Clay Fabric/Microstructure

Clay Fabric/Microstructure

Wen-An Chiou

Materials Characterization Centerand

Department of Chemical Engineering and Materials ScienceUniversity of California, Irvine

Irvine, CA 92697-2575USA

Pan-American Advanced Studies Institute on Transmission Electron Microscopy in Materials Science

July 13, 2006

Table of Contents• Introduction

– Soil, sediment and clay materials– Definition– Signature of clay fabric– Concepts of clay fabric– Review of previous research

• Methodology of clay fabric research– Dehydration– Clay fabric semi-quantification

• Clay fabric case studies– Deep sea sediments– In-situ clay fabric– Clay fabric from different sediments/environments

• Processes and mechanism of clay fabric• Summary• Future research

Soil, Sediment, and Clay Materials

• Soil, to engineers, is any unconsolidated materials composed of discrete solid particles and interstitial gas and/or liquids (Sowers and Sowers, 1961). More specifically, soil has been described as a particulate, multi-phase system.– Particulate: system of soil particles (solids).– Multi-phase: mineral phase (solids) plus fluid phase

(air, gas, and liquid – the pore fluid).– Pore fluid and its constituents (salts and organic

compounds) will affect the processed of interaction between the phases – referred as chemical interaction.

• Sediment, to the geologists, is a deposit formed by the agents of water, wind, or ice and a product of chemical, biological, and physical weathering of solid material on the earth’s surface.

• Soil: generally denotes those residual materials the accumulate during weathering.

• Soil and sediment are often used interchangeably depend on the context of discussion.

• The most important and active constituents of a soil (sediment) are the clays (clay minerals).

What is Clays and Clay Minerals

• Clays are:

• Size: less than 2 um in general

• Chemistry: phyllosicates with different cations

• Crystal structure: monoclinic, triclinic

• Morphology: various, depend on the species of clay

• Common clay minerals: kaolinite, smectite (montmorillonite), and illite.

• Other clay minerals: Halloysite, chlorite, vermiculite, attapulgite, sepiorlite, palygorsite, miexed-layer clays, and allophane.

• Prior to 1923, clays were thought to be amorphous (Hadding, 1923)

• Reference: Clay mineralogy: Grim, 1968

Special Properties of Clays

• Physical Significance of the Atterberg Limits – a very useful characteristics of assemblages of soil particles, which in turn is very useful in engineering practice.

• The limits are based on the concept of a fine-grained soil can exist in any of four various states depending on its water content.

Very small particles – provide very large surface areaNegatively charged surface – provide very active surface for chemical interaction

(From Bennett and Hulbert, 1986)

What is Clay Fabric/Microstructure

• Fabric: Any material structure consisting of connected parts; a fabrication or framework.

• Structure: The configuration of elements, parts, or constituents in such an entity; organization; arrangement.

• Fabric: As used by sedimentary petrologists, refers to “the orientation in space of the elements of which a sedimentary rock is composed” (Gary et al., 1972).

• Fabric element: may be a single crystal, a detrital fragment, a fossil or any component that behaves as a single unit with respect to an applied force.

• Clay Fabric: the special distribution, orientation, and particle-to-particles relationship of the <4um solid particles (mainly clay minerals) in the sediment.

• Clay microstructure: the arrangement of atoms in the crystallographic structure of an individual clay particle.

Diagram of the fundamental particle units called domains that comprise the “building blocks” of clay fabric in sediments and rocks. (From Bennett et al., 1991)

Signature of Clay Fabric/Microstrucutre

• Microstrucutre preserved in fine-grained sediments and rocks may be used to identify and recognize specific sedimentary environments and processes, thus serving as a distinct imprint or “signatures” of that environment.

• A few important questions:

– What microfabrics produced in various modern environment?

– What are the actual processes and mechanisms that determine clay signatures?

– What fabric signatures are found in shales and how may they used to provide clues to ancient environments?

– Can shale be classified according to fabric types, which, in turn, may be used to interpret shale properties?

Major processes and mechanisms that determine microfabric signatures in the macro and micro geological environments

Processes and mechanisms represents a continuum during the developmental history of clay sediment and shale microstructure (From Bennett et al., 1991)

Importance of Clay Fabric

• In geological science:– Understanding of geological

environments, economic geology, environmental geology, engineering geology

• In civil engineering:– Understanding the soil

properties, soil behavior, foundation and slope stability, geotechnical properties

• In materials science:– Understanding the material

properties, manufacturing

• In soil sciences: – soil stability, agriculture,

Background- Historical Review (Concept of Clay Fabric)

• The physicochemistry of clays is critical in determining the nature of clay fabric, especially in the early stage of formation.

• Early Concept:– Perhaps the earliest concepts of clay fabric from engineering and ge

ological perspectives were presented by Terzaghi (1925).– Casagrande (1932)– “Terzaghi-Casagrande honeycomb structure”

• Clay mineral stick to each other at point of contact, with forces sufficiently strong to construct a honeycomb structure, which permits large amount o

f water to be enclosed in the voids.

Early Concepts of Clay Fabric

• minerals of chemically sensitive clays are Goldschmidt (1926) and Lambe (1953)

• Flaky arranged in an unstable “cardhouse structure”.• Lambe (1953), particle orientation in a dispersed system is a p

arallel arrangement(oriented fabric), whereas in a flocculated system, it is random (cardhouse fabric).

• Mitchell (1956) pointed out important differences between dispersed and flocculated clays in relation to their geotechnical properties.

Cardhouse, of saltwater Cardhouse of freshwater

Later Concepts and Observations

• During the last 20-35 years, studies of clay fabric have gained momentum with the advent of the electron microscopy, and renewed interest has resulted in numerous investigation of clay particle arrangement in sediment by direct observation.

• Rosenqvist set the stage for EM studies of clay fabric conducted in the 1960s.

• Aylmore and Quirk (1960) proposed the term turbostratic arrangement for a fabric consisting of domains or stacks

• Keller (1964) described a more open structure, which called as “bookhouse” fabric or book structure.

• Clay fabric from different environments: fresh water, lake, marine and deep sea sediments.

• Clay fabric of laboratory simulated geotechnical condition.

Van Olphen proposed various modes of particles association when clay particles flocculate: FF, EF, and EE. EE and EF produce agglomerates (called “floc”). The FF association is termed “aggregation”.

Although apparently those structures proposed by van Olphens have not been found for natural sediments. The modes, however, do afford a means of reference for the study of fabric in natural clay-water system.

Techniques and Instrumentation for Clay Fabric Research

Sampling and subsampling

Fixation

Dehydration

Embedding

Microtoming

Electron microscopy observation

Subsampling

Schematic diagram showing the viewing orientation of embedded specimen. (A) Side or parallel to core axis view. (B) Top or normal to core axis view. (C) Random view.

Analytical technique and instrumentation for clay fabric analysis: Dehydration technique

Analytical technique and instrumentation for clay fabric analysis: Impregnation, sectioning, and TEM

Quantification of Clay Fabric: A Simple Technique

• Orientation Analysis• Measurement of elongation

direction of grain projections.• The direction of elongation was

assumed to be that of two parallel lines with minimum separation that can be drawn tangent to the grain projection.

• General view: 2-3 traverse of 200 counts of clay particles at low magnification

• Confidence: 3-4 traverse of 100 counts on medium or higher magnification

• Use computer program, pay attention to the grey level setting

Graphic Computation and Degree of Orientation

• Specific orientation of each particle was assigned to one of the eighteen 10o intervals between 0 and 180o.

• Rose (polar coordinate) diagrams were used for the purpose of comparison.

• Degree of orientation: similar to the idea of grain size analysis

• Graphic computation: to characterizing degree class orientations by using the formula developed by Folk and Ward (1957) for computing sorting. “Degree of Orientation” ~”Sort the Particle Orientation”

Clay particle orientation analysis using simple point count technique

Point counting – the simplest, fastest, and most accurate measurementStatistical number for calculation

Case Study I – Clay Fabric of Sediments from Middle America Trench

Purpose: to delineate the interrelationship between tectonic, sedimentation, and geotechnical properties is particular important for areas subjected to the dynamic affects of convergence.

Study Area and Sampling Site

• Sample: DSDP Leg 66, within the Middle America Trench complex provided an unique opportunity to investigate these inter-relationship across the trench.

• Geological setting: Site 488, approximately 4 km from the trench axis at the base of the trench inner slope. Drilling penetrated 313 m of middle to upper Quaternary clayey silts, which overlie 115 m of lower to middle Quaternary clayey silts interbedded with sand.

Geotechnical Property Summary Profiles, Site 488

- However, it has identified a nearly horizontal coherent reflection occurring very near the depth of the geotechnical properties anomaly at 235m. Depth Porosity H2O DensitySurface 70% 47% 1.53 mg/m3 210 m 49% 27% 1.75235 m 35% 14.5% 2.05 300 m 45% 33% 1.88400 m 15% 1.98

- No anomalies in geotechnical properties are evident in the upper 200 m, which can be correlated with the thrust fault evident on the site survey seismic reflection profiles.

- Core obtained through this zone show steeply dipping and truncated beds as well as inclined fractures, which suggest deformation resulting from tectonic or mass movement processes.

Clay Fabric Reflects Geotechnical Properties

180 m (above), preferentially oriented clay fabric consisting of domains of clay particles. Probably results from natural consolidation processes due to increasing overburden pressure.

220 m and 235 m (anomaly zone), well defined domains which appear randomly oriented. Clay fabric appears more disrupted with fewer well-defined domains and an increase in electron dense particles.

385 m (below zone of deformation), well-defined and highly oriented domains probably indicative of normal gravitational compaction processes.

Case Study II – In Situ Clay Fabric of Gassy

Submarine Sediment • Objectives: • (1) to delineate clay fabric microfeatures in sedim

ent samples obtained by the use of a pressure core barrel;

• (2) to reveal clay fabric in the natural marine sediments;

• (3) to evaluate the difference in clay fabric between pressure core barrel samples and conventional core samples, and to enhance the understanding of the clay fabric of fine-grained gassy sediments.

Study Area and Sampling Sites – Mississippi Delta

• One of the most thoroughly investigated marine environments in the world.

• One of the most productive

areas in petroleum exploration.

Pressurized core barrel samples were brought up, and sampling was carried out in a hyperbaric chamber.

The most critical steps in preparing samples for EM studies are the techniques employed in the dehydration of wet specimens and the process of embedding a specimen with an appropriate medium.

Special pressure vessel for replacing interstitial water with immediate fluid before critical point drying under equivalent in situ down hole pressure.

Technique of Dehydrating Gassy Sediment at In-Situ Pressure

Clay Fabric Analysis

• Method (A) – down-hole pressure maintained until the sample was critical point dried

• Clay fabric characterized by relatively well-oriented clay particles or domains, with random structures occurring only locally.

• Domains: FF clay platelets forming a nearly perfect stack. Domains appear to vary over a considerable range of size.

• Large voids: between clay particles and domains, and could have been occupied by gases and/or interstitial water.

• Rose diagram and clay orientation frequency curve reveal the statistical calculation of well-oriented clay fabric.

• SEM observation also reveal fairly well-oriented nature of clay platelets although may not show as distinctly as those in TEM micrograph.

• Statistical analysis of clay orientation is difficult based on SEM results.

• Method (B) – Downhole press

ure was released before complete dehydration of the sediment.

• Clay fabric appears from semi-oriented to fairly nonoriented microfeatures. Random fabric were pedominant in most cases.

• Domains: numerous EE contacts, and the size of individual domains decreases. Particles do not appear to be in contact with other particles, but seem to be “floating” in space.

• Rose diagram: random arrangement.

• Frequency curve: widely spread orientation spectrum.

• Method (C) Conventional method, the downhole pressure was released before any process of dehydration of the sample.

• Clay fabric: typified by highly random arrangement of clay particles or domains.

• Domains: appear to be “floating” in space.

• Voids: large and appear to be very well connected showing “channel-like” feature. A few large “pocket-like” pore spaces.

• Orientation analysis: showing highly random clay particle arrangement.

• SEM show similar clay microfeatures.

Method (C): Conventional method

Clay Fabric and Shear Strength

Vane shear tests were performed:– (1) while at downhole pre

ssure (in a hyperbaric chamber)

– (2) in the laboratory after depressurization (conventional technique).

– Clay sediment with preferred orientation displays high shear strength, while clay sediment with random microstructure has lower shear strength.

Clay Fabric of Eckernford Bay, Baltic Sea

Surface sediments (195 cm) showing microfabric of open pore network With aggregates of more closely packed particles.

Clay Fabric and Biological Components

Clay Fabric of Eckernford Bay, Baltic Sea, Sampling depth 1 mm, showing particle agglomerates about a stained microbial cell.

Clay Fabric and Clay DiagenesisThe microfabric of authigenic mineralization in a red clay of the Northwest Pacific deep-sea basin (22.5 m subbottom) as observed by TEM.

Note the lacy. Fine divided crystals that form a very porous network. The mineral “smectite” appears to have developed by expansion of the crystal network pressing larger crystals radially outward while reorienting the larger particles in a direction normal to the direction of stress.

Clay Fabric and Clay/Mineral Diagenesis

Clay Fabric and Mineral Diagenesis

Formation of Biosediment Aggregates

(From Bennett et al., 1991)

Major Processes and Possible Mechanisms

Physicochemical Processes



Bioorganic Processes



Burial-diagenesis


SUMMARY

A number of subjects that related to clay fabric has been presented and discussed in this presentation:

(1) The introduction of the concept of clay fabric, and the role of clay fabric in science and engineering.

(2) The brief review of the development of clay fabric research, and the methods of preparing samples for clay fabric study in a TEM.

(3) The results of clay fabric study from different examples, and possible mechanism of the formation of clay fabric.

(4) The demonstration of the importance of clay fabric in the different fields of science/engineering, e.g., slope stability in civil engineering, the understanding of mineralization and mineral diagene

sis in geosciences.

Future Research

• Clay fabric so far only partially investigated, many unknowns are awaiting for further research. For examples:

• (1) The clay fabric in the suspension, and at the interface between suspended and surface sediments.

• (2) The relation of clay fabric to bulk and geotechnical properties of soils/sediments.

• (3) The relationship between clay fabric and different environments,

• (4) The mechanism of the formation and transformation of clay fabric though a few models has been proposed by previous researchers.

• (5) The role of clay fabric on the surface of different rocks.• (6) The interface between clay fabric and bioshpere.

ReferencesMany figures and micrographs present herewith were taken from the following books, papers cited in those books:

(1) W.;A. Chiou, 1981, Clay Fabric of Gassy Submarine Sediments: Ph.D. Dissertation, Texas A&M University, College Station, Texas.

(2) Bennett, R. H. and M. H. Hulbert, 1986, Clay Microstructure, International Human Resources Development Corp.

(3) Bennett, R. H., W. R. Bryant and M. H. Hulbert, 1991, Microstructure of Fine-Grained Sediments, From Mud to Shale, Springer-Verlag.

(4) Deep Sea Drilling Report, Leg, 488, 1981(?).

Muchas Gracias

Thank You

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Clay Fabric/Microstructure