Structural Features of Hydrated Representatives of the Labuntsovite Group: Refinement of Crystal Structures of Intermediate Members of the Tsepinite-Na–“Tsepinite-Ba”–Tsepinite-K Solid Solution

Two mineral species of the labuntsovite group from Khibiny (sample 1) and Lovozero (sample 2) alkaline massifs are analyzed by single crystal X-ray diffraction and Raman spectroscopy. They are intermediate members of the tsepinite-Na–“tsepinite-Ba”–tsepinite-K solid solution, which are characterized by a high degree of hydration and a low concentration of extra-frame cations. Parameters of monoclinic cells are: a = 14.5086(6) Å, b = 14.2174(6) Å, c = 7.8712(3) Å, β = 117.119(4)°, V = 1444.09(11) Å³ (sample 1) and a = 14.2582(4) Å, b = 13.7541(6) Å, c = 7.7770(2) Å, β = 116.893(4)°, V = 1360.20(9) Å³ (sample 2). Crystal chemical formulas (Z = 2) are: |^A[Na_0.84K_0.6[(H₂O)₄]_5.2]^BK_0.2^CBa_0.25^D[Ca_0.35Na_0.15Fe_0.025(H₂O)]| {M1(Ti_1.2Nb_0.8)M2(Ti_1.1Nb_0.9)(O,OH)₄(Si₄O₁₂)₂} for sample 1 and |^A[Na_1.6(H₂O,H₃O)_0.4]^BK₂^C(Ba_0.51Sr_0.21)^{D [Mn}_0.3Ca_0.2(H₂O)](H₂O)₄|{M1(Ti_1.94Nb_0.06)M2(Ti_1.88Nb_0.12)(O,OH)₄(Si₄O₁₂)₂} for sample 2. The high degree of hydration of sample 1 is expressed in the presence of proton hydrate complexes and tetrahedral [H₂O]₄-associates. Labuntsovite group minerals (LGMs) are characterized by so-called block isomorphism due to different distributions of D cations. Our analysis of heteropolyhedral MT-frameworks in LGMs allows us to determine the topological features of cation networks.