• journal_title：The Canadian Mineralogist
• Contributor：Andrew M. McDonald ; George Y. Chao
• Publisher：Mineralogical Association of Canada
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.48.2.267
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：48
• issue：2
• firstpage：267
• section：Articles

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(Sr_21.16Na_1.17Ca_0.21)_∑22.54Ba_4.00 (Zr_25.33Ti_0.93)_∑26.26(Si_77.02B_0.98)_∑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) ų 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 [F_o > 4σ(F_o)]. 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.