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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