| Summary: | 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/mm<sup>2</sup>; the Mohs hardness is 5. <i>D</i>(meas.) is 2.97(1) g·cm<sup>−3</sup>, <i>D</i>(calc.) is 3.04 g·cm<sup>–3</sup>. Odikhinchaite is optically uniaxial (–), ω = 1.638(2), ε = 1.630(2). The IR spectrum shows the presence of the <sup>IV</sup>Mn<sup>2+</sup>O<sub>4</sub> polyhedra, H<sub>2</sub>O molecules and CO<sub>3</sub><sup>2–</sup> anions. The chemical composition is (electron microprobe, H<sub>2</sub>O determined by the modified Penfield method, CO<sub>2</sub> determined by selection sorption of gaseous annealing products; wt%): Na<sub>2</sub>O 9.25, K<sub>2</sub>O 0.59, CaO 12.77, MnO 5.49, FeO 0.75, MgO 0.24, La<sub>2</sub>O<sub>3</sub> 0.38, Ce<sub>2</sub>O<sub>3</sub> 0.39, Nd<sub>2</sub>O<sub>3</sub> 0.15, Al<sub>2</sub>O<sub>3</sub> 0.07, SiO<sub>2</sub> 44.80, ZrO<sub>2</sub> 11.13, TiO<sub>2</sub> 0.07, Nb<sub>2</sub>O<sub>5</sub> 4.17, Cl 0.69, CO<sub>2</sub> 0.90, H<sub>2</sub>O 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) Å<sup>3</sup>. 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><sup>N</sup></i><sup>1</sup>(Na<sub>2.58</sub>Ca<sub>0.42</sub>)<i><sup>N</sup></i><sup>2</sup>[Na<sub>2.37</sub>Ca<sub>0.51</sub>(H<sub>2</sub>O)<sub>0.12</sub>]<i><sup>N</sup></i><sup>3</sup>(Sr<sub>2.00</sub>K<sub>0.45</sub>Na<sub>0.35</sub><i>REE</i><sub>0.20</sub>)<i><sup>N</sup></i><sup>4</sup>Na<sub>3</sub><i><sup>N</sup></i><sup>5</sup>[(H<sub>2</sub>O)<sub>1.8</sub>Na<sub>1.2</sub>]}{<i><sup>Z</sup></i>Zr<sub>3</sub><i><sup>M</sup></i><sup>1</sup>Ca<sub>6</sub><i><sup>M</sup></i><sup>2</sup>(Mn<sub>2.49</sub>Fe<sup>2+</sup><sub>0.51</sub>)[<i><sup>M</sup></i><sup>3</sup>Nb(OH)<sub>1.82</sub>O<sub>1.18</sub>](<i><sup>M</sup></i><sup>4</sup>SiOH)[Si<sub>3</sub>O<sub>9</sub>]<sub>2</sub>[Si<sub>9</sub>O<sub>27</sub>]<sub>2</sub><i><sup>X</sup></i><sup>1</sup>[(CO<sub>3</sub>)<sub>0.53</sub>Cl<sub>0.47</sub>]<i><sup>X</sup></i><sup>2</sup>[(H<sub>2</sub>O)<sub>0.6</sub>(O,F)<sub>0.4</sub>]<i><sup>XM</sup></i><sup>4</sup>(CO<sub>3</sub>)<sub>0.15</sub>. 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).
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