Degradation and chemical alteration of illitic minerals under experimental and natural weathering conditions.

GEHLKEN, P.-L. (2005):
Degradation and chemical alteration of illitic minerals under experimental and natural weathering conditions.

Illites are the most abundant clay minerals in sedimentary rocks and soils, but up to now little is known about the chemical and mineralogical changes which illitic minerals have been undergone during weathering.
In this study weathering effects on the structures of illitic minerals are studied on viewpoints of dissolution rate, mass balance and texture in laboratory experiments as well as in soil profiles.
The weathering experiments are carried out on specimens with grain sizes less than 2 µm of several very pure samples of illites and micas with various compositions. Chemical compositions of the illitic minerals are determined by RFA and FTIR spectroscopy. The chemical characterisations of a part of the samples were carried out previously with AAS, an electron microprobe and infrared spectroscopy by Gehlken 1987. The mineral powders are treated with dilute organic (acetic acid) and inorganic acids (HCl, H2SO4) at different pH values and low temperatures in open and closed systems. After a certain period of reaction Si, Al, Fe, Mg, Na and K in the solution are measured with AAS and ICP-MS. The chemical and structural changes in the acid residue are examined by RFA and FTIR spectroscopy.
The experimental results which are compared to some infrared measurements on illites and micas (Gehlken 1987, Gehlken & Tarrah 1992) reveal a strong dependence of illite/mica degradation on the octahedral composition. Al-rich micas and muscovitic illites show the greatest resistance to the attack of acids which is manifested only by a small release of K, Mg, and Fe, that is less than 1 % of the initial content. Contrary to this, illites and micas with higher proportions of Mg and Fe in the octahedral sheet complex do much less resist the acids. The portion of released Mg, Fe, and K into the solution amount up to 40 % of the initial content. Infrared spectroscopic investigations demonstrate that the treatment with acids can lead to characteristic variations of infrared frequencies especially in the range between 510 cm-1 to 540 cm-1. The degree of the frequency shift in the range of 540 – 510 cm-1 is a function of the total octahedral Mg and Fe contents in illitic micas (Flehmig & Gehlken 1988).
Natural weathering effects of illites and the degradation processes could be studied on the base of the actual mineral composition by quantitative mineralogical phase analysis with FTIR spectroscopy and means of chemical analysis with AAS and RFA on the grain size fractions < 0.63 µm and 0.63 – 2 µm of selected young soil profiles.
The laboratory experiments are in good agreement with weathering processes of nature. Within same periods and with similar chemical and physical acting intensities of weathering illitic minerals containing higher amounts of Mg and Fe are more easily decomposed than Al-rich illites and muscovites. The degradation is accompanied with an appreciable release of K along with Mg and Fe from the illite structures and leads in the upper horizon of the soil profiles to the formation of mixed layer minerals and dioctahedral vermiculite structures with polymer aluminium hydroxo aquo complexes in the interlayer region (Flehmig et al. 1990).