Crystal structures of two new low-symmetry calcium-deficient analogs of eudialyte
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The crystal structures of two new low-symmetry (sp. gr. R3) representatives of the eudialyte group from Mont Saint-Hilaire (Quebec, Canada) and the Lovozero massif (Kola Peninsula, Russia) were studied by single-crystal X-ray diffraction analysis and refined to R = 0.068 and 0.054 using 2899 reflections with F > 5σ(F) and 2927 reflections with F > 3σ(F), respectively. The idealized formulas of these representatives are Na13(Ca3Mn3)Zr3(Fe, Mn)3(□)(Si)[Si3O9]2[Si9O27]2(O, OH, Cl)3 · 2H2O and Na15(Ca3Mn3)Zr3(Fe, Zr)3(Si)(Si) · [Si3O9]2[Si9O27]2O2(OH, F, Cl)2 · 2H2O. Both minerals are analogs of oneillite and are characterized by a low Ca content. The distinguishing features of the mineral from Quebec are that the M(4) site is essentially vacant (>50%) and Ca atoms occupy one independent site in the six-membered ring, whereas another site is occupied by Mn along with a small impurity of Na. In the mineral from the Lovozero massif, both the M(3) and M(4) sites are occupied predominantly by silicon, while Ca atoms are distributed between both octahedral sites of the six-membered ring, one of these sites being occupied predominantly by Mn.
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