1 - Lecture notes for Clay Mineralogy 


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 Clay Minerals Society produces a glossary of clay terms, which is a good source to go to for many words used in clay science. You can download the glossary from here.

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)

where:

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

(interpolate A as necessary).

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.

spint time equations

Hathaway, J.C. (1956) Procedure for clay mineral analysis used in the sedimentary petrology laboratory of the U.S. Geological Survey. Clay Minerals Bulletin, 3, 8-13.

Important assumptions are made when a clay mineralogist reports <2 μm esd. They include:

  1. Centrifuge acceleration and deceleration are assume to be linear and constant
  2. The initial distance is determined by the top water level in the centrifuge tube (from the rotor center).
  3. The final distance is the level of accumulated sediment in the centrifuge tube (from the rotor center).
  4. A parrticle density of 2.65 g / cm3 is assumed
  5. The density and viscosity of water are also a function of temperature. At aobut 20°C water has a viscosity of about 1 cP. A more exact form of the equation above would include a temperature term for the viscosity of water.

A spreadsheet using Stokes Law allows calculating time for an operationally defined esd particle assuming a temperature and settling distance in water. The example below shows a settling time of almost 4 hours for 2 µm esd particles at depth of 5 cm in a graduated cylinder. The terminal velocity of very fine clay particles settling in room temperature water can be extremely slow. For example a 0.2-µm esd particle takes almost 15 days in a graduated cylinder. Using a centrifuge can reduce the settling time. Using initial and final rotor dimensions of 15 and 20 cm, respectively and spinning at 5000 RPM takes about 5 minutes to settle 2 µm esd particles. Stoke’s law is nonlinear, so take the time to download the spreadsheet stokes.xls. Parameterize the values for your lab and experiment by changing variables of particle size, temperature, depths, and rotor speeds to gain a sense for how Stoke’s law works.


Stokes

Colloids and 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 environments.

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?

The field of clay mineralogy is truly a multi-disciplinary science. If you were to attend a meeting for 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.

Clay mineral production for Georgia

Statistics released by the U.S. Geologic Survey:

Fullers Earth:

pet waste absorbent
oil absorbent
pesticide carrier
fertilizers
drilling mud
cement
paint
filtering
catalyst in oil refining

Value = US $85 million

Other:

barite
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
marble

Value = US $ 116 million

Kaolin and Bauxite:


Value = US $1.05 billion

Paper filler and coating
Cement
Food additives
Ceramic raw material (including fracking propents)
Pharmaceuticals and medicines
Fertilizers
Filler in rubber and plastic
Insecticide carriers
Cosmetics
Extender in ink and paint
Textiles
Detergents
Cracking catalysts
Tanning leather
Plaster
Foundries
Adhesives
Polishing compounds
Ceramic propants for hydraulic fracturing

1st 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.

 


Where do you get information about clay mineralogy?


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.