Determination of porosity and pore connectivity in feldspars from soils of granite and saprolite

Abstract

Pores in feldspars, the most common silicates, are of great interest for the estimation of internal weathering reactions. Here, we compare three different methods to characterize the porosity of feldspars from four soils of granite and saprolite developed on granites in Germany (Black Forest, Harz mountains). Besides the classical mercury intrusion porosimetry (MIP), we used a technique based on Wood's metal intrusion. The liquid metal was pressed at 100 to 150°C with approximately 500 bar argon pressure into the pores, and the distribution of the solidified metal in feldspar grains was analyzed using electron microscopy. In addition, the temperature-controlled water release from water-presaturated feldspars was analyzed using Karl-Fischer titration (KFT).Volume fractions of connective pores determined by the three methods were qualitatively in good agreement, considering that single grains are analyzed using Wood's metal intrusion and KFT but larger batches of feldspar were analyzed using MIP.. With MIP, relatively high porosities between 7.8 and 22.3 vol.% were determined in feldspars, much larger than usually found in granite rocks. These high values were confirmed by KFT analyses. Back-scattered electron images of feldspars impregnated with Wood's metal demonstrated that, at the initial stage of alteration, pores propagated from cracks and outer surfaces into the interior of feldspars. Highly altered feldspar species show a patchy distribution of pores, and pores are preferentially formed in the Na-rich phase of alkali feldspars. Enhanced topographical images indicated that some of the connective small pores were not intruded by liquid Wood's metal at the applied pressure of 500 bar, probably because of the filling of pores with secondary minerals and the ink-bottle effect. The ink-bottle effect also leads to an overestimation of small pores by MIP. As a consequence, the specific surface area calculated from MIP data is significantly lower than the value determined by nitrogen adsorption. The pore size distribution obtained by MIP was found to be unimodal with maxima in pore radii between 80 and 390 nm for most of the samples (an exception was the St. Blasien Cv-horizon with two broad maxima at 20 and 540 nm). Organic carbon of up to 2.2 g/kg was found in all studied feldspars. This implies that pores within feldspars are, or at least have been, accessible for dissolved organic matter. The volume fraction of pores that are disconnected to the surface (inclusions) was estimated to be 1.2 to 2.7 vol.% based on the water release of above 700°C, as analyzed using KFT. Secondary minerals, which play an important role in weathering reactions by sealing pores and locally buffering chemical reactions, were identified by X-ray diffraction, thermal analysis, and infrared spectroscopy. Kaolinite, gibbsite, and sericite were found with an overall portion of up to 4.6 vol.% but were strongly varying in abundance. Copyright © 2006 by Lippincott Williams & Wilkins, Inc.