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Duration = 15 mins. N. Sivakugan 1 Copyright©2001 Elements of Earth 8-35 km crust % by weight in crust O Si Al Fe Ca Na K Mg other = 49.2 = 25.7 = 7.5 = 4.7 = 3.4 = 2.6 = 2.4 = 1.9 = 2.6 82.4% 12500 km dia SIVA 2 Copyright©2001 Soil Formation Parent Rock Residual soil ~ in situ weathering (by physical & chemical agents) of parent rock Transported soil ~ weathered and transported far away by wind, water and ice. SIVA 3 Copyright©2001 Parent Rock ~ formed by one of these three different processes igneous formed by cooling of molten magma (lava) e.g., granite sedimentary formed by gradual deposition, and in layers e.g., limestone, shale metamorphic formed by alteration of igneous & sedimentary rocks by pressure/temperature e.g., marble SIVA Copyright©2001 Residual Soils Formed by in situ weathering of parent rock SIVA 5 Copyright©2001 Transported Soils Transported by: SIVA Special name: wind “Aeolian” sea (salt water) “Marine” lake (fresh water) “Lacustrine” river “Alluvial” ice “Glacial” 6 Copyright©2001 SIVA 7 Copyright©2001 Basic Structural Units Clay minerals are made of two distinct structural units. hydroxyl or oxygen oxygen aluminium or magnesium silicon 0.26 nm Silicon tetrahedron SIVA 0.29 nm Aluminium Octahedron 8 Copyright©2001 Tetrahedral Sheet Several tetrahedrons joined together form a tetrahedral sheet. tetrahedron hexagonal hole SIVA 9 Copyright©2001 Tetrahedral & Octahedral Sheets For simplicity, let’s represent silica tetrahedral sheet by: Si and alumina octahedral sheet by: Al SIVA 10 Copyright©2001 Different Clay Minerals Different combinations of tetrahedral and octahedral sheets form different clay minerals: 1:1 Clay Mineral (e.g., kaolinite, halloysite): SIVA 11 Copyright©2001 Different Clay Minerals Different combinations of tetrahedral and octahedral sheets form different clay minerals: 2:1 Clay Mineral (e.g., montmorillonite, illite) SIVA 12 Copyright©2001 Kaolinite Typically 70-100 layers joined by strong H-bond no easy separation Al Si Al Si Al Si Al Si SIVA 0.72 nm joined by oxygen sharing Copyright©2001 Kaolinite used in paints, paper and in pottery and pharmaceutical industries (OH)8Al4Si4O10 Halloysite kaolinite family; hydrated and tubular structure (OH)8Al4Si4O10.4H2O SIVA 14 Copyright©2001 Montmorillonite also called smectite; expands on contact with water Si Al Si easily separated by water joined by weak van der Waal’s bond SIVA Si Al Si Si Al Si 0.96 nm 15 Copyright©2001 Montmorillonite A highly reactive (expansive) clay swells on contact with water (OH)4Al4Si8O20.nH2O Bentonite high affinity to water montmorillonite family used as drilling mud, in slurry trench walls, stopping leaks SIVA 16 Copyright©2001 Illite Si joined by K+ ions fit into the hexagonal holes in Si-sheet Al Si Si Al Si 0.96 nm Si Al Si SIVA 17 Copyright©2001 Others… Chlorite A 2:1:1 (???) mineral. Si Al Al or Mg Vermiculite montmorillonite family; 2 interlayers of water Attapulgite chain structure (no sheets); needle-like appearance SIVA 18 Copyright©2001 A Clay Particle Plate-like or Flaky Shape SIVA 19 Copyright©2001 Clay Fabric edge-to-face contact Flocculated SIVA face-to-face contact Dispersed 20 Copyright©2001 Clay Fabric Electrochemical environment (i.e., pH, acidity, temperature, cations present in the water) during the time of sedimentation influence clay fabric significantly. Clay particles tend to align perpendicular to the load applied on them. SIVA 21 Copyright©2001 SIVA 22 Copyright©2001 Scanning Electron Microscope common technique to see clay particles qualitative plate-like structure SIVA 23 Copyright©2001 Others… X-Ray Diffraction (XRD) to identify the molecular structure and minerals present Differential Thermal Analysis (DTA) to identify the minerals present SIVA 24 Copyright©2001 Casagrande’s PI-LL Chart 60 U-line Plasticity Index 50 montmorillonite illite A-line 40 30 kaolinite 20 halloysite 10 chlorite 0 0 10 20 30 40 50 60 70 80 90 100 Liquid Limit SIVA 25 Copyright©2001 SIVA 26 Copyright©2001 Specific Surface surface area per unit mass (m2/g) smaller the grain, higher the specific surface e.g., soil grain with specific gravity of 2.7 10 mm cube spec. surface = 222.2 mm2/g SIVA 1 mm cube spec. surface = 2222.2 mm2/g 27 Copyright©2001 Isomorphous Substitution substitution of Si4+ and Al3+ by other lower valence (e.g., Mg2+) cations results in charge imbalance (net negative) positively charged edges + + + _ _ _ _+ + _ + _ _ _ __ + _ _ _ _ _ _ _ _ _ _ _ _ _ SIVA negatively charged faces Clay Particle with Net negative Charge 28 Copyright©2001 Cation Exchange Capacity (c.e.c) known as exchangeable cations capacity to attract cations from the water (i.e., measure of the net negative charge of the clay particle) measured in meq/100g (net negative charge per 100 g of clay) milliequivalents The replacement power is greater for higher valence and larger cations. Al3+ > Ca2+ > Mg2+ >> NH4+ > K+ > H+ > Na+ > Li+ SIVA 29 Copyright©2001 A Comparison Mineral Specific surface (m2/g) 10-20 C.E.C (meq/100g) 3-10 80-100 20-30 Montmorillonite 800 80-120 Chlorite 80 20-30 Kaolinite Illite SIVA 30 Copyright©2001 Cation Concentration in Water cation concentration drops with distance from clay particle clay particle + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ ++ + + SIVA - + cations + - ++ + + + + + + + + + -+ + + + + - + + + + + + + -+ + + + + + -+ + + double layer + + + + + + free water 31 + Copyright©2001 Adsorbed Water A thin layer of water tightly held to particle; like a skin 1-4 molecules of water (1 nm) thick more viscous than free water SIVA - adsorbed water 32 Copyright©2001 Clay Particle in Water adsorbed water SIVA 1nm 50 nm - double layer - water free water 33 Copyright©2001 SIVA 34 Copyright©2001 Summary - Clays Clay particles are like plates or needles. They are negatively charged. Clays are plastic; Silts, sands and gravels are non-plastic. Clays exhibit high dry strength and slow dilatancy. SIVA 35 Copyright©2001 Summary - Montmorillonite Montmorillonites have very high specific surface, cation exchange capacity, and affinity to water. They form reactive clays. Montmorillonites have very high liquid limit (100+), plasticity index and activity (1-7). Bentonite (a form of Montmorillonite) is frequently used as drilling mud. SIVA 36