Journal of Conference Abstracts

The Dehydration of Ca-Montmorillonite: an In Situ X-ray Diffraction Study

Helen Bray (bray@esc.cam.ac.uk) & Simon A. T. Redfern (satr@e. sc.cam.ac.uk)

Dept. Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ.

Smectites, so-called "swelling clays" are of great interest in the field of Earth sciences. Commonly found in shale formations during oil and gas exploration, they are associated with borehole instability. In addition, smectites transported down subducting lithospheric slabs are an important water reservoirs controlling sub-surface fluid migration and partial melting of the overlying mantle. Smectites are also common components of commercial clays used by the ceramic industry. The relationship of the properties of fired ceramic products to those of raw materials can only be understood by the temperature-dependence of the raw material itself.

The kinetics of low T dehydration of one montmorillonite sample have been studied by EDXRD. Non-isothermal and isothermal experiments revealed new features of the reaction kinetics, including the variation in equilibrium water as a function of temperature. We were able to observe, for the first time, changes in the peak shape and position of the 001 basal reflection, which we have related to both the changes in the bulk water content and also the variation in the heterogeneity of the H₂O distribution along the z-axis. These results have been correlated with TG and FT-IR experiments, leading to a new kinetic model for the dehydration of interlayer water in Ca-montmorillonite. An alternative method of determining reaction kinetics via time-dependent Landau-Ginzburg theory is being developed and compared with the more widely used empirical rate laws.

XRD and Microprobe Analysis on Authigenic Phyllosilicates in Permian Silica-Rich Volcanic Rocks of Eastern Germany

G. A. Brecht (gbrecht@gfz-potsdam.de) & C. Breitkreuz (cbreit@gfz-potsdam.de)

GeoForschungsZentrum Potsdam, Telegrafenberg A26, 14473 Potsdam, Germany.

X-ray diffraction and microprobe measurements reveal mainly a hydrothermal origin for the authigenic clay minerals in Permian silica-rich volcanic rocks of E Germany. This is one result of our current study that aims (A) at characterise the nature of authigenic clay minerals in silica-rich volcanic rocks, and (B) to reconstruct the post-depositional thermal history of the area using temperature-sensitive characteristics of clay minerals. Samples were taken from surface outcrops N of Magdeburg and the area of Halle as well as from drill cores of the NE-German Basin down to 8 km depth. For the latter, Benek et al. (in press) calculated about 34700 km³ of silica-rich lavas and ignimbrites to have formed during the Lower Permian.

Calibrated illite (001) and chlorite (002) crystallinity data (using the crystallinity index scale of Warr and Rice 1994) indicate conditions ranging from the diagenetic zone to the anchizone. Although the crystallinity of the chlorite is generally better than that of the illites (i.e. narrower peaks), no correlation exists between the crystallinity of these two minerals. This lack of microstructural equilibrium may be suggestive of dissimilar growth histories during the low temperature alteration stages of the volcanics. Analysis of the variation in d-spacings of illites and chlorites, on both air-dried and ethylene glycolated samples, show the content of expandable mixed layer smectites to be less than 5% and indicate a formation temperature >200°C.

In contrast microprobe analyses from chlorites give temperatures up to 325°C depending on the method of chlorite geothermometry applied. These are 50-100°C higher than the recent well temperatures. Furthermore, no depth dependence of crystallinity and chlorite-geothermometry temperatures have been detected in the core samples. This and the low smectite content suggest a hydrothermal origin for the phyllosilicates. The data obtained sofar suggests that in silica-rich volcanic rocks, under hydrothermal conditions, the chlorite crystallinity reflects more closely formation temperatures, whereas illite crystallinity remains low.

Benek et al. Tectonophysics (in press).

Warr & Rice J. Metam. Geol., 12, 141-152 (1994).