Hydroterskite: A New Mineral Species From the Saint-Amable Sill, Quebec, and A Comparison With Terskite and Elpidite

Abstract

Hydroterskite, ideally Na 2 ZrSi 6 O 12 (OH) 6 , is a new mineral species from the Saint-Amable sill at the Demix-Varennes quarry, near Varennes, Quebec, Canada. Hydroterskite crystals are short prismatic in habit, up to 3 mm in width, and bounded by the {100}, {010}, and {001} prisms. The crystals are translucent, pale gray in color, with a white streak and vitreous luster. Hydroterskite is brittle and splintery, with good cleavages on the prismatic planes. It is biaxial negative, α = 1.562 ± 0.002, β = 1.567 ± 0.002, γ = 1.571 ± 0.002, 2 V meas. = 86(3)°, 2 V calc. = 83°. Dispersion could not be observed, and there is no pleochroism. The optical orientation is X = c , Y = b , and Z = a . The average result of three analyses of a single grain (oxide wt.%) gave Na 2 O 7.82, K 2 O 0.07, CaO 0.62, FeO 0.89, MnO 0.71, Al 2 O 3 0.08, La 2 O 3 0.12, Ce 2 O 3 0.24, SiO 2 59.82, HfO 2 0.11, ThO 2 1.15, ZrO 2 15.00, TiO 2 1.15, Nb 2 O 5 1.12, F 0.11, H 2 O (by stoichiometry) 8.79, for a total (−0.05 O=F) of 96.84. Magnesium, Y, Sr, and Cl were sought but not detected. The empirical formula on the basis of 18 anions with 6 OH − is (Na 1.543 K 0.009 Ca 0.068 La 0.005 Ce 0.009 ) ∑1.634 (Zr 0.744 Ti 0.088 Nb 0.052 Th 0.005 Hf 0.003 Fe 0.076 Mg 0.002 Mn 0.061 Al 0.010 ) ∑1.041 Si 6.087 O 12 (OH 5.963 F 0.035 Cl 0.002 ) ∑6 . The structure of hydroterskite is orthorhombic in the centric space group Pnca , with a 13.956(6), b 14.894(7), c 7.441(4) A, V 1546.8(2.0) A 3 , and Z = 4. It has been refined to an R index of 0.036 on the basis of 1080 observed reflections. There are two crystallographically distinct Na sites, both with eight-fold co-ordination: [ Na1 O 6 (OH) 2 ] is a hexagonal dipyramid polyhedron, whereas [ Na2 O 6 (OH) 2 ] is a bifurcated pentagonal dipyramid. The Zr atom is in a regular octahedral coordination. Silicon, in tetrahedral coordination with oxygen, forms spiral chains parallel to the c axis. These chains are cross-linked by edge-sharing [ZrO 6 ] octahedra and [NaO 8 ] polyhedra. All three H atoms were located. The crystal-structure determination of terskite in the literature was most likely incorrectly assigned to an acentric space-group. The hydroterskite crystal structure is compared to the structures of terskite, elpidite, and yusupovite.