Rundqvistite-(Ce), Na 3 (Sr 3 Ce)(Zn 2 Si 8 O 24 ), a new mineral from the Darai-Pioz alkaline massif, Tien-Shan Mountains, Tajikistan: mineral description and crystal structure

Rundqvistite-(Ce), ideally Na 3 (Sr 3 Ce)(Zn 2 Si 8 O 24 ), is a new mineral from the Darai-Pioz alkaline massif, Tien-Shan Mountains, Tajikistan. The mineral occurs as elongated grains up to 0.1 mm long and up to 0.03 mm thick embedded in quartz–pectolite aggregate in a silexite-like peralkaline pegmatite. Associated minerals are quartz, fluorite, pectolite, baratovite, aegirine, leucosphenite, neptunite, reedmergnerite, orlovite, sokolovaite, mendeleevite-(Ce), odigitriaite, pekovite, zeravshanite, kirchhoffite and garmite. The mineral is colourless with a vitreous lustre and a white streak, brittle, D meas. is 3.70(2) and D calc. is 3.709 g/cm 3 . Rundqvistite-(Ce) is monoclinic, space group P 2 1 / c , a = 5.1934(16), b = 7.8934(16), c = 26.011(5) Å, β = 90.02(3)° and V = 1066.3(4) Å 3 . The chemical composition of rundqvistite-(Ce) is SiO 2 40.17, La 2 O 3 2.64, Ce 2 O 3 7.55, Pr 2 O 3 0.80, Nd 2 O 3 2.43, Sm 2 O 3 0.33, Eu 2 O 3 0.09, Gd 2 O 3 0.24, Tb 2 O 3 0.18, Dy 2 O 3 0.21, PbO 1.03, SrO 19.83, FeO 0.37, ZnO 13.08, CaO 2.55, Na 2 O 8.04, total 99.54 wt.%. The empirical formula calculated on 24 O apfu (atoms per formula unit) is Na 3.10 Sr 2.29 Ca 0.54 Pb 0.06 (Ce 0.55 La 0.19 Nd 0.17 Pr 0.06 Sm 0.02 Gd 0.02 Eu 0.01 Tb 0.01 Dy 0.01 ) Σ1.04 Zn 1.92 Fe 0.06 Si 8.00 O 24 Z = 2. The structural formula based on refined site-occupancies is (Na 2.94 Sr 0.06 ) Σ3.00 [(Sr 2.23 Ca 0.54 Pb 0.06 Na 0.13 ) Σ2.96 Ln 3+ 1.04 ] Σ4.00 [(Zn 1.92 Fe 2+ 0.06 ) Σ1.98 Si 8 O 24 ], where Ln 3+ 1.04 = (Ce 0.55 La 0.19 Nd 0.17 Pr 0.06 Sm 0.02 Gd 0.02 Eu 0.01 Tb 0.01 Dy 0.01 ) Σ1.04 . The crystal structure of rundqvistite-(Ce) was refined to R 1 = 2.76% on the basis of 3184 unique reflections [ F > 4σ| F |]. In rundqvistite-(Ce), the main structural unit is a (Zn 2 Si 8 O 24 ) 12– sheet parallel to (100). In the sheet, the Si and Zn tetrahedra form four-, five- and eight-membered rings. The interstitial cations at the Na and M (1–3) sites sum to [Na 3 (Sr 3 Ce)] 12– apfu. The Na and M(1–3) polyhedra share common edges to form a layer. Rundqvistite-(Ce) is a structural analogue of vladykinite, ideally Na 3 Sr 4 (Fe 2+ Fe 3+ )Si 8 O 24 . Rundqvistite-(Ce) and vladykinite are related by the following substitution: [8] Ce 3+ + [4] (Zn 2+ ) 2 ↔ [8] Sr 2+ + [4] (Fe 2+ Fe 3+ ). The mineral is named after Dmitry Vasilievich Rundqvist (1930–2022), a prominent Russian geologist and an expert on the geology of ore deposits, metallogeny and mineralogy of Precambrian rocks.