Yusupovite, Na2Zr(Si6O15)(H2O)3, a new mineral species from the Darai-Pioz alkaline massif and its implications as a new microporous filter for large ions

Yusupovite, ideally Na2Zr(Si6O15)(H2O)(3), is a new silicate mineral from the Darai-Pioz alkaline massif in the upper reaches of the Darai-Pioz river, area of the joint Turkestansky, Zeravshansky, and Alaisky ridges, Tajikistan. Yusupovite was found in a pegmatite composed mainly of reedmergnerite, aegirine, microcline, and polylithionite. It occurs as prismatic grains about 2 mm in size embedded in reedmergnerite; associated minerals are quartz, pectolite, zeravshanite, mendeleevite-(Ce), fluorite, leucosphenite, a pyrochlore-group mineral, neptunite, telyushenkoite, moskvinite-(Y), and shibkovite. Yusupovite is colorless, transparent with a white streak, has a vitreous luster, and does not fluoresce under ultraviolet light. Cleavage is perfect on {110}, parting was not observed. Mohs hardness is 5. Yusupovite is brittle with a splintery fracture. The measured and calculated densities are 2.69(2) and 2.713 g/cm(3), respectively. Yusupovite is optically biaxial (+) with refractive indices (lambda = 589 nm) alpha = 1.563(2), beta = 1.565(2), gamma = 1.577(2); 2 V-meas = 42(3)degrees, 2V(calc) = 45 degrees, strong dispersion: r>v. Yusupovite is monoclinic, C2/m, a = 14.5975(4), b = 14.1100(4), c = 14.4394(4) angstrom, beta = 90.0399(4)degrees, V = 2974.1(3) angstrom(3). The six strongest reflections in the X-ray powder diffraction data [d (angstrom), I, (hkl)] are 7.05, 100, (020); 3.24, 96, (420); 3.10, 69, (241, (2) over bar 41); 5.13, 53, (202, (2) over bar 02); 6.51, 42, (201, (2) over bar 01); 3.17, 34, (042). The chemical composition (electron microprobe) is: Nb2O5 0.39, SiO2 58.84, ZrO2 16.55, HfO2 0.30, FeO 0.01, Y2O3 3.05, Cs2O 2.58, K2O 0.95, Na2O 8.91, H2Ocalc 7.40, total 98.98 wt%, with H2O calculated from structure refinement. The empirical formula (based on 17.5 O apfu) is (Na1.76K0.12Cs0.11)(Sigma 1.99) (Zr0.82Y0.17Nb0.02Hf0.01)(Sigma 1.02)(Si6.01O14.98)(H2O)(2.52,) Z = 8. The crystal structure of yusupovite was refined to R-1 = 3.46% based on 4428 observed reflections. In the crystal structure, there are six Si sites occupied by Si, two M sites occupied mainly by Zr with minor Y and Hf. Si tetrahedra form an epididymite Si6O15 ribbon along [010]. Epididymite ribbons and Zr-dominant M octahedra share common vertices to form a heteropolyhedral Si-Zr-O framework. There are six interstitial sites partly occupied by alkali cations Na, K, and Cs. The three [7]-coordinated Na sites are occupied by Na at 95, 84, and 78%. The three A sites are occupied by K and Cs at 12, 18, and 16%. There are 10 W sites occupied by H2O groups at 18-84%. Due to (K,Cs), Na and H2O disorder, the symmetry of yusupovite decreases from orthorhombic, space group Pbcm (elpidite), to monoclinic, space group C2/m, and the b unit-cell parameter of yusupovite is doubled compared to the corresponding cell parameter in elpidite, b(yus) = 2a(elp). Yusupovite, ideally Na2Zr(Si6O15)(H2O)(3), is a dimorph of elpidite, Na2Zr(Si6O15)(H2O)(3).