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ABSTRACT Witanachchi, Channa Devinda ABSTRACT Witanachchi, Channa Devinda. Isovolumetric Weathering of Granite in Wake County, North Carolina. (Under the direction of Dr. Stanley W. Buol). Saprolite, formed by chemical weathering of rocks near the earth’s surface, holds water, serves as a parent material of soils, and is a medium for waste disposal. Saprolite formation consumes CO2 and may stabilize atmospheric CO2 levels. This dissertation examined the influence of joint orientation on isovolumetric weathering of saprolite developed on the Rolesville granitic batholith at Knightdale, North Carolina. Rock -3 density (ρs) (µ α 0.05) was 2.62±0.01 g cm . Mass altered per unit volume (mA/VT) of saprolite was taken as the difference between ρs and primary mineral mass remaining per 0 unit volume (m1 R/VT). Altered mass lost per unit volume (mAL/VT) was taken as the difference between ρs and bulk density (ρb). Altered mass retained per unit volume (mAR/VT) was taken as (mA/VT) – (mAL/VT). Saprolite with steeply-dipping joints showed a uniformly sandy texture. The distribution (mass percent) of sand-, silt-, and clay-sized particles (µ α 0.05) was 82.4±2.7, 10.3±1.8, and 2.3±2.5, respectively, on a whole saprolite basis, and ρb (µ α 0.05) was 1.66±0.06 g cm-3. Saprolite with horizontally-oriented unloading joints was extensively altered and occurred between horizontal slabs of unweathered rock. The saprolite was composed of sandy layers alternating with clayey layers on the scale of approximately 1 to 2 cm. The distribution of sand-, silt-, and clay-sized particles (µ α 0.05) in the saprolite was 50.1±10.4, 3.1±0.5, and 46.8±10.5, respectively, on a whole saprolite basis. Bulk density -3 (µ α 0.05) was 1.55±0.01 g cm . The mean content of sand-, silt-, and clay-sized particles in the two saprolites differed statistically at α = 0.001, and mean bulk density differed at α = 0.01. The fine-earth fraction of saprolite with steeply-dipping joints was characterized (µ α 0.05) by pH of 5.8±0.2, mass percent Fe2O3 of 0.21±0.09, cation exchange capacity (CEC) at pH 7.0 of 3.95±0.88 cmol+ kg -1, and percent base saturation (% BS) of 36.66±9.93. The fine-earth fraction of saprolite with horizontal joints was characterized (µ α 0.05) by pH of + –1 5.1±0.2, mass percent Fe2O3 of 2.68±0.28, CEC at pH 7.0 of 8.28±0.91 cmol kg , and % BS of 19.73±9.22. The means of pH, mass percent Fe2O3, and CEC in the two saprolites differed statistically at α = 0.001, and the means of % BS differed at α = 0.05. The differences in mean values of individual extractable bases are not significant at α = 0.05. -3 Density of unweathered granite (µ 0.05) was 2.62±0.01 g cm . Calculated mean (µ α 0.05) -3 values of mA/VT, mAL/VT, and mAR/VT (all in g cm ) in saprolite with steeply-dipping joints were 1.17±0.12, 0.96±0.06 and 0.21±0.05, respectively. Corresponding values in saprolite with horizontal joints were 1.85±0.15, 1.08±0.02 and 0.77±0.17, respectively. Calculated mean (µ α 0.05) values of mAL/mA were 0.82±0.03 for the former saprolite and 0.58±0.06 for the latter, indicating greater leaching losses in the former. Differences in the calculated means of mA/VT, mAR/VT, mAR/mA and mAL/mA in the two saprolites are statistically significant at α = 0.001, and mAL/VT differed at α = 0.05. Saprolite with steeply dipping joints was composed predominantly of plagioclase and potassium feldspar. Saprolite with horizontal joints contained approximately equal proportions of potassium feldspar and kaolinite (or halloysite). Nordstrandite occurred in both types of saprolite. 0 Saprolite was classified based on the relative proportions of (m1 R/VT) 100/ρs, (mAR/VT) 100/ρs, and (mAL/VT) 100/ρs. Saprolite with steeply-dipping joints classified as ‘moderately altered, highly leached”, and saprolite with horizontal joints classified as ‘severely altered, moderately leached’. Joint orientation appears to be a significant variable in saprolite formation. ISOVOLUMETRIC WEATHERING OF GRANITE IN WAKE COUNTY, NORTH CAROLINA by Channa Devinda Witanachchi A Dissertation Submitted to the Graduate Faculty of North Carolina State University In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy Department of Soil Science Raleigh 2004 Approved by __________________________________________ (Chairman of Advisory Committee) ____________________ ____________________ ____________________ Biography The author received the first eleven years of formal education in Sri Lanka. He came to the United States to attend the last year of high school and received a high school diploma from Vista High School, in Vista, North San Diego County, California. He received a Bachelor of Arts degree in geology from Occidental College, Los Angeles. Subsequently, he attended the geology program at Bryn Mawr College, Pennsylvania, and received a Master of Arts degree in geology. At Bryn Mawr, the title of his thesis was 'Metamorphism and Deformation in the Wissahickon Schist, Southeastern Pennsylvania', done under the supervision of Dr. Maria L. Crawford. This was followed by one year of study in the graduate geology program at Emory University, in Atlanta, and a year in the graduate geology program at Duke University, in Durham, North Carolina. The author joined the Soil Science program at North Carolina State University in 1994 to further pursue his interests in agronomy and the environmental sciences, and served as a Research Assistant in the Pedology Laboratory for three years. Subsequently he worked at the North Carolina Geological Survey’s Piedmont Office. He is presently employed by the Division of Water Quality at the North Carolina Department of Environmental and Natural Resources (NCDENR). ii Acknowledgements The author wishes to express appreciation for the guidance received from his advisor, Dr. Stanley W. Buol. Much was learned about soils and global agriculture through our many conversations. Many thanks are due to Dr. Aziz Amoozegar, Dr. Mike Vepraskas, and Dr. Edward Stoddard for their support in many ways, including the service in the author’s advisory committee. Special thanks are due to Dr. Stoddard for extensive use of the X-ray equipment at the Department of Marine, Earth and Atmospheric Sciences, for his encouragement of cross-disciplinary studies, and for helping locate a study site for the research. Special thanks are also due to Dr. Amoozegar for help with bulk density determinations. Thanks are due to Kim Hutchinson for answers to many laboratory questions, to Peggy Longmire for assistance with centrifugation, and Roberta Harraway-Miller for assistance with determination of particle-size distribution and cation exchange capacity. The study site at Wake Stone Corporation’s quarry in Knightdale, Wake County, North Carolina was made available through the kind permission of Mr. John R. Bratton. Thanks are extended to all staff at the quarry for their assistance, especially to geologist David Lee. iii The author thanks the Soil Science Department of North Carolina State University for support through a research assistantship. iv TABLE OF CONTENTS List of Tables ……………………………………………………………….. XIV List of Figures ……………………………………………………………….... XVI Chapter 1 INTRODUCTION AND OBJECTIVES 1 Chapter 2 PREVIOUS WORK ……………………………... …………. 21 2.1 Weathering profiles ………………………………………….. 21 2.2 Weathering depth .…………………………………………… 28 2.3 Saprolite texture ……………………………………………… 31 2.4 Factors influencing weathering ……………………………… 35 2.5 Mechanisms of mineral alteration in saprolite ………………. 41 2.6 Major chemical reactions in saprolite ………………………. 44 2.7 Feldspar Weathering ………………………………………... 46 2.8 Previous work on quantification of weathering ……………... 51 Chapter 3 A MASS BALANCE APPROACH OF WEATHERING 57 3.1 Mass alteration, retention and loss ………………………….. 57 3.2 Bulk density ………………………………………………….. 64 3.3 Particle size distribution as a tool in the study of 65 isovolumetric weathering …………………………………… 3.4 Interpreting particle size distributions of isovolumetrically 69 weathered regolith in terms of alteration of primary mineral mass ………………………………………………………… Chapter 4 STUDY SITE 74 Chapter 5 MATERIALS AND METHODS 77 v 5.1 Sample selection and preparation …………………………… 77 5.2 Soil reaction ……………………………………………..….. 80 5.3 Analysis of free iron ….……….……………………………. 81 5.4 Extractable Cations …………………………………………. 81 5.5 Cation exchange capacity …………………………………… 81 5.6 Particle size distribution …………………………………….. 82 5.7 Bulk density …………………………………………………. 82 5.8 Mineralogical analyses of randomly-oriented specimens of 82 sand- and silt-sized fractions and oriented specimens of clay- sized fractions of saprolite using X-ray diffraction ………… 5.9 Petrographic examination of grain mounts of the sand-sized 84 fraction of saprolite …………………………………………. 5.10 Statistical Analyses ………………………………………….. 87 Chapter 6 PHYSICAL CHARACTERISTICS OF REGOLITH 88 6.1 Mass distribution of sand-, silt-, and clay-sized particles …… 88 6.2 Particle size distribution of sand subfractions ……….………. 92 6.3 Bulk density …………..…………………………………….. 93 Chapter 7 MASS ALTERATION AND ITS PARTITIONING 96 BETWEEN SAPROLITE AND ITS ENVIRONMENT 7.1 Calculating mass altered per unit volume ….……………….. 96 7.2 Calculating altered mass lost per unit volume ……………… 101 7.3 Calculating altered mass retained per unit volume ………… 101 7.4 Variation of mA/VT, mAL/VT, mAR/VT, mAL/mA and mAR/mA 104 with weathering environment ………………………….…… vi Chapter 8 CHEMICAL CHARACTERISTICS OF REGOLITH 108 8.1 Soil reaction (pH) ……………………………………………. 110 8.2 Cation exchange capacity (CEC) ……………………………. 110 8.3 Extractable bases ……………………………………………. 111 8.4 Percent base saturation (% BS) …………………………….. 112 8.5 Mass percentage of citrate-bicarbonate-dithionite extractable 112 (free) iron ………………………………………. Chapter 9 REGOLITH MINERALOGY 114 9.1 Petrographic examination of grain mounts of the sand – sized 114 fraction of saprolite …………………………………………. 9.2 X-ray diffraction ……………………………………….…... 117 9.3 Distribution of quartz and feldspar ………………………….
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