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Converged surface roughness parameters—A new tool to quantify rock surface morphology and reactivity alteration

Cornelius Fischer^*,**, and Andreas Lüttge^*,

^* Department of Earth Science, MS-126, Rice University, 6100 Main Street, Houston, Texas 77005, U.S.A.
^** Geowissenschaftliches Zentrum der Universität Göttingen, Abt. Sedimentologie and Umweltgeologie, Goldschmidtstrasse 3, D-37077 Göttingen, Germany
Department of Chemistry, Rice University, 6100 Main Street, Houston, Texas 77005, U.S.A.

Corresponding author, e-mail: cornelius{at}rice.edu

The quantification of surface topography is essential in understanding the processes of dissolution and precipitation occurring at the rock-water interface. As a new approach to quantify rock surface alteration in the micrometer to nanometer scale we utilize the convergence of well-known surface roughness parameters. This approach allows the quantification of rock surface size and amplitude as well as its state of alteration during fluid-rock interaction. Vertical scanning interferometry (VSI) is our tool of choice for measuring rock surface topography because of its high vertical resolution and large field of view.

Here, we present a case study to demonstrate the potential of surface roughness convergence for quantifying rock surface alteration during weathering. Black slates show different concentrations of organic matter (OM) due to different oxidative weathering ranks. Roughness and surface size data indicate that the original smooth slate surface with deviations of only some tens of nanometers was altered to a rough surface with pore diameters at a scale of some hundreds of nanometers up to several microns. Surface roughness data of a sample profile of three weathering stages (small, medium, large OM decrease) primarily indicate an increase of the OM’s surface area and roughness during weathering. However, during further weathering, surface area and roughness were decreased. From these data we conclude that those parts of the OM that do not directly adjoin to the slate’s clay minerals have a higher reactivity. This means that during ongoing OM weathering the rock surface reactivity and topography are controlled by the extent of OM degradation. Because both reactivity and topography of the observed surface will alter during reaction, it must be concluded that a constant term of "reactive" surface area must not be used to calculate the dissolution rates.