlore (two-fold elementary fluorite unit cell – 2C) and murataite (3C) blocks (modules). Later it was confirmed by X-ray structural analysis of single crystals of murataite varieties [3-5]. The four-, five-, and six-coordinated sites are capable to incorporate small Ti, Al, and Fe and other corrosion product cations, the larger sites with CN = 7 and 8 are occupied with Ca, Mn, Zr, REE and An cations. During melt crystallization the firstly segregated polytype is the phase with the highest content of the pyrochlore modules. Thus, the content of Ans, REEs, and Zr reduces in the row: M7 – M5 – M8 – M3 while the Ti, Fe, and Al concentrations increase. The highest concentrations of the An, REE, and Zr located in the core of the grains yield their lowest release from ceramics [6].The work was supported from RFBR (project #18-29-12032\18 mk) 1. T.S. Ercit, F.C. Hawthorne. Canad. Miner. 33 (1995) 1223-1229. 2. V.S. Urusov et al. Crystallogr. Repts. 52 (2007) 37–46. 3. S.V. Krivovichev et al. Minerals as Advanced Materials II. S. Krivovichev (Ed.). Berlin, Heidelberg: Springer-Verlag, 2012, pp. 293–304. 4. A.S. Pakhomova et al. Zeitschrift für Kristal. 228 (2013) 151–156. 5. A.S. Pakhomova et al. Eur. J. Mineral. 28 (2016) 205–214. S.V. Stefanovsky et al. J. Alloys Compds. 444-445 (2007) 618–620.