Nb-Fe-RICH ZIRCONOLITE GROUP MINERALS IN CALCITE CARBONATITES OF THE BELAYA ZIMA MASSIF (EASTERN SAYAN)

Nb-Fe-RICH ZIRCONOLITE GROUP MINERALS IN CALCITE CARBONATITES OF THE BELAYA ZIMA MASSIF (EASTERN SAYAN)

Two compositional varieties of Nb-Fe-zirconolites have been found in calcite carbonatites of the Belaya Zima alkaline massif: «laachite» Ca2 Zr2 Nb2 TiFe2+O14 (Nb2 O5 > 28 wt. %; TiO2 < 14 wt. %) and «NbFe3+-zirconolite» Ca2 Zr2 NbTi2 Fe3+O14 (Nb2 O5 < 25 wt. %; TiO2 > 16 wt. %). Maximum...

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Bibliographic Details
Main Authors: V.V. Sharygin, A.G. Doroshkevich, E.A. Khromova
Format: Article
Language:English
Published: South Urals Federal Research Center of Mineralogy and Geoecology UB RAS 2016-12-01
Series:Минералогия
Subjects:
zirconolite group
nb-fe-zirconolite
laachite
fluorcalciopyrochlore
carbonatite
belaya zima alkaline massif
eastern sayan
Online Access:https://journal.mineralogy.ru/wp-content/uploads/2019/09/2016-4-1.pdf
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Summary:Two compositional varieties of Nb-Fe-zirconolites have been found in calcite carbonatites of the Belaya Zima alkaline massif: «laachite» Ca2 Zr2 Nb2 TiFe2+O14 (Nb2 O5 > 28 wt. %; TiO2 < 14 wt. %) and «NbFe3+-zirconolite» Ca2 Zr2 NbTi2 Fe3+O14 (Nb2 O5 < 25 wt. %; TiO2 > 16 wt. %). Maximum contents of minor components in these minerals are (wt. %): ThO2 2.2; MnO 1.5; MgO 0.6; Ta2 O5 1.1; HfO2 0.6; Y2 O3 0.5; REE2 O3 5.7; FeO 9.4. Chemical zoning patterns of individual grains are evident of heterovalent Nb5+ + Fe2+ ↔ Ti4+ + Fe3+ isomorphism; the core-to-rim evolution of Ca2 Zr2 Nb2 TiFe2+O14 → Ca2 Zr2 NbTi2 Fe3+O14 was revealed in «laachite» in contrast to reverse trend in «Nb-Fe3+-zirconolite». The content of hypothetical CaREEZr2 NbTi2 (Mn,Fe)2+O14 end-member in both phases is < 15 mole %. «Laachite» occurs in carbonatites enriched in fluorcalciopyrochlore and depleted in Na-Fe-silicates (central part of the massif), whereas «Nb-Fe3+-zirconolite» was found in carbonatites with abundant Na-Fe-amphiboles and aegirine and low amount of fluorcalciopyrochlore (near the contact with silicate rocks). The occurrence of diverse Nb-Fe-zirconolites can be related to local T-X-f O 2 conditions of calcite carbonatite crystallization. The nomenclature of the zirconolite group is briefly discussed, namely, possible allocation of Nb-Fe-zirconolites into individual subgroup (laachite, hypothetical minerals CaREEZr2 NbTi2 (Mn,Fe)2+O14 and Ca2 Zr2 NbTi2 Fe3+O14).
ISSN:2313-545X
2782-2044

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