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Rogermitchellite is a new, complex species of zirconosilicate found in igneous breccia at the Poudrette quarry, Mont Saint-Hilaire, Quebec. It is associated with aegirine, annite, galena, a labuntsovite-group mineral, manganoneptunite, microcline, pyrrhotite, sodalite and zircon. Paragenetically, the mineral crystallizes after aegirine and before microcline. Isolated, single crystals are colorless to grey, prismatic along [001], and slightly flattened perpendicular to this direction, with a maximum dimension of 0.1 × 0.25 mm. It develops as euhedral crystals displaying the prominent forms prism {100} and pinacoid {001}, with a minor, very shallow ditrigonal scalenohedron {10l}. The mineral is transparent with a vitreous luster and a white streak and is non-luminescent. The Mohs hardness is 5, and no cleavage is evident. It is brittle with a conchoidal to splintery fracture. The calculated density is 3.34 g/cm3. Rogermitchellite is non-pleochroic, uniaxial positive, with ω 1.640(1) and ε 1.663(1). Four analyses made on one crystal gave, as an average: Na2O 3.45, CaO 0.10, SrO 18.54, BaO 5.18, B2O3 (calc.) 2.34, SiO2 39.12, ZrO2 26.39, TiO2 0.63 and H2O (calc.) 6.96, for a total of 102.71 wt.%. The empirical formula, based on 288 anions and the sum of (Si + B) = 90 apfu, is: Na12(Sr21.16Na1.17Ca0.21)∑22.54Ba4.00 (Zr25.33Ti0.93)∑26.26(Si77.02B0.98)∑78B12O246(OH)24·18H2O or, ideally, Na12Sr24Ba4Zr26Si78B12O246(OH)24·18H2O. The mineral is trigonal, space group P3̅c1, a 26.509(4), c 9.975(2) Å, V 6070.6(1) Å3 and Z = 1. The strongest six lines on the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 2.760(100)(442), 3.761(90)(402), 1.991(70)(444), 3.150(50)(441), 5.762(40)(400) and 3.924(30)(312). The crystal structure was refined to R = 4.62% and wR2 = 12.73% for 1588 reflections [Fo > 4σ(Fo)]. It consists of pairs (Si3O9) rings (offset 30° relative to one another along [001]) linked to isolated ZrO6 octahedra in a motif similar to that in benitoite. Additional (Si3O9) and [Si2BO7(OH)2] rings along with ZrO6 octahedra complete a three-dimensional heteropolyhedral framework. Channels along [001] within this framework are occupied by Ba, Na, (Sr + Na) and H2O. Rogermitchellite has chemical and crystal-structure features common to benitoite, wadeite and catapleiite, and should be considered as a structural hybrid of these minerals. It possesses additional features (presence of essential B, Sr, H2O; trigonal symmetry) that make it unique.<br/>