Voronkovite, Na15(Na,Ca,Ce)3(Mn,Ca)3Fe3Zr3Si26O72(OH,O)4Cl · H2O, a new mineral species of the eudialyte group from the Lovozero alkaline pluton, Kola Peninsula, Russia

This paper describes a new member of the eudialyte group named in honor of Alexander Alexandrovich Voronkov (1928–1982), the prominent Russian crystallographer. The new mineral has been found in the Shkatulka hyperalkaline pegmatite body at Mt. Alluaiv, Lovozero alkaline pluton, Kola Peninsula, Russia. The mineral is associated with microcline, sodalite, nepheline, aegirine, terskite, lomonosovite, vuonnemite, shkatulkalite, manganoneptunite, and sphalerite. Voronkovite occurs as rounded, poorly faced crystals up to 2–5 mm across. The new mineral is light brown, with vitreous luster and conchoidal fracture. The streak is white. Voronkovite is transparent and brittle; the Mohs hardness is 5; cleavage or parting is not observed. D(meas) = 2.97(2) g/cm3 (volumetric method); D(calc) = 2.95 g/cm3. The new mineral is uniaxial, positive, pleochroic from lemon yellow along X to brownish pink along Y, and is not luminescent in UV light. Voronkovite easily dissolves and gelates in acid at room temperature. The new mineral is trigonal, space group R3, a = 14.205(7), c = 30.265(15) Å, V = 5289(8) Å3, Z = 3. The strongest reflections in the X-ray diffraction pattern [d, Å (I)(hkl)] are 2.970(100)(315), 4.316(85)(205), 2.848(84)(404), 3.221(43)(208), 3.536(41)(027), 3.039(41)(119). The chemical composition (electron microprobe, H2O determined with chemical analysis) is as follows, wt %: 15.84 Na2O, 0.28 K2O, 3.08 CaO, 1.76 SrO, 4.65 MnO, 3.53 FeO, 0.93 La2O3, 1.36 Ce2O3, 0.68 Nd2O3, 0.15 Al2O3, 49.48 SiO2, 0.33 TiO2, 14.11 ZrO2, 0.23 HfO2, 0.91 Nb2O5, 0.44 Cl, 0.21 F, 1.56 H2O, 0.19 -O = (Cl,F)2; total is 99.34. The empirical formula calculated on the basis of Si + Al = 26 (Z = 3) is as follows: (Na13.96Sr0.54K0.19)Σ14.69(Na1.64Ca0.92Ce0.26 La0.18)Σ3.00(Mn2.06Ca0.81Nd0.13)Σ3.00(F1.54Zr0.60Na0.48Nb0.21Ti0.13Hf0.04)Σ3.00Zr3.00(Si1.91Al0.09)Σ2.00(Si24O72)[(OH)2.98 O1.02]Σ4(Cl0.39F0.35)Σ0.74 · 1.23H2O. The simplified formula is Na15(Na,Ca,Ce)3(Mn,Ca)3Fe3Zr3Si26O72(OH,O)4Cl · H2O. The IR spectrum of the mineral and description of its crystal structure are given. The position of voronkovite in the crystallochemical taxonomy of eudialyte group is discussed, and the relationship to other members, oneillite and raslakite, are characterized. The type material is deposited at the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.