ALBERT

Description: Yusupovite, ideally Na 2 Zr(Si 6 O 15 )(H 2 O) 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 ( = 589 nm) α = 1.563(2), β = 1.565(2), = 1.577(2); 2 V meas = 42(3)°, 2 V calc = 45°, strong dispersion: r 〉 v. Yusupovite is monoclinic, C 2/ m , a = 14.5975(4), b = 14.1100(4), c = 14.4394(4) Å, β = 90.0399(4)°, V = 2974.1(3) Å 3 . The six strongest reflections in the X-ray powder diffraction data [ d (Å), I, ( hkl )] are 7.05, 100, (020); 3.24, 96, (420); 3.10, 69, (241, 41); 5.13, 53, (202, 02); 6.51, 42, (201, 01); 3.17, 34, (042). The chemical composition (electron microprobe) is: Nb 2 O 5 0.39, SiO 2 58.84, ZrO 2 16.55, HfO 2 0.30, FeO 0.01, Y 2 O 3 3.05, Cs 2 O 2.58, K 2 O 0.95, Na 2 O 8.91, H 2 O calc 7.40, total 98.98 wt%, with H 2 O calculated from structure refinement. The empirical formula (based on 17.5 O apfu) is (Na 1.76 K 0.12 Cs 0.11 ) 1.99 (Zr 0.82 Y 0.17 Nb 0.02 Hf 0.01 ) 1.02 (Si 6.01 O 14.98 )(H 2 O) 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 Si 6 O 15 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 H 2 O groups at 18–84%. Due to (K,Cs), Na and H 2 O disorder, the symmetry of yusupovite decreases from orthorhombic, space group Pbcm (elpidite), to monoclinic, space group C 2/ m , and the b unit-cell parameter of yusupovite is doubled compared to the corresponding cell parameter in elpidite, b yus = 2 a elp . Yusupovite, ideally Na 2 Zr(Si 6 O 15 )(H 2 O) 3 , is a dimorph of elpidite, Na 2 Zr(Si 6 O 15 )(H 2 O) 3 .