Home > Mineral Abstracts > CanMin Vol. 43, pp. 747-58 (2005)




Vol. 43, pp. 747-58 (2005)



Bobtraillite,
(Na,Ca)13Sr11(Zr,Y,Nb)14Si42B6O132(OH)12 · 12H2O,
a new mineral species from
Mont Saint-Hilaire, Quebec:
Description, structure determination and
relationship to benitoite and wadeite


Andrew M. McDonald
Department of Earth Sciences, Laurentian University,
Sudbury, Ontario, P3E 2C6, Canada


George Y. Chao
Ottawa-Carleton Geoscience Centre,
Department of Earth Sciences, Carleton University,
Ottawa, Ontario, K1S 5B6, Canada



ABSTRACT

     Bobtraillite is a new, complex zirconsilicate 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 (<0.1mm) rosettes. It is transparent with a vitreous luster and a white streak, and is non-luminescent. The Mohs hardness is 5½, 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 w 1.627(1) and e 1.645(1).

     Seven analyses made on one crystal gave:

Na2O - 5.62
BaO - 0.40
ZrO2 - 25.32

CaO - 1.1
Ba2O3 (calc.) - 3.38
HfO2 - 0.48

SrO - 17.76
SiO2 - 40.51
Nb2O5 - 1.32

H2O (calc.) - 5.25
Total 102.30 wt.%,

     corresponding to: (Na11.20Ca1.22)S12.42(Sr10.59Ba0.16)S10.75(Zr12.69Y0.63Nb0.61Hf0.14)S14.07 Si41.64B6O132 · 12H2O on the basis of 156 anions, or ideally, (Na,Ca)13Sr11(Zr,Y,Nb)14 Si42B6O132(OH)12 · 12H2O.

     The mineral is trigonal, space group P3c1, a 19.720(1), c 9.9788(5)Å, V 3360.7(1) Å3 and Z=1.

     The strongest six lines on the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are:

2.752(100)(332)
3.13(70)(331)
3.96(51)(212)

3.76(49)(302)
6.46(38)(210)
5.43(33)(211)

     The structure of bobtraillite was refined to R = 3.49% and wR2 = 10.88% for 1575 reflections [Fo > 4s(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 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.

Keywords: bobtraillite, new mineral species, zirconsilicate, crystal structure, cyclosilicate, wadeite, benitoite, igneous breccias, Mont Saint-Hilaire, Quebec.


© 2005  The Canadian Mineralogist