Odikhinchaite, Na9Sr3[(H2O)2Na]Ca6Mn3Zr3NbSi (Si24O72)O(OH)3(CO3)·H2O, a New Eudialyte-Group Mineral from the Odikhincha Intrusion, Taimyr Peninsula, Russia

Abstract

The new eudialyte-group mineral, odikhinchaite, was discovered in a peralkaline pegmatite vein hosted by melteigite at the Odikhincha ultrabasic alkaline–carbonatite intrusion, Taimyr Peninsula, Krasnoyarsk Krai, Russia. Associated minerals are orthoclase, albite, aegirine, cancrinite, ancylite-(Ce), catapleiite, and wadeite. Odikhinchaite occurs as dense rosette-like aggregates up to 11 mm across, consisting of split lamellar individuals. The mineral is translucent to transparent, deep purple, with vitreous luster. Odikhinchaite is brittle, with uneven fracture; distinct cleavage on (001) is observed. Hardness determined by the micro-indentation method is equal to 430 kgf/mm2; the Mohs hardness is 5. D(meas.) is 2.97(1) g·cm−3, D(calc.) is 3.04 g·cm–3. Odikhinchaite is optically uniaxial (–), ω = 1.638(2), ε = 1.630(2). The IR spectrum shows the presence of the IVMn2+O4 polyhedra, H2O molecules and CO32– anions. The chemical composition is (electron microprobe, H2O determined by the modified Penfield method, CO2 determined by selection sorption of gaseous annealing products; wt%): Na2O 9.25, K2O 0.59, CaO 12.77, MnO 5.49, FeO 0.75, MgO 0.24, La2O3 0.38, Ce2O3 0.39, Nd2O3 0.15, Al2O3 0.07, SiO2 44.80, ZrO2 11.13, TiO2 0.07, Nb2O5 4.17, Cl 0.69, CO2 0.90, H2O 2.22, –O = Cl –0.16, total 99.72. The crystal structure was solved using single-crystal X-ray diffraction data. Odikhinchaite is trigonal, space group R3m; the unit-cell parameters are: a = 14.2837(2) Å, c = 30.0697(3) Å, V = 5313.04(12) Å3. The new mineral is isostructural with other 12-layered members of the eudialyte group with the space group R3m. Its crystal chemical formula is (Z = 3): {N1(Na2.58Ca0.42)N2[Na2.37Ca0.51(H2O)0.12]N3(Sr2.00K0.45Na0.35REE0.20)N4Na3N5[(H2O)1.8Na1.2]}{ZZr3M1Ca6M2(Mn2.49Fe2+0.51)[M3Nb(OH)1.82O1.18](M4SiOH)[Si3O9]2[Si9O27]2X1[(CO3)0.53Cl0.47]X2[(H2O)0.6(O,F)0.4]XM4(CO3)0.15. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 11.42 (64) (101), 4.309 (41) (205), 3.405 (53) (131), 3.208 (45) (208, 036), 3.167 (44) (217), 2.978 (100) (315), 2.858 (86) (404).