Abstract text (incl. figure legends and references)
Illite is one of the most abundant clay minerals on earth, yet its structure remains not fully resolved. Illites of the Remlinger sandstone display a fibrous growth around the detrital grains and form a meshwork within the pores of the sandstone. The meshwork and particles trapped therein during fluid flow lead to a dramatic decrease in permeability, which should be prevented in geothermal systems using for example such sandstones.
In autoclave experiments, the interaction of the Remlinger sandstone with two different natural brines, Bad Nauheimer and Gerolsteiner, were compared to an unaltered sandstone as reference, in order to determine possible changes in structure, stacking or shape of the illite fibers.
The surface of fluid-altered sandstone plugs was characterized using environmental scanning microscopy (ESEM). Grinded material of rim and core was examined by X-ray powder diffraction (XRPD). In addition, particles filtered from the fluid after the autoclave experiment were investigated employing transmission electron microscopy (TEM). Moreover, 3D electron diffraction experiments were performed using Automated Diffraction Tomography (ADT) (Kolb U., 2012; Rozhdestvenskaya et al., 2011).
Based on a 50 nm illite fiber, the structure of illite could be solved during TEM analysis using a single ADT data set. The reconstructed reciprocal space also revealed one-dimensional diffuse scattering, indicating a disorder in the stacking of the illite layers. The degree of disorder is dependent on the potassium content of the fluid, since the K+-ions of the interlayer can be released via the (010) habit plane. This reduces the stability of the structure, resulting in a high fragmentation and detachment of individual illite layers. Such mobile layer fragments can then be transported by mobile fluids to interact and entangle in the sandstone.
These new insights into the structure of illite fibers show a distinct dependence of the arrangement and local agglomeration of the fibers in the pores on the potassium content of the fluid (brine) used. The increased migration and interweaving of the fibers associated with a low potassium content can lead to severe clogging of the pores and thus to a reduction in fluid permeability. Altered sandstones with illitic pore filling are therefore less suitable for long-term geothermal projects.
Kolb U. 2012: Automated Diffraction Tomography. Uniting Electron Crystallography and Powder Diffraction. Springer, 314-326.
Rozhdestvenskaya E., Mugnaioli E., Czank M., Depmeier W., Kolb U., Merlino S. 2011: Essential Features of the Polytypic Charoite-96 Structure Compared to Charoite-90. Mineral. Magazine, 75, No. 6, 2833-2846.