Publication Type:
Journal ArticleSource:
The Canadian MineralogistThe Canadian Mineralogist, Mineralogical Association of Canada, Ottawa, ON, Canada, Volume 51, Number 4, p.597-612 (2013)ISBN:
0008-4476Keywords:
alkalic composition, anions, Canada, cations, chemical composition, crystal chemistry, crystal structure, eastern canada, electron probe data, feldspathoid rocks, formula, FTIR spectra, garnet group, hydrogarnet, hydroxyl ion, igneous rocks, infrared spectra, lattice parameters, metals, Mont-Saint-Hilaire Quebec, nesosilicates, niobium, orthosilicates, Pegmatite, plutonic rocks, powder method, quebec, Rouville County Quebec, silicates, single-crystal method, solid solution, space groups, spectra, substitution, thorite, titanium, unit cell, x-ray diffraction data, zircon groupAbstract:
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 (sub 0.21) Nb (sub 0.20) Ti (sub 0.18) Ca (sub 0.13) Y (sub 0.10) REE (sub 0.09) Fe (sub 0.03) Zr (sub 0.0 1) Sr (sub 0.01) Mn (sub 0.01) K (sub 0.01) Na (sub 0) . (sub 01) ) (sub Sigma ) (sub 1.00) [(Si (sub 0.49) [] (sub 0.41) Al (sub 0.08) P (sub 0.01) S (sub 0.01) ) (sub Sigma ) (sub 1. 00) (O (sub 2.33) F (sub 0.02) )](OH) (sub 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 (sub 2) O (sub 5) ) and Ti (0.19 apfu Ti, 7.41 wt.% TiO (sub 2) ), and evidence for the (SiO (sub 4) ) (super 4-) (OH) (sub 4) (super 4-) "hydrogarnet" substitution. The crystal structure was solved and refined to R = 3.40% and wR (super 2) = 9.73% for 68 reflections with F (sub o) > 4sigma (F (sub o) ). The studied thorite is slightly metamict, tetragonal, space group I4 (sub 1) /amd, with a 7.058(1) Aa, c 6.2260(12) Aa, V 310.15(11) Aa (super 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 (sub axial) = 2.428(5) Aa] and four equatorial O atoms [A-O (sub eq) = 2.322(6) Aa], which define a bisdisphenoid with = 2.374 Aa. 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) Aa. A partially occupied H site (31%) is located 0.980 Aa away from the O atom, forming (O (sub 4) H (sub 4) ) (super 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 AO (sub 8) bisdisphenoid, relaxation and shortening of A-O bonds as a result of the (SiO (sub 4) ) (super 4-) (OH) (sub 4) (super 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) (sub 4) (super 4-) -dominant "thorogummite" with intermediate compositions defined by Th(SiO (sub 4) ) (sub 1) (sub -x) (OH) (sub 4) (sub x) , reported compositions indicate otherwise and it is suggested that the name "thorogummite" be abandoned.
Notes:
GeoRef, Copyright 2018, American Geological Institute.<br/>2014-004231<br/>thorogummite
