Minerals introduction to earth science

Patterns in Nature: Minerals & Prelude A: Rock Groups Chemical bonding: Focus on covalent bonds Mineral polymorphs Physical properties of minerals... Factors that determine the intern

Earth Materials: Minerals

Chapter 5 Patterns in Nature: Minerals &

Prelude A: Rock Groups

Chemical bonding: Focus

on covalent bonds

Mineral polymorphs

Physical properties of minerals

charged cation cation.

Chlorine picks up an electron becoming a negatively charged anion

anion.

Atomic Bonding

Bonding between sodium and chlorine in halite

is based on these charge differences.

Sharing Electrons: Covalent Bonding

Shared electrons Nucleus

Factors that determine the internal

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

✦ 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.

Physical properties of minerals

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

Euhedral crystals of quartz

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

Quartz geode

Anhedral crystals formed by crowding during growth

Anhedral quartz crystals formed by crowding during growth Crystal terminations of

euhedral quartz

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

Cleaves into long flexible fibers

-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”)

Asbestos

Asbesto s

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?

3-directional

cleavage

✦ 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.

Quartz does not

cleave, but breaks along smooth,

curved, glassy surfaces

Called

“conchoidal”

(glassy) fracture

Conchoidal Fracture in Quartz

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”

CO 2 bubbles

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

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.

Important Non-silicate 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)

The Common Rock-forming Minerals

Average composition of the Earth’s crust.

Percent of elements by WEIGHT

Question: What minerals would you expect to be most abundant on Earth?

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

The Common Rock-forming Minerals

1 silicon (Si) atom

4 oxygen (O) atoms

Basic Building Block of Silicate Minerals:

The Silicon-Oxygen Tetrahedron

An anion with charge of -4

Tetrahedra link up by forming covalent bonds between oxygen atoms:

Single silicon tetrahedron:

A silicon atom

covalently-bonded to four oxygens.

Two tetrahedra can join

by sharing an electron between adjacent oxygen atoms

Silicates: The Common Rock-forming Minerals

Basic Building Block:

The Silicon-Oxygen Tetrahedron

Oxygen atom

Silicon atom

The Common rock-forming minerals

Silicon-oxygen tetrahedra can be arranged into:

Single chains: Pyroxene Double chains: Amphibole Sheets: Micas

Silicate chains and sheets

Not electrically neutral! Unsatisfied

negative charges

of oxygens located at the edges of chains,

or between sheets are neutralized by coordinating metallic ions at those sites.

Balancing Charges in Silicates: Role of Metal Cations

Ionic Substitution

Ions of similar size (ionic radius) and charge can substitute for one another in a mineral.

Definition of a rock:

A rock is:

Prelude Chapter: Rocks

1) Comprised of one or more minerals

There are three types of rocks:

Sedimentary (formed by the breakdown of other rocks)

Metamorphic (formed when preexisting rocks

are heated under pressure

Prelude Chapter: Rocks

collection of one or more

minerals

Prelude Chapter: Rocks

rock minerals mineral

Prelude Chapter: Rocks

So far we have:

rock

collection ofone or more

minerals

mineral

A collection

of one or moretypes of atoms minerals

Prelude Chapter: Rocks

Rocks and minerals

● Some rocks composed entirely of one mineral

limestone (calcite)

Prelude Chapter: Rocks

● 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)