ROGERMITCHELLITE, Na12(Sr,Na)24Ba4Zr26Si78(B,Si)12O246(OH)24{middle dot}18H2O, A NEW MINERAL SPECIES FROM MONT SAINT-HILAIRE, QUEBEC: DESCRIPTION, STRUCTURE DETERMINATION AND RELATIONSHIP WITH HFSE-BEARING CYCLOSILICATES

Abstract

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 {10 l }. 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/cm 3 . Rogermitchellite is non-pleochroic, uniaxial positive, with ω 1.640(1) and e 1.663(1). Four analyses made on one crystal gave, as an average: Na 2 O 3.45, CaO 0.10, SrO 18.54, BaO 5.18, B 2 O 3 (calc.) 2.34, SiO 2 39.12, ZrO 2 26.39, TiO 2 0.63 and H 2 O (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: Na 12 (Sr 21.16 Na 1.17 Ca 0.21 ) ∑22.54 Ba 4.00 (Zr 25.33 Ti 0.93 ) ∑26.26 (Si 77.02 B 0.98 ) ∑78 B 12 O 246 (OH) 24 ·18H 2 O or, ideally, Na 12 Sr 24 Ba 4 Zr 26 Si 78 B 12 O 246 (OH) 24 ·18H 2 O. The mineral is trigonal, space group P 3 c 1, a 26.509(4), c 9.975(2) A, V 6070.6(1) A 3 and Z = 1. The strongest six lines on the X-ray powder-diffraction pattern [ d in A(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 w R 2 = 12.73% for 1588 reflections [ F o > 4σ( F o )]. It consists of pairs (Si 3 O 9 ) rings (offset 30° relative to one another along [001]) linked to isolated ZrO 6 octahedra in a motif similar to that in benitoite. Additional (Si 3 O 9 ) and [Si 2 BO 7 (OH) 2 ] rings along with ZrO 6 octahedra complete a three-dimensional heteropolyhedral framework. Channels along [001] within this framework are occupied by Ba, Na, (Sr + Na) and H 2 O. 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, H 2 O; trigonal symmetry) that make it unique.