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Nikitina L.P., Babushkina M.S., Chernova O.G., Ovchinnikov N.O., Yakovleva A.K., Yakovlev Yu.N., Goilo E.A.

Structural state of amphibole from the Archaean rocks of the Kola Superdeep Borehole and their homologues at the surface

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Abstract. Trioctahedral ferromagnesium micas from the SG-3 Archaean rocks and their surface homologues constituting the amphibolite-gneiss complex in the Pechenga structure southern framing (Allarechka block) have been studied. The mineral structure, the nature and degree of structural defects have been investigated by X-ray powder diffractometry, infrared and Mossbauer spectroscopy. The chemical composition of micas was determined by the wet chemistry and microprobe methods. The study showed that with the same fm values, the mica structures from the SG-3 Archaean rocks have higher values of the unit cell volume over those for synthetic micas of the phlogopite-annite series. This is evidence of the mica structural "looseness" and is unexpected for a mineral from deep zones (7900-11350 m) taking into account the presence of trivalent cations to 0.5 AU in the mica octahedral layer. The presence of defects related to the composition non-stoichiometry (the presence of crystalline hydrate water molecules in the alkaline cation site and availability of vacancies in the octahedral sites M1) and defects of "order-disorder" type have been established. Non-stoichiometry of the composition is equally typical of the micas from the SG-3 Archaean rocks and surface homologues and is likely to reflect the metamorphism specific regime of the Archaean complex under amphibolite facies conditions (increased partial water pressure, lack of alkalies). At the same time the content of crystalline hydrate water molecules in the mica structure from the SG-3 Archaean rocks decreases slowly with depth. A various degree of cation order in the octahedral sites of the mica structure from the SG-3 Archaean rocks (disorder or close to disorder distribution) and from the homologous rocks (order distribution with prevailing ferrous iron occupation of the sites M1) has been established. The difference in the order degree most likely results from the different velocity of the Archaean rocks lift in the process of drilling as well as from the erosion processes. This fact supports the assumption that some structural peculiarities of minerals do not alter when extracting the core from the borehole. This provides preservation of the information on the crystalline substance state at depths of the Archaean complex cut by the borehole.

Printed reference: Structural state of amphibole from the Archaean rocks of the Kola Superdeep Borehole and their homologues at the surface / L.P. Nikitina, M.S. Babushkina, O.G. Chernova, N.O. Ovchinnikov, A.K. Yakovleva, Yu.N. Yakovlev, E.A. Goilo // Vestnik of MSTU. 2007. V. 10, No 1. P. -.

Electronic reference: Structural state of amphibole from the Archaean rocks of the Kola Superdeep Borehole and their homologues at the surface / L.P. Nikitina, M.S. Babushkina, O.G. Chernova, N.O. Ovchinnikov, A.K. Yakovleva, Yu.N. Yakovlev, E.A. Goilo // Vestnik of MSTU. 2007. V. 10, No 1. P. -. URL: http://vestnik.mstu.edu.ru/v10_1_n26/articles/04_nikit.pdf.

(In Russian, p.11, fig. 3, tables 8, ref 13, Adobe PDF)