Abstract ID/ No de résumé: 616
EXSOLUTION IN NIOBIAN RUTILE: NEW PROCESSES AND NEW PHASES
CERNY, P., and CHAPMAN, R., Geological Sciences, Univ. Manitoba, Winnipeg, MB, R3T 2N2, cernyp@ms.umanitoba.ca; UHER, P., Geological Institute, Slovak Academy of Sciences, 842 26 Bratislava; SIMMONS, Wm.B., Geology and Geophysics, Univ. New Orleans, New Orleans, LA 70148
Room for the session: 400A
Board if it is a poster: C-49
Date of the presentation (mm/dd): 05/20
Beginning of the presentation: 11:00
Number of symposium, special session or general session: G11
Lieu de la présentation: 400A
# du tableau si c'est un poster : C-49
Date de la présentation (mm/dd): 05/20
Début de la présentation: 11:00
No du symposium, de la session spéciale ou de la session générale: G11
Electron-microprobe and XRD examination of niobian rutile from several worldwide localities revealed new phenomena of exsolution and breakdown in this mineral which is relatively widespread in granites and granitic pegmatites. (i) At Praivį (Slovak Republic), niobian rutile, associated with titanian columbite and a Fe3+NbO4-rich titanian ixiolite, displays very fine (1 to 3 µm) lamellar exsolution of niobian armalcolite to pseudobrookite (6 to 10 wt.% Nb2O5, 0.5 to 5.5 wt.% Ta2O5), and later aggregates of niobian ilmenite, ilmenite and titanian hematite. (ii) At Håverstad and in central Iveland (Norway), Fe3+-dominant niobian rutile shows a two-stage exsolution: extremely fine lamellae (1 to 5 µm) of Ti-rich titanian ixiolite/columbite in Nb-rich rutile segregate into coarse blebs of Ti-poorer titanian columbite/ixiolite embedded in Nb-poorer rutile; the exsolved orthorhombic phase contains 8.98 wt.% Sc2O3. (iii) In the Eptevann pegmatite (Norway), coarse heterogeneous blebs of titanian columbite/ixiolite, exsolved from a Fe3+-dominant niobian rutile, contain 12.50 wt.% Sc2O3. (iv) In the Tesirogi pegmatite (Japan), complex intergrowths of two-stage exsolution products, similar to those from Norway, also display lamellae of ilmenite, indicative of Fe2+ excessive over the columbite-type substitution in the parent phase. (v) In the McGuire pegmatite (Colorado), the breakdown of niobian rutile starts with exsolution of extremely fine (1 to 3 µm) lamellae of a phase corresponding to Fe2+Fe3+NbTi2O9. This phase readily breaks down to extremely fine-grained aggregates which probably consist of rutile and columbite. In spatial progression from these aggregates, exsolution in the primary niobian rutile advances into progressively coarser intergrowth of rutile and columbite, both displaying progressively lower degrees of mutual solid solution. (vi) In a sample from Ilmen Mts. (Russia), extremely Mn-poor and Fe3+-rich heterogeneous niobian rutile contains lamellae of very Mn-rich "wolframoixiolite" with 30.10 wt.% WO3. Overall, niobian rutile exsolves apparently metastable phases in early stages, followed by fine lamellar (or granular) Ti-rich ixiolite/columbite which undergoes coarsening and further exsolution. Recrystallization and possibly metasomatism of the coarse exsolution products conclude the process.
Abstract ID/ No de résumé: 617
ZIRCONIUM AND HAFNIUM IN MINERALS OF THE COLUMBITE FAMILY
Cerny, P., Geological Sciences, Univ. Manitoba, Winnipeg, MB, R3T 2N2; ERCIT, T.S., Research Div., Canadian Museum of Nature, Ottawa, ON K1P 6P4; SMEDS, S.-A., Earth Sciences, Uppsala Univ., S-752 36 Uppsala; GROAT, L.A., Geological Sciences, Univ. British Columbia, Vancouver, BC, V6T 2B4; CHAPMAN, R., Geological Sciences, Univ. Manitoba, Winnipeg, MB, R3T 2N2
Room for the session: 400A
Board if it is a poster: C-50
Date of the presentation (mm/dd): 05/20
Beginning of the presentation: 11:00
Number of symposium, special session or general session: G11
Lieu de la présentation: 400A
# du tableau si c'est un poster : C-50
Date de la présentation (mm/dd): 05/20
Début de la présentation: 11:00
No du symposium, de la session spéciale ou de la session générale: G11
Zirconium is rarely analyzed for in minerals of the broader columbite family, and no data are available for hafnium. The present survey was prompted by high Zr contents accidentally discovered in ixiolite from Sweden and Northwest Territories. Specimens examined come from the Tanco pegmatite, Bernic Lake, s.e. Manitoba, Red Cross Lake, n.e. Manitoba, Sidensjö area, central Sweden, Quadeville area, Ontario, and the Little Nahanni pegmatite group, Northwest Territories. Compositionally complex, Sn- and Ti-rich and/or highly disordered minerals (ixiolite group, wodginite group, and an undefined phase) typically contain elevated Zr+Hf contents, whereas compositionally simple, ordered minerals (columbite group sensu stricto, tapiolite series) are very poor in these elements. - The highest contents of ZrO2 (9.55 wt.%) and HfO2 (1.15 wt.%) are found in microscopic grains ( 100 µm) with ixiolite stoichiometry, enclosed in (or possibly exsolved) from columbite. Wodginite contains appreciable but lower ZrO2 ( 2.13 wt.%) and HfO2 ( 0.71 wt.%), whereas columbite has ZrO2 0.60 wt.% and HfO2 0.08 wt.% (with both commonly below detection limits). Wodginite has consistently low but locality-dependent Zr/Hf (by wt.) averaging from 2.7 (Red Cross Lake) to 1.9 (Tanco), whereas associated and coexisting columbite phases show values >6.3, and ixiolite averages at 5.0 (Sidensjö) and 7.5 (Little Nahanni). These Zr/Hf ratios show no uniform relationship to those of associated zircon but are significantly lower than the latter. The sum of Zr+Hf tends to be positively correlated with Sn+Ti, Ta/(Ta+Nb) and Mn/(Mn+Fe) but reverse trends or lack of correlation are observed locally. Also, correlation with the sequence of parent pegmatite zones and with zoning of individual crystals is not uniform at different localities. - Overall, minerals of the ixiolite and wodginite groups prove to be efficient carriers of, and fractionation agents for Zr and Hf, in a wide range of pegmatite categories including the most fractionated, pollucite-bearing complex bodies in which zircon is extremely rare. The conditions shifting Zr+Hf from the marginally stable 8-fold coordination in the generally widespread silicate phase into the srilankite-like octahedral sites of the examined oxide minerals are not clear.
Total number: 2