Silicate Clays
• Clay constitutes those particles that are
smaller than 0.002 mm in diameter.
• It constitutes mainly of secondary minerals
formed as a result of decomposition of
original minerals.
• Clay has no genetic significance. It is the
product of weathering and has been
formed by hydrothermal action or has
been deposited as sediment.
• The coarser particles of the clay especially
those between 0.002 and 0.001mm
contain a few primary minerals like mica,
talc, hematite and quartz. The proportion
of these particles, however, is
comparatively small.
• In general the term clay implies natural,
earthy, fine-grained material which
develops plasticity when mixed with a
limited amount of water.
• By plasticity is meant the property of the
moistened material to be deformed under
application of pressure with the deformed
shape being retained when the pressure is
removed.
Different meanings of clay
• Soil particle - <0.002mm in diameter
• Soil texture - soil dominated by clay
particles
• Minerals - of specific composition,
– may not be of clay-size, eg. Mica
– Clay-size minerals like gypsum, carbonates,
or quartz are not clay minerals.
• Chemically clays are essentially silica,
alumina and water frequently with
approximate quantities of iron, alkalis and
alkali earth.
• Essential elements: Si, Al and H2O
• Optional elements:
– Fe, Mg - If present remain in the structure
– Ca, Na and K - May or may not be in the
structure
Textural Triangle
• Much of the soil clays are colloidal in size.
There is no sharp line of demarcation
between clay and colloidal clay but
particles less than 0.0002 mm in diameter
are considered colloidal clay. The colloidal
clays exhibit Brownian movement and
have considerably higher surface area.
• Basic structure of clay minerals
• Two structural units are involved in most of the
silicate minerals:
– Tetrahedral silica sheets (Silica tetrahedral units)
– Octahedral alumina sheets (Alumina octahedral units)
• The tetrahedral silica sheets are attached to the
octahedral alumina sheet in the structure.
Tetrahedral silica sheet
• The tetrahedral sheet is build of silica
tetrahedrons. The silica unit consists of a sheet
of oxygen atoms that are held together by silicon
atoms.
• The Si atoms are surrounded by four oxygen
atoms, three in the sheet and one above. The
oxygen atom above the sheet provides the
means of linking a silica unit to an alumina unit.
• The O- to –O distance in the silica tetrahedral
sheet is 2.55 Ao. The thickness of the unit is
4.93 Ao .
The Simplest Structure
• Tetrahedron
– One silicon ion
surrounded by four
oxygen atoms.
• Tetrahedral sheets
– Tetrahedral sheets
are joined by
shared oxygen
atoms
Basic units
Tetrahedral Sheet
Octahedral alumina sheet
• The alumina unit consists of two sheets of
closely packed oxygen atoms or hydroxyl groups
which are held together by alumina atoms in
such a way that one alumina is surrounded by
six oxygen atoms or hydroxyl groups.
• The normal O – to – O distance is 2.60 Ao and
the common OH- to –OH distance is about 3 Ao .
• The thickness of the unit is 5.05 Ao in the clay
mineral structure.
Octahedral alumina sheet
• When alumina is present in the octahedral unit,
only two – thirds of the possible positions are
filled to balance the structure (gibbsite). These
types of minerals are called dioctahedral
minerals.
• When divalent ions like Mg2+ is present in place
of trivalent aluminum (Al3+), all positions are
filled to balance the structure which is the brucite
structure as shown below and the minerals of
these types are called trioctahedral minerals.
The Eight-sided Octahedron
• Octahedron
– Six oxygen with a
central Al3+ atom
• Octahedral sheet
– Octahedral sheets are
linked together by
shared oxygen
Octahedral Sheet
Octahedral alumina sheet
Trioctahedral sheet
Isomorphous Substitution
• The replacement of one atom by another
of similar size in a crystal lattice without
disrupting or changing the crystal structure
of the mineral.
(http://sis.agr.gc.ca/cansis/glossary/isomor
phous_substitution.html)
Isomorphous Substitution
• In tetrahedral sheets-
– Al3+ can be substituted for Si4+
• In octahedral sheets-
– Mg2+ can be substituted for Al3+
– Al3+ can be substituted for Mg2+
• Importance-
– Causes unbalanced internal (negative) charge in the
layers.
– Unbalanced negative charges are compensated by
the absorption of ions on the edges of clay particles
Isomorphous Substitution
Classification of silicate clays
• A. Amorphous e.g. Allophane group
• B. Crystalline
– 2 layer type (1:1)
– 3 layer type (2:1)
– Mixed layer type (2:1:1)
– Chain structure type
2 layer type (1:1 type)
i) Equidimensional (Kaolin group)
• Kaolinite
• Nacrite
• Dickite
• Anauxite
ii) Elongate (halloysite group)
• Endellite
• Metahalloysite
3 layer type: I. Expanding lattice
type
(i) Equidimentional • (ii) Elongate
Smectite groups Smectite groups
• Montmorillonite • Nontronite
• Bidellite • Saponite
• Sauconite • Hectorite
Vermiculite group
• Vermiculite
3 layer type: II. Non-Expanding
lattice type
• Micas
– Muscovite
– Biotite
– Phlogopite
c) Mixed layer type
Chlorite group
d) Chain structure type
•Attapulgite
•Sepiolite
•Palygorskite
