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The crystal chemistry of a unique Nb-Ti-rich thorite from Mont Saint-Hilaire (Quebec) has been examined by a combination of single crystal and powder X-ray diffraction, electron microprobe analyses, and Fourier-transform infrared spectroscopy. The average of 9 compositions gave (Th0.21Nb0.20Ti0.18Ca0.13Y0 10REE0.09Fe0.03Zr0.01Sr0 01Mn0.01K0.01 Na-0.01)(Sigma 1.00) [(Si-0.49 square 0.41Al0.08P0.01S0.01)(Sigma 1.00)(O2 33F0.02)](OH)(1.70). This is the first example in the literature of a zircon-group mineral containing elevated concentrations of Nb (0.20 apfu, 13.33 wt.% Nb2O5) and Ti (0.19 apfu Ti, 7.41 wt.% TiO2), and evidence for the (SiO4)(4-) <-> (OH)(4)(4-) "hydrogarnet" substitution. The crystal structure was solved and refined to R = 3.40% and wR(2) = 9.73% for 68 reflections with F-o > 4 sigma(F-o). The studied thorite is slightly metamict, tetragonal, space group I4(1)/amd, with a 7.058(1) angstrom, c 6.2260(12) angstrom, V 310.15(11) angstrom(3), and Z = 4. It is isostructural with other zircon-group minerals and has a unit cell which is 4% smaller than that of thorite sensu stricto, a result of the incorporation of high field-strength elements of smaller radii. The structure consists of one eight-coordinated metal site (A = Th, Zr, U, REE, Y, Nb, Ti, etc.), one tetrahedral site (T), one O site, and one variably-occupied H site. The A site is coordinated by four axial O atoms [A-O-axial = 2.428(5) angstrom] and four equatorial O atoms [A-O-eq = 2.322(6) angstrom], which define a bisdisphenoid with < A-O > = 2.374 angstrom. The T site in MSH thorite is only partially occupied by Si (33% vacant) and coordinated by four O with T-O = 1.641(5) angstrom. A partially occupied H site (31%) is located 0.980 angstrom away from the O atom, forming (O4H4)(4-) groups when the T site is vacant. Removal of the center of symmetry in the structure allows for the possibility of the presence of bimodal T-O and A-O bond lengths, leading to both short Si-O bonds and longer square-OH bonds, as well as the shorter A-O bonds required for Nb and Ti. Accommodation of Nb and Ti into the thorite structure may be facilitated by increased distortion of the AO(8) bisdisphenoid, relaxation and shortening of A-O bonds as a result of the (SiO4)(4-)<-> (OH)(4)(4-) substitution, and the likely presence of defects (O vacancies) in regions which have undergone slight metamictization, resulting in short-range ordering of Nb, Ti, and Th. Although it is possible that a metastable, limited solid solution exists between thorite and (OH)(4)(4-)-dominant "thorogummite" with intermediate compositions defined by Th(SiO4)(1-x)(OH)(4x), reported compositions indicate otherwise and it is suggested that the name "thorogummite" be abandoned.

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