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

• Main Authors: Yuliya D. Gritsenko, Nikita V. Chukanov, Sergey M. Aksenov, Igor V. Pekov, Dmitry A. Varlamov, Leonid A. Pautov, Svetlana A. Vozchikova, Dmitry A. Ksenofontov, Sergey N. Britvin
• Format: Article
• Language: English
• Published: MDPI AG 2020-11-01
• Series: Minerals
• Subjects: odikhinchaite; new mineral; eudialyte group; crystal structure; peralkaline pegmatite; Odikhincha intrusion
• Online Access: https://www.mdpi.com/2075-163X/10/12/1062

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. <i>D</i>(meas.) is 2.97(1) g·cm⁻³, <i>D</i>(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 CO₃^2– 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 <i>R</i>3<i>m</i>; the unit-cell parameters are: <i>a</i> = 14.2837(2) Å, <i>c</i> = 30.0697(3) Å, <i>V</i> = 5313.04(12) ų. The new mineral is isostructural with other 12-layered members of the eudialyte group with the space group <i>R</i>3<i>m</i>. Its crystal chemical formula is (<i>Z</i> = 3): {<i>^N</i>¹(Na_2.58Ca_0.42)<i>^N</i>²[Na_2.37Ca_0.51(H₂O)_0.12]<i>^N</i>³(Sr_2.00K_0.45Na_0.35<i>REE</i>_0.20)<i>^N</i>⁴Na₃<i>^N</i>⁵[(H₂O)_1.8Na_1.2]}{<i>^Z</i>Zr₃<i>^M</i>¹Ca₆<i>^M</i>²(Mn_2.49Fe²⁺_0.51)[<i>^M</i>³Nb(OH)_1.82O_1.18](<i>^M</i>⁴SiOH)[Si₃O₉]₂[Si₉O₂₇]₂<i>^X</i>¹[(CO₃)_0.53Cl_0.47]<i>^X</i>²[(H₂O)_0.6(O,F)_0.4]<i>^XM</i>⁴(CO₃)_0.15. The strongest lines of the powder X-ray diffraction pattern [<i>d</i>, Å (<i>I</i>, %) (<i>hkl</i>)] 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).