Hydroterskite, ideally Na2ZrSi6O12(OH)6, is a new mineral species from the Saint-Amable sill at the Demix-Varennes quarry, near Varennes, Québec, 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, 2Vmeas. = 86(3)°, 2Vcalc. = 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 Na2O 7.82, K2O 0.07, CaO 0.62, FeO 0.89, MnO 0.71, Al2O3 0.08, La2O3 0.12, Ce2O3 0.24, SiO2 59.82, HfO2 0.11, ThO2 1.15, ZrO2 15.00, TiO2 1.15, Nb2O5 1.12, F 0.11, H2O (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 (Na1.543K0.009 Ca0.068La0.005Ce0.009)∑1.634(Zr0.744Ti0.088Nb0.052Th0.005Hf0.003Fe0.076Mg0.002Mn0.061Al0.010)∑1.041Si6.087O12(OH5.963F0.035Cl0.002)∑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) Å, V 1546.8(2.0) Å3, 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: [Na1O6(OH)2] is a hexagonal dipyramid polyhedron, whereas [Na2O6(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 [ZrO6] octahedra and [NaO8] 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.

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