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  • Rother, Gernot  (13)
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  • 1
    Language: English
    In: Geochimica et Cosmochimica Acta, May 15, 2013, Vol.109, p.400(14)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2013.02.012 Byline: Alexis K. Navarre-Sitchler (a)(b), David R. Cole (c), Gernot Rother (d), Lixin Jin (b)(e), Heather L. Buss (f), Susan L. Brantley (b) Abstract: During weathering, rocks release nutrients and store water vital for growth of microbial and plant life. Thus, the growth of porosity as weathering advances into bedrock is a life-sustaining process for terrestrial ecosystems. Here, we use small-angle and ultra small-angle neutron scattering to show how porosity develops during initial weathering under tropical conditions of two igneous rock compositions, basaltic andesite and quartz diorite. The quartz diorite weathers spheroidally while the basaltic andesite does not. The weathering advance rates of the two systems also differ, perhaps due to this difference in mechanism, from 0.24 to 100mmkyr.sup.-1, respectively. The scattering data document how surfaces inside the feldspar-dominated rocks change as weathering advances into the protolith. In the unaltered rocks, neutrons scatter from two types of features whose dimensions vary from 6nm to 40[mu]m: pores and bumps on pore-grain surfaces. These features result in scattering data for both unaltered rocks that document multi-fractal behavior: scattering is best described by a mass fractal dimension (D.sub.m) and a surface fractal dimension (D.sub.s) for features of length scales greater than and less than [approximately equal to]1[mu]m, respectively. In the basaltic andesite, D.sub.m is approximately 2.9 and D.sub.s is approximately 2.7. The mechanism of solute transport during weathering of this rock is diffusion. Porosity and surface area increase from [approximately equal to]1.5% to 8.5% and 3 to 23m.sup.2 g.sup.-1 respectively in a relatively consistent trend across the mm-thick plagioclase reaction front. Across this front, both fractal dimensions decrease, consistent with development of a more monodisperse pore network with smoother pore surfaces. Both changes are consistent largely with increasing connectivity of pores without significant surface roughening, as expected for transport-limited weathering. In contrast, porosity and surface area increase from 1.3% to 9.5% and 1.5 to 13m.sup.2 g.sup.-1 respectively across a many cm-thick reaction front in the spheroidally weathering quartz diorite. In that rock, D.sub.m is approximately 2.8 and D.sub.s is approximately 2.5 prior to weathering. These two fractals transform during weathering to multiple surface fractals as micro-cracking reduces the size of diffusion-limited subzones of the matrix. Across the reaction front of plagioclase in the quartz diorite, the specific surface area and porosity change very little until the point where the rock disaggregates into saprolite. The different patterns in porosity development of the two rocks are attributed to advective infiltration plus diffusion in the rock that spheroidally fractures versus diffusion-only in the rock that does not. Fracturing apparently diminishes the size of the diffusion-limited parts of the spheroidally weathering rock system to promote infiltration of meteoric fluids, therefore explaining the faster weathering advance rate into that rock. Author Affiliation: (a) Geology and Geological Engineering, Colorado School of Mines, CO, United States (b) Center for Environmental Kinetics Analysis, Earth and Environmental Systems Institute Pennsylvania State University, PA, United States (c) Department of Earth Sciences, The Ohio State University, Columbus, OH, United States (d) Geochemistry and Interfacial Sciences Group, Chemical Sciences Division, Oak Ridge National Laboratory, TN, United States (e) Department of Geological Sciences, University of Texas at El Paso, El Paso, TX, United States (f) School of Earth Sciences, University of Bristol, UK Article History: Received 24 June 2012; Accepted 7 February 2013 Article Note: (miscellaneous) Associate editor: Jiwchar Ganor
    Keywords: Terrestrial Ecosystems ; Porphyry ; Porosity ; Metamorphic Rocks
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 2
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 May 2013, Vol.109, pp.400-413
    Description: During weathering, rocks release nutrients and store water vital for growth of microbial and plant life. Thus, the growth of porosity as weathering advances into bedrock is a life-sustaining process for terrestrial ecosystems. Here, we use small-angle and ultra small-angle neutron scattering to show how porosity develops during initial weathering under tropical conditions of two igneous rock compositions, basaltic andesite and quartz diorite. The quartz diorite weathers spheroidally while the basaltic andesite does not. The weathering advance rates of the two systems also differ, perhaps due to this difference in mechanism, from 0.24 to 100 mm kyr , respectively. The scattering data document how surfaces inside the feldspar-dominated rocks change as weathering advances into the protolith. In the unaltered rocks, neutrons scatter from two types of features whose dimensions vary from 6 nm to 40 μm: pores and bumps on pore–grain surfaces. These features result in scattering data for both unaltered rocks that document multi-fractal behavior: scattering is best described by a mass fractal dimension ( ) and a surface fractal dimension ( ) for features of length scales greater than and less than ∼1 μm, respectively. In the basaltic andesite, is approximately 2.9 and is approximately 2.7. The mechanism of solute transport during weathering of this rock is diffusion. Porosity and surface area increase from ∼1.5% to 8.5% and 3 to 23 m g respectively in a relatively consistent trend across the mm-thick plagioclase reaction front. Across this front, both fractal dimensions decrease, consistent with development of a more monodisperse pore network with smoother pore surfaces. Both changes are consistent largely with increasing connectivity of pores without significant surface roughening, as expected for transport-limited weathering. In contrast, porosity and surface area increase from 1.3% to 9.5% and 1.5 to 13 m g respectively across a many cm-thick reaction front in the spheroidally weathering quartz diorite. In that rock, is approximately 2.8 and is approximately 2.5 prior to weathering. These two fractals transform during weathering to multiple surface fractals as micro-cracking reduces the size of diffusion-limited subzones of the matrix. Across the reaction front of plagioclase in the quartz diorite, the specific surface area and porosity change very little until the point where the rock disaggregates into saprolite. The different patterns in porosity development of the two rocks are attributed to advective infiltration plus diffusion in the rock that spheroidally fractures versus diffusion-only in the rock that does not. Fracturing apparently diminishes the size of the diffusion-limited parts of the spheroidally weathering rock system to promote infiltration of meteoric fluids, therefore explaining the faster weathering advance rate into that rock.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 3
    Language: English
    In: Energy and Fuels, 24 February 2015, Vol.29(3)
    Description: The production of natural gas has become increasingly important in the United States because of the development of hydraulic fracturing techniques, which significantly increase the permeability and fracture network of black shales. The pore structure of shale is a controlling factor for hydrocarbon storage and gas migration. In this work, we investigated the porosity of the Union Springs (Shamokin) Member of the Marcellus Formation from a core drilled in Centre County, PA, USA, using ultrasmall-angle neutron scattering (USANS), small-angle neutron scattering (SANS), focused ion beam scanning electron microscopy (FIB-SEM), and nitrogen gas adsorption. The scattering of neutrons by Marcellus shale depends on the sample orientation: for thin sections cut in the plane of bedding, the scattering pattern is isotropic, while for thin sections cut perpendicular to the bedding, the scattering pattern is anisotropic. The FIB-SEM observations allow attribution of the anisotropic scattering patterns to elongated pores predominantly associated with clay. The apparent porosities calculated from scattering data from the bedding plane sections are lower than those calculated from sections cut perpendicular to the bedding. A preliminary method for estimating the total porosity from the measurements made on the two orientations is presented. This method is in good agreement with nitrogen adsorption for both porosity and specific surface area measurements. Neutron scattering combined with FIB-SEM reveals that the dominant nanosized pores in organic-poor, clay-rich shale samples are water-accessible sheetlike pores within clay aggregates. In contrast, bubblelike organophilic pores in kerogen dominate organic-rich samples. Developing a better understanding of the distribution of the water-accessible pores will promote more accurate models of water-mineral interactions during hydrofracturing.
    Keywords: Bio-Inspired, Mechanical Behavior, Carbon Sequestration ; Engineering ; Chemistry
    ISSN: 0887-0624
    E-ISSN: 1520-5029
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  • 4
    Language: English
    In: Energy and Fuels, 16 June 2016, Vol.30(6)
    Description: Pores within organic matter (OM) are a significant contributor to the total pore system in gas shales. These pores contribute most of the storage capacity in gas shales. Here we present a novel approach to characterize the OM pore structure (including the porosity, specific surface area, pore size distribution, and water accessibility) in Marcellus shale. By using ultrasmall and small-angle neutron scattering, and by exploiting the contrast matching of the shale matrix with suitable mixtures of deuterated and protonated water, both total and water-accessible porosity were measured on centimeter-sized samples from two boreholes from the nanometer to micrometer scale with good statistical coverage. Samples were also measured after combustion at 450 °C. Analysis of scattering data from these procedures allowed quantification of OM porosity and water accessibility. OM hosts 24–47% of the total porosity for both organic-rich and -poor samples. This porosity occupies as much as 29% of the OM volume. In contrast to the current paradigm in the literature that OM porosity is organophilic and therefore not likely to contain water, our results demonstrate that OM pores with widths 〉20 nm exhibit the characteristics of water accessibility. In conclusion, our approach reveals the complex structure and wetting behavior of the OM porosity at scales that are hard to interrogate using other techniques.
    Keywords: Bio-Inspired, Mechanical Behavior, Carbon Sequestration ; Engineering ; Chemistry
    ISSN: 0887-0624
    E-ISSN: 1520-5029
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  • 5
    Language: English
    In: Reviews in Mineralogy and Geochemistry, 2015, Vol.80(1), pp.331-354
    Keywords: Igneous And Metamorphic Petrology ; Andesites ; Antilles ; Appalachians ; Argentina ; Bedrock ; California ; Caribbean Region ; Crystalline Rocks ; Diorites ; Electron Microscopy ; Fractals ; Granites ; Greater Antilles ; Igneous Rocks ; Measurement ; Metagranite ; Metaigneous Rocks ; Metamorphic Rocks ; Neutron Methods ; North America ; Piedmont ; Plutonic Rocks ; Porosity ; Puerto Rico ; Quartz Diorites ; Regolith ; Samples ; Silicate Rocks ; South America ; United States ; Virginia ; Volcanic Rocks ; Volcaniclastics ; Weathering ; West Indies;
    ISSN: 1529-6466
    E-ISSN: 19432666
    Source: CrossRef
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  • 6
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 June 2010, Vol.74(12)
    Description: Journal Article.
    Keywords: Geosciences ; Water ; Weathering ; Rocks ; Particles ; Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
    Source: SciTech Connect (U.S. Dept. of Energy - Office of Scientific and Technical Information)
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  • 7
    In: Earth Surface Processes and Landforms, 30 June 2013, Vol.38(8), pp.847-858
    Description: Weathering disaggregates rock into regolith – the fractured or granular earth material that sustains life on the continental land surface. Here, we investigate what controls the depth of regolith formed on ridges of two rock compositions with similar initial porosities in Virginia (USA). , we predicted that the regolith on diabase would be thicker than on granite because the dominant mineral (feldspar) in the diabase weathers faster than its granitic counterpart. However, weathering advanced 20× deeper into the granite than the diabase. The 20 × ‐thicker regolith is attributed mainly to connected micron‐sized pores, microfractures formed around oxidizing biotite at 20 m depth, and the lower iron (Fe) content in the felsic rock. Such porosity allows pervasive advection and deep oxidation in the granite. These observations may explain why regolith worldwide is thicker on felsic compared to mafic rock under similar conditions. To understand regolith formation will require better understanding of such deep oxidation reactions and how they impact fluid flow during weathering. Copyright © 2012 John Wiley & Sons, Ltd.
    Keywords: Regolith Thickness ; Fluid Transport ; Neutron Scattering ; Geomorphology ; Nanoporosity
    ISSN: 0197-9337
    E-ISSN: 1096-9837
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  • 8
    Language: English
    In: American Mineralogist, 01 January 2011, Vol.96(4)
    Description: We used small-angle and ultra-small-angle neutron scattering (SANS/USANS) to characterize the evolution of nanoscale features in weathering Rose Hill shale within the Susquehanna/Shale Hills Observatory (SSHO). The SANS/USANS techniques, here referred to as neutron scattering (NS), characterize porosity comprised of features ranging from approximately 3 nm to several micrometers in dimension. NS was used to investigate shale chips sampled by gas-powered drilling ('saprock') or by hand-augering ('regolith') at ridgetop. At about 20 m depth, dissolution is inferred to have depleted the bedrock of ankerite and all the chips investigated with NS are from above the ankerite dissolution zone. NS documents that 5--6% of the total ankerite-free rock volume is comprised of isolated, intraparticle pores. At 5 m depth, an abrupt increase in porosity and surface area corresponds with onset of feldspar dissolution in the saprock and is attributed mainly to peri-glacial processes from 15 000 years ago. At tens of centimeters below the saprock-regolith interface, the porosity and surface area increase markedly as chlorite and illite begin to dissolve. These clay reactions contribute to the transformation of saprock to regolith. Throughout the regolith, intraparticle pores in chips connect to form larger interparticle pores and scattering changes from a mass fractal at depth to a surface fractal near the land surface. Pore geometry also changes from anisotropic at depth, perhaps related to pencil cleavage created in the rock by previous tectonic activity, to isotropic at the uppermost surface as clays weather. In the most weathered regolith, kaolinite and Fe-oxyhydroxides precipitate, blocking some connected pores. These precipitates, coupled with exposure of more quartz by clay weathering, contribute to the decreased mineral-pore interfacial area in the uppermost samples. These observations are consistent with conversion of bedrock to saprock to regolith at SSHO due to: (1) transport of reactants (e.g., water, O{sub 2}) into primary pores and fractures created by tectonic events and peri-glacial effects; (2) mineral-water reactions and particle loss that increase porosity and the access of water into the rock. From deep to shallow, mineral-water reactions may change from largely transport-limited where porosity was set largely by ancient tectonic activity to kinetic-limited where porosity is changing due to climate-driven processes.
    Keywords: Geosciences ; Physics Of Elementary Particles And Fields ; Ankerite ; Chlorine Compounds ; Dissolution ; Drilling ; Feldspars ; Fractals ; Fractures ; Geometry ; Illite ; Kaolinite ; Neutrons ; Oxygen Compounds ; Porosity ; Quartz ; Scattering ; Shales ; Small Angle Scattering ; Surface Area ; Tectonics ; Water ; Weathering ; Geology
    ISSN: 0003-004X
    E-ISSN: 1945-3027
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  • 9
    Language: English
    In: Chemical Geology, 09 October 2013, Vol.356, pp.50-63
    Description: Soils developed on the Oatka Creek member of the Marcellus Formation in Huntingdon, Pennsylvania were analyzed to understand the evolution of black shale matrix porosity and the associated changes in elemental and mineralogical composition during infiltration of water into organic-rich shale. Making the reasonable assumption that soil erosion rates are the same as those measured in a nearby location on a less organic-rich shale, we suggest that soil production rates have on average been faster for this black shale compared to the gray shale in similar climate settings. This difference is attributed to differences in composition: both shales are dominantly quartz, illite, and chlorite, but the Oatka Creek member at this location has more organic matter (1.25 wt.% organic carbon in rock fragments recovered from the bottom of the auger cores and nearby outcrops) and accessory pyrite. During weathering, the extremely low-porosity bedrock slowly disaggregates into shale chips with intergranular pores and fractures. Some of these pores are either filled with organic matter or air-filled but remain unconnected, and thus inaccessible to water. Based on weathering bedrock/soil profiles, disintegration is initiated with oxidation of pyrite and organic matter, which increases the overall porosity and most importantly allows water penetration. Water infiltration exposes fresh surface area and thus promotes dissolution of plagioclase and clays. As these dissolution reactions proceed, the porosity in the deepest shale chips recovered from the soil decrease from 9 to 7% while kaolinite and Fe oxyhydroxides precipitate. Eventually, near the land surface, mineral precipitation is outcompeted by dissolution or particle loss of illite and chlorite and porosity in shale chips increases to 20%. As imaged by computed tomographic analysis, weathering causes i) greater porosity, ii) greater average length of connected pores, and iii) a more branched pore network compared to the unweathered sample. This work highlights the impact of shale–water–O interactions in near-surface environments: (1) black shale weathering is important for global carbon cycles as previously buried organic matter is quickly oxidized; and (2) black shales weather more quickly than less organic- and sulfide-rich shales, leading to high porosity and mineral surface areas exposed for clay weathering. The fast rates of shale gas exploitation that are ongoing in Pennsylvania, Texas and other regions in the United States may furthermore lead to release of metals to the environment if reactions between water and black shale are accelerated by gas development activities in the subsurface just as they are by low-temperature processes in our field study.
    Keywords: Pyrite Dissolution ; Organic Matter ; Shale Gas ; Trace Metals ; Neutron Scattering ; Computed Tomography ; Geology
    ISSN: 0009-2541
    E-ISSN: 1872-6836
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  • 10
    Language: English
    In: Soil Science Society of America Journal, 01 January 2015, Vol.79(1)
    Description: 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 mu m) and specific surface area (SSA) during weathering. The internal surface of each unweathered rock is characterized as both a mass fractal and a surface fractal. The mass fractal describes the distribution of pores ( approximately 300 nm to approximately 5 mu m) along grain boundaries and triple junctions. The surface fractal is interpreted as the distribution of smaller features (1-300 nm), that is, the bumps (or irregularities) at the grain-pore interface. The earliest porosity development in the granite is driven by microfracturing of biotite, which leads to the introduction of fluids that initiate dissolution of other silicates. Once plagioclase weathering begins, porosity increases significantly and the mass + surface fractal typical for unweathered granite transforms to a surface fractal as infiltration of fluids continues. In contrast, the mass + surface fractal 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 granite and diabase is likely due to the different mechanisms of solute transport across the primary silicate reaction front.
    Keywords: Bio-Inspired, Mechanical Behavior, Carbon Sequestration ; Agriculture
    ISSN: 0361-5995
    E-ISSN: 1435-0661
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