The crystal structure of laptevite-(Ce), NaFe²⁺(REE₇Ca₅Y₃)(SiO₄)₄(Si₃B₂PO₁₈)(BO₃)F₁₁, a new mineral species from the Darai-Pioz alkaline massif, Northern Tajikistan

Abstract

Laptevite-(Ce), ideally NaFe²⁺(REE₇Ca₅Y₃) · (SiO₄)₄(Si₃B₂PO₁₈)(BO₃)F₁₁, is a new member of the vicanite group discovered in the moraine of the Darai-Pioz glacier, Alai Mountain Range, Tien-Shan, Tajikistan. The crystal structure of laptevite-(Ce), trigonal, a = 10.804(2) Å , c = 27.726(6) Å , V = 2802.6(2) Å 3, sp. gr. R3m, Z = 3, D_calc. = 4.660 · 103 kg/m³, has been refined to R₁ = 0.0387 for 924 unique (F_o > 4σF) reflections, collected on a Bruker single-crystal P4 diffractometer with a 4K CCD detector and Mo-K_a X-radiation. The simplified formula is NaFe²⁺(REE,Ca,Y)₁₅(SiO₄)₄ [(Si,B,P)₆O₁₈](BO₃)F₁₁. In the crystal structure of laptevite-( Ce), there are four tetrahedrally coordinated T sites. The T(1) site is occupied by Si and minor B, 〈T(1)-O〉 = 1.594 Å ; the T(2, 3) sites are occupied by Si, 〈T(2,3)-O〉 = 1.64 Å ; the T(4) site is occupied by B, P and minor Si, with 〈T(4)-O〉 = 1.51 Å . The [3]-coordinated T(5) site has composition B_0.84⃞_0.16, with 〈T(5)-O〉 = 1.42 Å . Cations at the T(1 + 4), T(2, 3) and T(5) sites form the complex anions (Si₃B₂PO₁₈), (SiO₄)₄ and (BO₃), respectively. The [7-10-coordinated M(1-5) sites are occupied by REE, Ca, Y and minor Sr, with 〈M(1-5)-O〉 = 2.37-2.62 Å . The octahedrally coordinated M(6) site is occupied by Fe²⁺ > Mn > Ti, with 〈M(6)-O〉 =2.13 Å . The [10]-coordinated M(7) site has composition Na_0.75REE_0.25, with 〈M(7)-O〉 = 2.64 Å . The T and M polyhedra form a framework. Laptevite-(Ce) is isostructural with other vicanite-group minerals: vicanite- (Ce), okanoganite-(Y), hundholmenite-(Y) and proshchenkoite-(Y). Vicanite-group structures differ in the dominant cation species at the T(1, 3-6) and M(3-7) sites.