Earth Materials: Minerals
Today’s Lecture:
Chapter 5. Patterns in
Nature: Minerals &
Prelude A: Rock Groups
Chemical bonding: Focus
on covalent bonds
Mineral polymorphs
Physical properties of
minerals
Common rock-forming
“silicate” minerals
Introduction to rocks & the
rock cycle
Atomic Bonding
 Ionic Bonding
Example: Table Salt: Sodium (Na) and Chlorine (Cl)
Sodium gives up an
electron becoming
a positively-charged
charged cation.
Chlorine picks up an electron
becoming a negatively charged
anion.
Bonding between sodium and chlorine in halite
is based on these charge differences.
Sharing Electrons: Covalent Bonding
Nucleus
Shared electrons
Factors that determine the internal
structure of minerals:
1) Composition of magma or fluids from which the
minerals form.
2) Conditions under which the mineral forms:
 Temperature
 Pressure
Minerals comprised of the same elements in the
same proportions can possess markedly
different internal structures.
For example:
Higher pressure -> Denser packing of atoms -> Different mineral
Mineral Structure & Conditions of Formation
Different minerals w/ same chemical composition ,
but differing structures, are called “polymorphs”
Graphite (a form of pure carbon)
 Soft gray material, e.g., pencil lead
 Crystal structure: sheets of carbon
Diamond (also pure carbon)
 Forms deep in Earth at high pressures,
& is hardest substance known to humans
 Crystal structure: dense & compact
Identifying Minerals
To identify minerals, we use their physical and optical
properties. Some properties are more diagnostic than
others, so we try to use a combination when making a
determination. Useful properties include:
 Color
 Luster
 Hardness
 Streak
 Crystal form
 Cleavage
 Fracture
 Reaction to acid
 Taste
 Smell
 Magnetization
 Optical properties
 Elasticity
 Specific gravity
Physical properties of minerals
 Color
Obvious, but often misleading. Slight impurities in a mineral
can change its color.
Example: Quartz (when pure it is colorless), but there are
many color varieties which result from small amounts of other
elements.
Physical properties of minerals
 Luster
The appearance of light reflected from minerals.
Examples:
Metallic luster vs. nonmetallic luster
Glassy (vitreous) luster
Resinous luster
Physical properties of minerals
 Hardness
Very useful! Measures a mineral’s resistance to
scratching. We use Moh’s hardness scale (below)
for comparisons.
Crystal Form Reflects the Internal Arrangement of Atoms
Crystal form in halite
(salt; NaCl) is cubic
A law of mineralogy:
Constancy of angles between crystal faces
 Crystal Form
The shape of a well-formed crystal reflects
directly the orderly internal arrangement of
Its constituent atoms.
Well-formed crystals that grow without
interference are called “euhedral”.
Quartz
(SiO2)
Anhedral crystals form when
crystals don’t have room
to grow and bump into each other
feldspar in an igneous rock
Irregular boundaries between crystals
due to interference during growth
Anhedral crystals
formed by crowding
during growth
Quartz geode
Crystal terminations of
euhedral quartz
Anhedral quartz crystals
formed by crowding
during growth
Euhedral crystals of amphibole in a volcanic rock
Cleavage: Tendency to break along
preferred planes of weakness.
Cleavages represent directions of
weaker bonding between atoms.
2-directional cleavage in mica
In mica, atoms are arranged in
weakly-connected sheets
Asbestos
Cleaves into long flexible fibers
Asbestos
Asbestos
-group of silicate minerals that readily separate into fibers
that are: thin, flexible, heat resistant, chemically inert
=> many uses
- mainly three types:
chrysotile (“white asbestos”)
crocidolite (“blue asbestos”)
amosite (“brown asbestos”)
3-directional
cleavage
Cleavage in both Halite
(salt) & calcite (lime)
is in three directions.
But the angles between
cleavages are different
for these minerals.
 Halite has a cubic
cleavage.
Calcite cleaves into
rhombohedra.
Can you spot which is
which in the samples
to the left?
In Summary….
 Cleavage:
The tendency of a mineral to break along planes of weak bonding
in the crystal structure. The number and angles between cleavgae
faces are very useful properties for identification.
Calcite
(rhombs)
Halite
Mica (sheets)
(cubes)
Conchoidal Fracture in Quartz
Bond strengths
are equal in all
directions.
No preferred
directions of
weakness.
Quartz does not
cleave, but breaks
along smooth,
curved, glassy
surfaces.
Called
“conchoidal”
(glassy)
fracture
Conchoidal fracture
in volcanic glass
Streak: Color of mineral
in its powdered form
Hematite: Iron oxide
Carbonate minerals, like calcite, dissolve
in acid and release carbon dioxide
“The Acid Test”
CO2 bubbles
Important Non-silicate Minerals
Halides
Halite (Na, Cl: NaCl)
-> common table salt
Sulfates
Gypsum (Ca,S,O,H: CaSO4-H2O)
-> calcium sulfate + water, main ingredient of
plaster & other building materials
Oxides
Hematite (Fe, O: Fe2O3)
-> steel
Important Non-silicate Minerals
Carbonates
Calcite (Ca, C, O: CaCO3)
Dolomite (Ca, Mg,C, O: CaMg(CO3)2
 Found together in sedimentary rock limestone.
 Main ingredient to cement, roads & building stones.
The Common Rock-forming Minerals
Over 4000 minerals: only few dozen are abundant, making
up most rocks of Earth’s crust
=> rock-forming minerals
Only 8 elements make up most of crust’s minerals &
represent over 98% of the continental crust
The two most abundant elements:
 Silicon (Si)
 Oxygen (O)
Question: What minerals would
you expect to be most abundant on Earth?
Percent of elements by WEIGHT
Average composition of the Earth’s crust.
The Common Rock-forming Minerals
Earth’s Crust
Primarily Si & O followed in abundance by
Fe, Mg, Ca, Na, K, etc.
Dark colored (mantle and oceanic crust)
Olivine (Si, O, Fe, Mg)
Pyroxene (Si, O, Fe, Mg, Ca)
Amphibole (Si, O, Fe, Mg)
Light colored (crust, esp. continental crust)
Quartz (SiO2) - Hard, transparent
Feldspar (Si, O, Al, K, Na, Ca) - Hard, white, gray, pink
Clay (Mostly come from weathering feldspar)
Calcite (CaCO3, shells) Limestone - Used for cement
Basic Building Block of Silicate Minerals:
The Silicon-Oxygen Tetrahedron
An anion with charge of -4
O
1 silicon (Si) atom
4 oxygen (O) atoms
2-
4SiO4
4+
Si
O
O
2-
O
2-
2-
Silicon tetrahedron has
An overall charge of -4
Silicates: The Common Rock-forming Minerals
Basic Building Block:
The Silicon-Oxygen Tetrahedron
Tetrahedra link up by forming
covalent bonds between oxygen atoms:
Single silicon tetrahedron:
A silicon atom covalentlybonded to four oxygens.
Oxygen atom
Silicon atom
Two tetrahedra can join
by sharing an electron
between adjacent oxygen
atoms
The Common rock-forming minerals
Silicates
Silicon-oxygen tetrahedra can be arranged into:
Double chains: Amphibole
Single chains: Pyroxene
Sheets: Micas
Balancing Charges in Silicates: Role of Metal Cations
Silicate chains and sheets
Unsatisfied
Not electrically neutral!
Iron (Fe)
Magnesium (Mg)
Potassium (K)
Sodium (Na)
Aluminum (Al)
Calcium (Ca)
negative charges
of oxygens
located at the
edges of chains,
or between
sheets are
neutralized by
coordinating
metallic ions at
those sites.
Ionic Substitution
Ions of similar size (ionic radius) and charge
can substitute for one another in a mineral.
Prelude Chapter: Rocks
Definition of a rock:
A rock is:
1) Comprised of one or more minerals
2) Naturally occurring
There are three types of rocks:
Igneous (formed by cooling from magma)
Sedimentary (formed by the breakdown of other rocks)
Metamorphic (formed when preexisting rocks
are heated under pressure.
Prelude Chapter: Rocks
rock
Prelude Chapter: Rocks
collection of
one or more
rock
minerals
Prelude Chapter: Rocks
rock
minerals
mineral
Prelude Chapter: Rocks
So far we have:
rock
minerals
mineral
collection of
one or more
minerals
A collection
of one or more
types of atoms
Prelude Chapter: Rocks
Example:
Granite & its
constituent
minerals:
Quartz
Amphibole (hornblende)
Feldspar
Prelude Chapter: Rocks
Rocks and minerals
 Some rocks composed entirely of one mineral
limestone (calcite)
 Most rocks have more than one kind of mineral
granite
 Some rocks contain non-mineral matter
coal (has organic debris)
obsidian (volcanic glassy rock -> not crystalline)