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Abstract:

Bobtraillite is a new, complex zirconosilicate found in igneous breccias and nepheline syenite pegmatites at the Poudrette quarry, Mont Saint-Hilaire, Quebec. It is associated with a burbankite-group mineral, donnayite-(Y), clinoamphibole, albite, aegirine, pyrrhotite, pyrite, annite, analcime, microcline, a white mica (muscovite?), yellow titanite, clinopyroxene and calcite. Crystals are commonly grey to brown in color, blocky to prismatic, elongate along [001], with a maximum length of 2 mm. It is generally found as isolated single crystals or simple parallel intergrowths with two or three members, rarely in small (0.1 mm) rosettes. It is transparent with a vitreous luster and a white streak, and is non-luminescent. The Mohs hardness is 51/2; no cleavage is evident. It is brittle and has an uneven to conchoidal fracture. The calculated density is 3.16 g/cm3. Bobtraillite is nonpleochroic, uniaxial positive, with 1.627(1) and 1.645(1). Seven analyses made on one crystal gave: Na2O 5.62, CaO 1.11, SrO 17.76, BaO 0.40, B2O3 (calc.) 3.38, Y2O3 1.15, SiO2 40.51, ZrO2 25.32, HfO2 0.48, Nb2O5 1.32 and H2O (calc.) 5.25, total 102.30 wt.%, corresponding to (Na11.20Ca1.22)12.42 (Sr10.59Ba0.16)10.75 (Zr12.69Y0.63Nb0.61Hf0.14) 14.07Si41.64B6O132 (OH)12&bull12H2O on the basis of 156 anions, or ideally, (Na,Ca)13Sr11(Zr,Y,Nb)14 Si42B6O132(OH)12 &bull12H2O. The mineral is trigonal, space group P3c1, a 19.720(1), c 9.9788(5) A, V 3360.7(1) A3 and Z = 1. The strongest six lines on the X-ray powder-diffraction pattern [d in A(I)(hkl)] are: 2.752(100)(332), 3.13(70)(331), 3.96(51)(212), 3.76(49)(302), 6.46(38)(210), and 5.43(33)(211). The structure of bobtraillite was refined to R = 3.49% and wR2 = 10.88% for 1575 reflections [Fo 4(Fo)]. Isolated ZrO6 octahedra are linked by (Si3O9) rings to form a pinwheel-like motif similar to that found in wadeite. Additional (Si3O9) and [Si2BO7 (OH)2] rings complete a two-dimensional, giant pinwheel motif along [001]. Bonding between three-membered rings and ZrO6 octahedra along [001] completes the structure in three dimensions. Channels along [001] are occupied by Na, H2O and Sr. Bobtraillite shares chemical and crystal-structure features with benitoite, wadeite and catapleiite, and should be considered as a structural hybrid of these minerals. The presence of essential B, Sr, H2O and its trigonal symmetry make it unique. We discuss the group classification of minerals based on frameworks containing isolated MO6 and three-membered rings of tetrahedra. The mineral is paragenetically late, arising from unusual, hydrous fluids enriched in B, Sr, Na and Zr. There are similarities between the fluid resulting in the crystallization of bobtraillite in igneous breccias and that responsible for the assemblages exotic minerals in altered marble xenoliths at Mont Saint-Hilaire. The name of the new species honors Robert ("Bob") J. Traill, formerly mineralogist at the Geological Survey of Canada, Ottawa.

Notes:

Compilation and indexing terms, Copyright 2018 Elsevier Inc.<br/>2005299221277<br/>Bobtraillite<br/>Concoidal fracture<br/>Non-luminescent crystals<br/>Structural hybrid