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Abstract <p>The crystal structure of seidite-(Ce) was studied on the material from the holotype deposit (Yubileynaya pegmatite, Lovozero alkaline massif, Kola Peninsula, Russia) using Cu<i>K</i>α radiation. The studied crystal of seidite-(Ce) has a monoclinic symmetry, <i>P</i>2<sub>1</sub>/<i>m</i>, <i>a</i> = 7.2416(7) Å, <i>b</i> = 24.218(5) Å, <i>c</i> = 14.3760(8) Å, β = 90.049(8)<sup>ο</sup>, <i>V</i> = 2521.3(6) Å<sup>3</sup>. The crystal structure was solved on a single crystal by direct methods and refined to <i>R</i><sub>1</sub> = 0.30 on the basis of 2982 independent reflections with <i>I</i> &gt; 2σ<i>I</i>. The model obtained by the single-crystal method was confirmed by the Rietveld method (<i>R</i><sub>wp</sub> = 0.016, <i>R</i><sub>Bragg</sub> = 0.011 for 3197 reflections). The model of the crystal structure of the mineral differs from the one proposed earlier. It is based upon the triple [(SrCe)(TiO)[Si<sub>8</sub>O<sub>20</sub>(OH)<sub>2</sub>]F]<sup>4–</sup> layers parallel to (010) and consisting of two mirror-symmetric silicon-oxygen [Si<sub>4</sub>O<sub>10</sub>(OH)] layers formed by ten-membered tetrahedral rings with the ratio of Q<sup>2</sup>&#xa0;: Q<sup>3</sup> = 2 : 3. The layers are stacked together through TiO<sub>5</sub>F octahedra and chains of face-sharing (Ce,Sr)O<sub>8</sub> bicapped trigonal prisms parallel to the <i>c</i> axis. The Na and K atoms, alongside H<sub>2</sub>O molecules, occupy the intra- and interlayer spaces of the structure. The crystal chemical formula of the mineral can be written as (Na<sub>3.15</sub>K<sub>0.30</sub>Ca<sub>0.10</sub>)(Ce<sub>1.38</sub>Sr<sub>0.62</sub>)(TiO)[Si<sub>8</sub>O<sub>20</sub>(OH)<sub>2</sub>]F&#xa0;⋅&#xa0;H<sub>2</sub>O, and the idealized end-member formula as Na<sub>4</sub>(SrCe)(TiO)[Si<sub>8</sub>O<sub>20</sub>(OH)<sub>2</sub>]F ⋅ H<sub>2</sub>O. The data obtained suggest the existence of two or more polytypes of seidite-(Ce), as well as the structural relationship between the mineral and laplandite-(Ce).</p>

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Seidite-(Ce): New Structure Model, Refinement of the Formula, and the Problem of Polytypism

  • 期刊论文

  • 发表日期：

    2026年5月13日

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  • T. L. Panikorovskii,
  • V. N. Yakovenchuk,
  • S. V. Krivovichev,
  • S. N. Volkov

摘要

Abstract

The crystal structure of seidite-(Ce) was studied on the material from the holotype deposit (Yubileynaya pegmatite, Lovozero alkaline massif, Kola Peninsula, Russia) using CuKα radiation. The studied crystal of seidite-(Ce) has a monoclinic symmetry, P21/m, a = 7.2416(7) Å, b = 24.218(5) Å, c = 14.3760(8) Å, β = 90.049(8)ο, V = 2521.3(6) Å3. The crystal structure was solved on a single crystal by direct methods and refined to R1 = 0.30 on the basis of 2982 independent reflections with I > 2σI. The model obtained by the single-crystal method was confirmed by the Rietveld method (Rwp = 0.016, RBragg = 0.011 for 3197 reflections). The model of the crystal structure of the mineral differs from the one proposed earlier. It is based upon the triple [(SrCe)(TiO)[Si8O20(OH)2]F]4– layers parallel to (010) and consisting of two mirror-symmetric silicon-oxygen [Si4O10(OH)] layers formed by ten-membered tetrahedral rings with the ratio of Q2 : Q3 = 2 : 3. The layers are stacked together through TiO5F octahedra and chains of face-sharing (Ce,Sr)O8 bicapped trigonal prisms parallel to the c axis. The Na and K atoms, alongside H2O molecules, occupy the intra- and interlayer spaces of the structure. The crystal chemical formula of the mineral can be written as (Na3.15K0.30Ca0.10)(Ce1.38Sr0.62)(TiO)[Si8O20(OH)2]F ⋅ H2O, and the idealized end-member formula as Na4(SrCe)(TiO)[Si8O20(OH)2]F ⋅ H2O. The data obtained suggest the existence of two or more polytypes of seidite-(Ce), as well as the structural relationship between the mineral and laplandite-(Ce).