Clay Mineralogy and Soil Structure

Clay Mineralogy and Soil Structure1INTRODUCTIONCOARSE GRAINED SOILFINE GRAINED SOILContain minerals: Quartz and feldspar

Strong and electrically inert

Behaviour do not depend on : Nature of mineral presentBehaviour depend on Nature and characteristics of mineral presentClay mineralogy is the science dealing with the structure of clay minerals on microscopic, molecular and atomic scale.

It also includes the study of the mineralogical composition and electrical properties of the clay particles. CLAY MINERALOGYGRAVITATIONAL & SURFACE FORCES

In clayey soils surface forces are predominant than gravitational forces

PRIMARY VALENCE BOND

IONIC BONDOne atom join another atom either by adding or losing electronsEg: Nacl

COVALENT BOND - Develops b/w two atoms by sharing electrons in their outer shellEg: Oxygen Molecule

Primary valence bond stronger do not break

4HYDROGEN BOND

Bond b/w hydrogen cation and anions of two atoms of another elementEg: Bond b/w hydrogen atoms and oxygen atoms in a water molecule

SECONDARY VALENCE BONDS

Intermolecular bond which develop b/w atoms in one molecule to atoms in another molecule

Clay Minerals are Composed of two Basic Structural Units

Silica tetrahedral unitAlumina octahedral unitBasic Structural Units of Clay minerals7Tetrahedral unit

- Number of tetrahedral units combine to form sheet, with oxygen atom at the base of all tetrahedra8Octahedral Sheet Gibbsite sheet: Several octahedral sheets combine to form sheet

Isomorphous substitutionis the replacement of the central atom of the tetrahedral or octahedral unit by another atom during the formation of the sheets.Eg: One silicon atom in tetrahedral unit may be substituted by Al atomIncreases negative charge on particleSlight distortion of crystal lattice occursISOMORPHOUS SUBSTITUTIONDifferent Clay Minerals1:1 Clay Mineral (e.g., kaolinite, halloysite)

2:1 Clay Mineral (e.g., montmorillonite, illite)

Different combinations of tetrahedral and octahedral sheets form different clay minerals:

Basic structural unit consists of alumina sheet combined with silica sheetStructural units join together by hydrogen bond that develops b/w oxygen of silica sheet and hydroxyls of alumina sheetThickness 7 A0Si4Al4O10(OH)8. Platy shapeThe bonding between layers are van der Waals forces and hydrogen bonds (strong bonding).There is no interlayer swellingElectrically neutralEg: China clay

1. Minerals-Kaolinite

Thickness : 0.005 Specific surface: 15 m2/g

SEM (Scanning Electron Microscope) image of kaolinite clay 2. HalloysiteSimilar to koalinite but structural units are seperated by single molecular layer of waterProperties depend on layer of waterIf water removed by drying, properties of mineral changesTubular in shapeSoils with low mass density

Basic structural unit consist of an alumina sheet sandwhiched between two silica sheetsThickness: 10 A0

Structural unit joined by link b/w oxygen ions of two silica sheets and due to Vander waal forces.Negatively charged surfaces of silica sheet attract water in the space between two structural units- expansion of materialWater removed by heating b/w 200 to 300 oc High shrinkage and swelling characteristics

3. Montmorillonite Mineral

nH2O+cations

Lateral Dimension: 0.1 to 0.5 Thickness: 0.001 to 0.005

SEM image of Montmorillonite clay Similar to montmorilloniteLarge amt of isomorphours substitution of silica by Al- Large negative chargeLink b/w structural unit by Pottasium-Bond strongerSpace b/w structural unit smallerSwells less than montmorillonite but more than in KaoliniteLattice is stronger- less cleavage

4. ILLITE MINERAL

Lateral Dimension: 0.1 to 0.5 Thickness: 0.005 to 0.05 Specific surface 80 m2/g

SEM image of Illite clay Clay carry electric chargeElectrode dipped in beaker containing clay mixed with waterElectrode connected to electrical circuit containing battery and ammeter- defection in needle of ammeterFlow of current thru mediumOnly negative charges - measuredMagnitude of electric charge depend on surface area of particleElectrical charges on clay mineralsIsomorphous substitutionDissociation of hydroxyl ion into hydrogen ionsAdsorption of negative ions on clay surfaceAbsence of positive ions in lattice of clay surfacePresence of organic matterReasons for net negative chargeDiffused Double LayerClay minerals carry a net negative charge

Attraction of water molecules to soil surfaceNegative charged faces of clay particle attract positive ends of dipole water molecule Cations attract to clay particles and water dipoles attracted to these cations thru their negatively charged endsAttraction b/w water molecule and clay particle by hydrogen bondingClay- Water InteractionsCations attracted to clay particle also tries to move away due to their thermal energy

Net effect of force due to these attractive and repulsive forces force of attraction decreases with increase in distance from clay particle surface

Layer extending from clay particle surface to limit of attraction- Diffused Double Layer

Thickness- 400 A0Diffused Double layer

Adsorbed Water

Water held in diffused double layerWithin zone of influence, properties of adsorbed water different from normal water at the same temperature Near the surface- water has property of a solidAt middle- Resemble viscous liquidBeyond the layer- Properties of normal waterTo remove adsorbed water- heated to temperature more than 2000c

In coarse particles ,adsorbed water film are thin

In fine grained soil, adsorbed water layer- more thicker ( exceed size of grain)

- Adsorbed layer influence its physical properties eg: ability to deform plastically without cracking when mixed with varying amount of water

BASE EXCHANGE/CATION EXCHANGE

Cations attracted to negatively charged surface are not strongCations can be replaced by other cations (Exchangeable Cations) BASE EXCHANGEEg: H+ ions can be replaced by other cations like Na, K, Mg etcQuantity of exchangeable cations in soil- BASE EXCHANGE CAPACITY

The geometrical arrangement of soil particles with respect to one another is known as soil structure.

The soils in nature have different structures depending upon the particle size and the mode of formation.SOIL STRUCTURESoil Structure for Coarse Grained Soil

Single grained structure

Sand and gravel particles which settle under gravitational forces take an equilibrium position. Each particle is in contact with those surrounding it.Depending upon the relative position of soil particles, the soil may be a loose structure or dense structure.

gravitational forces are more predominant than surface forces28Honey comb structure

It is possible for fine sands or silts to be deposited such that the particles when setting develop a particle-to-particle contact that bridges over large voids in the soil mass. usually develops when the particle size is between 0.002 mm and 0.02 mm.

Soil Structure for Clay

Flocculated structures

Consists of flocs of soil instead of individual soil particlesIf the inter particle force are attractive, then particles want to come together, making flocculated clay.

Dispersed clay If the final inter particle forces are repulsive, the particles want to separate from each other when the boundary confinements are removed. The soils in dispersed structure generally have a low shear strength, high compressibility and low permeability.

Composite structures

Coarse grained skeletonSpace between the bulky grains is occupied by clayey particles

Clay-matrix structureBulky grains appear floating without touching one another