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Minerals 2017, 7(11), 215; doi:10.3390/min7110215

Subsolidus Evolution of the Magnetite-Spinel-UlvöSpinel Solid Solutions in the Kovdor Phoscorite-Carbonatite Complex, NW Russia

1
Kola Science Centre, Russian Academy of Sciences, 14 Fersman Street, Apatity 184209, Russia
2
Geological Institute of Kola Science Centre, Russian Academy of Sciences, 14 Fersman Street, Apatity 184209, Russia
*
Author to whom correspondence should be addressed.
Received: 19 September 2017 / Revised: 31 October 2017 / Accepted: 3 November 2017 / Published: 9 November 2017
(This article belongs to the Special Issue Fundamentals and Frontiers in Mineralogy)
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Abstract

The Kovdor phoscorite-carbonatite ore-pipe rocks form a natural series, where apatite and magnetite first gradually increase due to the presence of earlier crystallizing forsterite in the pipe marginal zone and then decrease as a result of carbonate development in the axial zone. In all lithologies, magnetite grains contain (oxy)exsolution inclusions of comparatively earlier ilmenite group minerals and/or later spinel, and their relationship reflects the concentric zonation of the pipe. The temperature and oxygen fugacity of titanomagnetite oxy-exsolution decreases in the natural rock sequence from about 500 °C to about 300 °C and from NNO + 1 to NNO − 3 (NNO is Ni-NiO oxygen fugacity buffer), with a secondary positive maximum for vein calcite carbonatite. Exsolution spinel forms spherical grains, octahedral crystals, six-beam and eight-beam skeletal crystals co-oriented with host magnetite. The ilmenite group minerals occur as lamellae oriented along {111} and {100} planes of oxy-exsolved magnetite. The kinetics of inclusion growth depends mainly on the diffusivity of cations in magnetite: their comparatively low diffusivities in phoscorite and carbonatites of the ore-pipe internal part cause size-independent growth of exsolution inclusions; while higher diffusivities of cations in surrounding rocks, marginal forsterite-rich phoscorite and vein calcite carbonatite result in size-dependent growth of inclusions. View Full-Text
Keywords: Kovdor; phoscorite; carbonatite; titanomagnetite; exsolution; temperature; zonation Kovdor; phoscorite; carbonatite; titanomagnetite; exsolution; temperature; zonation
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MDPI and ACS Style

Ivanyuk, G.Y.; Kalashnikov, A.O.; Pakhomovsky, Y.A.; Bazai, A.V.; Goryainov, P.M.; Mikhailova, J.A.; Yakovenchuk, V.N.; Konopleva, N.G. Subsolidus Evolution of the Magnetite-Spinel-UlvöSpinel Solid Solutions in the Kovdor Phoscorite-Carbonatite Complex, NW Russia. Minerals 2017, 7, 215.

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