Abstract

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 Na0.01)Σ1.00 [(Si0.490.41Al0.08P0.01S0.01)Σ1.00(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)44− “hydrogarnet” substitution. The crystal structure was solved and refined to R = 3.40% and wR2 = 9.73% for 68 reflections with Fo > 4σ(Fo). The studied thorite is slightly metamict, tetragonal, space group I41/amd, with a 7.058(1) Å, c 6.2260(12) Å, V 310.15(11) Å3, 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–Oaxial = 2.428(5) Å] and four equatorial O atoms [A–Oeq = 2.322(6) Å], which define a bisdisphenoid with <A–O> = 2.374 Å. 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) Å. A partially occupied H site (31%) is located 0.980 Å 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 □–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 AO8 bisdisphenoid, relaxation and shortening of A–O bonds as a result of the (SiO4)4−↔ (OH)44− 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)44−-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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