CLAYS Ask a
Geologist
Background | Name | Sources | Uses | Substitutes and Alternative Sources
MII Photos Claystone Kaolinite Kaolinite
Background
The term clay refers to a number of earthy materials that are composed of minerals
rich in alumina, silica and water. Clay is not a single mineral, but a number of
minerals. When most clays are wet, they become "plastic" meaning they can be
formed and molded into shapes. When they are "fired" (exposed to very high
temperatures), the water is driven off and they become as hard as stone. Clay is
easily found all over the world. As a result, nearly all civilizations have used some
form of clay for everything from bricks to pottery to tablets for recording business
transactions.
The minerals that make up clay are so fine that until the invention of X-ray
diffraction analysis, these minerals were not specifically known. Under extremely
high magnification, one can see that clay minerals can be shaped like flakes, fibers,
and even hollow tubes. Clays can also contain other materials such as iron oxide
(rust), silica, and rock fragments. These impurities can change the characteristics of
the clay. For example, iron oxide colors clay red. The presence of silica increases
the plasticity of the clay (that is, makes it easier to mold and form into shapes).
Clays are categorized into six categories in industry. These categories are ball clay,
bentonite, common clay, fire clay, fuller’s earth, and kaolin.
Name
Sources
Clays are common all over the world. Some regions, as might be expected, produce
large quantities of specific types of clay. It is estimated that the state of Georgia has
kaolin clay reserves of 5 to 10 billion tons. The United States is self-sufficient so it
imports only small amounts of clay from Mexico, Brazil, United Kingdom, Canada,
and assorted other nations. The United States exports nearly half of its production
worldwide.
The nations producing the most significant amounts of the various clays are as
follows:
Kaolin: Brazil, United Kingdom, and the United States are the dominant producers
of high quality kaolin.
Ball clays: Major producers of ball clays are Germany, the United States, United
Kingdom, the Czech Republic, China, and France.
Fire clays: Major fire clay producing countries are Germany, and the United
States.
Bentonite: Major producers of bentonite are the United States, Germany, Turkey,
and Greece.
Fuller’s earth: Major producers of fuller’s earth are the United States (attapulgite,
smectite), Spain (attapulgite, sepiolite), and Senegal (attapulgite).
Uses
The United States both imports and exports clays and clay products. It is estimated
that the United States consumes about 37.6 million tons of clays each year.
Ball clays are good quality clays used mostly in pottery but are also added to other
clays to improve their plasticity. Ball clays are not as common as other clay
varieties. One third of the ball clay used annually is used to make floor and wall
tiles. It is also used to make sanitary ware, pottery, and other uses.
Bentonite is formed from the alteration of volcanic ash. Bentonite is used in pet
litter to absorb liquids. It is used as a mud in drilling applications. It is also used in
other industrial applications such as the "pelletizing" of iron ore.
Common clay is used to make construction materials such as bricks, cement, and
lightweight aggregates.
Fire clays are all clays (excluding bentonite and ball clays) that are used to make
items resistant to extreme heat. These products are called refractory products.
Nearly all (81%) of fire clays are used to make refractory products.
Fuller’s earth is composed of the mineral palygorskite (at one time this mineral was
called "attapulgite"). Fuller’s earth is used mostly as an absorbent material (74%),
but also for pesticides and pesticide-related products (6%).
Kaolinite is a clay composed of the mineral kaolin. It is an essential ingredient in
the production of high quality paper and some refractory porcelains.
Substitutes and Alternative Sources
When necessary, calcium carbonate and talc can be used in place of clay as filler in
some applications. However, clay is so abundant in all its forms that such
substitutions may only be necessary if the alternative materials are less expensive
than clay (which is not very likely).
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