Organic matter as an important cause for mineral diversity in peralkaline pegmatitesтезисы доклада Тезисы

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[1] Chukanov N. V., Pekov I. V., Ermolaeva V. N. Organic matter as an important cause for mineral diversity in peralkaline pegmatites // IV International symposium “Mineral diversity: research and preservation”. — Sofia, EARTH AND MAN NATIONAL MUSEUM, 2007. — P. 7–10. Organic substances (OS) are usual components of peralkaline pegmatites. OS can form macroscopic segregations growing on microporous hetero-framework zeolite-like silicates with Ti, Nb or Zr (playing the role of catalysts of OS formation), or intergrowths with minerals of REE, Th, U etc. The formation of organic substances is possible only under reductive conditions arising after mass crystallization of aegirine, in accordance with the scheme: Na2O + 2FeO + 4SiO2 + CO2  2NaFe3+Si2O6 + CO; Na2O + 2FeO + 4SiO2 + 0.5CO2  2NaFe3+Si2O6 + 0.5C. Oxygen-bearing aromatic substances act as complexing agents towards Th, REE, U, Zr, Ti, Nb and facilitate transportation of these elements under low-temperature hydrothermal conditions in the form of water-soluble associates. Microheterogeneous aggregates with OS can contain alkaline-earth and other elements in the form of carboxylate salts M2+(ArCOO-)2 (where Ar is a polycyclic aromatic radical; M = Ca, Sr, Ba, Pb, REE) and microscopic inclusions of mineral phases with Th, U, REE, Nb, Ti. OS from peralkaline pegmatites of Khibiny and Lovozero massifs (Kola peninsula) have been investigated using IR spectroscopic, electron microprobe and X-ray powder diffraction. The following mineral phases have been detected in microheterogeneous aggregates with OS: REE carbonates, thorite, steacyite, Na,Th-silicates Na2-5ThTiSi8(O,OH)24·nH2O and Na2Th3Si8O23·nH2O, Th- and U-silicates (Th,REE,Ca)1-xSi(O,OH)4·nH2O, (Th,REE,Ca)(Ti,Nb)SiO6·nH2O, NaUSi6O14(OH)·nH2O, Na2U2Si3O11·nH2O, Na2USi4O11·nH2O, (K,Ba)1.5(Ca,Na)(Th,REE)(Ti,Nb)3(Si,Al)8(O,OH)x·nH2O, (Th,REE)(Ti,Nb)(Si,P)O6·nH2O, NaCaK0.5MnTh4REE1-x(Ti,Nb)2Si8(O,OH)x·nH2O, (Na,Ca,K)2(Zn,Ti)2Th4REE(Si,Al)8(O,OH)x·nH2O, an analog of seidite-(Ce) with Th>REE, the phases Na5REE2(Si,P)6O18·nH2O, (Na,Ca)4(Mn,Fe)(Y,HREE,Th,U)2Si12O30·nH2O, alkaline niobosilicates with (Nb+Ti):Si varying from 3:2 to 3:1, rhabdophane-(Ce), rhabdophane-(Nd), monazite-(La), sazhinite-(Ce), vitusite-(Ce) etc. Separation of Y (+HREE), Ce, La, Nd, Th and U between organic and different mineral phases is an important distinctive feature of microheterogeneous aggregates with OS from in peralkaline pegmatites, and an important cause for unusual mineral diversity in these formations.

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