We are going to continue our discussion on
crystal structures of different ceramic compounds.
Last time, we have started preliminary discussion
on silicates, the silica bearing compounds.
We will come back to that discussion a little
later. However, before that, let me go back
and discuss some of the features of the crystal
structures, which I have not discussed so
far.
The very first thing we want to discuss is
a consolidated picture of what we have already
discussed. That is, a list of different compounds,
primarily oxides, some sulphides may be and
their crystal structures. We can classify
them according to the formula of compounds,
their complexity of the structural features
and then the coordination numbers and so on.
Some are the structures we have already discussed.
Rock salt structures we have discussed and
its basic features, what we are presenting
here is a consolidated picture of all the
different oxides we come across and their
structural features.
So, the simplest form are simplest structure
type is actually rock salt. Its general formula
is basically MO a divalent metal with oxygen.
In case of rock salt structure or sodium chloride
structure, as we have discussed earlier, if
the basic anionic packing is closed packed
structure of F C C, that is face centered
cubic are cubic close packed structure. We
discussed about the coordination number, either
cation or the anion and what it is presented
here, what is the coordination number, if
the cation is of the coordination number of
the anion. So, it happens so, that in rock
salt structure, both cation and the anion
have a six fold coordination.
We also mentioned earlier that there are a
skeleton of oxygen ions and the interstitial
positions are primarily occupied by the cations.
So, there are octahedral sites as well as
tetrahedral sites. For each oxygen atom, there
are two tetrahedral sites in the structure
and one octahedral site. This all octahedral,
all octahedral means all the available octahedral
sites are occupied by the cations in this
particular structure. Examples of this kind
of oxides, if the mention, this is magnesium
oxide, calcium oxide, barium oxide, strontium
oxide, iron oxide, nickel oxide, manganese
oxide, cobalt oxide, cadmium oxide etcetera.
So, all these oxides of the general formula
m o, normally specializes or they have the
rock salt structure.
Next, we also discussed this particular structure
is cesium chloride structure and of course,
there are not too many compounds or there
are hardly any oxides, which has this cesium
chloride structure, but for the sake of completion,
this is a simple cubic. This is only one of
the rare compounds having a simple cubic structure
and formula of course is c s c l and in this
case, the cation having eight fold coordination
and also the anion, anion and cation both
have this eight fold coordination, right.
All the cubic sites are filled up. So, because
both of them have the same coordination and
it is a simple cubic structure, so it is a
cubic coordination. So, all the cubic sites
are basically filled up by either cesium or
the bromine or in this case, another compound
is cesium iodide. So, this is one simple crystal
structure type. This we did not discuss earlier,
but the structure type, there are many, some
oxides do crystallize in this particular structure.
This is a zinc sulphide. Zinc sulphide, in
fact has two different polymorphs; one is
called zinc blende and the other is called
wurtzite. The basic difference, the main difference
is here. The packing in one case or the symmetry
in one case is face centered cubic.
It is also a close packed structure as also
the H C P. Both H C P and F C C has a close
packed structure, but one polymer has a face
centered cubic structure and other has a hexagonal
structure. Otherwise, they are all same. The
anion and the cation, both have four fold
coordination, and that means tetrahedral coordination.
Both in the case zinc blende structure, the
cubic structure as well as the hexagonal structure.
In both the cases, half of the tetrahedral
sites are occupied by the cation. Half of
the tetrahedral sites are occupied by the
cation, because the ratio you can see, the
ratio of the cation and anion is same.
The tetrahedral sites in a close packed structure
are actually double the number of oxygen or
anions. So, only half is filled up and the
examples are zinc sulphide. This is also zinc
sulphide. One form of zinc sulphide is called
wurtzite and another form is zinc blende.
Beryllium oxide crystallizes in zinc blende
structure. Silicon carbide, silicon carbide
is one of the carbides commonly used in ceramics,
the traditional structural ceramics. Again,
just like zinc sulphide, it has two different
polymorphs. Silicon carbide has the cubic
structure as well as the F C C structure,
the hexagonal structure. Zinc oxide has a
hexagonal structure. So, this is another group
of oxides, quite important oxides having either
zinc blende or wurtzite structure.
Fluorite, we have discussed earlier. Calcium
fluorite, the structure type is calcium fluorite
and the mineralogical name or calcium fluorite
is actually fluorite. It is a simple cubic
structure and to some extent, similar to cesium
chloride. However, the coordination numbers
are slightly different for the cation. It
is eight fold and for the anion, it is only
four fold. So, this is different compared
to the cesium chloride structure. Half of
the cubic sites, that means, the eight fold
coordination sites are been filled in the
fluorite structures. So, in that, all the
cubes or body centered of the cube is not
filled up. Only the alternate sites, alternate
cubes are filled up with the cations and therefore,
you have a large number of oxides. All of
them are of m o 2 type oxides, m o 2, z r
o 2, c e o 2, thorium dioxide, uranium dioxide,
hafnium dioxides and a host of, this is praseodymium
and then you have host of rare earth oxides.
Many of these rare earth oxides do have this
fluorite structures and they have certain
important properties, which we will discuss
later.
Then, in addition to fluorite, we have another
structure type is called anti fluorite. If
you look at the chemical formula, these compounds,
one is c a f 2, whereas the other is m 2 o.
So, these are, you can see these are monovalent
of metals, like lithium or alkali metals like
mostly, lithium oxide, sodium oxide, potassium
oxide, rubidium oxide. So, all these do have
a so called anti fluorite structures and this
is not simple cubic.
This is F C C close packed cubic and this
is different. This is just the reverse. Reverse
of this. Here, the cation has fourfold coordination,
whereas the anion has eight fold coordination
and all tetrahedral sites are filled up, which
is quite obvious in the fluorite, because
of the particular formula, only half of the
cubic sites are cubic interstitial sites are
occupied, whereas here, all the tetrahedral
sites are occupied. because the ratio is m
2 o. So, there is a double number of oxygen.
The metal cations is double the number of
anions. So, all the tetrahedral sites are
actually filled up.
To continue with this list, we have a rutile
structure. This is the formula m o 2. We have
discussed earlier and the type of compounds
which crystallizes in this particular structure
is t i o 2, s n o 2, g e o 2, germanium dioxides,
manganese dioxide, t i o 2 again. I think
there is a mistake. We have repeated. It should
be tellurium. This is tellurium dioxide. So,
these are distorted F C C structure. It is
not a completely or exactly F C C or it is
not a cubic. In fact, we will see, the symmetry
is towards tetragonal, not F C C. But, for
our purpose, at this point of time, it is
F C C and coordination number of the cation
is 6 and that of anion is 3. Half the octahedral
sites are occupied in this case.
Corundum we have also discussed. The chemical
formula of the structure type is m 2 o 3,
askew oxides and this is H C P. The symmetry
is hexagonal and cation having coordination
number of 6, whereas the anion has a coordination
number of 4. Only two-third of the octahedral
sites, two-third of the octahedral sites is
occupied by the cation, because of the formula
here. It is 3 is to 2. So, we have only two-third
and rest one-third is vacant. The oxides,
which actually crystallizes or have this particular
structure is a l 2 o 3, f e 2 o 3, m n 2 o
3, t i 2 o 3, c r 2 o 3 and so on.
Next the complex oxides. We have discussed
both of them in the last class. The general
formula of a b o 3 and we have a cubic structure,
cubic close packed or face centered cubic,
in other words. They have three different
elements here. One anion and two cations.
So, the coordination number here is 12 6 and
6. So, one of the cations has a twelve fold
coordination, whereas the other has six fold
coordination and they anion has a six fold
coordination. Half the octahedral sites are
actually occupied in this particular system.
In fact, this is a very important group of
oxides having many different functional properties.
We will discuss them later, particularly electrical
properties. Many of them have very interesting
electrical properties. We can modify even
the electrical properties and most of them
are known for their high dielectric content
material. Barium titanate, strontium titanate,
cobalt titanate, barium zirconate and barium
stagnate, different kinds of basically titanates,
zarconetes and stagnates, most of them have
this perovskite structure. We have two varieties
of spinel structure. The general formula is
a b 2 o 4. Again, it is a cubic symmetry;
cubic close packed and the ratio of, the cation
and anion coordination numbers, not the ratio.
They are actually the coordination number
of different ions. Cation is 4 and another
cation is 6 and anion is 4.
So, we have two different cations occupying
two different sites, because we have tetrahedral
sites and octahedral sites. In perovskite,
there is none of the tetrahedral sites or
occupied, whereas in spinal, both the tetrahedral
and octahedral sites are occupied. One-eighth
of the available of the tetrahedral sites
are occupied by A atoms and half of the available
octahedral sites are occupied by B atoms.
So, that is what you call the direct spinel
structure and the examples are, magnesium
aluminate, iron aluminate, zinc aluminate
and zinc ferrite. These are spinel structures
with this kind of distribution a and b, distributed
between tetrahedral and octahedral sites.
Last variety in this series is inverse spinel
and the chemical formula or general formula
is b a b o 4. Here it was a b 2 o 4. Incidentally,
a is a divalent and b is trivalent. Here,
a is divalent and b is tetravalent. B here
is a trivalent and 2 of the cations have been
distributed between octahedral and tetrahedral
and a has gone inside the octahedral site.
So, what has happened is, b has come out from
the octahedral to the tetrahedral. b was here
in the spinel structure, normal spinel structure
or direct or it is mostly called normal spinel.
Not direct spinel. In the normal spinel structure,
it is actually b atoms are in the octahedral
sites, whereas in the inverse spinel, b atom
is actually distributed between octahedral
and tetrahedral. So, half of b atom is occupying
one-eighth of the tetrahedral site. In half
of the octahedral available sites, a atoms
are there plus half of b.
So, there is redistribution of the a and b
ions between the tetrahedral and octahedral
sites. The example are iron, magnesium ferrite,
iron nickel ferrite and then iron; in fact,
it is not iron, it is manganese ferrite, it
is nickel ferrite and magnesium ferrite and
then of course, and there is another titanate
is also there. So, magnesium, magnesium, magnesium
is on this side and also in this side and
titanium, it forms a titanate. So, these are
the different kind of oxides available to
us and most of them are used for different
advanced applications.
One of the major oxides is zirconium dioxides,
z r o 2. It has many different important properties,
which we will be discussing later. So, it
may be worthwhile to know little bit about
the structure of this particular oxide and
trying to give little bit emphasis on the
zirconia structure. Zirconia, z r o 2, we
have seen that, just before this slide, it
was indicated that zirconia have a cubic fluorite
structure. But, all zirconia do not have the
cubic fluorite structure. We have a cubic
modification. So, this is the distribution
of atoms as for as the cubic fluoride structure
is concerned. So, this is stable at a very
high temperature. About above 2200 degree
centigrade, whereas, room temperature, zirconia
has a monoclinic structure.
So, there is a not so symmetric structure.
It is a distorted structure. We discussed
about this different symmetric in a few minutes.
So, zirconia has three different polymorphs.
One is monoclinic and is tetragonal and then
the other one is cubic. This is the low temperature
modifications and room temperature is about
1100 degree centigrade. From 1100 degrees
to about 2200 degree centigrade, this particular
structure is stable. That is tetragonal and
then above 2200, it becomes cubic. So, higher
is the temperature, the symmetry becomes more
regular or it becomes, the structure becomes
more symmetric. That is the normal tendency
in any polymorphic transformations.
So, these are the polymorph transformation.
Polymorphic, polymorphs of zirconia, that
is the cubic tetragonal and monoclinic in
the descending order of the temperature. We
will discuss those things at a later stage,
because zirconia is one compound, which has
very important properties, both from the point
of view of electrical properties as well as
from the mechanical properties. So, mechanical
properties which is the tough ceramics, one
can toughen zirconia, what we called the transformation
toughing. So, these polymorphs can be designed
properly. The micro structure can be designed
in such a way that the material becomes quite
tough. It is not so brittle. If you feel I
am very fast, research paper has come out
on zirconia. As far as toughing is concerned,
at that time, people talked about ceramics
steel. So, it was equivalent to steel, applications
of steel or metallic materials can be replaced
by zirconia. So, on the other hand, zirconia
also has very interesting electrical properties.
It is one of the very few oxides having oxygen
ion conductivity. A very high level of oxygen
ion conductivity and that is again useful
from industrial point of view, extensively
used industrially for many different purposes.
We will see that at later stage. So, zirconia
is a very important member of structural ceramics,
sorry, the advanced ceramics materials.
Well, coming back again, we have discussed,
while the structure, many a times we were
talking about the symmetry, symmetry in the
distribution or in the atomic arrangement
of the different materials. Let us try to
look at it very briefly, what are the different
crystal systems available. Sometimes, you
talk about hexagonal crystal structures; sometimes
cubic crystal structures and we have also
discussed about monoclinic, for example zirconia.
So, there are different kinds of symmetric
elements and it may not be possible to discussing
everything here. However, very briefly, let
me tell you there are 7 crystal systems. There
are 7 crystal systems depending on what is
the relationship between the 6 parameters,
what we call the lattice parameters. 6 parameters
in the sense, we have three dimensional picture
and a three access system, a b c or x y z
axis and along the x y z, we have unit vectors
called a b c. So, then you have different
ways of expressing this volume. The symmetry
may be different in different structures.
So, alpha beta gamma are the angles and it
will be c c a and a b. So, these are the axis
you can see. This is a axis, this is b axis
and this is c axis. So, alpha is the angle
between a and c, beta is the angle between
a and b, beta is the angle between b and c
and alpha is the angle between the a and b.
So, these are a b c axis and alpha, beta,
gamma, are the angles between them. So, if
that be so, you can imagine various different
relationships between them and this is what
we called triclinic system.
The list I will give you in a minute, what
is the relationship between a b c and alpha
beta gamma. So, you have triclinic system;
you have monoclinic system and then again
monoclinic as two different forms. One is
called the primitive form and other is called
base center form. Well, in all these cases,
the corners are occupied by the atoms or ions.
All the corners are occupied by the atoms
or ions and in addition, in some of the cases,
we have extra ions or extra atoms, like in
this case, it is what we call base center.
Only there are two faces. Two faces of these
two sides. Opposites are occupied by the two
atoms or ions.
So, in this case, orthorhombic, these are
all orthorhombic. Four varieties of orthorhombic
are available. One is the simple or the primitive
cell, then we have a base center and then
we have a body center here and then the fourth
one is face centers. So, in addition to the
corner positions, the face center positions
are also occupied by the atoms. So, depending
on the atomic arrangement, one can imagine
these kinds of unit cells and definition of
the unit cell is, these unit cells once repeated
in all the three different directions completely
fills up the volume and the vectors are repeated
in all the three different directions. Here
again, we have a tetragonal symmetry and we
have only body centered. Not the others. In
case of tetragonal, only body center. In case
of monoclinic, only base center.
Hexagonal, of course it is a primitive cell.
There is no other body center, face center
or base center. So, it is only one variety
of hexagonal cell and we will discuss and
will just point out what is the relationship
and how hexagonal system is different from
cubic or other systems. We have a rhombohedral
here. There is no again. Only primitive cell
is there. There is no base center, face center
or body center. In case of cubic, three varieties
are there. So, it is the primitive. There
is no atom inside or either in the body center
or face center, whereas, this is a face center
cubic structure. We have discussed this. All
the face center cubic and hexagonal we have
discussed extensively and there is a body
center. Body center also we have discussed
earlier. So, in all, there are 14 kind of
systems, what we call crystal systems. Sometimes,
it is called Bravais Lattices. So, any kind
of structures can be described by one of these
14 systems. So, any structure, any atomic
arrangement, what you can think of, are actually,
really, in reality whatever exists, they can
be described by these 14 bravais lattices.
Their relationship is given in the next slide,
how this a b c and alpha, beta, gamma are
related. In the triclinic system, a, b and
c, none are equal. So, the unit vectors are
different. So, they are completely different
and then alpha not equal to beta not equal
to gamma and none of them are 90 degrees.
So, it is not an orthogonal system metal at
all. So, the angle between any of the two
axis is really not 90 degrees different from
that and that is how a triclinic system is
described. It is a very least amount of symmetry
is available there in this kind of a system
and some of the compounds we will be discussing
about the formulae of these compounds later
on, but these are some ceramic systems like
kyanite, basically aluminum silicate, then
albite and feldspar. Feldspar we have mentioned
earlier. It is one of the major components
or raw material and is used white wares or
white clay products.
So, then we come to the second system. It
is called monoclinic. The diagram or the axis,
relationship of the axis is given in the previous
slide. Here it is, a not equal to b not equal
to c. Once again, the unit cell vector in
three different directions is not equal. They
are 
all different. Alpha and gamma are equal to
90 degrees, whereas, beta is not 90 degrees.
So, that is the relationship between the axis
as well as the angles in the monoclinic system.
Two of the most important examples are what
we call monazite. Monazite actually is a mineral,
which contains thorium, thorium phosphate
and orthoclase silicate. So, these have a
monoclinic structure.
Orthorhombic relationship is like this, a
not equal to b not equal to c, whereas, all
the angles 90 degrees. So, all the angles
are 90 degrees, but the axis unit vectors
or lattice parameters are different. Olivine
and brookite, they are again two different
silicate minerals, which have this kind of
a structure. Of course, these are only few
examples. There are many, many different compounds,
which may have orthorhombic structure or any
of these structures or symmetry structure.
Tetragonal, a equal to b not equal to c, but
in this case, again alpha equal to beta equal
to gamma equal to 90 degrees. All the angles
are 90 degrees. Only one axis is different.
a and b is equal, but c axis is not same as
a b.
So, the two components, which has this kind
of a structure is zircon. Zircon is basically
a zirconium silicate structure, z r o 2, s
i o 2 and rutile, we have already discussed.
So, rutile has basically a tetragonal structure,
tetragonal symmetry. Hexagonal, hexagonal
has a equal to b not equal to c. Similar to
tetragonal, but only difference here compared
to any other, we can see alpha equal to beta
equal to 90 degrees but gamma is 120 degrees.
So, it is a slightly different geometry than
any of the other one. So, hexagonal has this
kind of a relationship between their axis.
Sometimes, hexagonal is also described in
a slightly different way. Instead of a b c,
sometimes it is also described their as a
1, a 2, a 3 and c. Anyway, we are not going
to discuss the details of that, so high quart,
one form of silica, high quartz and beryllium.
Once again silicate minerals do have a hexagonal
structure and many elements like copper, do
have hexagonal symmetry. So, many of the metals
do have a zinc copper, zinc, sorry, I am making
a mistake, not copper. Copper is a F C C.
Zinc actually has a hexagonal structure. Copper
is a F C C structure.
Rhomohedron or sometimes is also referred
to as trigonal. It is called a b c. Here,
a equal to b equal to c and then alpha equal
to beta equal to gamma and none of them are
90 degrees. So, illuminate, it is another
mineral. It is actually iron titanate and
that is illuminate and calcite is calcium
carbonite. So, rhomohedral is the structure,
which is available in this kind of compounds.
Cubic is of course the simplest to describe
and the most symmetric material I think, where
a equal to b equal to c and alpha equal to
beta equal to gamma equal to 90 degrees. So,
there are many, many oxides. Only two of them
is given here, magnetite and garnet. Garnet,
again a general formulae, and there are many
varieties of garnets.
So, that is seven crystal systems and they
apply to any, whenever we are discussing the
crystal structure, they will follow, they
will one kind of systems will be fitted into
that structure.
Well, I now come back once again for the discussion
on the silicate structure. It is one of the
most exciting and interesting subject as far
as the structural features are concerned.
This I discussed last time. Basic unit of
any silicate structure or silica is s i o
4. That means, a silicon is coordinated tetrahedrally
by oxygen ions. This is a unit which actually
repeats itself in many different ways. So,
it has a unsatisfied bond of 4 or charge of
4, so s i o 4 with 4 minus. So, this is the
basic unit of any silicate structure and even
silica glass. Then they can be combined in
different ways. So, 2 silicon can share one
oxygen and the other oxygen’s can be available
for binding to other cations. So, silicon
is being shared or a oxygen is shared between
two silicon ions.
So, then this becomes one unit and the formula
will be s i 2 o 7 6 minus. So, it has 6 negative
charges, which can be satisfied by combination
with other cations other than silica. It may
be aluminum, it may be magnesium, or it may
be calcium. So, some of these things will
come and join and satisfy the unsatisfied
bonds here. It can form a ring of this nature.
Two silicon ions once again sharing a oxygen
and out of the 4 oxygen, 2 are actually shared.
Here, none of the oxygen’s are shared between
the silicon ions, whereas here, one oxygen
is being shared by 2 silicon ions. In this
structure, 2 of the 4 oxygen ions of each
tetrahedron is being shared by silicon.
So, there is a ring formation, what we call
a 3 member ring formation and the overall
charge, unsatisfied charge is 6 minus. So,
S i 3 O 9 6 minus. Then we have a slightly
bigger ring. Here, is a 6 member ring. Instead
of a 3 member ring, we can have a 6 member
ring and this can be a repeating unit for
the other cations to join in and form the
total structure. So here, once again 2 of
the oxygen ions are shared by silicon ions
and the overall group formula is S i 6 O 12
with 12 negative charges. So, more number
of cations can join in and form a much bigger
molecule.
Instead of a forming a ring, still 2 of the
oxygen ions can be shared by silicon in this
forming a chain. So, it is called pyroxene
structure and here again, two of the four
oxygen ions are being shared by silicon and
rest will be connected or joined to or bonded
to other cations. This is another structure.
Again, the basic unit is silicon oxygen tetrahedron
and you can see, it is what we call a double
chain. This is one chain and this is another
chain and then there is a kind of cross linkage.
Here, this oxygen is shared by silicon of
one chain with the silicon of other chain.
As a result, in this 4 oxygen of this particular
tetrahedral, all the 3 are shared. This is
shared, this is shared and also this is shared
between two neighboring silicon ions.
So, in this particular tetrahedron, all the
3 are shared, whereas next one only 2 are
shared because there is no cross linking.
So, cross linking is been done at certain
intervals. Not all of them or cross linked
between the chains. So, both kind of, 2 bridging
oxygen’s and 3 bridging oxygen’s are available
for the different tetrahedrons. So, effectively
2.5 out of 4 is actually shared and this particular
structure is called amphiboles. The unit formula
is about depending on what is the number of
tetrahedron in a chain, you will get a value
of . So, it is variable. So, there will be
different kind of structures or minerals having
different kinds of formula.
These are all summarized here. We have started
with this and then we have 2 silicon and oxygen
tetrahedral are shared. It has a three member
ring, six member ring, single chain and double
chain. So, the same thing has been summarized
in this case.
This is also another summary. Of course, an
additional structure is also there. The only
additional information is these are called
olivines in mineralogical name. Here, these
are all beryl. This kind of structure is available
in beryl, one kind of mineral, silicate mineral.
Then pyroxene and amphibole have been discussed.
The only new thing here is mica. This is sheet
structure. These are single tetrahedron, 3
4 6 rings, tetrahedron rings. 3 rings, 4 rings
and 4 rings will be not be discussed here.
4 ring is also possible. 6 rings we have discussed.
So, there are different kinds of rings. Then
we have single chain. This is one view of
that and this is other view. Then you have
double chain. That also we have discussed.
What we have not discussed so far is this
kind of a structure. So, this chain extends
indefinitely in, particularly in x and y directions.
So, it is horizontally extending in the form
of a sheet. So, sheet structure, this is what
we call the sheet structure of silicate. So,
we have many different forms of silicates.
We have olivines, we have beryl type of structures,
pyroxenes, amphiboles and mica. Mica is suddenly
a very useful product or useful mineral and
the structure is what we call a sheet structure.
Here, only 2 of the chains are joined together,
whereas in the sheet structure, a large number
of chains are getting joined together to form
a complete horizontal structure of silicon
oxygen sheet. So, that is another very interesting
property of silica and there are many varieties
of sheet structures. I will give some of them
and most important sheet structure silicate
is kaolinite or sometimes the compound is
called kaolinite and the mineral is called
kaolin. So, that is once again a naturally
occurring mineral and is very very useful
and one of the major components of traditional
ceramics. Most of the white clay actually
contains kaolin and when it is mixed with
some iron oxide and other things, it becomes
red.
But otherwise, it is a very white clay and
less is the amount of iron oxide and white
is the clay. So, this has a very important
structure and has been studied in quite detail.
The basic description is like this. It is
a sheet structure, so you have a sheet here.
It is along the vertical direction. This is
not the horizontal thing. Earlier, we have
seen a kind of planer configuration. This
is you are seeing from the top. So, this looks
like this.
But, if you are looking from the side, it
will look like this. So, it is a kind of civil
engineering terminology, elevation. So, this
is the sheet. These are silicon. There are
three oxygen on the side and one oxygen on
this side. So, one oxygen is protruding out
here in this and so there is, one of the oxygen
is on this layer, whereas 3 oxygen’s are
on this layer and in between, you have smaller
ions, the silicon ions. So, these are silicon
ions. So, this is total is the silicon oxygen
layer or silicon oxygen sheet. On top of that,
vertically above, you have another layer and
in this case, the kaolinite formula, the chemical
formula is actually hydrogen alumina silicate.
So, it has silica, S i O 2, A l 2 O 3 and
some hydroxyl ions. So, it is actually a kind
of interpenetrating layer between and S i
O 2. So, there are, this layer actually is
a layer of aluminum, aluminum hydroxide layer.
So, these ions, although they have not been
differentiated between this silicon and this
aluminum, these are all aluminum ions. These
are all aluminum ions and this layer is actually
aluminum hydroxide layer. So, these are hydroxyl
ions and this white ones are oxygen ions.
So, in this particular layer, there is both
oxygen and hydroxyl and another distinctive
feature of this structure is, aluminum is
having an octahedral coordination, whereas
silicon has a tetrahedral coordination. So,
aluminum has a octahedral coordination, but
not all the anions are oxygen. Only a part
of it is oxygen and partly hydroxyl. So both
hydroxyl and oxygen together form the octahedral
and in between, at the center of the octahedral,
aluminum ions sits. So, you have a interpenetrating
or a joint. Two layers are joined together
and one is the tetrahedral layer of silicon
oxygen and another is a octahedral layer of
aluminum, oxygen and hydroxyl. So, this is
the composition or this is the structure.
That is how you can describe the structure
of kaolinite. So, it is a layer structure.
A double layer of kind of thing; a silicon
oxygen layer and aluminum hydroxide layer,
two of them are joined together very strongly
and there is ionic bonding. So, these layers
are quite strongly bonded and totally it is
a sheet type like structure. So, this is the
description of kaolinite structure and mostly
they will be available in flaky form.
Next, we come to some few more sheet structure.
I just, I will not discuss in greater details,
but I will give you some idea, how this kaolinite
structure is different from other sheets structures.
This we have discussed. This is your silicon
oxygen, this layer and here is the silicon.
This is aluminum and you can see there are
6, totally 6. So, this is octahedron here.
This is tetrahedron. So, this is the aluminum
hydroxyl layer and here is silicon oxygen
layer. Now, this forms the unit cell. Then
in the vertical direction, this repeats. This
repeats itself. So, this, this, this and this,
are exactly identical. So, these layers are
kind of stacked one over the other and forms
a total volume or fills up the total volume.
So, this is the basic description of kaolinite
structure and this is our unit cell, from
the lower end of this unit to the upper end
of the next unit. That is your, what we call
the unit cell dimension in the c direction.
This is the c direction and the seat is in
the x and y direction or a and b directions.
So, the layer is about 7.2 angstroms and this
layer and this layer are bounded together
by van der walls bond. There is no other chemical
bond, except van der walls bond and so, this
bonds are very, very weak bonds. This is very
strong bond, because they have a ionic bonding
between these different ions. So, this is
difficult to separate. However, this van der
walls bond is very weak bond. So, this layer
can be separated out quite easily from the
other one. But, they do have some bond. So,
there are weak, very weak bond in the c direction,
but they are very strongly bond in x y direction.
So, these are the different layers, atomic
molecular layers of the kaolinite and that
is how it is formed. Compared to that, if
you go to mica, it is also sheet silicate
structure, but in the structure of mica is
quite different. Not quite different, it is
slightly different in fact. If you look at
this geometry and this geometry, you will
find this top portion here is the exact mirror
image of the bottom portion. This portion
and this position are exact mirror image of
each other. So, whereas up to this, is actually
kaolinite structure. It is very close to the
kaolinite structure. So, as if 2 silicon oxygen,
2 kaolin structures are inverted together
and bonded together. So, this is actually
a more symmetric structure. In the sense,
this side also you have silicon oxygen and
on the top side also you have a silicon oxygen
and in between, you have aluminum oxygen hydroxyl.
So, 1 aluminum hydroxide layer is basically
sandwiched between two silicon oxygen tetrahedral
layers. So, they are all seeds in the x y
z direction. This is extending x and y directions,
but in the vertical direction, c directions,
this is called the c directions, they are
actually repeating itself in an inverted manner.
So, unlike kaolinite, it is one layer of silicon
oxygen and over that, a aluminum hydroxide
layer. Whereas, in mica structure, what we
call the double silicate, double silicate
structure, we have one silicon oxygen and
then aluminum hydroxide is sandwiched between
another silicon oxygen layer. So, two silicon
oxygen layers sandwiching the aluminum hydroxide
layer, that forms the unit cell. So, you have
a much longer or a much longer unit cell.
Here about 10 angstroms and then that repeats.
Here, this unit repeats itself, where in case
of mica, this total unit repeats itself. So,
there are three layers. However, there are
some unsatisfied bonds.
For that, in between, you have some larger
ions. Here is, they are joined together by
only hydroxyl ions, sorry, van der waals bond
and so they are very weak bonds. Here of course,
it is not exactly van der waals bonds, but
in between, we have some positive ions. These
are alkali ions and they are replaceable ions.
Between the layers, they are replaceable ions.
These ions are normally alkali ions, like
potassium or sodium. Mostly potassium or sodium
and depending on whether it is a potassium
ion or sodium ion, you have two different
compounds or mica. In case of muscovite mica,
its potassium and sodium mica is also available.
So, these are the difference between two layers
or two different kinds of sheet silicate structures.
One is a kaolinite structure and other is
a mica structure. There are many varieties
of silicates and there is huge number of silicates
available and most of them are naturally occurring
and one can of course synthesize to some extent.
But, they are difficult to synthesize. So,
most of them are actually naturally occurring
silicate structures.
Well, we have discussed just now about the
silicate structures, where silica or silicon
is getting combined, silicon tetrahedron,
silicon oxygen tetrahedron is getting combined
with many other cations, but silica as such
is also very important ceramic raw materials,
S i O 2, where it is all silicon oxygen tetrahedron
only. So, this structure is sometimes called,
mostly called framework structure. So, in
all the three different directions, there
are only silicon oxygen tetrahedron and they
of course, have some crystalline formation,
so that, we have very definite crystal structures,
very definite geometric. So, these things
we will discuss in the next class, because
time is up and. Thank you so much.
