But why clay-

It is important to know where we are coming from , since the course of study in MNA/IMACS centers on

clay and clay minerals basically

How are clays formed Clays are formed from weathering of silica rich rocks

e.g Granite

Processes of Formation-

-Weathering of the silica rich rock

-bedrock weathering

-transporting of the weathered rocks

-Sedimentary processes

-Incorporation of Carbonates

Requirements - ReactionTime

- Igneous Rocks (Intrusive Igneous Rocks)

-Geological Factor

-Agents of Transportation

Agents of Weathering

-Alteration of Chemical composition

-Pressure

-Temperature

Types of Clays –Based on Origin

1-Residual Clays-not far from parent rock

2-Sedimentary Clays -far from parent material

Residual Clay (primary clays)–Non plastic,white,e.g kaolin

Sedimentary Clay (secondary clays)-Plastic,grey,darker,smaller paricles and uniform e.g ball clay,fire clay

Residual clays are formed basically by surface weathering This gives clays in 3 ways

-chemical decomposition of rocks

-solution of rocks(limestone)containing clayey impurities which are insoluble

-disintergration and solution of shales

But the most common process of clay formation is the decompostion of feldspar

SOME INTRUSIVE ROCKS Coarse Grains-slow cooling

Clays are not formed from Extrusive rocks

Diorite

Granite

Granite pegmatite

Gabbro

Mechanisms of clay minerals formation -Inheritance

-Neoformation

-Transformation

The 3 mechanisms operating in 3 geological systems

-Weathering

-Sedimentary

- Diagenetic hydrothermal

Inheritance Another area reaction

Stable

Natural deposit

Previous stage in rock cycle reaction

Dominace in sedimentary environments

Neoformation

Clays precipitation from solution

Reaction of amorphous material

Neoformation Secondary precipitates of calcite , sulphates , Fe

Affirmation of permafrost

Structural and mineralogical changes are irreversible and affect present soils

Understanding of quartenary environment-needs assessment of possible frozen ground

Transformation Possesion of inherited structure

Chemical reactions-ions exchanged and modification of cations (layer transformation)

1)ion exchange between losely bound ion and those of environments

2)layer transformation –modification of arrangement of octahedral, tetrahedral and interlayer cations

Dominance in diagenetic-hydrothermal environments

The best mechanism Layer transformation

Result of this type of reaction are better preserved in geological records

Environment of formation 1)Weathering zone

Upper zone of the earth crust –T and P varies

Short reaction time

2)Sedimentary environment-long reaction time(sedimentation,subsidence)

-near or below sea level

Sediment-water interface

-low temperature

Pressure (1 kb )

Diagenetic hydrothermal

-zones in contact with hot water

-wide range of environmental conditions

Incresing energy required Mechnism/ environments

inheritance neoformation

transformation

sedimentary

weathering

Diagenesis-hydrothermal

CONTROVERSIAL ISSUE OF DETRITAL VS DIAGENESIS ORIGIN OF CLAYS

DETRITAL OR DIAGENESIS

Detrital-reflection of character of source material, inheritance of basic clay mineral lattice

Diagenesis-adsorption of cation and its modification

Application of XRD in Detrital vs Diagenesis controversy -Collection of thousands of samples

-to illustrate the variety of clay under similar environmental conditions

-variety of environments in which same clay minerals occur

XRD OF ORGANIC AND PYRITE RICH SHALES -No particular clay mineral is restricted to a particular

environments

-dominance of kaolinite in fluviate environment

-illite ,monmorillionite abundance,frequency,sole clay mineral in all environments

-chlorite not a dominant mineral

-non marine shales are seldom,if ever monomineralic

interpretation -clay minerals do not originate from depositional

environments

-they are detrital in origin

-though they are altered(degraded) in fluviate and subaerial environments

But little evidence of alteration in major basin of deposition to show that it is a major factor in defining the ultimate mineralogic character of clay

TYPES OF CLAYS BASED ON GROUPS -KAOLINITE

-MONTMORILLIONITE

-SMECTITE

-ILLITE

-CHLORITE

BASED ON GENERAL COMPOSITION AND PROPERTIES -KAOLIN-consist mainly of kaolinite

-BENTONITE-majorly monmorillionite,hardens when mixed with water

-BALL CLAY-kaolinite, mica

-FIRE CLAY-kaolinite, feoxide,magnesia,alkalines,can resist high temp

-COMMON CLAY-no resistance to heat, has impurities than fire clay

-ARMENIAN BOLE-attapulgite

Bibliography Clay mineral formation and transformation in rocks

and soils by D.D EBERL , US Geological survey

A discussion on the origin of clay minerals in sedimentary rocks by Charles .E Weaver, Shell Oil Co,Houston,Texas

Identification of Clay minerals by Xray Diffraction Analysis by George W Brindley

The origin of clay minerals in soilsand weathered rocks, www.springer.com

Clay types, geologic origins by ARTS 186 Directory

NOW THAT WE KNOW WHERE WE ARE COMING FROM , WE CAN IDENTIFY WHERE WE ARE GOING TO

THANKS FOR LISTENING

MERCI POUR VOTRE ATTENTION