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Engineering Geology

ECIV 2204

Chapter (2) Minerals

Instructor : Dr. Jehad Hamad

2017-2016

Chapter 2:

Minerals:

Building Blocks of Rocks

Earth’s Molten Stage

During the early formation of the Earth it was molten

During this stage the heavier elements such as iron and

nickel, sank to the deeper interior of the Earth.

This left a thin layer of lighter materials on the surface that is

called the crust.

The majority of the Earth’s mass lies below the crust

Chemical Analysis

8 elements make up 98.6% of the crust

These 8 elements make up the solid materials of the Earth’s

crust and are known as rocks and minerals

A mineral: is solid inorganic material of the Earth that has

both a known chemical composition and a crystalline structure

that is unique to that mineral

A rock : is a solid aggregate of one or more minerals that

have been cohesively brought together by a rock-forming

process.>

(A)The percentage by weight

of the elements that make up

Earth's crust.

(B) The percentage by weight

of the elements that make up

the whole Earth.

The Earth is composed of rocks. Rocks are aggregates of minerals. So minerals are the basic building blocks of the Earth.

Currently there are over 4,000 different minerals known and dozens of new minerals are discovered each year. Our society depends on minerals as sources of metals, like Iron (Fe), Copper (Cu), Gold (Au), Silver (Ag), Zinc (Zn), Nickel (Ni), and Aluminum (Al), etc., and non-metals such as gypsum, limestone, halite, and clay. Many minerals of great economic importance and their distribution, extraction, and availability have played an important role in history.

Minerals are composed of atoms. We'll start our discussion with the geological definition of a Mineral.

Rocks and Minerals

Rocks - Mixtures of Minerals

Mixtures or aggregates of minerals are called rocks. There are three basic kinds of rocks,

each type is determined by the process by which the rock forms.

Igneous Rocks - form by solidification and crystallization from liquid rock, called magma.

Sedimentary Rocks - form by sedimentation of mineral and other rock fragments from

water, wind, or ice and can also form by chemical precipitation from water.

Metamorphic Rocks - form as a result of increasing the pressure and/or temperature on a

previously existing rock to form a new rock.

Each of these rock forming processes results in distinctive mineral assemblages and textures

in the resulting rock. Thus, the different mineral assemblages and textures give us clues to

how the rock formed. An understanding of the rock forming processes is one of the goals of

this course

Definition of a Mineral

• A mineral is Naturally formed - it forms in nature on its own (some

say without the aid of humans] ,Solid ( it cannot be a liquid or a gas)

• With a definite chemical composition (every time we see the same

mineral it has the same chemical composition that can be expressed by

a chemical formula).

• It has a characteristic crystalline structure (atoms are arranged within

the mineral in a specific ordered manner).

• Usually inorganic, although a mineral can be formed by an organic

process.

Formation of Minerals

Minerals are formed in nature by a variety of processes. Among them are:

• Crystallization from melt (igneous rocks)

• Precipitation from water (chemical sedimentary rocks, hydrothermal ore

deposits)

• Biological activity (biochemical sedimentary rocks)

• Change to more stable state - (the processes of weathering, metamorphism,

and diagenesis).

• Each process has specific temperature and pressure conditions that can be

determined from laboratory experiments. Example: graphite and diamond.

A mineral is a naturally occurring inorganic

solid possessing a definite chemical

structure that gives it a unique set of

physical properties.

Thus, ice is a mineral, but liquid water is not (since it is not solid).Halite (salt) - is naturally formed, is solid, does have a definite chemical composition that can be expressed by the formula NaCl, and does have a definite crystalline structure. So it is a minerals

Examples•Glass - can be naturally formed (volcanic glass called obsidian), is a solid, its chemical composition, however, is not always the same, and it does not have a crystalline structure. Thus, glass is not a mineral.

Ice - is naturally formed, is solid, does have a definite chemical

composition that can be expressed by the formula H2O, and does have

a definite crystalline structure when solid.

Most rocks are aggregates composed of two

or more minerals.

The building blocks of minerals are

elements.

An atom is the smallest particle of matter

that still retains the characteristics of an

element.

Each atom has a nucleus containing protons

and neutrons.

Orbiting the nucleus of an atom are

electrons.

The number of protons in an atom's nucleus

determines its atomic number and the

name of the element.

Atoms bond together to form a compound

by either gaining, losing, or sharing

electrons with another atom.

Atoms make-up minerals

An atom is the smallest component of matter

Each element is defined by the number of protons

The atom must be electrically neutral

The number of protons equals the number of electrons

AtomsSince minerals (in fact all matter) are made up of atoms, we must

first review atoms. Atoms make up the chemical elements. Each chemical element has nearly identical atoms. An atom is composed of three different particles:

•Protons -- positively charged, reside in the center of the atom called the

nucleus

•Electrons -- negatively charged, orbit in a cloud around nucleus

•Neutrons -- no charge, reside in the nucleus.

Each element has the same number of protons and the same number of

electrons.

•Number of protons = Number of electrons.

•Number of protons = atomic number.

•Number of protons + Number of neutrons = atomic weight.

Isotopes are variants of the same element

but with a different mass number (the

total number of neutrons plus protons

found in an atom's nucleus).

Some isotopes are unstable and disintegrate

naturally through a process called

radioactive decay.

Isotopes: النظائر are atoms of the same element with differing

numbers of neutrons. i.e. the number of neutrons may vary

within atoms of the same element. Some isotopes are unstable

which results in radioactivity.

•Example:

0 K (potassium) has 19 protons. Every atom of K has 19 protons.

Atomic number of K = 19. Some atoms of K have 20 neutrons,

others have 21, and others have 22. Thus atomic weight of K

can be 39, 40, or 41. 40K is radioactive.

Structure of AtomsElectrons orbit around the nucleus in different shells, A Stable electronic configuration for an atom is one 8 electrons in outer shell Thus, atoms often loose electrons or gain electrons to obtain stable configuration. Noble gases have completely filled outer shells, so they are stable.. Others like Na, K loose an electron. This causes the charge balance to become unequal. and produce charged atoms called ions. Positively charged atoms are called cations. Elements like F, Cl, O gain electrons to become negatively charged.

Negatively charged ions are called anions.

The drive to attain a stable electronic configuration in the outermost shellAlong with the fact that this sometimes produces oppositely chargedions, results in the binding of atoms together. When atoms becomeattached to one another, we say that they are bonded together.

The configuration of electrons determines if

an atom will respond with another atom

The sodium atom has one electron on its outer ring. The Chlorine atom has 7 electrons on its outer

ring. The two atoms share electrons forming an ionic bond.

Cations and Anions

The one electron on the

outer shell is given up

This leaves the sodium atom

with more protons, +1

The one electron is added to

Chlorines outer shell

This leaves the chlorine

atom with one more

electron, -1

cationanion

Types of bonding:

• Ionic Bonds - caused by the force of attraction between ions of opposite charge.

Example Na+1 and Cl-1. Bond to form NaCl (halite or salt). Ionic bonds

are moderately strong

• Covalent Bonds –• Electrons are shared between two or more atoms so that each atom has a stable electronic configuration (completely filled outermost shell) part of the time.

Example: H has one electron, needs to 2 to be stable. O has 6 electrons in its outer shell, needs 2 to be stable. So, 2 H atoms bond to 1 O to form H2O, with all atoms sharing

electrons, and each atom having a stable electronic configuration part of the time.

Covalent bonds are very strong bonds

• Metallic Bonds -- Similar to covalent bonding, except innermost electrons are also shared.

In materials that bond this way, electrons move freely from atom to atom and are constantly being shared. Materials bonded with metallic bonds are excellent conductors of electricity because the electrons can move freely through the material.

• Van der Waals Bonds -- a weak type of bond that does not share or transfer electrons.

Usually results in a zone along which the material breaks easily (cleavage). Good examples'sgraphite and micas like biotite and muscovite.

Ionic Substitution (Solid Solution)

Ionic substitution - (also called solid solution), occurs because some elements (ions) have the same size and charge, and can thus substitute for one another in a crystal structure.

Examples:

Olivines Fe2SiO4 and Mg2SiO4. Fe+2 and Mg+2 are about the same size, thus they

can substitute for one another in the crystal structure and olivine thus can have a

range of compositions expressed as the formula (Mg,Fe)2SiO4.

Composition of Minerals

The variety of minerals we see depend on the chemical elements available to form

them. In the Earth's crust the most abundant elements are as follows:

O, Oxygen 45.2% by weight

Si, Silicon 27.2%

Al, Aluminum 8.0%

Fe, Iron 5.8%

Ca, Calcium 5.1%

Mg, Magnesium 2.8%

Na, Sodium 2.3%

K, Potassium 1.7%

Ti ,Titanium 0.9%

H, Hydrogen 0.14%

Mn, Manganese 0.1%

P, Phosphorous 0.1%

Eight elements make-up 99% of the Earth’s rust

Silicon and oxygen make-up 70 % of the Earth’s crust

Note that Carbon (one of the most abundant elements in life) is not

among the top 12.

Because of the limited number of elements present in the Earth's crust

there are only about 4000 minerals known. Only about 50 of these

minerals are common. The most common minerals are those based on Si

and O: the Silicates. Silicates are based on SiO4 tetrahedron. 4 Oxygens

covalently bonded to one silicon atom

Properties of Minerals

Physical properties of minerals allow us to distinguish between minerals and thus

identify them, as you will learn in lab. Among the common properties used are:

• Habit – shape• Color

• Streak (color of fine powder of the mineral)

• Luster – metallic (reflection of light)

• Cleavage (planes along which the mineral breaks easily)

• Density (mass/volume)

• Hardness:

The properties of minerals include crystal

form, luster, color, streak, hardness,

cleavage, fracture, and specific gravity.

In addition, a number of special physical and

chemical properties (taste, smell, elasticity,

malleability, feel, magnetism, double

refraction, and chemical reaction to

hydrochloric acid) are useful in identifying

certain minerals.

Each mineral has a unique set of properties

that can be used for identification.

The eight most abundant elements found in

Earth's continental crust (oxygen, silicon,

aluminum, iron, calcium, sodium,

potassium, and magnesium) also make up

the majority of minerals.

The most common mineral group is the

silicates.

All silicate minerals have the silicon-oxygen

tetrahedron as their fundamental building

block.

In some silicate minerals the tetrahedra are

joined in chains; in others the tetrahedra

are arranged into sheets, or three-

dimensional networks.

Each silicate mineral has a structure and a

chemical composition that indicates the

conditions under which it was formed.

The nonsilicate mineral groups include:

the oxides (e.g., magnetite, mined for iron),

sulfides (e.g., sphalerite, mined for zinc),

sulfates (e.g., gypsum, used in plaster and

frequently found in sedimentary rocks),

native elements (e.g., graphite, a dry

lubricant),

halides (e.g., halite, common salt and

frequently found in sedimentary rocks),

and

carbonates (e.g., calcite, used in portland

cement and is a major constituent in two

well-known rocks: limestone and marble).

The term ore is used to denote useful metallic

minerals, like hematite (mined for iron) and

galena (mined for lead), that can be mined for

a profit, as well as some nonmetallic

minerals, such as fluorite and sulfur, that

contain useful substances.

igneoussedimentary

formed by cooling of molten

magma (lava)

formed by gradual deposition, and

in layerformed by alteration of igneous

& sedimentary rocks by

pressure/temperature

e.g., limestone, shale

e.g., marble

e.g., granite

metamorphic

Rocks formed by one of these three different processes

Clay minerals are made of two distinct structural units.

0.26 nm

oxygen hydroxyl or

oxygen

0.29 nm

Silicon tetrahedron Aluminium Octahedron

Several tetrahedrons joined together form a tetrahedral sheet.

tetrahedron

For simplicity, let’s represent silica tetrahedral sheet by:

Si

and alumina octahedral sheet by:

Al

Different combinations of tetrahedral and octahedral sheets form different clay minerals:

Different combinations of tetrahedral and octahedral sheets form different clay minerals:

Clay Mineral (e.g., montmorillonite, illite)

Si

Al

Si

Al

Si

Al

Si

Al

joined by strong H-bond

no easy separation

0.72 nm

Typically 70-100 layers

joined by oxygen sharing

Si

Al

Si

Si

Al

Si

Si

Al

Si

0.96 nm

joined by weak

van der Waal’s bond

easily separated by

water

Si

Al

Si

Si

Al

Si

Si

Al

Si

0.96 nm

joined by K+ ions

fit into the hexagonal holes in

Si-sheet

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Cation concentration drops with distance from clay particle

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Crystal Structures:

Can be made up of atoms of one or more

kinds of elements.

Crystals are classified according to six

major groups, with subdivisions of each.

A crystal is composed of a structural unit that is repeated

in three dimensions. This is the basic structural unit of a

crystal of sodium chloride, the mineral halite.

(A)The geometric shape of a

tetrahedron with four equal sides.

(B) A silicon and four oxygen

atoms are arranged in the shape

of a tetrahedron with the silicon in

the center. This is the basic

building block of all silicate

minerals.

Compare the dark colors of the ferromagnesian silicates

augite (right), hornblende (left), and biotite to the light-

colored nonferromagnesian silicates.

Compare the light colors of the nonferromagnesian silicates mica (front center), white and pink orthoclase (top and center), and quartz, to the dark-colored ferromagnesian silicates.

Structure of silicates

Isolated tetrahedrons

Chain silicates

Sheet silicates

Framework silicates

Nonsilicates – make up 8% of Earth’s crust

Carbonates

Sulfates

Oxides

Sulfides

Halides

Phosphates

Hydroxides

Native elements

Physical Properties of Minerals

Color

A visual measure.

Not very useful for identification as color of minerals varies

considerably.

Streak

This is the color of the mineral when it is finely powdered.

Rubbed across a piece of tile, leaving a fine powder of the

mineral on the tile.

Hardness

Resistance of the material to being scratched.

Measured using the Mohs hardness scale, which compares the

hardness of the mineral to 10 reference minerals.

MINERALS ARE MADE UP OF SINGLE

ELEMENTS OR COMPOUNDS

ELEMENTS A SUBSTANCE THAT CANNOT BE

BROKEN DOWN TO ANY SIMPLER

SUBSTANCE

EIGHT MOST COMMON ELEMENTS IN THE EARTH’S CRUST:

1.) OXYGEN 46% 3-8.)THE REMAINING 25% IS

COMPOSED OF Al, Fe, Ca, Na, K, Mg

2.) SILICON 29%

A MINERAL IS:

1.) NATURALLY OCCURRING

2.) DEFINITE CHEMICAL COMPOSITION

3.) INORGANIC SOLID

4.) CRYSTALLINE STRUCTURE

5.) DEFINITE SET OF CHEMICAL AND PHYSICAL

PROPERTIES

MOST MINERALS ARE COMPOSED OF TWO OR

MORE ELEMENTS

HALITE (NaCl)QUARTZ (SiO2)

• OXYGEN AND SILICON COMBINE READILY WITH EACH OTHER

AND WITH OTHER ELEMENTS TO FORM THIS FAMILY OF

MINERALS

• MOST COMMON FAMILY OF MINERALS AND MAKE UP OVER 90%

OF ALL MINERALS

• THIS IS BECAUSE OXYGEN AND SILICON ARE THE MOST

COMMON ELEMENTS IN THE EARTH’S CRUST

THE PHYSICAL PROPERTIES OF MINERALS ARE A

REFLECTION OF THE INTERNAL ARRANGEMENT OF

THEIR ATOMS Physical Properties

PhysicalProperties

LEAST USEFUL PROPERTY, CHANGE IN THE CHEMICAL

FORMULA WILL VARY THE COLOR OF THE MINERAL.

(QUARTZ) SiO2

ROSE QUARTZ

CONTAINS TITANIUM OXIDE AND MANGANESE

OXIDE TO MAKE A ROSE COLOR

CRYSTAL QUARTZ

Color

METALLIC- Looks like metal in the way the mineral reflects light

(galena or pyrite) LUSTER

- THE WAY A MINERAL SHINES IN REFLECTED LIGHT.

LUSTER

THE COLOR OF A MINERALS POWDER

RUB THE MINERAL AGAINST A

STREAK PLATE AND OBSERVE

THE POWDER’S COLOR

HOW DO YOU FIND THE

STREAK COLOR OF A

MINERAL?

Breaking Pattern:

CLEAVAGE- Tendency of a mineral to separate along planes of

weakness

ONE PLANE OF WEAKNESS

BASAL CLEAVAGE (MICA)

TWO PLANES OF CLEAVAGE AT

90* (ORTHOCLASE)

FRACTURE- NO PLANES OF WEAKNESS SO THE MINERAL BREAKS

ALONG IRREGULAR SURFACES

CONCHOIDAL- SURFACES ARE SOMEWHAT

ROUNDED, SHELL-LIKE (QUARTZ)

Resistance to be scratched; is very useful since a mineral’s

hardness is constant

• Scale to measure hardness was developed by friedrich moh in 1812

• Hardness is measured on a scale from 1 being the lowest to

10 being the highest.

• To determine the hardness of a mineral you must try to

scratch the mineral against a glass plate

Hardness