CRYSTAL STRUCTURE DETERMINATION OF A UNIQUE Nb-Ti-RICH THORITE FROM MONT SAINT-HILAIRE, QUEBEC, WITH COMMENTS ON ‘THOROGUMMITE’

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 (Th 0.21 Nb 0.20 Ti 0.18 Ca 0.13 Y 0.10 REE 0.09 Fe 0.03 Zr 0.01 Sr 0.01 Mn 0.01 K 0.01 Na 0 . 01 ) Σ 1.00 [(Si 0.49 □ 0.41 Al 0.08 P 0.01 S 0.01 ) Σ 1.00 (O 2.33 F 0.02 )](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.% Nb 2 O 5 ) and Ti (0.19 apfu Ti, 7.41 wt.% TiO 2 ), and evidence for the (SiO 4 ) 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 > 4σ( F o ). The studied thorite is slightly metamict, tetragonal, space group I 4 1 / amd , with a 7.058(1) A, c 6.2260(12) A, V 310.15(11) A 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) A] and four equatorial O atoms [ A –O eq = 2.322(6) A], which define a bisdisphenoid with A –O> = 2.374 A. 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) A. A partially occupied H site (31%) is located 0.980 A away from the O atom, forming (O 4 H 4 ) 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 A O 8 bisdisphenoid, relaxation and shortening of A –O bonds as a result of the (SiO 4 ) 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(SiO 4 ) 1 −x (OH) 4 x , reported compositions indicate otherwise and it is suggested that the name “thorogummite” be abandoned.