Glauconite Feasibility to Numerical Stratigraphic Studies: 57Fe Mossbauer Evidence

O. V. Yakovleva Institute of Precambrian Geology and Geochronology, RAS, nab. Makarova 2,

St. Petersburg 199034, Russia

I. M. Gorokhov Institute of Precambrian Geology and Geochronology, RAS, nab. Makarova 2,

St. Petersburg 199034, Russia

Glauconite contains radioactive 87Rb and 40K and may be used for dating sedimentary rocks. Yet many Rb-Sr and/or K-Ar dates of glauconite do not correspond to the age of sedimentation, but are representative of more recent points in time. This paper is an attempt to find a new crystallochemical criterion of glauconite feasibility to numerical stratigraphic studies. The fine structure of Precambrian glauconites from East Europe and Siberia has been studied by 57Fe Mössbauer spectroscopy. Based on the Rb-Sr and K-Ar dates, glauconites are classified in two groups: (i) minerals with isotopic dates conforming to their
stratigraphic ages, and (ii) "rejuvenated" minerals which have lost radiogenic 87Sr and 40Ar.

A chemical alteration appears to be responsible for the "rejuvenation" of glauconite. Several alternatives may be discussed: (i) substitution of Al3+ for Si4+ within the tetrahedral layer with simultaneous occupation of vacant interlayer positions with K+ to comply with electroneutrality; (ii) substitution of Al3+ for Fe3+ within the octahedral layer; (iii) substitution of Al3+ for Fe2+ and Mg2+ within the octahedral layer accompanied either by entering Al3+ into tetrahedrons in lieu of Si4+ or by the expulsion of K+ from a quantity of interlayer positions, and (iv) oxidation of Fe2+ within the octahedral layer with the charge compensation as in (iii). To check these potentialities, the relative weights of diverse environments of cis-octahedral Fe3+ by the cations of all structural layers were calculated for glauconites in accordance with their structural formulae. The "random" and "ordered" distributions of cations are considered. The latter is characterized by the maximum relative weight of homogeneous, i.e. (3R3+) and (3R2+), cation combinations around each octa-cation.

The Mössbauer spectra are computer fitted to five quadrupole doublets. The doublet with a quadrupole splitting (QS) value of 0.27-0.35 mm/s is assigned to Fe3+-ions which are in homogeneous octahedral environments and not adjacent to Al3+ containing tetrahedrons. The doublet with QS=0.50-0.80 mm/s is assigned to Fe3+ in all other environments. For "non-
rejuvenated" glauconites the measured proportion of homogeneous environments for Fe3+ is consistent with that calculated for the ordered distribution of octa-cations. For "rejuvenated" glauconites this measured proportion is far above that calculated for any type of distribution. This fact is likely due to higher Fe2+/Fe3+ ratios in primordial glauconites which later were oxidized and reordered again, with losing their radiogenic 87Sr and 40Ar. The simulating calculations confirm this possibility. Such a "memory" in the fine structure provides a means for discriminating between "rejuvenated" glauconites and those suitable for numerical stratigraphic studies.

This work was supported by the I.S.F. and R.F.G. Grant MLO-300.