Soil Science Society of America Journal Soil Sci. Soc. Am. J. 79:55–73 doi:10.2136/sssaj2014.04.0135 Received 6 Apr. 2014. *Corresponding author (eab204@psu.edu). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. How Oxidation and Dissolution in Diabase and Granite Control Porosity during Weathering Soil Chemistry Weathering extends to shallower depths on diabase than granite ridgetops despite similar climate and geomorphological regimes of denudation in the Virginia (United States) Piedmont. Deeper weathering has been attributed to advective transport of solutes in granitic rock compared to diffusive transport in diabase. We use neutron scattering (NS) techniques to quantify the total and connected submillimeter porosity (nominal diameters between 1 nm and 10 mm) and specifc surface area (SSA) during weathering. The internal sur- face of each unweathered rock is characterized as both a mass fractal and a surface fractal. The mass fractal describes the distribution of pores (~300 nm to ~5 mm) along grain boundaries and triple junctions. The surface frac- tal is interpreted as the distribution of smaller features (1–300 nm), that is, the bumps (or irregularities) at the grain–pore interface. The earliest poros- ity development in the granite is driven by microfracturing of biotite, which leads to the introduction of fuids that initiate dissolution of other silicates. Once plagioclase weathering begins, porosity increases signifcantly and the mass + surface fractal typical for unweathered granite transforms to a surface fractal as infltration of fuids continues. In contrast, the mass + surface frac- tal does not transform to a surface fractal during weathering of the diabase, perhaps consistent with the interpretation that solute transport is dominated by diffusion in that rock. The difference in regolith thickness between gran- ite and diabase is likely due to the different mechanisms of solute transport across the primary silicate reaction front. Abbreviations: FIB, focused ion beam; NS, neutron scattering; RZ, reaction zone; SANS, small-angle neutron scattering; SAP, saprolite zone; SEM-EDS, scanning electron microscopy with energy dispersive spectrometer; SLD, scattering length density; SSA, specifc surface area; USANS, ultra-small-angle neutron scattering; UWR, unweathered rock; WR, weathered rock; m-CT, microcomputed X-ray tomography. W eathering, the transformation of intact rock into soil through physical and chemical reactions, is a key process that afects the CO 2 cycle, soil formation, and nutrient uptake into ecosystems. Upon reaction with meteoric fuids, pristine parent rocks transform from relatively nonporous mate- rial to porous weathered material (soil and saprolite), which we term here regolith (Brantley and White, 2009; Buol and Weed, 1991; Pavich et al., 1989). Te earliest weathering-related mineral-fuid interactions are thought to be largely controlled by the distribution of the connected porosity and the topography of the pore interface at the nanometer scale (Hochella and Banfeld, 1995). When interconnected, these pores likely allow solute transport only by difusion in relatively pristine, low-po- rosity crystalline rocks. For instance, in unweathered granite rocks with low poros- ity, microcracks and elongated voids can be the most important pathways of solute transport (Sausse et al., 2001). During weathering and rock disaggregation, solute Ekaterina Bazilevskaya* Earth and Environmental Systems Inst. Penn State Univ. University Park, PA 16802 Gernot Rother Geochemistry and Interfacial Sciences Group Chemical Sciences Division Oak Ridge National Laboratory Oak Ridge, TN 37831 David F.R. Mildner NIST Center for Neutron Research National Inst. of Standards and Technology Gaithersburg, MD, 20899 Milan Pavich U.S. Geological Survey Eastern Geology and Paleoclimate Science Center 12201 Sunrise Valley Drive MS 926a Reston, VA 20192 David Cole School of Earth Science Ohio State Univ. Columbus, OH 43219 Maya P. Bhatt Central Dep. of Environmental Science Tribhuvan Univ. Kathmandu, Nepal Lixin Jin Dep. of Geological Sciences Univ. of Texas El Paso, TX 79968 Carl I. Steefel Earth Sciences Division Lawrence Berkeley National Laboratory 1 Cyclotron Road Berkeley, CA 94720 Susan L. Brantley Earth and Environmental Systems Inst. Penn State Univ. University Park, PA 16802 Published January 13, 2015