INDUSTRIAL GEOLOGY

SGL 358

BRIGHT STAR UNIVERSITY

CHAPTER FOUR

Clay

Clay usually consists of a mixture of various minerals and therefore has a wide
variety of properties. It is difficult to establish a general definition. There are
definitions based on aspects including

− grain size,

− properties,

− geological origin and

− industrial end uses.

Clay is usually defined as

− a material with a grain size <2μm, or

− an earthy, very fine-grained mineral (or mineral mixture) which usually

becomes plastic in the presence of water (some clay minerals are non-plastic).

Clays defined in this way primarily consist of a mixture of certain typical

minerals, the clay minerals. These include phyllosilicates, such as illite,
kaolinite, halloysite, chlorite, the mixed layer-minerals, and the smectites (Na-,
Ca-montmorillonite, beidellite, saponite, stevensite, hectorite). These are the
most economically and industrially important clay minerals.

Clays are associated with accessory minerals in varying proportions (in

particular, mica, quartz and feldspar).

In general, clay minerals are formed by the alteration or weathering of other

silicates such as feldspars, pyroxenes and amphiboles.

Clay minerals are systematically classified depending on the structural link

between the SiO4- tetrahedral layers and the Al-octahedral layers.

Water forms relatively independent layers in the clay minerals of these groups
and occupies the space between the bound layers consisting of octahedral and
tetrahedral layers. The degree of ordering of the lattice has a major influence
on the technical properties of clay minerals. The major replacement of certain
ions in the lattice by other ions with similar ionic radii gives rise to flaws in the
crystal lattices and changes the mineral properties.
 1) Kaolin and kaolinitic clays

The term kaolin used in an industrial and marketing sense refers to a fine-
grained natural product that primarily consists of kaolin group minerals -
primarily kaolinite and more rarely halloysite, dickite and nacrite. It may also
contain mica, quartz, feldspar, smectite etc... In English, the term „china clay“
is used for kaolin derived from primary deposits.

Kaolinite

− is chemically inert over a wide pH range (4 – 9),

− has a white or almost white colour,

− has good covering properties,

− is soft and non-abrasive,

− has small grain size,

− is plastic (exception: flint clay),

− is refractory (fusion point 1,850 °C) and fires white,

− has low electrical and thermal conductivity,

− has low absorption capacity, low cation exchange-capacity, good dispersion

capacity and

− can be activated by acid treatment.

Kaolinitic clays are deposited in continental environments (limnic, brackish,

fluviatile) and are the erosion products of kaolinitic weathering products. The
mineralogical composition of kaolinitic clays varies over a wide range: the main
components are kaolinite, mica/illite, and quartz; small amounts of organic
material often give rise to a dark-grey to black colour, although this disappears
when fired. The kaolinite in kaolinitic clays is usually more or less characterised
by a high degree of disordering. Because of its marked plasticity, kaolinitic clays
are primarily used by the ceramics industry.

End uses

Kaolin and kaolinitic clays have an enormous range of end uses.

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Kaolin is used for the following applications amongst others as a filler, extender
and adsorption medium including in the manufacture of:

− paper

− rubber products

− plastics

− paint, laquers, inks

− insecticides

− adhesives

− mineral fertilizers

− pharmaceutical and cosmetic products

− detergents

− graphite and coloured pencils

− asbestos products

− thermoplastic roofing materials

− linoleum and linoleum adhesives

− textiles

for the coating of paper as a binder for the production of:

− grinding disks

− foundry moulds

− granulated animal feed mixtures

− welding electrodes

as a ceramics raw material for:

− porcelain

− white and coloured earthenware, stoneware

− refractory ceramics

− electronic porcelain

− ceramic monoliths (catalyser carrier in cars)

− ceramic membranes (for industrial filter processes)

− catalysers (cracking processes)

− other industrial ceramics

as a raw material for the synthesis of:

− aluminium

− aluminium sulphate

− aluminium phosphate

− zeolites

− alloys (sialon)

− fibre glass

− cordierite

− mullite

− propping agent in oil wells

for the production of special cement (white cement, refractory cement, acid-
resistant cement). Kaolinitic clays are used in particular in fine ceramics for:

− earthenware, whiteware, sanitary ceramics

− wall and floor tiles

− electronic porcelain

in the refractory ceramics industry as:

− binding clay

− chamotte (grog)
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in agriculture as:

− binding clay in animal feeds

in chemistry as:

− filler, e.g., in plastics.

2) Illite

Illite is not a firmly defined mineral but a collective term for a group of mica-
like triple layer clay minerals, which are widespread in argillaceous weathering
products. Illite contains less alkali but more water than mica.

Unlike smectite, illite does not undergo any inner crystalline expansion.

End uses

The most important end uses of illitic clays are compiled in the next table.

Substitutes

Depending on the end use and the local conditions in each case, illite and illitic
clays can in part also be substituted by other mineral raw materials: e.g., in
ceramics by kaolin, in moulding materials by smectite, in landfill basal seals by
other impermeable clays, as a filler in paper by calcium carbonate.

3) Smectite (bentonite, montmorillonite)

Smectites are expandable triple layer silicates which can incorporate inorganic
and organic cations between the layers and therefore change the distance
between the sheets in the c-direction. By far the most common smectite
mineral is montmorillonite. Rocks with high smectite contents (usually
montmorillonite), are primarily traded as bentonite.
Structural formulae of the most important smectites after JASMUND & LAGALY (1993) and an
overview of the range of synonyms.

Active or bleaching earths are the name for all clays whose adsorption
capacity enable them to bleach vegetable oils, mineral oils, fats and lubricants,
and purify wine, juices, syrups etc. They can be made up of smectites.

When treated with 3-8 % soda solution, Ca-bentonites can be activated to Na-
bentonites (activated bentonite). When treated with acid, they can be
activated to H-bentonites (acid clay). Natural H-bentonites („acid earths “)
occur in Japan and Canada.
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Deposit genesis

Smectites are created:

− as residual deposits (weathering) above primarily basic magmatic rocks in

alkali environments in the presence of Mg,

− by hydrothermal alteration, and

− by the deposition of transported fine material (marine and non-marine).

Smectite deposits occur primarily in late Mesozoic and Cenozoic formations.

They are frequently marked by extreme variations in thickness and facies. The
main industrially used smectites are Na, Ca, Mg and H-bentonites. Because
bentonite deposits are usually lensoidal, the spacing of the exploration
boreholes should not exceed 100 m; borehole separations of 25-15 m may be
necessary to reliably confirm the size of reserves.

End uses

As shown in the next table, bentonite can be used for an enormous range of
applications.

Bentonite is primarily used in industries in industrially developed countries.

 4) Brick clay, loam

Brick clay and loam are formed from mixtures of a wide range of clay minerals
and other minerals depending on their genesis. Because of their high
proportion of fluxing agents (alkalis, iron compounds, lime) brick clays and
loams start to melt at temperatures of 950 to 1,200 °C.
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Because the physical properties, which are based on the type of clay minerals,
are very important for their industrial use, the mineralogical composition of
the raw material and its grain size distribution are very significant factors.

Loam consists of very variable grain sizes from particles <0.002 mm (usually
“clay minerals”) to silt (0.002-0.06 mm) and sand fractions (0.06-2.0 mm) right
up to gravel (up to around 20 mm). Loam often contains iron compounds
which give it a yellow-brown to brown colour, as well as carbonate. Loam is
not as plastic as clay. Clay-rich loam is called „rich “and loam with a minor
amount of clay is called „poor “or “lean”.

Grain size-composition of loam, after FINNERN et al. (1994).

End uses

Brick clay and loam is suitable as a raw material for the production of fired
bricks (backing bricks as solid bricks, horizontal coring bricks or vertically
perforated bricks, of facing bricks and clinker) chimney bricks, sewer bricks,
water engineering clinker, tunnel clinker, road construction clinker, noise
absorption bricks, sealing bricks, roofing tiles, and drainage pipes and cable
protection ducts.

Loam can be used in non-fired form as a construction material (building loam)

and sometimes as a soil improver in sandy, clay-free soils.