MINERALOGY

INTRODUCTION
• Mineralogy is a branch of geology which deals with
various aspects of minerals such as their individual
properties, their chemical composition etc.
• A mineral is a naturally occurring inorganic solid
substance characterised with a definite chemical
composition and a definite atomic structure. The
minerals generally occur in the form of natural
aggregates called rocks which make the crust of the
earth.
• The quality of rocks depends on their mineral
composition.

SIGNIFICANCE OF MINERALOGY IN
CIVIL ENGG
• Since all civil engg projects are constructed on
the surface of the earth which is made up of
rocks but rock is a aggregate of minerals so it
is essential that a civil engineer should have
the sound knowledge about the minerals.
• The competence, strength and quality of the
rock depends on the composition of the
mineral components.

PHYSICAL PROPERTIES OF MINERALS
 Forms
 Colour
 Streak
 Luster
 Cleavage
 Fracture
 Hardness
 Specific gravity

Forms
• Depending on the internal structure of the minerals
they have different forms.
1. Fibrous- e.g. Asbestos
2. Columnar- e.g. Hornblende
3. Granular- e.g. Chromites
4. Crystalline- e.g. Quarter
5. Massive- e.g. Feldspar
6. Bladed- e.g. Kainite
7. Foliated- e.g. Mica
8. Tabular- e.g. Calcite
9. Reniform- e.g. Hematite

1. Isolated individual
mineral crystals
a. bladed-elongated
flattened crystals
looking like a knife
blade
b. acicular-thin needle
like crystals
c. capillary-hair like or
thinner

Groups of distinct crystals
a. dendritic-
resembling small veins
on a tree leaf
b. radiated-crystals
appearing in a radial
pattern
c. drusy-a surface
containing very small
crystals

Groups of distinct crystals
occurring in parallel or
spherical form
a. columnar-column
like crystals
b. bladed-many flat
bladed crystals
c. fibrous-parallel fibres
d. colloform-includes
botryoidal ( resembling
a bunch of grapes),
reniform (kidney-
shaped masses)

Aggregate of scales or
lamellae
a. foliated
(micaceous)-easily
splits into thin leaves
or sheets
b. plumose-scaly-
featherlike structure

Colour
Colour in minerals is caused by the absorption, or lack of absorption, of various
wavelengths of light. Most minerals have a distinctive colour that can be used
for identification. In opaque minerals, the colour tends to be more consistent,
so learning the colours associated with these minerals can be very helpful in
identification. Translucent to transparent minerals have a much more varied
degree of colour due to the presence of trace minerals. Therefore, colour alone
is not reliable as a single identifying characteristic.

Colour......................
Most minerals are
coloured by a limited
number of metals
present as impurities.
The most common
elements affecting
colour are:
chromium, iron,
manganese, titanium
and copper.
It is chromium which produces the
intense red of ruby and the
brilliant green of emerald.
Minerals tend to
occur in a range of
colours, and colour
patterns which help
to identify them

Streak
• A mineral's streak is the colour
it has when ground to a
powder. Some minerals that
occur in a range of colours
always have the same streak,
thus streak is considered a
more stable indicator. The
great majority of minerals
have a white streak, but there
are a few well-known minerals
for which the streak is an
important property.

Lustre
• Lustre is a description of the way light interacts with the
surface of a crystal. Broadly it can be divided into two-
Metallic and Non-metallic lustre.
• Metallic: very high reflectance, opaque
• Sub metallic: medium reflectance, opaque

• Adamantine: very
high reflectance,
transparent, very
gemmy crystals
• Glassy (Vitreous):
high reflectance,
transparent or
translucent
Resinous: medium reflectance, translucent,
the look of resins such as dried glue or
chewing gum

• Waxy: medium
reflectance,
translucent or
opaque, the look of
resins such as dried
glue or chewing gum
• Pearly: low
reflectance,
translucent or opaque
• Dull: no reflectance,
opaque, just a non-
reflective surface of
any kind
• Earthy: the look of
soil or clay
turquoise blue
botryoidal smithsonite

Some other Lustres
Silky Oily
Crystalline matte

Cleavage
• Cleavage is a plane of weakness of a mineral along which
it breaks easily producing a smooth plane. It may be
described as perfect, excellent, good, poor, etc. Mica is a
good example – breaking along very closely spaced flat
planes that yield thin "sheets."

Weak Bonding Yields Planer Cleavage

Amphibole Cleavage ~120/60°

Fracture
• Fracture describes the nature of its broken
surface which is not parallel to cleavage plane.
• All minerals break but fracture describes a
break when the resulting surface is not
smooth and flat.
• A fresh fractured surface shows the true
colour of the mineral.
• Different types of fractures are even, uneven,
hackly, splintery, conchoidal etc .

• A few specific types of
fracture are:
• 1. Conchoidal- a smooth,
curved breakage—quartz
• 2. Fibrous or Splintery
• 3. Hackly-jagged with
sharp edges
• 4. Irregular-rough
surfaces

Hardness
• It is the resistance offered by
the mineral to abrasion. This
physical property is controlled
by the chemical composition
and crystalline structure of a
mineral.

Specific Gravity (Density)
• Density refers to the mass per unit volume. Specific Gravity is
the relative density-
• weight of substance
the weight of an equal volume of water
• In cgs units density is grams per cm3, and since water has a
density of 1 g/cm3, specific gravity would have the same
numerical value as density, but no units. The weight of the
equal amount of water is found by finding the difference
between the weight of the mineral in air and the weight of
the mineral in water.
• The average specific gravity for a metallic mineral is about 5.5,
while 2.5 is the average for a nonmetallic mineral.

Polymorphism and polymorphs
• Substances having the same chemical composition
but different crystal structures.
– e.g. diamond and graphite
• Both minerals are composed of pure carbon, but
diamond is the high pressure polymorph of graphite.
• This gives rise to extremely different physical
properties.

Diamond vs. Graphite Crystal Structures
From: http://www.molecules.org/elements.html#diamond
Hardness: 10 Hardness: 1-2

STUDY OF PHYSICAL PROPERTIES OF
MINERALS
1 Crystal system Monoclinic
2 Colour Colourless
3 Form and habit Laminated, foliated
4 Streak White
5 Cleavage Perfect basal
6 Lustre Pearly
7 Transparency Transparent
8 Fracture Even
9 Hardness 2.5-3
10 Sp. gravity 2.77 to 2.78
11 Composition (H,K) AlSiO4
12 Name Muscovite
13 Variety Sericite, domourite and gibertite are secondary micas
14 Occurrence It occurs in igneous rocks like granites and pegmatites.
Also in sedimentary and metamorphic rocks.
15 Uses Muscovite is widely used for insulation in electrical
industry, Ground mica is used as filler, it is also used as
heat resistant supports and windows.

Classification of minerals
• Rock forming
Rock forming may be silicate or non silicate.
Silicate minerals contains quartz families,
Feldspar, Mica, Pyroxene, Amphibole, Olivine,
Garnet and non silicate minerals contain only
carbonate group
• Ore forming
 Ore forming minerals are non silicate minerals
which contains oxide, sulphide and sulphate
group.