Published on March 2, 2014
Topic 4: Clay Mineralogy A short series of lectures prepared for the Third year of Geology, Tanta University 2013- 2014 by Hassan Z. Harraz firstname.lastname@example.org 1
OUTLINE OF TOPIC 4: 2 March 2014 ORIGIN OF CLAY MINERALS CLAY MINERALS ATOMIC STRUCTURE Basic Structural Units TYPES OF CLAY MINERALS: 1) Silicate Clays (crystalline): a) Kaolinite b) Halloysite c) Smectite d) Illite e) Vermiculite f) Chlorite g) Attapulgite (Chain Structure Clay Minerals) h) Mixed Layer Clays 2) Sesquioxide/oxidic clays 3) Amorphous clays (non-crystalline) “Activity” of silicate clays Generalized Chemical Weathering Chemical Weathering Products Uses of Clay Clay Fabric IDENTIFIED CLAY MINERALS SPECIAL TERMS Prof. Dr. H.Z. Harraz Presentation Clay Mnerals 2
Elements of Earth 8-35 km crust % by weight in crust O Si Al Fe Ca Na K Mg other 12500 km dia = 49.2 = 25.7 = 7.5 = 4.7 = 3.4 = 2.6 = 2.4 = 1.9 = 2.6 82.4% 3
Soil Formation Parent Rock ~ formed by one of these three different processes 1) Igneous: formed by cooling of molten magma (lava) e.g., Granite, Basalt Residual soil Transported soil ~ in situ weathering (by physical & ~ weathered and transported chemical agents) of parent rock far away 2) Sedimentary: formed by gradual deposition, and in layers e.g., Sandstone, limestone, shale 3) Metamorphic: formed Transported by: Special name: Wind “Aeolian” Sea (salt water) “Marine” Lake (fresh water) “Lacustrine” River “Alluvial” Ice by alteration of igneous & sedimentary rocks by pressure/temperature e.g., schist, marble “Glacial” 4
Origin of Clay Minerals “The contact of rocks and water produces clays, either at or near the surface of the earth” (from Velde, 1995). Rock +Water Clay For example, The CO2 gas can dissolve in water and form carbonic acid, which will become hydrogen ions H+ and bicarbonate ions, and make water slightly acidic. CO2 + H2O H2CO3 H+ + HCO3 The acidic water will react with the rock surfaces and tend to dissolve the K ion and silica from the feldspar. Finally, the feldspar is transformed into kaolinite. Feldspar + hydrogen ions + water clay (kaolinite) + cations, dissolved + silica 2KAlSi3O8 + 2H+ + H2O Al2Si2O5(OH)4 + 2K+ + 4SiO2 Note that: The hydrogen ion displaces the cations. The alternation of feldspar into kaolinite is very common in the decomposed granite. The clay minerals are common in the filling materials of joints and faults (fault gouge, seam) in the rock mass. 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 5
2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 6
2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 7
CLAY MINERALS Clay minerals exhibit colloidal behaviour. That is, their surface forces have greater influence than the negligible gravitational forces. Clay is a particle size i.e., Micelle: meaning particle of silicate clay Clay particles are smaller than 2 microns. Their shapes can be studied by an electron microscope. Predominant make-up is Secondary minerals Clay minerals are Phyllosilicate minerals Composed of tetrahedral and octahedral “sandwiches” Tetrahedron: central cation (Si+4, Al+3) surrounded by 4 oxygens Octahedron: central cation (Al+3,Fe+2, Mg+2) surrounded by 6 oxygens (or hydroxyls) Sheets combine to form layers Layers are separated by interlayer space Water, adsorbed cations 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. 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 8
A Clay Particle Plate-like or Flaky Shape 9
Basic Structural Units oxygen Silicon tetrahedron Clay minerals are made of two distinct structural units silicon Tetrahedron and Tetrahedral sheets Aluminium Octahedron Connected tetrahedra, sharing oxygens hydroxyl or oxygen SEM view of clay All have layers of Si tetrahedra and layers of Al, Fe, Mg octahedra, similar to gibbsite or brucite aluminium or magnesium Octahedron and Octahedral Sheets Connected octahedra, sharing oxygens or hydroxyls All clay mineral are made of different combinations of the above two sheets: tetrahedral sheet and octahedral sheet.
Basic Unit-Silica Tetrahedra Tetrahedral Sheet (Si2O10)-4 1 Si Replace four Oxygen with hydroxyls or combine with positive union 4O Tetrahedron Plural: Tetrahedra Several tetrahedrons joined together form a tetrahedral sheet. Here is a tetrahedral sheet, formed by connecting several tetrahedons. Note the hexagonal holes in the sheets. hexagonal hole (Holtz and Kovacs, 1981)
Basic Unit-Octahedral Sheet 1 Cation 6 O or OH Gibbsite sheet: Al3+ Al2(OH)6, 2/3 cationic spaces are filled One OH is surrounded by 2 Al: Dioctahedral sheet Different cations Brucite sheet: Mg2+ Mg3(OH)6, all cationic spaces are filled One OH is surrounded by 3 Mg: Trioctahedral sheet 13 (Holtz and Kovacs, 1981)
Tetrahedral & Octahedral Sheets For simplicity, let’s represent silica tetrahedral sheet by: Si and alumina octahedral sheet by: Al Mitchell, 1993
Different Clay Minerals All clay mineral are made of different combinations of the above two sheets: tetrahedral sheet and octahedral sheet. Different combinations of tetrahedral and octahedral sheets form different clay minerals: 1:1 phyllosilicate Clay Mineral (e.g., kaolinite, halloysite) 2 March 2014 2:1 phyllosilicate Clay Mineral (e.g., montmorillonite, illite) Prof. Dr. H.Z. Harraz Presentation Clay Minerals 15
TYPES OF CLAY MINERALS 1) Silicate Clays (crystalline) 2) Sesquioxide/oxidic clays 3) Amorphous clays (non-crystalline) 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 16
1) Silicate Clays (crystalline) Different types of silicate clays are composed of sandwiches (combinations) of layers with various substances in their interlayer space. 2:1 two tetrahedral sheets to one octahedral sheet 1:1 one tetrahedron sheet to one octahedral sheet Mitchell, 1993 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 17
a) Kaolinite Typically 70100 layers Al Si Al layer 0.72 nm Si joined by strong H-bond no easy separation Al Si Al Si joined by oxygen sharing
a) Kaolinite 1:1 phyllosilicate Minerals Si4Al4O10(OH)8 Platy shape The bonding between layers are van der Waals forces and hydrogen bonds (strong bonding). There is no interlayer swelling Width: 0.1~ 4m Thickness: 0.05~2 m Hydrogen bonds in interlayer space strong Nonexpandable Low cation exchange capacity (CEC) Particles can grow very large (0.2 – 2 µm) Effective surface area = 10 – 30 m2/g External surface only Kaolinite is used for making paper, paint, pottery and pharmaceutical industries 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 19
a) Kaolinite 2 March 2014 17 m Trovey, 1971 ( from Mitchell, 1993) • Mineral particles of the kaolinite subgroup consists of the basic units stacked in the c direction. • The bonding between successive layers is by both van der Waals forces and hydrogen bonds. • Kaolinite is the purest of clays, meaning that it varies little in composition. It also does not absorb water and does not expand when it comes in contact with water. Thus, kaolinite is the preferred type of clay for the ceramic industry.
1.Silicate Clays kaolinite Kaolinite • Kaolinite clays have long been used in the ceramic industry, especially in fine porcelains, because they can be easily molded, have a fine texture, and are white when fired. • These clays are also used as a filler in making paper. good road base good foundation good for pottery; China clay (porcelain) easy to cultivate, but need manure or fertilizer Dominant clay mineral in highly weathered soils 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 21
b) Halloysite • 1:1 phyllosilicate Minerals • Si4Al4O10(OH)8·4H2O • A single layer of water between unit layers. • kaolinite family; hydrated and tubular structure while it is hydrated • The basal spacing is 10.1 Å for hydrated halloysite and 7.2 Å for dehydrated halloysite. Trovey, 1971 ( from Mitchell, 1993) 2 m • If the temperature is over 50 C or the relative humidity is lower than 50%, the hydrated halloysite will lose its interlayer water (Irfan, 1966). Note that this process is irreversible and will affect the results of soil classifications (GSD and Atterberg limits) and compaction tests. • There is no interlayer swelling. 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 22
c) Montmorillonite also called smectite; expands on contact with water Si Al Si easily separated by water Si Al Si A highly reactive (expansive) clay swells on contact with water 0.96 nm (OH)4Al4Si8O20.nH2O high affinity to water Bentonite: joined by weak van der Waal’s bond Si Al Si montmorillonite family used as drilling mud, in slurry trench walls, stopping leaks
c) Montmorillonite Montmorillonite or smectite is family of expansible 2:1 phyllosilicate clays having permanent layer charge because of the isomorphous substitution in either the octahedral sheet (typically from the substitution of low charge species such as Mg2+ , Fe2+, or Mn2+ for Al3+) The most common smectite clay is Montmorillinite, with a general chemical formula : (0.5Ca,Na)(Al,Mg,Fe)4(Si,Al)8O20(OH)4.nH2O 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). Montmorillinite is the main constituent of bentonite, derived by weathering of volcanic ash. Bentonite has the unsual property gives rise to interesting industrial used. Montmorillinite can expand by several times its original volume when it comes in contact with water. This makes it useful as a drilling mud (to keep drill holes open), in slurry trench walls, stopping leaks and to plug leaks in soil, rocks, and dams. Most important is as drilling mud in which the montmorillonite is used to give the fluid viscosity several times that of water. It is also used for stopping leakage in soil, rocks, and dams. Montmorillinite, however, is a dangerous type of clay to encounter if it is found in tunnels or road cuts. Because of its expandable nature, it can lead to serious slope or wall failures. 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 24
c) Montmorillonite Film-like shape. There is extensive isomorphous substitution for silicon and aluminum by other cations, which results in charge deficiencies of clay particles. Always negative due to isomorphous substitution Layers weakly held together by weak O-O bonds or cation-O bonds Cations adsorbed in interlayer space Interlayer cations hold layers together: In dry soils, bonding force is strong and hard clods form; deep cracks In wet soils, water is drawn into interlayer space and clay swells. n·H2O+cations n·H2O and cations exist between unit layers, and the basal spacing is from 9.6 Å to (after swelling). Maximum Swelling The interlayer bonding is by van der Waals forces and by cations which balance charge deficiencies (weak bonding). (Holtz and Kovacs, 1981) 5 m There exists interlayer swelling, which is very important to engineering practice (expansive clay). High Cation Exchange Capacity (CEC) High effective surface area = 650 – 800 m2/g Internal surface area >> external Expandable……..Most expandable of all clays Width: 1 or 2 m Thickness: 10 Å ….. About ~1/100 width
2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 26
Swelling Clays The interlayer in montmorillonite or smectites is not only hydrated, but it is also expansible; that is, the separation between individual smectite sheets varies with the amount of water present in the soil. Because of this, they are often referred to as "swelling clays". Soils having high concentrations of smectites can undergo as much as a 30% volume change due to wetting and drying or these soils have a high shrink/swell potential and upon drying will form deep cracks. Bentonite 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 27
Main difference- ions that make up the middle of the sandwich Plummer et al., Physical Geology 9th edition, McGraw Hill Inc, Fig. 2.19b 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 28
Cat-litter in action Plummer et al., Physical Geology 9th edition, McGraw Hill Inc, Box 02.04.f1 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 29
d) Illite Si Al Si joined by K+ ions Si Al Si fit into the hexagonal holes in Si-sheet Si Al Si Trovey, 1971 ( from Mitchell, 1993) 7.5 m 0.96 nm
d) Illite (Fine-grained micas, mica-like minerals) Illite is the most common clay mineral, often composing more than 50 percent of the claymineral suite in the deep sea. They are characteristic of weathering in temperate climates or in high altitudes in the tropics, and typically reach the ocean via rivers and wind transport. Illite type clays are formed from weathering of K and Al-rich rocks under high pH conditions. Thus, they form by alteration of minerals like muscovite and feldspar. Illite clays are the main constituent of shales. The Illite clays have a structure similar to that of muscovite, but is typically deficient in alkalies, with less Al substitution for Si. Thus, the general formula for the illites is: Si8(Al,Mg, Fe)4~6O20(OH)4·(K,H2O)2 OR KyAl4(Si8-y,Aly)O20(OH)4 , usually with 1 < y < 1.5, but always with y < 2. Because of possible charge imbalance, Ca and Mg can also sometimes substitute for K. The K, Ca, or Mg interlayer cations prevent the entrance of H2O into the structure. Thus, the illite clays are non-expanding clays. 1) Fewer of Si4+positions are filled by Al3+ in the illite. 2) There is some randomness in the stacking of layers in illite. 3) There is less potassium in illite. Well-organized illite contains 9-10% K2O. 4) Illite particles are much smaller than mica particles. 5) Ferric ion Fe3+ 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 31
d) Illite (Fine-grained micas, mica-like minerals) 2:1 phyllosilicate Minerals Flaky shape. The basic structure is very similar to the mica, so it is sometimes referred to as hydrous mica. Illite is the chief constituent in many shales. Some of the Si4+ in the tetrahedral sheet are replaced by the Al3+, and some of the Al3+ in the octahedral sheet are substituted by the Mg2+ or Fe3+. Those are the origins of charge deficiencies. The charge deficiency is balanced by the potassium ion between layers. Note that the potassium atom can exactly fit into the hexagonal hole in the tetrahedral sheet and form a strong interlayer bonding. The basal spacing is fixed at 10 Å in the presence of polar liquids (no interlayer swelling). Width: 0.1~ several m Thickness: ~ 30 Å As mica crystallizes from magma: Isomorphous substitution of Al+3 for Si+4 in tetrahedra high net negative charge K+ ions in interlayer space (Strongly binds layers) Non-expandable Minimum Swelling Surface area 70 -175 m2/g 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 32
e) Vermiculite Vermiculite is a 2:1 phyllosilicate clay mineral The octahedral sheet is brucite. Octahedral ions are Al, Mg, Fe The basal spacing is from 10 Å to 14 Å. It contains exchangeable cations such as Ca2+ and Mg2+ and two layers of water within interlayers. It can be an excellent insulation material after dehydrated. It is generally regarded as a weathering product of micas (Forms from alteration of mica): Weathering removes some K+ ions Replaced by hydrated cations in interlayer space Water molecules and cations bridge layers, so not as expandable as smectites Still have very high net negative charge High Cation Exchange Capacity (CEC) (highest of all clays) Expandable Surface area 600 – 800 m2/g Internal >> external Vermiculite is similar to montmorillonite, a 2:1 mineral, but it has only two interlayers of water. After it is dried at high temperature, which removes the interlayer water, expanded” vermiculite makes an excellent insulation material. Vermiculite is also hydrated and somewhat expansible though less so than smectite because of its relatively high charge. 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 33
Vermiculite Mitchell, 1993 Illite 2 March 2014 Vermiculite Vermiculite possesses the special property of expanding to between six and twenty times its original volume when heated to ~1,000oC. This process, called exfoliation, liberates bound water from between the mica-like layers of the mineral and literally expands the layers apart at right angles to the cleavage plane. Vermiculite is used to loosen and aerate soil mixes. Mixed with soil, it improves water retention and fertilizer release, making it ideal for starting seeds. Also used as a medium for winter storage of Prof. Dr. H.Z. Harraz Presentation Clay Minerals bulbs and flower tubers. 34
f) Chlorite 2:1 phyllosilicate Minerals Central cations in octahedral sheets is Fe or Mg Interlayer space occupied by a stable, positively charged octahedral sheet. Non-expandable. Minimum Swelling. 70 -100 m2/g surface area 2 March 2014 Gibbsite or brucite Prof. Dr. H.Z. Harraz Presentation Clay Minerals The basal spacing is fixed at 14 Å 35
g) Attapulgite (Chain Structure Clay Minerals) Attapulgite • chain structure (no sheets); needlelike appearance • They have lath-like or thread-like morphologies. • The particle diameters are from 50 to 100 Å and the length is up to 4 to 5 m. • Attapulgite is useful as a drilling mud in saline environment due to its high stability Trovey, 1971 ( from Mitchell, 1993) 4.7 m 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 36
h) Mixed Layer Clays • • • Different types of clay minerals have similar structures (tetrahedral and octahedral sheets) so that interstratification of layers of different clay minerals can be observed. Most than one type of clay mineral is usually found in most soils. Because of the great similarity in crystal structure among the different minerals, interstratification of two or more layer types often occurs within a single particle In general, the mixed layer clays are composed of interstratification of expanded waterbearing layers and non-water-bearing layers. Montmorillonite-illite is most common, and chlorite-vermiculite and chlorite-montmorillonite are often found. (Mitchell, 1993) 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 37
2) Sesquioxides / Oxidic Clays Ultimate weathering products Ultisols and Oxisols Very stable; persist indefinitely Yellow, red, brown Fe or Al as central cations Lack negative charge Don’t retain adsorbed cations Non-expandable Low cation exchange capacity (CEC) Low fertility: Often are net positive Often have enough Al or Mn to be toxic to plants High capacity to fix phosphorous so it is not available to plants Highly weathered so no more nutrients to release in weathering 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 38
Ultisol profile In heavily leached soils, sheets decompose into component Si tetrahedral and Al octahedral. Al octahedral often weather into gibbsite Al(OH)3 39 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals
3) Amorphous clays (non-crystalline, Allophanes and Imogolite) silicates These are structurally disordered aluminosilicates. They are normally derived from volcanic ash materials and constitute a major component of volcanic soils. Allophane and imogolite The formation of imogolite and allophane occur during weathering of volcanic ash under humid, temperate or tropical climate conditions. Allophane is X-ray amorphous and has no definite composition or shape. It is composed of hollow, irregular spherical particles with diameters of 3.5 to 5.0 nm. Allophane is often associated with clay minerals of the kaolinite group Imogolite has the empirical formula SiAl4O10.5H2O High internal negative charge High cation exchange capacity (CEC) High water-holding capacity Surface area 100 – 1000 m2/g 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 40
“Activity” of silicate clays refers to cation exchange capacity (CEC) Ability to retain and supply nutrients Fertility High activity clays: Less weathered ; high effective surface area smectite, vermiculite, mica (illite), chlorite Low activity clays: More weathered; less effective surface area kaolinite 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 41
What determines clay minerals in a given soil? Usually a mixture Climate Parent material Degree of weathering 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 42
Generalized Chemical Weathering Temperate Climates 3KAlSi3O8 + 2H+ + 12H2O KAlSi3O10(OH)2 + 6H4SiO4 + K+ (K-feldspar) (mica/illite) (silicic acid) Temperate Humid Climates: 2KAlSi3O8 + 2H+ + 3H2O 3Al2Si2O5(OH)4 + K+ (K-feldspar) (kaolinite) Humid Tropical Climate: Al2Si2O5(OH)4 + 5H2O 2Al(OH)3 + 2K+ + 4H4SiO4 (kaolinite) 2 March 2014 (gibbsite) Prof. Dr. H.Z. Harraz Presentation Clay Minerals 43
Clays: Important Chemical Weathering Products Clay Mineral Species are a function of: environmental conditions at the site of weathering available cations produced by chemical degradation 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 44
Generalized relationships: Ultisols Oxisols Kaolinite, oxidic clays Alfisols Mollisols Mica, vermiculite, smectite Vertisols Andisols 2 March 2014 Amorphous Prof. Dr. H.Z. Harraz Presentation Clay Minerals 45
Chemical Weathering Products As the age of sedimentary rocks increases clay mineral assemblages in the subsurface transform through diagenesis to illite + chlorite 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 46
Uses of Clay - Drilling Mud deep oil is at high pressure Cooling and cleaning the drill Drilling mud slurry “Gushers” used to be common until the use of drilling mud was implemented Bentonite and other clays are used in the drilling of oil and water wells. The clays are turned into mud, which seals the walls of the boreholes, lubricates the drill head and removes drill cuttings. 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 47
Uses of Clay - Contaminant Removal Clay slurrys have effectively been used to remove a range of comtaminants, including P and heavy metals, and overall water clarification. Schematic of montmorillonite absorbing Zn 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 48
Clay Fabric edge-to-face contact face-to-face contact Flocculated Dispersed The term fabric is used to describe the geometric arrangement of the clay particles. Flocculated and Dispersed are the two extreme cases. Flocculated fabric gives higher strength and stiffness. 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. 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 49 49
Scanning Electron Microscope common technique to see clay particles qualitative Clay particles are smaller than 2 microns. Their shapes can be studied by an electron microscope. plate-like structure 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 50 50
2.1 X-ray diffraction Mitchell, 1993 • The distance of atomic planes d can be determined based on the Bragg’s equation. BC+CD = n, n = 2d·sin, d = n/2 sin where n is an integer and is the wavelength. • Different clays minerals have various basal spacing (atomic planes). For example, the basing spacing of kaolinite is 7.2 Å. 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 51
2.2 Differential Thermal Analysis (DTA) • Differential thermal analysis (DTA) consists of simultaneously heating a test sample and a thermally inert substance at constant rate (usually about 10 ºC/min) to over 1000 ºC and continuously measuring differences in temperature and the inert material T. For example: Quartz changes from the to form at 573 ºC and an endothermic peak can be observed. T • Endothermic (take up heat) or exothermic (liberate heat) reactions can take place at different heating temperatures. The mineral types can be characterized based on those signatures shown in the left figure. (from Mitchell, 1993) Temperature (100 ºC)
2.2 DTA (Cont.) •If the sample is thermally inert, T •If the phase transition of the sample occurs, T Crystallize Time t Melt Time t Endothermic reactions take up Exothermic reactions liberate heat from surroundings and heat to surroundings and therefore the temperature T therefore the temperature T decreases. increases. T= the temperature of the sample – the temperature of the thermally inert substance. 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 53
Others… 1.Specific surface (Ss) 2.Cation exchange capacity (cec) 3.Plasticity chart(Casagrande’s PI-LL Chart) 5. Potassium determination Well-organized 10Å illite layers contain 9% ~ 10 % K2O. 6. Thermogravimetric analysis It is based on changes in weight caused by loss of water or CO2 or gain in oxygen. Sometimes, you cannot identify clay minerals only based on one method. 60 U-line Plasticity Index 50 montmorillonite illite A-line 40 30 kaolinite 20 halloysite 10 0 0 10 20 30 40 chlorite 50 60 Liquid Limit 70 80 90 100 54
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 1 mm cube 10 mm cube spec. surface = 222.2 mm2/g 2 March 2014 spec. surface = 2222.2 mm2/g Prof. Dr. H.Z. Harraz Presentation Clay Minerals 55
Specific Surface Specific surface surface / volume Specific surface surface / mass Surface related force Gravationa Preferred Surface related forces: van der Waals forces, capillary forces, etc. l force Demonstration of capillary force between Large particle and small particle. Example: 1 1 1 cm cube , 2 . 65 g / cm Ss 6 1 cm 2 1 cm 2 . 65 g / cm 3 3 2 . 3 10 1 1 1 m cube , 2 . 65 g / cm Ss 2 March 2014 6 1 m 4 m /g 3 2 Ss is inversely proportional to the particle size 2 1 m 2 . 65 g / cm 3 3 2 .3 m / g 2 3 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 56
Isomorphous Substitution The clay particle derives its net negative charge from the isomorphous substitution and broken bonds at the boundaries. substitution of Si4+ and Al3+ by other lower valence (e.g., Mg2+) cations, i.e. Lower charge cations replace higher charge cations as central cation (e.g., Mg+2 replaces Al+3). leaves net negative charge (results in charge imbalance (net negative)) positively charged edges + + + + _ _ _ _ + _ negatively charged faces + _ _ _ __ + _ _ _ _ _ _ _ _ _ _ _ _ _ Clay Particle with Net negative Charge 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 57 57
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+ The negatively charged clay particles can attract cations from the water. These cations can be freely exchanged with other cations present in the water. For example Al3+ can replace Ca2+ and Ca2+ can replace Mg2+. 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 58 58
A Comparison Mineral Kaolinite Illite Montmorillonite Chlorite 2 March 2014 Specific surface (m2/g) C.E.C (meq/100g) 10-20 80-100 800 80 3-10 20-30 80-120 20-30 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 59 59
Cation Concentration in Water cation concentration drops with distance from clay particle The negatively charged faces of clay particles attract cations in the water. The concentration of the cations decreases exponentially with the increasing distance from the clay particle. The negatively charged clay surface and the positively charged cations near the particle form two distinct layers, known as “electric double layer” or simply “double layer”. clay particle + + + + + ++ + ++ + + + + + + + + + + + ++ + + + + + + + + + + + ++ ++ + + 2 March 2014 + - + cations + - ++ + + + + + + + + + + -+ + + + - + + + + ++ -+ + + + + + + -+ + + double layer + Prof. Dr. H.Z. Harraz Presentation Clay Minerals + + + + + free water 60 60 +
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 - 2 March 2014 - adsorbed water Prof. Dr. H.Z. Harraz Presentation Clay Minerals 61 61
Clay Particle in Water adsorbed water - 2 March 2014 1nm 50 nm - double layer water - free water Prof. Dr. H.Z. Harraz Presentation Clay Minerals 62 62
Origins of Charge Deficiencies 1) Imperfections in the crystal lattice -Isomorphous substitution. • The cations in the octahedral or tetrahedral sheet can be replaced by different kinds of cations without change in crystal structure (similar physical size of cations). For example, Al3+ in place of Si4+ (Tetrahedral sheet) Mg2+ instead of Al3+(Octahedral sheet) unbalanced charges (charge deficiencies) • This is the main source of charge deficiencies for montmorillonite. • Only minor isomorphous substitution takes place in kaolinite. 2) Imperfections in the crystal lattice - The broken edge: • The broken edge can be positively or negatively charged. 3) Proton equilibria (pH-dependent charges): M OH H M OH OH M OH 2 (Pr otonation ) M O H 2 O ( Deprotonat ion ) • Kaolinite particles are positively charged on their edges when in a low pH environment, but negatively charged in a high pH (basic) environment. 4) Adsorbed ion charge (inner sphere complex charge and outer sphere complex charge: • Ions of outer sphere complexes do not lose their hydration spheres. The inner complexes have direct electrostatic bonding between the central atoms. 2 March 2014 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 63
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