Formation of bituminous substance in the presence of Nb-rich microporous silicates pegmatites of the Khibiny massif
Chukanov N.V.*, Pekov I.V.**,***, Perepelitsina E.O.*, Ermolaeva V.N.***,
* Institute of Problems of Chemical Physics RAS, Chernogolovka, Russia
** Lomonosov Moscow State University, Moscow, Russia;
*** Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia
Among silicate minerals, there is a family of Nb-rich oxosilicates (with Nb > Ti, Nb+Ti ³ Si) which take an intermediate position between silicates s.s. and oxides. Their specific feature is the presence of oxygen atoms which are bonded only to Ti, Nb and large cations, but are not to Si atoms. Crystal structures of these minerals are characterized by presence of pyrochlore module. Numerous finds of Nb-rich silicates (fersmanite, komarovite, and insufficiently studied related minerals) have been made in agpaitic pegmatites of Khibiny massif (Pekov et al., 2003; Pekov et al., 2004; Pekov, Podlesnyi, 2004), where they are associated with bituminous substances.
Catalytic ability of Nb centers in different reactions of organic synthesis, including polymerization processes under homogenous conditions is well known (Srinrivasan, Farona, 1988; Nakayama et al., 2008). In particular, microporous sodium niobosilicate АМ-11 with the ratio Si:Nb=4.6 catalyzes reactions of dehydroxilation and polycondensation of methanole (Philippou et al., 2001; Brandao et al., 2001).
In pegmatites of the Khibiny massif, bituminous substances are spatially aligned to the oxosilicates, that is considered by us as an indication of the possibility of analogous processes in nature. In order to check of this assumption, we studied the paragenetic relationships of niobosilicates with bituminous substances in Khibiny pegmatites.
Studied samples of bitumens originate from zeolitic zones of the pegmatite at Koashva mountain (sample 1), feldspat-sodalite-natrolite vein of the Kukisvumchorr mountain (Sokolova, 1965 – sample 2), two closely located aegirine-natrolite veins of similar type outcropped at the Kirovskii underground mine (samples 3A and 3B) and pegmatite «Belovitovoe» (Pekov, Podlesniy, 2004) located in the Kirovskii mine (sample 4). All the samples have been found in associations with Nb-, Ti- and Zr-silicates.
BSE-images of studied objects have been obtained with digital scanning electron microscope CamScan MV2300 (analysts: A.A. Mukhanova and K.V. Van). Most mineral inclusions in bitumen from samples 1 and 3B are thorite and thorium niobosilicate (figure 1). The stoichiometry of thorium niobosilicate inclusions can be described by the general formula (Na,Ca,K)1-2(Th,REE)4-3(Nb,Ti)(Si2O7)3·nH2O. Niobosilicate inclusions in sample 3B are characterized by the highest content of Nb. Its empirical formula can be written as (Na2.11Ca1.27Sr0.27K0.21Ce0.06La0.03)S3.97(Nb2.79Ti0.51Fe3+0.09)S3.39Si1.00O12.01F1.87·nH2O, what is close to the double formula of pyrochlore in which some part of Nb is replaced for Si.
Figure 1. Left: intergrowth of niobium oxosilicate (grey phase in the center) with bitumen (black) containing ingrowths of thorite and thorium niobosilicate (white areas). Sample 1. BSE image, width 0.6 mm. Right: overgrowing of mosaic aggregate of Th- and Nb-silicates (light area in the central part of the image) by bituminous substance (black) with inclusions of thorite and thorium niobosilicate. Sample 1. BSE image, width 0.4 mm.
In sample 1 bitumen is growing on a crystal of titanosilicate lomonosovite (figure 2). In sample 2 bitumen is poor-mineralized and contains only rare inclusions of mineral phases, for example Nb silicate inclusions to 10-15 μm in size.
IR spectra of samples were pressed in tablets with KBr and registered with two-beam spectrophotometer Specord 75 IR in the wave-number range 400-4000 sm-1. A pure KBr pellet was placed in the reference beam. IR spectra of bitumens from intergrowths with niobium silicates were compared with bitumen spectrum from granitic pegmatite Glasberget, Sweden, which sample were kindly presented by V.V. Gordienko. In the latter spectrum, characteristic bands of aliphatic hydrocarbon groups prevail (most strong are bands of wagging vibrations of CH2-groups at 1382 sm-1, scissoring vibrations of CH2-groups at 1463 sm-1, and C-H-stretching vibrations of aliphatic groups at 2852, 2923 and 2953 sm-1. In this spectrum there are also characteristic bands of (CH2)n-chains with n = 3 (810 sm-1), n = 4 (748 sm-1) and n > 4 (725 sm-1) (see Loghinov et al., 1979; Chukanov, Kumpanenko, 1988). IR-spectra of Khibina bitumens associated with niobium silicates fundamentally differ from IR-spectrum of aliphatic bituminous substance. In spectra of Khibiny bitumens bands of C-H-stretching vibrations in the range 2800-3200 sm-1 are significantly weaker, but instead strong band at 1585 sm-1 is observed corresponding to vibrations of aromatic rings. The band at 952 sm-1 with shoulder at 890 см-1 in spectrum of bitumen from sample 1 is due to the inclusions of diortosilicate (apparently, thorium niobosilicate noted above).
For the extraction of soluble part of bitumen, small-grained samples 1 (487 mg) and 2 (29 mg) have been placed in vessels with 30.0 and 9.24 g tetrahydrofuran (THF) respectively. Extraction was carried out during 24 hours. Thereafter the solution was repeatedly passed through Schott filter, the solvent has been evaporated and rests after evaporation has been weighed. Soluble in TGP part of the bitumen is 3.23 wt. % for sample 1 and 10.73 wt. % for sample 2.
Soluble in TGP parts of the both samples were analyzed using the method of exclusion chromatography. Gel-chromatograms (figures 7 and 8) have been obtained at the temperature of 35°C with the liquid chromatographer Waters GPCV-2000 supplied by the light-dispersion detector DAWN HELEOS II, THF being used as eluent. In table 1, numerical mean and weighted mean molecular masses (MM) of both samples are given.
Table 1. Numerical mean (Mn) and weighted mean (Mw) molecular masses of the parts of bitumens soluble in THF.
Note: *By the data of exclusion chromatography. **From light scattering data.
As one can see from figure 3, distributions of MM for both samples are polymodal that is especially strong pronounced for soluble in THF part of sample 2, in which local maxima of the MM distribution corresponds to fractions with Mn 352 (52% of substance), 568 (25%) and 1079 (23%). Besides, in this solution the trace quantities of high-molecular compounds with Mn >> 1000 are present. In sample 1 76% of soluble part consist from main fraction with Mn = 496 and 20% – from fraction with Mn > 1000.
Mn parameters calculated from light-scattering data are significantly more than analogous parameters detected by chromatographer. It means, that bitumen molecules in solution are agglomerated. Soluble part of sample 1 and high-molecular fraction of soluble part of sample 2 show the most pronounced tendency to the formation of molecular associates.
Figure 3. Chromatograms of bitumens from samples 1 (1) and 2 (2). Readings from refractometer (in conventional units) are lied off on the axis of ordinates.
Another example of close association of Nb-rich oxosilicate with organic substances in Khibiny pegmatites is connected with minerals of the komarovite-natrokomarovite series, which crystallized on the hydrothermal stage of pegmatite formation. All investigated samples of them contain dispersed inclusions of the bituminous substance detectable by IR-spectra. In particular, both IR-spectrum of natrokomarovite from sample 4 and IR-spectrum of niobium silicate from sample 3A contain a set of characteristic bands of aromatic carboxylates in the range 1570-1655 cm-1.
Close spatial association of phases with (Nb,Ti) ³ Si and high-molecular organic compounds in Khibiny pegmatites allows us to assume that natural processes of bitumen formation (including polymerization and polycondensation with the formation of unsaturated hydrocarbons and, probably, phenols) could be catalyzed by Nb-rich silicates. In this case polymodal MM distribution for soluble part of bitumens can indicate the presence of several channels of polymerization reactions (for example, with different catalytic centers).
This study was supported by RFBR grant No. 09-05-12001-ofi_m.
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