Sorokhtina N.V., Kogarko L.N., Kononkova N.N.

Variation in perovskite composition from Guli intrusive, Polar Siberia.

Vernadskiy Institute RAS. Moscow

 

Perovskite-group minerals are common accessory phases of Guli phoscorites and carbonatites. Pseudo cubic crystals of these minerals contain a many inclusions of rock-forming and accessory minerals, apatite, phlogopite, Ca-Mg and alkali-rich carbonates, calzirtit, pyrophanite etc.

The crystals of perovskite have not homogeneous chemical composition and contain variable amounts of Nb (0.29-14.61 wt% Nb₂O₅), Ce (1.14-4.64 wt% Ce₂O₃), Fe (0.87-4.65 wt% Fe₂O₃). All compositions of the Guli perovskite-group minerals fall into the perovskite field of the CaTiO₃-NaNbO₃-Na_0.5Ce_0.5TiO₃ classification diagram [1]. Nb and REE shows large variations and three distinct types of perovskite can be identified, niobian perovskite from phoscorites, niobian ceroan perovskite from phoscorites and early calcite carbonatites and very poor in Nb and REE perovskite from rare metal-rich calcite carbonatites. In some cases, the subhedral crystals of calzirtite are replaced by niobian ceroan perovskite containing up to 7 wt.% ZrO₂. Sr-rich members of the perovskite-group minerals are not found in Guli. The content of SrO varies from 0.12 to 0.27 wt.%. The Nb/Ta ratio in the niobian perovskite varies from phoscorites is 110 and from carbonatites 123, respectively. REE show positive correlation with Na, Nb show negative correlation with Ti. Nb may be situated in perovskite according to the following isomorphic scheme: Nb⁵⁺ + Fe³⁺ ↔ Ti⁴⁺ + Zr⁴⁺. The major trend of compositional evolution of perovskite-group minerals is directed to decreasing of Nb, REE and Na contents.

 

References

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Williams, C.T.,Kogarko L.N. New data about rare-metal mineralisation of carbonatites of the Guli massif (Polar Siberia) // Geochemistry. 1996. P 483-491(in Russian).