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Solid solutions of niobium in synthetic titanite

Ruslan P. Liferovich and Roger H. Mitchell
Solid solutions of niobium in synthetic titanite
The Canadian Mineralogist (October 2006) 44, Part 5: 1089-1097


Synthetic (Ca (sub 1-x) Na (sub x) )(Ti (sub 1-x) Nb (sub x) )OSiO (sub 4) and Ca(Ti (sub 1-2x) Nb (sub x) Al (sub x) )OSiO (sub 4) solid solutions have been prepared for x = 0.1 and 0.2 by ceramic methods and their crystal structure determined by Rietveld analysis. At ambient conditions, the isomorphic capacity of (F,OH)-free titanite is approximately 0.25 apfu Nb in a single-site scheme of substitution (2 (super VI) Ti (super 4+) <--> (super VI) Al (super 3+) + (super VI) Nb (super 5+) ) and approximately 0.22 apfu Nb in a two-site scheme ( (super VII) Ca (super 2+) + (super VI) Ti (super 4+) <--> (super VII) Na (super +) + (super VI) Nb (super 5+) ). All cations located at the VIIX and VIY sites are disordered. Analysis of tetrahedron bond-lengths indicates the absence of Al (super 3+) replacing Si (super 4+) in coordination tetrahedra. All Nb-doped varieties of titanite adopt space group A2/a. Thus, both single-site and complex multivalent schemes of substitution destroy the coherence of the off-centering of chains of octahedra typical of the CaTiOSiO4 end member, resulting in a P2 (sub 1) /a<-->A2/a phase transition. The (Ca (sub 1-x) Na (sub x) )(Ti (sub 1-x) Nb (sub x) )OSiO (sub 4) scheme of substitution incorporates the larger cations at both the (super VII) X and (super VI) Y sites, whereas the Ca(Ti (sub 1-2x) Nb (sub x) Al (sub x) )OSiO (sub 4) scheme involves only (super VI) Y-site (Al (super 3+) , Nb (super 5+) ) cations, with a slightly smaller "average" radius. Unit-cell dimensions change insignificantly and vary sympathetically with the change of average radii of the cations in the (Ca (sub 1-x) Na (sub x) )(Ti (sub 1-x) Nb (sub x) )OSiO (sub 4) series and vary insignificantly in the Ca(Ti (sub 1-2x) Nb (sub x) Al (sub x) )OSiO (sub 4) series. Both Nb-doped titanite and pure CaTiOSiO (sub 4) consist of distorted polyhedra. The seven-fold coordination polyhedra and octahedra in Nb-doped titanite are slightly less stretched as compared to those in pure CaTiOSiO (sub 4) . The SiO (sub 4) tetrahedron is compressed in Nb-doped titanite as compared to that in the pure CaTiOSiO (sub 4) . The experimental data obtained suggest that the existence of a titanite analogue with more than 50 mol.% of NaNbOSiO (sub 4) end member is unlikely. The solid solution involving the smaller (super VI) (Al,Nb) cations theoretically could be stabilized at high pressure, suggesting the existence of a potentially new species dominated by the Ca(AlNb)OSiO (sub 4) end member. The synthetic titanite compositions may be suitable for the sequestration of radioactive waste containing 94Nb.

ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 44, Part 5
Title: Solid solutions of niobium in synthetic titanite
Affiliation: Lakehead University, Department of Geology, Thunder Bay, ON, Canada
Pages: 1089-1097
Published: 200610
Text Language: English
Summary Language: French
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 43
Accession Number: 2007-038820
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200710
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