Clay mineralogy is the study of clays and clay minerals.
WHAT IS CLAY AND
WHAT IS A CLAY MINERAL?
The term clay is often operationally defined. The term clay denotes both (1) a particle size range and (2) a set of material physical properties. The upper limit for the clay particle size range varies depending upon the discipline that is operationally using the term (e.g., geology, engineering, soil science). In geology, the term clay includes all particles that are <2 μm (recall: 1 μm or micron = 10-6 meter), which is about the size of a Prokaryotic cell or 1/100 of a human hair, or the wavelength of infrared radiation). Sometimes the limit is reported at <4 μm (such as the engineering field). When the term "clay sized" particles is used, there is no connotation about composition. Clay sized material can constitute any material as long as it's within the particle size range of <2 μm. Bruce Railsback has produced a nice figure that depicts the "size of things".
The operational definition of clay also includes rheological properties (i.e., plasticity). Clay is often described as a fine grain material that is plastic when wet and hardens when dries. There is nothing inherent about composition in the term clay, although clay is often composed of clay minerals (see more below).
Be careful of the following caveat when you
hear the term "<2 μm". Many times in clay mineralogy,
people will talk about "the clay fraction" as being equivalent
to the <2 μm fraction. What is meant by this expression is
that the material has an "equivalent spherical diameter" or esd of <2 μm. This is
an operational definition based on Stokes' Law, which
describes the terminal velocity or rate of particle movement
in a fluid, for a given set of physical conditions. The
terminal velocity for a <2 μm particle in water can be
extremely slow, therefore its settling time in a normal
gravitation field can be extremely long (hours to days).
The time (tminutes) for an ideal spherical particle was formulated by Folk (1974, ISBN #0-914696-03-3) given the depth of settling in water (cm), diameter (mm) of particle, and density of particle (g/cm3).
tminutes = depth (cm) / (1500 * A * d2 (mm)
Where A is
a constant depending upon water viscosty, which is
temperature dependent (see below).
You can calculate and record the time (tminutes) for a 2 μm esd particle with a density of 2.65 g/cm3 to settle 10 cm in a graduated cylinder using the following equation.
tminutes = 10
cm / (A * 0.006)
A = 3.23 if
suspension water temperature = 16°C
A = 3.57 if suspension water temperature = 20°C
A = 3.93 if suspension water temperature = 24°C
You’ll find wait times
of ~7 hours. So plan accordingly!
tminutes: ______________ minutes; thours = tminutes/60 = _____________ hours
To speed things up, we resort to a centrifuge
to increase the forces involved. Stokes' formula is presented
in a usable form by Hathaway (1956) and is found below.
Hathaway, J.C. (1956) Procedure for
clay mineral analysis used in the sedimentary petrology
laboratory of the U.S. Geological Survey. Clay Minerals Bulletin,
Important assumptions are made when a clay mineralogist reports <2 μm esd. They include:
nanoparticles - Colloids and nanoparticles occur when material
becomes so small it can be considered a molecular aggregate.
The surface charges play an important role. Colloids are
operationally defined as fine material that stays in
suspension with its surrounding medium (solid, liquid or gas). As you will learn later on, for the
case of water solutions, the properties of a suspension are
dependent upon the concentrations and types of dissolved
ions in the solution. Since the mid-1990's the terms nanoparticle or nanocrystalline have come into popular use. These
terms denote particles that have crystalline order in the
nanometer size range (10-9 m). These materials
are commonly detected by electron optical methods and are
now routinely recognized with the advent of
second-generation electron microscopes.
Biochar is also an earth surface clay sized material
produced by natural and anthropogenic pyrolysis (burning) of
biomass. Biochar is found in many earth surface
Here is a brief comment about scale and resolution (the profoundness of which, perhaps will only be told by the test of time…). Many advances in science (hence civilization as we know it) have come about by making better our ability to spatially resolve (e.g., discovery of telescopes and optical microscopes). As our ability to image and describe the order/disorder and composition of materials across different scales improves, then so will our understanding of materials improve. We are just starting to understand the nature of the nanoscale. The next step in resolution is seeing the world on the picoscale.
Clay Minerals - The term clay mineral is most commonly used to denote a family of hydrous alumino-silicates (more specifically phyllosilicates). Most clay minerals are found in nature with particle sizes in the <4 μm range. They are chemically and structurally similar to other phyllosilicates, such as the true and brittle micas. We will learn much about clay minerals from the macroscopic study of true micas.
There are many other materials of geological and biological importance that are clay sized, however they are not "clay minerals" by the above definition. These other clay-sized minerals and materials include other silicates such as quartz and zeolites, as well as non-silicates such as the hydrous sulfates, hydroxides, oxyhydroxides, hydrous oxides, amorphous compounds, organic compounds, Prokaryotes, and viruses. Because their existence is intimately associated with clay minerals, they are included into the domain of clay mineralogy. Here is a link to Bruce Railsback's page on "The size of things"
I prefer not to get too anal about the above terms. If you ask every clay mineralogist their definition of a clay and clay mineral, then you'll likely get a different answer from each person, each time. The philosophy here is to be inclusive of all materials and strive to understand their fundamental structure and chemical nature. The better you understand minerals that are clay-size (which will be the subject of everything that follows) then the better you can understand their behavior in the environment.
Why are clay minerals so important?
If we look at the volume of material at the earth's surface we see that clay minerals constitute about 16% of its total. A 20 km thickness is considered the "surface" because it is the region from which we extract natural resources (and dump our waste...). The diagram below graphically explains how the value of 16% is obtained.
If 20% of the upper 20 km rocks
on Earth are igneous and metamorphic, then that leaves 80% of
all upper rocks as sedimentary. If 50% of these sedimentary
rocks are sandstones and limestones by volume, then that
leaves 40% of all upper rocks as shales. If 60% of these
shales are composed of framework silicates (quartz, feldspars,
etc), then that leaves the remaining clay minerals consituting
16% of all surface rocks. Clays minerals are also abundant in
soils and hydrothermal alteration zones associated with
igneous and metamorphic rocks. This analysis admittedly ignores organics and water,
but the point is that clay minerals are clearly a major
component of the Earth's surface.
Who studies clays?
field of clay mineralogy is truly a multi-disciplinary
science. If you were to attend a meeting
clay mineralogy, you are just as likely to encounter a
geologist as an engineer, a chemist, an agronomist, a
pharmacologist, a biochemist, a microbiologist, or
a material scientist. Anyone of these people would also
equally come from academic, industrial, and governmental
offices. Such a meeting would also have students and
international participants from all around the world.
Since this course is taught at the University of Georgia, I've added some interesting clay related facts about the state.
catalyst in oil refining
Value = US $85 million
bauxite --> synthetic mullite - Nation's leading manufacturer of ceramic propants for hydraulic fracturing
common clays --> bricks, cement, floor and wall tile
fire clays --> fire bricks and flue linings
iron oxides --> orange and red pigments
Micas --> pearlescent pigments
Value = US $ 116 million
Kaolin and Bauxite:
Value = US $1.05 billion
filler and coating
Ceramic raw material (including fracking propents)
Pharmaceuticals and medicines
Filler in rubber and plastic
Extender in ink and paint
Ceramic propants for hydraulic fracturing
National Ranking for kaolin production:
4th in total value of minerals produced.
Employment impact for Georgia.
8,500 are employed in Georgia's mining industry.
The clay mineral literature transcends a wide range of disciplines. The dedicated journals include:
In addition to dedicated journals to clay mineral studies you will likely find articles published in other journals that are listed below. This is, by far... not a complete list.
Societies that focus on clay science.
How do we effectively deal with materials that are not amenable to study by techniques that normally include our eyes (such as optical microscopy, where the limit resolution is a few um)? That will be the subject of the remainder of this course.