Hello everyone, welcome to fresh lesson in
an engineering geology.
Today we are going to discuss
about the chemical characteristics of minerals.
If you recall, last time around we talked
about, we
talked about geometric and the physical and
optical characteristics of minerals.
Today we are going to
have a look at the chemical features that
gives raise to those physical properties.
Before we begin with
the, with the subject matter of the particular
lesson, we are going to consider the question
set of the
previous lesson; I am going to give the answers.
.
The first question that I asked was on by
birefringence.
What is meant by birefringence?
Now,
birefringence is an optical characteristic
by which there is a double refraction of a
waveform of
electromagnetic waves that tries to cross
a chemical, a mineral crystal.
Now, the second part of the first question
was; whether you expect a halite crystal to
be birefringent?
Now, I said in the last lesson that if you
have got a crystal that is isotropic, then
it is not going to, it is
not going to exhibit birefringence and as
a result halite is also not going to be birefringence
crystal
because halite is an isotropic mineral.
.Now, the second question that was asked was;
you have given an XRD pattern for a sample
and you
were asked to identify one of the constitute
minerals of the sample for which the XRD pattern
was
provided.
Now, what I have done is I have put the XRD
pattern that was given to you in the last
question set on the left here and that is
shown, I am going to actually I am go to just
draw a line on the
top of the XRD pattern provided to you because
the line thickness was not adequate so that
the
resolution may not actually pick the input
XRD pattern.
Now, this is the XRD pattern that was given
to you.
So as usual, intensity is on the vertical
scale and on
the horizontal scale, we have got the angle
to theta that was explained in the last lesson.
Now, what I
have done here; I have put side by side the
XRD signatures of three clay forming minerals
on the plot
shown on the right side on the right bottom
of the slide here.
So, let me actually highlight here the
pattern that you would expect for a Montmorillonite
crystal and not for the Montmorillonite, actually
kaolinite crystal.
So, this is the signature of a typical kaolinite
crystal.
Now, what you can what you can see is that
for the constituent minerals, for the mineral
for the
unknown mineral that is provided to you, can
see there is a peek at about 12 degree value
of two theta
and in kaolinite also we have got a peek at
a similar angle and also you can see that
another intense
XRD signal was obtained at about 25 degree
value of two theta.
Now, for kaolinite also, we have got a
distinct signal at 25 degree value of 2 theta.
So from this, we can approximately say that
one of the
constituent minerals of this particular sample
is kaolinite.
So, that takes care of last questions set.
Now, we get back to the subject matter of
this particular, this particular lesson.
..
Now, what are the objectives of this lesson?
We are going to actually list a set, we are
going to list
major chemical constituents of the earth's
surface, we are going to identify the chemical
structure of
silicate minerals, carbonate minerals, oxide
and sulfide minerals and these are the major
building
blocks of the crust of the earth as we will
see in the course of this lesson and finally,
we are going to
identify the sources and chemical characteristics
of common minerals.
In other words, how these types
of individual minerals form; we are going
to briefly discuss those items.
..
Now, what is the composition, what is the
chemical composition of the surface of the
earth?
As we
have seen, the surface of the earth is called
crust of the geosphere and what is the constituent
that?
A
large part of the earth’s crust is composed
of oxygen and the second most abundant building
block of
the earth surface is silicon as is shown on
the pie chart of this particular slide; there
are several other
constituents.
The next most abundant one is aluminum and
then you have got iron, calcium, sodium, potassium
magnesium, titanium, hydrogen and finally
you have got the remaining chemical constituent
that are
clubbed together as others.
So, the most abandoned minerals are composed
of them silicon and
aluminum.
..
So, it is logical then to consider how the
structure, how we are going to, what kind
of chemical
structure we are going to get for silicate
minerals.
Now, in order to do that, what we are going
to do?
We are going to look at the building block
of the silicate minerals.
The basic building block of silicate
minerals is silica tetrahedral.
Schematically, it is shown on the right; you
can see that you have got a
silicon atom, you have got a silicon atom
near the center of the tetrahedral and at
the four corners of the
tetrahedral, there are oxygen, oxygen negative
ions in the corners of the tetrahedral, in
the corners of
the tetrahedral.
So, silica atom is at the center and there
are four oxygen ions at the corners of the
tetrahedral.
Now, this is a schematic; you should realize
that the cartoon that is shown on the right
of the slide is
actually schematic; in order to prepare this,
we did not consider the realistic size of
the atoms of silicon
and oxygen.
If you do that, then the picture that we are
going to get is kind of shown on the left
side of
the slide here.
..
This one is a depiction considering the geometric,
I mean considering the size of the individual
atoms
that is with which the silica tetrahedral
is composed.
Now, what we got here?
You can see that the
silicon ion is at the center or actually silicon
atom is at the center, this is the silicon
and then much
larger oxygen ions are shown like this; this
is the same thing, I mean the structure of
this one is same as
the tetrahedral but I just changed the relative
size of the ions here on the right because
it is difficult to
comprehend what is the exact look of the tetrahedra
from the figure that is shown on the left.
But the
figure on the left gives you an idea about
the relative size of the silicon ion of the
silicon atoms and
oxygen atoms.
So, then you can, to reiterate what we wanted
to say is that silicate minerals will be composed
of
different combinations of unit structures
and these unit structures are essentially
tetrahedral in shape
and they are composed of a silica ion at the
center and four oxygen ions at the corner
of the tetrahedral.
Now, let us look at how these individual building
blocks combine to form different types of
silicate
minerals.
..
Now, the first sub group of the silicate groups
of minerals that we are going to consider
here will be
single chain and double chain.
Single structure is shown on the left of the
slide here and here what we
got is you have got silica tetrahedra oriented
in an opposite manor, in an opposite manor
like this.
.
So, what happens here is that for each silica
tetrahedral, you have got two oxygen minus
ions is shared
with other tetrahedral.
So, what you got here is essentially a 1:3
combination of silicon and oxygen and
then you have got each of these chains, they
are going to actually combine with each other
with
.different types of cations.
Now, what happens here then; you have got
silica and three oxygens and the chemical
structure, I mean
the chemical formulation of a single chain
silicate mineral will be having a radical
which has got this
type of, which has got a composition of SiO3.
Now, let us consider what is meant by a double
chain.
Double chain minerals as shown schematically
on the right of this particular slide and
what you can see here is the three of the
oxygen’s here, actually
rather two oxygen of each tetrahedral is shared
again with other tetrahedra in the same chain.
But what
we got here which is unlike the previous one
is that another oxygen for the, another oxygen
here, they
are also shared with a opposite, with a sort
of like a mirror image of a change which falls
on the other
side.
So this chain, this chain here is combined
with another chain by sharing of oxygen at
atoms like
these.
So, it is obvious in this, it is obvious that
in this case you have got more number of oxygen
atoms
oxygen ions are shared amongst different silica
tetrahedra and as a result, the silicon to
oxygen ratio is
different in case of a double chain and here
the ratio between silicon and oxygen is instead
of 1:3 in
case of single chain; here, what we got here
is a 4:11 distribution of silicon and oxygen.
So, what you
got here is that you are going to get radicals
that are going to be Si4O11; this type of
radicals you keep
on appearing in the chemical composition of
double chain silicate minerals.
So, we are going to look at the examples,
individual examples of these types minerals
in due course of
this particular lesson.
..
Now, let us move on to the next types, other
types of silicate minerals.
You imagine that what we did,
we began with an individual silicate tetrahedral,
then we could combine this tetrahedra in a
combination of the single chain as we began,
then we got a double chain structures and
if we extend
this particular concept a little bit further,
then we are going to get a structure which
is known as sheet
silicate.
So, what do you got in sheet silicate is that
you have got a two dimensional network essentially
of, two dimensional network of silicon tetrahedral.
.
.So, here for instance, all this individual
triangles indicate silicate tetrahedra and
this particular
framework, these particular structure extents
horizontally both ways in the plain of the
tablet here as
well as vertically in those directions almost
indefinitely.
So, this particular type of structure is called
sheet silicate.
As the individual tetrahedra, they combine
with each other and they form individual
sheets.
So, here you can see more number of oxygen
ions are shared between different shade between
different tetrahedra and as a result, the
oxygen percentage, oxygen as a percentage
of silica will be even
smaller in comparison with a double chain
silicate mineral.
Now, a particular type of silicate mineral,
you will get a structure which is not in a
definite extension of
a two dimensional, two dimensional mesh in
a plain but it is a ring like structure as
shown on the right
here and for obvious reason, this type of
minerals is called ring silicates.
Here, you have got basically a
series of silica tetrahedra that combine with
each other to form a ring like structure.
These minerals are
relatively less abounded in comparison with
other types of silicate minerals that we have
so far
discussed.
Now, if we extend the concept of sheet like
structure, a step further and we consider
in three
dimensional of structure instead of a two
dimensional one, then we are going to end
up with the type of
silicate which is known as frame work silicate.
So, in this case, what you get is get tetrahedra
combined
with each other in a three dimensional sense
and you have got a solid form out of combination
of
silicate tetrahedra in three dimensional,
in all the three directions instead of the
two directions that we
consider in case of sheet silicates.
In addition to all this things, we can also
have we can also have a self-contained silica
tetrahedra.
So, in
addition to all the types we consider here;
we can have single tetrahedra, many type many
types of
minerals as we will see later on will be comprised
of single tetrahedra instead of a combination
of
several tetrahedra as we have considered here
or there are minerals which are composed of
double
tetrahedra and double tetrahedral minerals
or more rare than the other types of minerals,
silicate
minerals that we have discussed in this cash.
So, that actually gives you chemical structures
essentially
of different types of silicate minerals.
..
Now, let us consider individual rock forming
minerals which classify as different types
of silicate
structures that we just now discussed.
So, the first one that we are going to consider,
the simplest one is
the isolated tetrahedra.
So, in this case, what you got is you would
not have any oxygen sharing and the
structure and the chemical formula that you
are going to get for this types of minerals
is going to be
either X2SiO4 as in case of Olivine mineral
or X3Y2 (SiO4)3 as in case of garnet group
of minerals.
.
.By the way I want to actually mention here
that isolated tetrahedra are perhaps the most
stable and they
are the, perhaps they are the hardest and
the most stable minerals of the silicate;
the form the most
stable and hardest group of minerals of the
silicate family of minerals.
Now, you can see that the
olivine has got a hardness of 6.5 to 7 and
garnet also has got a hardness of 6.5 to 7.
Now, let us move on to the single chain.
In single chain we noticed earlier that you
have got one
oxygen share and the chemical formula of this
class of minerals typically will look like
this; XYSi2O6
and by the way you may have already understood
that by X and Y, I am indicating cations of
different
types like a calcium or magnesium or potassium
or iron or aluminum or there are could other
types of
cations as fell, these are just few examples
of abundant cations.
So here, what you will get is a chemical formulation
comprised of a chemical formula like XYSi2O6
and this one, this types of formulation is
typical of the pyroxene group of minerals
and these minerals
have got a hardness of 5 to 6 and notice that
these minerals cleave at 93 degree angle,
they cleave more
or less in orthogonal plains.
So, if you want actually shatter a pyroxene
group of mineral or rather if
you want to actually break a mineral composed
of single silica, single chain silica silicates;
then you
are going to cleave the minerals at in plains
that are almost orthogonal to each other.
This is important
as we will see when we consider a double chain
mineral.
.
.We have already seen that double chain mineral,
in case of double chain mineral you have got
a, you
have got 2 to 3 oxygen atoms shared between
different tetrahedra and an example of this
class of
minerals is Amphibole.
One of the minerals of the Amphibole group
is Hornblende.
A hornblende has
got a hardness of 6 and notice the cleavage
angle in this case, this particular types
of minerals actually
cleaves at 56 degree angle.
.
So, this is the angle of cleavage.
Now, you should actually notice that double
chain minerals, cleavage
of double chain minerals is very similar to
120 degree or 60 degree and this is in contrast,
this is in
contrast with a single chain, single chain
which we discussed just in the previous slide
which cleaves,
cleavage of single chain is approximately
equal to 90 degree.
So, single chain minerals if you break them
apart, then you are going to get orthogonal
cleavage plains,
almost an orthogonal cleavage plain.
But in case of double chain, you will have
cleavage plains that are
oriented with respect each other at 120 degree
or 60 degree angles.
Chemical formula in this case; it
will contain radical which is going to have
this type of structure; Si8O22 and this is
10 negative
electrical charge.
Then we move on to ring silicates and in case
of ring silicates, what we have already seen
that again
many of the oxygen actually, there is a mistake
here; two of the oxygen atoms are shared in
case of ring
silicates and one of the example of ring silicate
is Tourmaline and Tourmaline has got a hardness
of 7 if
.you recall.
Now, I would also like to bring to your notice
that these types of crystals also have got
a trigonal
symmetry.
We noticed in our previous lesson that if
you have got or if you consider a Tourmaline
crystal, then the tourmaline crystal can actually,
it has got threefold symmetry when you consider
a
tourmaline crystal.
Let me just bring to your memory, the shape
of the tourmaline crystal.
Actually, this
is going to be like this and here in the other
side, it is going to be like this.
So, this type of crystal has
got one symmetry access here, another access
here and another access of symmetry - the
drawing is
becoming awful - another access of symmetry
is going to be on that angle.
So, these accesses of symmetry are again oriented
at angles of 120 degree with respect to each
other
and if you remember; this one, the crystal
here is going to be a prismatic form and in
three dimensions,
it is going to have a look approximately as
shown on the sketch that I have just drawn.
So, that kind of
takes care of ring silicates as well.
Reverting back to the slides, now the next
group of silicate minerals is the sheet silicate
and in case of
sheet silicate, what you got is all oxygen
atoms in this case are going to be, all oxygen
ions in this case
are going to be shared.
..
Now, you remember, I indicated in the last
slide, there was a mistake in the last slide
in which I was
saying that in case of ring silicate, you
have got all oxygen ion shared but that was
not correct and in
case of ring silicates, you have got just
like sheets, just like, just like a two dimensional,
I mean double
chain, you have got two, you have got two
oxygen ions shared but here what we have got,
you have got
all oxygen ions shared in case sheet silicates.
And, a very obvious example of sheet silicate
is the mica and mica has got a hardness of
2.5 to 3 and
then finally, we move to the framework silicate
and aluminosilicates and in this case again,
you have
got all oxygen ions shared that means covalent
bonds formed out of all oxygen ions and an
example of
that is quartz - it is got a hardness of 7,
plagioclase feldspar - it has got hardness
of 6 to 6.5 and the
formulas of plagioclase feldspar are also
mentioned on the slide there; these are basically
calcium
aluminosilicates or sodium aluminosilicates
or potassium aluminosilicates.
So, that kind of brings us to
the end of different subclasses of silicate
minerals.
..
Now, we need to look at how different silicate
minerals form?
What is the origin?
How the silicate
minerals form in the nature?
So, one of the sources of silicate minerals
are formed by cooling of
magma or metamorphism.
So, this is the primary source, so most of
the silicate minerals are formed by
cooling magma or from metamorphism.
Now, weathering, deposition and digenesis;
that also is source
or could be sources of silicate minerals.
And as we will see later on; sand stones,
siltstone, clay stone and shale, they form
in this manner and
by weathering deposition and digenesis actually
chemical composition of minerals could also
change in
the process and the third source of silicate
minerals is chemical precipitation.
Now, precipitation of
amorphous silica from sea water is the mechanism
that is responsible for this, for formation
of flint and
chart type of minerals.
So basically then, you could for, you could
get silicate rocks formed out of cooling of
magma
metamorphism or from weathering and consequent
alteration of the chemical characteristics
of the
original silicate mineral and also you could
get silicate rocks formed out of chemical
precipitation in
case of flint and chart.
That ends the discussion on silicate minerals.
..
The second most important mineral group perhaps
is the carbonate mineral groups.
The key building
block in this case is the CO3 2- radical and
this is basically a plainer triangle and in
this case, what you
got is the carbon atom is at the center; in
this case, you have got the carbon atom at
the center.
.
So, what you got in this case is carbon atom
at the center and then you have got oxygen
ions, carbon
ion at the center and oxygen ion in the three
corners of the triangle.
So basically, this is the, this is the
carbonate type of structure.
So, if you recall, then each one of these
things have got two negative, two
.negative charge and carbon will have four
positive charges.
So, the balance of this is negative of – 2.
So, a carbonate ion has got a charge of negative
-2.
So, this a plainer structure as is indicated
on the
slide.
Now, what are the typical properties of the
carbonate group of minerals?
One of the very important
properties is varying degree of solubility.
It actually solubilizes in water to a varying
degree.
Now, it
also reacts with mild hydrochloric acid.
So, these are the two key properties of carbonate
group of
minerals.
What are the examples of carbonate group of
minerals?
The most abandoned mineral perhaps of the
carbonate group is calcite and the other example
of the
carbonate group is dolomite and also you can
think about several others like limestone.
Now, I would
like to also mention here that calcite has
got a chemical structure which has got a formula
of XCO3.
For
example, you have got, you could get calcium
carbonate.
This is perhaps the most abandoned one of
that group or you could even get calcium,
magnesium carbonate which is actually of the,
which is
called the dolomite.
Now, aragonite is another mineral that also
has got a chemical formula of calcium carbonate,
I mean it
is the same think as CaCO3 same as calcite.
This also has got the same chemical formula
as calcite but
the structure, the structure or the geometry
of crystals of calcite and aragonite are totally
different as we
will see later on.
So, this type of, this type of different minerals
which is got the same chemical formulation
but different
physical appearance or different crystallographic
characteristics, this is called, this type
of mineral is
called polymorph.
So, aragonite is essentially a polymorph of
calcite.
Aragonite is mineral that is more
rare actually then calcite and that forms,
aragonite forms on the high pressure but low
temperature
environments.
..
What are the origins of carbonate minerals?
Carbonate minerals have got a biogenic origin
or it could
be deposited from the calcite particles from
pre-existing rocks by the action of water.
So, water can
solubilize calcium carbonate from pre-existing
rocks and you can deposit that through inorganic
process and that also could give rise to calcite
rocks.
Dolomites actually forms from a process that
is
known as dolomitization and this is basically
by replacement of calcium ion with magnesium
ion of the
pre-existing rock or from direct deposition
of water that is rich in magnesium ion.
So basically, carbonate minerals, they originate
from precipitation process; it could be either
organic, it
could be either an organic precipitation process
or it could be an inorganic process from sea
water or
from the action of water basically.
..
Now, as I indicated earlier, I gave you the
example of two polymorph crystals, 2 polymorph
minerals;
calcite and aragonite and these two types
of minerals is shown schematically here on
this slide.
You can
see that the one on the left, the one on the
left is of mineral calcite and this has got
a trigonal symmetry,
this is got a trigonal symmetry or 3 fold
symmetry really; whereas when you consider
mineral
aragonite, then you have got a 2 fold symmetry
only and this is essentially a rhombohedral
structure
which was discussed briefly in our previous
lesson.
.
.Now, basically in this case, what you have
got is the hardness of calcite is 3, whereas
hardness of
aragonite is 3.5 to 4 and I indicated the
crystallographic symmetry and it is also mentioned
here.
I also
indicated that aragonite mineral is more rare
than calcite, calcite is more abundant and
now the
question comes that and actually I also would
like to mention that this type of mineral,
actually
aragonite in a sense is an unstable mineral
under the normal condition of temperature
and pressure.
Then the question comes; how you get this
aragonite mineral at all?
Now, aragonite minerals actually
forms from calcite - could form actually,
could form - that is one of ways aragonite
minerals could
form is from the metamorphism of calcite rich
rock.
So, if you expose calcite reach rock, in a
high
pressure environment, you could end up with
a rock that is rich in aragonite mineral.
Now, that actually gives you an example also
of how different types of minerals change
in chemical
characteristics when you expose them to different
types of pressure temperature environment
and we
will examine in this one in more detail when
we consider metamorphism later on in this
course.
Now, the third example is given here is of
dolomite and dolomite is got a hardness of
3.5 to 4 and this
is got trigonal rhombohedral symmetry and
just actually quite compare, actually trigonal
rhombohedral
symmetry and the symmetry in this case is
quite comparable to the symmetry of calcite
as indicated on
the left, on the left most cartoon on this
slide.
..
Now, another mineral group that is also quite
abundant is formed of different types of oxides.
Now, as
the names suggests; in this case, you will
get oxygen ions O -2 anion present in the
chemical structure.
Examples of this class of minerals are oxides
of iron, hematite and magnetite.
Now, hematite has got a
chemical composition, hematite has got a chemical
composition that is composed of trivalent
iron,
whereas magnetite has got a chemical composition
that has got a bivalent as well as trivalent
iron in the
chemical structure.
So, they are essentially, they are very similar;
they are very similar to each other in a sense.
Let us look
at this thing because that could be of interest.
Now, hematite has got a dark red to blue black
in color,
very dark color in fact, it has got a deep
red streak, the specific gravity of hematite
is 5 to 6.5, hardness
is 6.5 and if you compare these properties
with magnetite; what you get is again a grey
black color
which is again a dark color, it is got a black
streak, a deep colored streak again quite
in comparison
with hematite and then it has got a hardness
of 6.5 to 6, then again the hardness is again
not very
different from the hardness of hematite, slightly
reward perhaps and it has got a specific gravity
of 5.2
that also well within the range these specific
gravity of hematite mineral.
Now, the basic difference that you get in
these two cases is that hematite exhibits
magnetism after
being heated.
Hematite in fact is weakly magnetic, whereas
magnetite is strongly magnetic and hence
the name of that particular mineral.
So, the very, the most obvious way of classifying
hematite mineral
from magnetite mineral is by looking at the
magnetic properties of these two types of
minerals.
..
Another relatively abundant group of minerals
are called sulfide minerals and here you have
got
bivalent sulfide, bivalent sulfide anion or
sulfide anion and these things, examples of
these things are
pyrites and galena.
So, if you recall, we looked at the crystal
structure of pyrite and galena in the
previous, in the previous lesson.
Pyrite in a sense is ferrous sulphate and
here what we got is a bivalent iron, bivalent
iron, ferrous
sulfide and galena is essentially lead sulfide.
Pyrite if you recall, it is got a golden color,
this thing also
call fool's gold because of this.
It is not a precious metal but it has got
an appearance which is similar to
gold, it has got greenish black streak, specific
gravity of 5 and hardness of 6.5.
By the way, the hardness that I am talking
about in this lesson is in the mho scale which
we discussed
in this previous lesson.
Galena on the other hand, it has got a dark
red color, dark grey streak, it is got a
hardness of 2.5 and specific gravity of 7.5.
..
Now, we look at the origins of oxides and
sulfides mineral, oxides and sulfides.
The oxides are formed
out of igneous or metamorphic rocks from volcanism
which is the source of many oxide minerals
and
iron oxide by the way, iron oxide forms out
of chemical precipitation.
Some of the ferrous minerals
also form because of biogenic action quite
in similar manner as of a biogenic carbonate
mineral.
Then
sulfides, sulfides actually form out of igneous,
out of volcanism or metamorphism or it could
be formed
out of mobilization of sulfide minerals and
redeposition through chemical action, chemical
weathering.
.
.Now this one, this cartoon here, this slide
here shows a few examples of unit sulfide,
unit crystals of
sulfide minerals, oxide and sulfide minerals.
Now, hematite we discussed; it is an oxide
mineral and it
is got an unit crystals that is shown on the
left.
Magnetite, it is again an oxide mineral composed
of
bivalent and trivalent iron and the crystal
is shown in the middle here and a sulfide
mineral galena is
shown on the right and we looked at this particular
mineral in detail when we were considering
the
geometric characteristics of mineral last
time in the last lesson.
.
So finally, summarizing what we learned in
this lesson; we discussed the chemical composition
of the
earth's crust, we discussed the chemical composition
of silicate, carbonate, oxide and sulfide
mineral,
we considered examples of these minerals and
these minerals composed a very large proportion
of
earth's crust, we looked at a list of special
characteristics of classes of minerals that
arise because of the
chemical composition.
For example, we looked at the cleavage angles
of a double chain and single
chain a silicate minerals.
.
.And, we end finally this particular lesson
with a question set and try to formulate the
answers of these
questions.
The first question is that what is meant by
polymorph?
Give an example of a polymorph
mineral of pure carbon, pure carbon.
Now second thing, second question that asked
is can you provide a reason for mechanical
strength of
quartz?
You give a quality reason why quartz is so
strong and in art.
How would you distinguish
hematite and magnetite minerals is the third
question.
Fourth question is how would you distinguish
olivine and quartz minerals and finally can
you provide a reason for acidity of subsurface
soils
containing sulfide minerals.
In fact, the ore bodies that contain sulfide
minerals that is actually a very common source
of mineral of
different economic minerals, sulfides are
very common source of several different economic
minerals
and these areas often affected by acidity
and consequent loss of fertility within the
area where the
waste, mine waste is dumped.
Now, what I ask here is that can you give
the reason why subsurface soils containing
sulfide minerals
would exhibit acidity?
Try these questions in your spare times and
I will also give you my versions of
the answers when we meet next time for the
next lesson.
Thank you very much.
.
.Hello, everyone and welcome to a fresh lesson
on the engineering geology.
Today we are basically
going to talk about origin and types of rocks
and also we are going to look at the different
types of
rocks that as commonly available, normally
found across the country of India.
But before we actually
proceed with the subject matter of today’s
lesson, we are going to consider the questions
that were
asked the last time when we met.
This is the question set that you already
have.
(Refer the slide time 1:15)
.The first question was; what is meant by
polymorph and I also asked you to give an
example of a pair
of polymorph of pure carbon mineral.
Now, polymorph is actually are a bunch of
minerals which have
the same kind of chemical composition or chemical
formula but they have got different
crystallographic geometry or different structure
really.
Now, two common two polymorphs of pure carbon
mineral are graphite which is essentially
a sheet,
which has got a sheet structure like what
we had for sheet silicates and then you have
got diamond
which is basically which has got a pair of
tetrahedra…(.)
((… Refer Slide Time: 55:55)) essentially
volcanic rocks and metamorphic rocks that
generate out of
volcanic rocks.
(Refer slide time: 55:54)
So, that in a nutshell gives you the inventory
of different types of rocks found across our
country.
((…Refer Slide Time: 56:18)) is still used
in getting inside about the rock forming processes.
What
Bowen proposed was the fact that minerals
actually change their characters as the temperature
comes
down and there are two classes of minerals;
one along the discontinuous series shown on
the left here,
these are the ones, this is the class of minerals
on the discontinuous series and then you have
got
another class of mineral that form a continuous
series as ((Audio not clear Refer Slide Time:
57:19))
.((… Refer Slide Time: 57:28)) essentially
volcanic rocks and metamorphic rocks that
generate out of
volcanic rocks.
(Refer slide time: 55:54)
So, that in a nutshell gives you the inventory
of different types of rocks found across our
country.
(Refer slide time: 58:25)
.To summarize the lesson then, what we learnt
in this lesson include the definitions of
general terms all
rock and soil, we have discussions on rock
forming processes, we also looked at a list
of main rock
forming minerals and examined their susceptibility
weathering characteristics or susceptibility
to
weathering.
We looked at different types of rocks; volcanic,
metamorphic and sedimentary, looked at
rock cycle and looked at rocks that are found
in different parts of our country.
(Refer slide time: 56:09)
.That brings us to the end of this presentation,
so we close with the question set which you
should try
answering in your spare time.
The first question is that how in general
would you expect the percentage
of oxygen to change in a silicate mineral
with weathering?
Second question is which of the two
minerals abundant on earth’s crust would
be more resistant to weathering; quartz or
feldspar?
Third
question is what is meant by digenesis and
fourth one is how magma can remain in a partially
molten
state.
Try answering these questions; I will help
you out with the answers when we meet in the
next time.
So,
that ends this lesson and thank you very much
for listening to me.
So, until I meet you next time
around, bye for now.
.
