Abstract:
Rastsvetaevite was found on Mt. Rasvumchorr (Khibiny alkaline massif, Kola peninsula) occurring in a hyperagpaitic pegmatite as irregularly shaped grains 1-2 mm in size and their aggregates, up to 2 cm across. It is associated with nepheline, sodalite, potassium feldspar, aegirine, scherbakovite, villiaumite, natrite, nacaphite, rasvumite, etc. Color reddish-pink, transparent, conchoidal fracture; brittle; luster vitreous; streak white. Mohs' hardness 5-6. Dmeas = 2.86(2) g/cm3 (volumetric method), Dcalc = 2.84 g/cm3. Optically uniaxial, positive, n0 = = 1.598(1), ne= 1.600(1). Trigonal, space group R3m, a = 14.249(1), с = 60.969(7) Å, V= 10720(3) Å3. Z= 3. The strongest lines of the powder diffraction pattern [d, Å (I)(hkl)] are: 2.987(100)(3.1.10), 4.345(81)(2.0.10), 2.861 (73)(408), 3.249(57)(2.0.16), 6.48(47)(108), 3.565(41 )(220). IR data are given in the paper. Chemical composition (wt %, electron probe, H2O content by Penfield method): Na2O 13.76, K2O 6.12, CaO 10.37, SrO 1.26, BaO 0.07, FeO 2.73, MnO 0.49, Ce2O3 0.11, Аl2О3 0.17, SiO2 50.72, ZrO2 11.48, НfО2 0.14, TiO2 0.39, Nb2O5 0.34, Ta2O5 0.05, Cl 1.33, H2O 0.89, -O = Cl2 0.30; total 100.12. Empirical formula (based on total of anions 152.0, Z= 3) is Na27.10(K.7.93Ba0.03)Σ7.96(Ca11.29Sr0.74Ce0.04) Σ12.07(Fe2.32Mn0.42)Σ2.74(Zr5.69Ti0.30Hf0.04)Σ76.03(Si51.53 Al0.20Nb0.16Ta0.01)Σ51.90O144O2.14(OH)1.86Cl2.29 ∙ 1.71H2O. Simplified formula: Na27K8Ca12Fe3Zr6Si52O144(O,OH, H2O)6Cl2 or Na27K8Ca12Fe3Zr6Si4[Si3O9]4[Si9O27]4(O,OH,H2O)6Cl2. Crystal structure of the mineral includes two types of modules (alluaivite-like and eudialytc-like) alternating along the с axis, and causing rastsvetaevite to have a double с period (relative to eudialyte) and pronounced ion-exchange properties. In crystal chemical systematics of cudialyte-group minerals rastsvetaevite forms a subgroup, one among three principal ones characterized by the 24-layered structural array. Rastsvetaevite is a late pegmatite mineral, the product of natural transformation of eudialyte. It is widespread in margins of Khibiny apatite deposits, in zones marked by intensive potassium metasomatism with replacement of standard eudialyte by its K-rich analogues. The mineral is named in honor of Ramiza K. Rastsvetaeva (b. 1936), prominent Russian crystallographer, who has deciphered crystal structures for many newly discovered minerals, including the above one. The type material is deposited at the Fersman Mincralogical Museum, Moscow.