Introduction to Geo sciences
Dr. Olympa Baro
Assistant Professor
Department of Civil Engineering
NIT Silchar
Unit 2: Minerology
Book of reference: A textbook of geology by P K Mukerjee
• Minerology is a branch of geology, which deals with the
various aspects related to minerals such as their
individual properties their mode of formation and mode
of occurrence
• Minerals are a substances that must meet five
requirements:
– naturally occurring
– inorganic
– solid
– definite chemical composition
– ordered internal structure
• "Definite chemical composition" means that all
occurrences of that mineral have a chemical composition
that varies within a specific limited range
• "Ordered internal structure" means that the atoms in a
mineral are arranged in a systematic and repeating
pattern
Study of crystals:
• Within the mineral kingdom 98% of the minerals are
crystalline
• The branch of mineralogy dealing with the study of
crystals is known as crystallography
• Crystals are solids bounded by smooth, more or less
plane surfaces arranged in a regular pattern and are
formed due to operation of inter atomic forces at the
time of consolidation of the mass from fluid state
under favorable conditions
• A mineral possessing both the external form
and internal atomic structure is said to be a
perfect crystal
• If a mineral has acquired the internal atomic
structure only without the development of the
corresponding external form it is called
crystalline
• If a mineral has developed neither external
form nor internal atomic structure it is called
as amorphous
Fundamental difference between crystal and
amorphous substance:
• In a crystal (or crystalline) there exists a regular
arrangement of atoms in space
• An amorphous substance exhibits a haphazard and
random disposition of atoms
Important terms:
• Faces: the smooth surfaces bounding a crystal
• Edges: the straight lines along which the adjacent
faces meet
• Solid angle: when three or more faces meet at a
point
Fig: Crystal faces, edges
and solid angle
• Interfacial angle: angle subtended between the
normals drawn on the two faces of a crystal
Symmetry in crystals
• A systematic and regular arrangement of faces, edges, etc., of
a crystal in space is defined as symmetry
1. Plane of symmetry: It is an imaginary plane
which passes through the centre of a crystal
which divides it into two equal portions which
are exactly the mirror images of each other
How many planes of symmetry will a cube have?
Fig: parallel and diagonal planes of symmetry
2. Axis of symmetry : An axis of symmetry or axis of rotation is
an imaginary line, passing through the crystal such that when
the crystal is rotated about this line, it presents the same
appearance more than once in one complete revolution
– Four fold or tretragonal symmetry
– Three fold symmetry or trigonal symmetry
– Two fold or binary symmetry
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3. Centre of symmetry: It is an imaginary point in the
crystal that any line drawn through it intersects the
surface of the crystal at equal distance on either side
Physical properties of minerals
• The physical characteristics of minerals
include traits which are used to identify and
describe mineral species
• These traits include colour, streak, lustre,
density, hardness, specific gravity, cleavage,
fracture, lustre, form and structure
Colour:
• The colour of a mineral is produced by the
wavelengths of light reflected not absorbed by a
mineral
• The presence of minute impurities effects the
colour of a mineral
• For example even the smallest impurity in quartz
such as additional trace elements or gaps or any
kind of inclusions causes lowest absorptions and
consequently causes resulting colour
Fig: Different colours of quartz due to inclusions of one mineral within the host
mineral
• Colour of a mineral is by no means a dependable or
distinguishable character
Streak
• Streak is the color of the mineral in powdered
form.
• Streak shows the true color of the mineral.
• The streak of metallic minerals tends to appear
dark because the small particles of the streak
absorb the light hitting them.
• Non-metallic particles tend to reflect most of the
light so they appear lighter in color or almost
white.
• Because streak is a more accurate illustration of
the mineral’s color, streak is a more reliable
property of minerals than color for identification.
Streak Test
• To test the streak color of a mineral you need a streak plate. A
streak plate is just a ceramic, unglazed tile. To test a mineral
rub the specimen across the tile as if trying to make a scratch.
Observe the color the residue left on the tile.
Fig: Streak of minerals
Hardness
• Hardness is a measure of the mineral’s resistance
to scratching.
• Mohs scale which is a set of 10 minerals of known
hardness is used to measure hardness of
unknown minerals.
• The softest mineral, talc, has a Mohs scale rating
of one. Diamond is the hardest mineral and has a
rating of ten.
• Softer minerals can be scratched by harder
minerals because the forces that hold the crystals
together are weaker and can be broken by the
harder mineral.
Cleavage
• Cleavage is the tendency of a mineral to break
along certain planes to make smooth surfaces.
• A mineral that naturally breaks into perfectly flat
surfaces is exhibiting cleavage.
• Not all minerals have cleavage. A cleavage
represents a direction of weakness in the crystal
lattice.
• Cleavage is the result of weaker bond strengths or
greater lattice spacing across the plane in
question than in other directions within the
crystal.
Fracture:
• If the mineral contains no planes of weakness, it will break
along random directions called fracture. Several different
kinds of fracture patterns are observed.
• Conchoidal fracture - breaks along smooth curved surfaces.
• Fibrous and splintery - similar to the way wood breaks.
• Hackly - jagged fractures with sharp edges.
• Uneven or Irregular - rough irregular surfaces.
Fig: Conchoidal fracture
Fig: Splintery fracture.
Fig: Hackly fracture
Fig: Uneven fracture
Specific Gravity
• Specific Gravity of a mineral is a comparison or ratio of the
weight of the mineral to the weight of an equal amount of
water.
• 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.
Magnetism
• Magnetism is the characteristic that allows a mineral to
attract or repel other magnetic materials. It can be difficult to
determine the differences between the various types of
magnetism, but it is worth knowing that there are distinctions
made.
Luster
• Luster refers to the general appearance of a mineral
surface to reflected light. Two general types of luster
are designated as follows:
1. Metallic - looks shiny like a metal. Usually opaque
and gives black or dark colored streak.
2. Non-metallic - Non metallic lusters are referred to as
a) vitreous - looks glassy - examples: clear quartz,
tourmaline
b) resinous - looks resinous - examples: sphalerite, sulfur.
c) pearly - iridescent pearl-like - example: apophyllite.
d) greasy - appears to be covered with a thin layer of oil -
example: nepheline.
e) silky - looks fibrous. - examples - some gypsum,
serpentine, malachite.
f) adamantine - brilliant luster like diamond.
Fig: Greasy lustre (Chrysocolla) Fig: Adamantine Luster (Anglesite)
Crystalline Structure
• Mineral crystals occur in various shapes and sizes. The
particular shape is determined by the arrangement of the
atoms, molecules or ions that make up the crystal and
how they are joined. This is called the crystal lattice.
• There are degrees of crystalline structure, in which the
fibers of the crystal become increasingly difficult or
impossible to see with the naked eye or the use of a hand
lens.
• Microcrystalline and cryptocrystalline structures can only
be viewed using high magnification.
• If there is no crystalline structure, it is called amorphous.
However, there are very few amorphous crystals and
these are only observed under extremely high
magnification.
• A small group of a repeating pattern of the
atomic structure is known as the unit cell of the
structure.
• A unit cell is the building block of the crystal
structure and it also explains in detail the entire
crystal structure and symmetry with the atom
positions along with its principal axes.
• The length, edges of principal axes and the angle
between the unit cells are called lattice constants
or lattice parameters.
Crystal Systems
• A Crystal System refers to one of the many
classes of crystals, space groups, and lattices.
In crystallography terms, lattice system and
crystal, the system are associated with each
other with a slight difference. Based on their
point groups crystals and space groups are
divided into seven crystal systems.
Triclinic System:
• It is the most unsymmetrical crystal system. All three
axes are inclined towards each other, and they are of
the same length.
• Some standard Triclinic Systems include Labradorite,
Amazonite, Kyanite, Rhodonite, Aventurine Feldspar,
and Turquoise.
Monoclinic System:
• It comprises of three axes where two are at right angles
to each other, and the third axis is inclined.
• All three axes are of different length. Based on the inner
structure the monoclinic system includes Basal
pinacoids and prisms with inclined end faces. Some
examples include Diopside, Petalite, Kunzite, Gypsum,
Hiddenite, Howlite, Vivianite and more.
• Orthorhombic System:
• It comprises of three axes and is at right angles to
each other. There are different lengths.
• Based on their Rhombic structure the orthorhombic
system includes various crystal shapes namely
pyramids, double pyramids, rhombic pyramids, and
pinacoids. Some common orthorhombic crystals
include Topaz, Tanzanite, Iolite, Zoisite, Danburite
and more.
Hexagonal System:
• It comprises four axes. The three
a1, a2 and a3 axes are all
contained within a single plane
(called the basal plane) and are at
120°. They intersect each other at
an angle of sixty degrees. The
fourth axis intersects other axes
at right angles.
• Crystal shapes of hexagonal
systems include Double Pyramids,
Double-Sided Pyramids, and
Four-Sided Pyramids. Example:
Beryl, Cancrinite, Apatite, Sugilite,
etc.
Trigonal System:
• Angles and axis in a trigonal system
are similar to Hexagonal Systems.
• At the base of a hexagonal system,
there will be six sides. In the
trigonal system there will be three
sides.
• Crystal shapes in a trigonal system
include three-sided pyramids,
Scalenohedral and Rhombohedra.
Some typical examples include
Ruby, Quartz, Calcite, Agate,
Jasper, Tiger’s Eyes and more.
Tetragonal Systems:
• It consists of three axes. The main axis varies in length; it
can either be short or long. The two-axis lie in the same
plane and are of the same length.
• Based on the rectangular inner structure the shapes of
crystal in tetragonal include double and eight-sided
pyramids, four-sided prism, trapezohedrons, and pyrite.
Cubic System:
• Cubic system is the most symmetrical one out of
the seven crystal system.
• All three angles intersect at right angles and are
of equal length.
• Crystal shapes of a cubic system based on inner
structure (square) include octahedron, cube, and
Hexaciscoherdron. Example: Silver, Garnet, Gold,
and Diamond.
