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1. Introduction of minerals and crystal

2. Physical properties of minerals
3. Crystallographic axes and angle, crystal system
4. Rock forming minerals and their engineering
significance

 A Mineral is a naturally occurring usually

abiogenic solid substance that is characterized with
specific chemical composition and often with a
definite atomic structure.

 A rock is an aggregate of minerals or non-minerals

and does not have specific chemical composition.

 A mineral that occurs solely as a rock in the nature,

is called rock forming mineral.

 The study of minerals is called as Mineralogy.

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Gold

Examples:

Azurite Hematite

Examples:

Calcite

Pyrite

Amethyst

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 A crystal is a solid material whose constituent

atoms, molecules, or ions are arranged in an
ordered pattern extending in all three dimensions.

 A crystal is bounded by smooth geometrical

surfaces called faces with specific and
characteristic orientations.

 Crystallography is the scientific study of crystals

and crystal formation.

Pyrite
 Examples (Cubic)

Quartz
Mica (Tabular) (Tubular)

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• Minerals are identified with their different

properties such as
Physical properties - such as color, luster, shape,
form, etc.

Optical properties – behavior in presence of light,

such as twinning, anisotropy

Chemical properties – characteristics of mineral

based on chemical analysis

1. Color
2. Streak
3. Luster
4. Transparency
5. Hardness
6. Cleavage and Parting
7. Fracture
8. Tenacity
9. Specific Gravity
10. Form /Habit

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 Color is the most obvious property of mineral but

it is often non-diagnostic.

 Streak is the color of mineral in its powder form.

 Color of mineral is determined by the impurities

included in it.

Tourmaline in different colors

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 Luster of mineral indicates how the light reflects

from the surface of mineral.
 Luster can be categorized as metallic an non-
metallic.
 Metallic luster – e.g., pyrite and galena
 Non-metallic luster :
Adamantine – sparking as in diamond
Vitreous – glassy as in quartz and most of silicates
Pearly – such as in Talc
Resinous – such as in garnet
Silky – common in fibrous minerals such as asbestos

 Examples:

Metallic (pyrite) Adamantine (diamond) Vitreous (amethyst)

Pearly (talc) Resinous (garnet) Silky (asbestos)

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 Transparency of a mineral describes the ability of

light passing through the mineral.

Transparent – e.g., muscovite

Translucent – e.g., jadeite, nephrite

Opaque – e.g., hematite, pyrite

Transparent
(muscovite)

Translucent
(jadeite)

Opaque
(hematite)

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• Hardness of a mineral shows how much it can resist

scratching. It is controlled by the chemical composition
and crystalline structure of a mineral.
Mohs hardness Mineral Chemical formula
1 Talc Mg3Si4O10(OH)2
2 Gypsum CaSO4·2H2O
3 Calcite CaCO3
4 Fluorite CaF2
5 Apatite Ca5(PO4)3(OH,Cl,F)
6 Orthoclase KAlSi3O8
7 Quartz SiO2
8 Topaz Al2SiO4(OH,F)2
9 Corundum Al2O3
10 Diamond C

 Cleavage is a plane in mineral

which is parallel to the crystal
face. It develops due to weakness
in the crystalline structure and
the minerals can easily break
down along this plane.

 Parting is a false cleavage. It is similar in appearance to

cleavage but is instead produced by structural defects.
Parting varies from crystal to crystal of same mineral while
the number of cleavages remains same for all the varieties of
one minerals.

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 Fracture is a plane where a mineral breaks down

because of external forces.

Even – smooth fracture follows the cleavage plane (chert)

Uneven – irregular fracture planes with very small
ridges and depressions (fluorite)
Conchoidal – shows smooth concentric curved lines
(quartz)
Hackly – broken surface is highly irregular (native
copper)
Splintry – rough wooden fracture (kyanite)
Earthy – fracture surface is smooth, soft and porous
(chalk)

• It is the behavior of mineral that is experienced while

breaking, bending, cutting, or crushing.

Brittle – can not resist the external force and breaks like
glass (quartz)
Ductile – can be bend easily and it can be made to thin
wire (gold)
Sectile – can be cut in small pieces smoothly (copper)
Malleable – can be made in different forms easily
(copper, gold)
Flexible – if mineral can be bent in any shape (Chlorite)
Elastic – shows elastic nature (asbestos)

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• It describes the density of a mineral.

Density = mass / volume

Specific gravity of a mineral =

weight of mineral in air/weight of mineral in water at 4oC

High specific gravity is a diagnostic property of a mineral,

for e.g., hematite (sp. gr.=5.26) , galena (sp. gr.=7.2-7.5)

• It defines the naturally occurring form of a mineral.

Crystalline – a mineral in crystal form

Amorphous – non crystalline / powdered form

Structural form :
Tabular – occurs in the form of a flattened (Barite)
Elongated – column-like crystals (quartz)
Bladed – blade-like form (Kyanite)
Lamellar – thick, flexible, leaf-like sheets (Vermiculite)
Foliated – thin, like sheet of paper (muscovite)

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Fibrous – composed of fibers (asbestos, gypsum)

Radiating – needle like crystals (pyrites)
Granular – occurs in form of small grains (magnetite)
Globular – looks rounded, bulb-like forms (hematite)
Reniform – shapes as human kidney (hematite)

Optical Properties of Minerals

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Optical Properties:
Optical properties of minerals describe the interaction of
polarized light rays with the minerals. These properties are
viewed in either plane polarized light (using only the lower
polarizer) or cross-polarized light (using both polarizers).

1. Colour and Pleochroism:

Many minerals may present a variety of colors, in the same or
different rocks, or even multiple colours in a single mineral
specimen called colour zonation. And pleochroism (color
change of grain as stage is rotated)

Refractive Index and Relief:

The speed of light slows down when it enters a
material from air. This change is described by the
material’s refractive index. Relief is a measure of
the relative difference between a mineral grain and
its surrounding
Extinction:
When you rotate the stage à extinction relative to the
cleavage or principle direction of elongation is
extinction angle; Parallel, inclined, symmetric
extinction

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Twinning and Extinction Angle:

Twinning is characteristic in thin section for several
common minerals – especially feldspars
The twins will go from light to dark over some angle
This is characteristic of the composition
Stage of the petrographic microscope is graduated in
degrees with a vernier scale to measure the angle of
extinction precisely.
Cleavage:
Parallel cracks in mineral related to crystal structure,
often diagnostic of a mineral. In thin sections cleavage is
developed during grinding of thin section.

Fracture:
Irregular cracks not related to atomic structure e.g. olivine

Inclusion:
If dots of other minerals are present in the mineral under
microscope, inclusion are present otherwise absent.
Isotropism:
If the minerals look dark in all times while rotating the
stage, such minerals are called isotropic minerals. E.g
Garnet.
Shape:
The minerals show commonly three kinds of shapes.
These are: Euhedral, Anhedral and Subhedral.

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Classification of crystals according to the degree of

crystallization

(i) (ii) (iii)

 Euhedral: Well-formed, with sharp, easily recognized faces

 Subhedral: Showing a less than well developed crystal with
respect to forms and symmetry
 Anhedral: A crystal with no plane faces.

Crystal Faces
Interfacial Angles
Crystallographic Axes
Axial Ratio

 Crystal Faces: A crystal is composed of

one or more than one types of regular
geometrical surfaces, called crystal faces.
 A crystal may have two to forty-eight
faces of similar geometrical shape.

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 The angle made by two adjacent faces is called

interfacial angle.

d
c b
a

b c
a d

 Symmetry of a crystal defines the geometrical

regularity on the body of crystal. If a crystal is said to
be symmetrical, its one half shows the mirror image of
other half.
 There are three different types of symmetries:
1. A plane of symmetry
2. An axis of symmetry
3. Centre of symmetry

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 If an imaginary plane passing through the center of a

crystal divides the crystal into two exactly similar
halves, that plane is called a plane of symmetry.
 These two halves of crystal are mirror image of each
other.

horizontal vertical diagonal

 It is an imaginary line in a crystal passing through its

center along which if the crystal is rotated by 360o, a
certain crystal face appears at the same position at least
twice.
 Two-Fold Symmetry – a crystal is said to have two-fold
symmetry, if it needs 180o of rotation to get similar face.
 Three-Fold Symmetry – It requires that a crystal must
be rotated by 120o to get the identical of reference face.
 Four-Fold Symmetry – In this, identical faces appear in
a crystal after 90o of rotation.
 Six-Fold Symmetry – in every 60o of rotation, an
identical face to the reference face comes.

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 A crystal is said to have center of symmetry, if an

imaginary line passing through its center meets exactly
similar faces or similar edges in two opposite directions
at equal distance.
 All crystals may not show a plane of symmetry and an
axis of symmetry, but they have center of symmetry.

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 A crystal form is a set of crystal faces that are

related to each other by symmetry.
Some of the important types of forms are describe
below:

Pedion: It is an open form made up of a

single face

It is an open form made up of two

parallel faces

Dome: It is an open form made up of two

nonparallel faces symmetrical with respect to
a symmetry plane
Sphenoid: It is an open form made up of two
nonparallel faces symmetrical with respect to
a 2- fold or 4-fold symmetry axis

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Disphenoid: It is an closed form composed of a

four-faced form in which two faces of the upper
sphenoid alternate with two of the lower sphenoid

Bipyramid: It is an closed form composed of

3, 4, 6, 8 or 12 nonparallel faces that meet at a
point

Orthorhombic bipyramed

Hexagonal bipyramid

Prism: It is an open form composed of 3, 4, 6, 8

or 12 faces, all of which are parallel to same
axis.
Orthorhombic prism Tetragonal prism

Ditetragonal prism

Dihexagonal prism
Hexagonal prism

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Rhombohedron: It is an closed
form composed of 6
rhombohedron faces.

Scalenohedron: It is an closed
form composed of 12 faces, each
face is a scalene triangle.
There are three pairs of faces
above and three pairs below in
alternating positions

Crystal Habit is a description of the shapes and

aggregates that a certain mineral is likely to form.
Acicular: Thin, sharp and slender
needle like crystal

Bladed: Blade-like, slender and

flattened e.g. Kyanite

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Foliated/ lamellar: Layered Fibrous: Extremely

structure, parting into thin slender prisms
sheets

Prismatic: Elongate, prism-like:

well-developed crystal faces
parallel to the vertical axis

Reticulated: Crystals
Tabular: More elongated than
forming net-like
equant, slightly longer than
intergrowths
wide, flat tablet-shaped

Stalactitic: Forming as
stalactites or stalagmites;
cylindrical or cone-shaped

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 All the crystals with similar symmetry are grouped into

one group, which is called a crystal class.

 Based on the characteristics of crystallographic axes, all

the crystals are categorized into six groups, called
crystal systems.
1. Isometric (Cubic) System
2. Tetragonal System
3. Orthorhombic System
4. Monoclinic System
5. Triclinic System
6. Hexagonal System

 If the crystals have following characteristics, they

are said to belong to Isometric or cubic system:
a. All three axes are essentially equal in length
b. Angle between every two axes is 90o
c. All three axes can be mutually interchangeable
a3+

a1-

a2- a2+

a1+

a3-

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 The crystals falling under Tetragonal System follow

the following properties:
a. Two axes are equal in length, horizontal, and
interchangeable
b. The third axis is vertical and is either longer or shorter
than the other two horizontal axes
c. All three axes are mutually perpendicular c+

a1-

a2- a2+

a1+

c-

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 The Orthorhombic System includes the crystals

which show following characteristics:
a. Axes are essentially unequal to each other in length
b. All axes are perpendicular to each other

c+

a-

b- b+

a+

c-

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 The crystals falling under Monoclinic System have

following characteristics:
a. Axes are essentially unequal to each other in length
b. One of the interfacial angle is not right angle, while
other two are equal to 90o
c. One crystallographic axis is always inclined.
c+

a-

b- b+

a+
c-

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 Crystals that have following properties are grouped

under Triclinic System
a. All crystallographic axes are unequal in length
b. Axes are inclined in various angles to each other

c+

a-

b- b+

a+
c-

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 If the crystals have following characteristics, they

are said to belong to Hexagonal system:
a. Three axes are horizontal, equal in length,
interchangeable, making angle 120o with each other
b. Forth axis is vertical and perpendicular to all three axes
c. Forth axis is unequal to other three axes.
c+

a3+ a1-

a2- a2+
a3-
a1+

c-

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 There are two divisions of Hexagonal System

1. Hexagonal Division
2. Trigonal (or Rhombohedral) Division

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 There are only few minerals (less than 100) form

the great amount of the rocks of the earth crust.
These most common minerals have been grouped
as rock forming minerals.

 Among these rock forming minerals, only about

25 minerals make almost 99.5% of the rocks.

 There are 10 groups of these most common rock

forming minerals.

About 8% of the earth crust is made up of silicate

minerals.
1. The Feldspar Group
(Na, K, Ca)AlSi3O8
2. Pyroxene Group
(Ca,Na,Al,Li)SiO3
3. Amphibole Group
(Ca,Mg,Fe,Mn,Na,K)7(Si4O11)(OH)2
4. Mica Group
(K,Na,Li,Mg,Fe)Al3Si3O10(OH)2

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Oxide minerals are second large occurring rock

forming minerals.
1. Quartz (SiO2)
It is hard and resistant to weathering. It has no cleavage and
partings. It has different colored varieties that have economic
importance.
2. Corundum (Al2O3)
It is very hard and resistant mineral. It has a number of gem
varieties.
3. Spinel (MgAl2SiO4)
It is also hard as quartz and resistant.

There are a few carbonate minerals, which are very

important rock forming minerals:
1. Calcite (CaCO3)
Limestone is composed of high percentage of calcite. Marble is
recrystallized form of calcite.
2. Dolomite [Ca,Mg(CO3)2]
It occurs in massive forms with extending for kilometers
across. It contains a significant amount of magnesium and can
be used economically.
3. Magnesite (MgCO3)
It is useful as refractory material and source of magnesium.

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 Topographical controls

 Strength of rock forming minerals

 Weathering characteristics

 Topographical controls
 Erosion rate of rock forming minerals
 Topographical breaks - steepness or smoothnes
 Karst Topography – controlled by Limestone
 Groundwater control

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 Strength of Rock Forming Minerals

 Hard and soft nature of different minerals
 Engineering design and site investigation controlled by
strength of minerals
 Quality of construction materials are controlled by
strength of minerals

 Chemical Weathering of Rock Forming Minerals

Olivine
Anorhite (Ca-Feldspars)
Pyroxene
Amphibole
Albite (Na-Feldspars)
Biotite
Orhoclase (K-Feldspar)
Muscovite
High stable
Clay Minerals
Quartz
Fe-Oxides

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 What is mineral? Describe the physical properties of

mineral.
 Explain Engineering Significance of Rock forming
Minerals.
 Define Moh’o Hardness scale. Describe elements of
crystals.
 What are optical properties of minerals? Describe
crystal system.

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