Letnikovite-(Ce), (Na□)Ca₂Ce₂[Si₇O₁₇(OH)]F₄(H₂O)_4, a new mineral from the Darai-Pioz alkaline massif, Tajikistan: mineral description, crystal structure and a new single [Si₇O₁₇(OH)] sheet

AbstractLetnikovite-(Ce), ideally (Na□)Ca2Ce2[Si7O17(OH)]F4(H2O)4, is a new mineral with no natural or synthetic analogues (IMA2022–132). The mineral occurs at the Darai-Pioz alkaline massif, Tien-Shan mountains, Tajikistan, in a silexite-like peralkaline pegmatite. Letnikovite-(Ce) occurs as isolated prismatic grains up to 0.03 × 0.1 mm in a quartz–pectolite aggregate. Associated minerals are quartz, fluorite, pectolite, baratovite, aegirine, leucosphenite, neptunite, reedmergnerite, orlovite, sokolovaite, mendeleevite-(Ce), odigitriaite, pekovite, zeravshanite, kirchhoffite and garmite. The mineral is colourless with a vitreous lustre and a white streak, and Dcalc. is 2.847 g/cm3. Letnikovite-(Ce) is monoclinic, C2/m, a = 7.4726(3), b = 22.9196(9), c = 13.9360(6) Å, β = 105.550(5)° and V = 2299.43(17) Å3. The chemical composition of letnikovite-(Ce) is SiO2 42.38, Gd2O3 0.16, Eu2O3 0.28, Sm2O3 0.07, Nd2O3 5.64, Pr2O3 1.69, Ce2O3 11.73, La2O3 2.24, PbO 1.22, SrO 5.77, FeO 0.32, CaO 11.87, MgO 1.14, Cs2O 0.57, K2O 0.65, Na2O 2.24, H2O 7.79, F 7.29, O = F –3.07, total 99.98 wt.%. The empirical formula calculated on 7 Si apfu is Na0.72K0.14Cs0.04Ca2.10Sr0.55Mg0.28Pb0.05Fe0.04(Ce0.71Nd0.33La0.14Pr0.10Eu0.02Gd0.01)Σ1.31Si7O21.84F3.81H8.58 for Z = 4. The structural formula is (Na0.72Ca0.16□1.12)Σ2(Cs0.04□0.96)Σ1(Ca1.83□0.17)Σ2(Mg0.28Fe0.04□0.68)Σ1(Ln3+1.31Sr0.55Ca0.09Pb0.05)Σ2[Si7O17(OH)]F3.81(H2O)3.79, where Ln3+1.31 = (Ce0.71Nd0.33La0.14Pr0.10Eu0.02Gd0.01)Σ1.31. The simplified formula is (Na,□)2Ca2(Ln3+,Sr)2[Si7O17(OH)]F4(H2O)4, where Ce is the dominant lanthanoid. The crystal structure was solved by direct methods and refined to an R1 index of 4.2%. In letnikovite-(Ce), the main structural unit is a layer which consists of the central heteropolyhedral Ca–Ce sheet and two adjacent single [Si7O17(OH)] sheets parallel to (001). In the Si–O–OH sheet, the Si tetrahedra form five-membered and ten-membered rings. This is the first occurrence of a single [Si7O17(OH)]7– sheet in a mineral. The layers connect via Na and Cs at the interstitial A(1,2) sites, H2O groups and hydrogen bonding. The mineral is named in honour of Felix Artem'evich Letnikov (born October 3rd,1934) in recognition of his outstanding contributions to the field of petrology and geochemistry of Precambrian rocks.