The crystal structures of four betalomonosovite samples from peralkaline pegmatites of the Khibiny alkaline complex, Kola Peninsula, Russia, were studied using single-crystal X-ray diffraction. The samples represent different chemical and structural varieties of betalomonosovite. The suggested general formula of betalomonosovite, based on the empirical data known to date, is Na5 + xTi4[Si2O7]2[PO3(OH)]2 − y[PO2(OH)2]yO2[(OH,F)2 − zOz], where 0 ≤ x ≤ 2, 0 ≤ y ≤ 1 and 0 ≤ z ≤ 1. Betalomonosovite is a transformation product of lomonosovite, Na10Ti4(Si2O7)2(PO4)2O4, due to the leaching of Na and protonation of O atoms of phosphate groups and Ti–O–Ti bridges. Distinctive features of betalomonosovite are: (a) the presence of H+ in the form of species-defining acid phosphate anion(s) [PO2(OH)2] ± [PO3(OH)]; (b) the presence of OH- anion at the Ti–O–Ti bridges in the O sheet of the HOH block and c) deficiency of Na (as compared to lomonosovite) in an amount related to the degree of protonation. A hypothetical end-member product of lomonosovite alteration with the simplified formula Na4Ti4[Si2O7]2[PO2(OH)2]2O2(OH)2 has not been reported in Nature yet. The ways and the degree of alteration may vary significantly, resulting in significant variations in both crystal structure and chemical composition, including the Na content, the degree of the substitution of O by OH, unit-cell dimensions and splitting of the P, O and/or Na sites.

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