What is a Mineral?

What Exactly is a Mineral?

• Compositionally and physically distinct substance

• Four criteria to be a true mineral…

Naturally Occurring Created through natural geologic processes

Synthetic diamond: not a mineral Raw diamond: mineral

InorganicThat is, they do not consist of carbon-hydrogen molecules that may also form crystalline substances through biological processes

Sugar: It looks like a mineral, but is it?

Nope. Note the C-H bonds.

SolidLiquid water Snowflake—frozen water

Not a mineral Mineral…believe it or not

Definite Chemical CompositionMinerals are complex assemblages of multiple elements

Chemical Formula

Types of atoms involved and relative proportions

Ex. Quartz

Chemical formulaSiO2

Ex #2: Elbaite

Chemical formulaNa(Li,Al)3Al6(BO3)3Si6O18(OH)4

Elements

• Pure chemical substance composed of one type of atom (i.e. specific # of protons…)

• 88 naturally occurring elements• Most common (i.e. compose 98% of the

earth’s crust—and therefore, minerals)– Oxygen, silicon, aluminum, iron, calcium,

magnesium, potassium, and sodium

Atomic Structure

• Atom– Basic unit of an element– Very, very, very small

(1/10,000,000,000 m)– Nucleus

– Protons (+)– Neutrons (neutral)

– Electrons (-)– “cloud” surrounding

nucleus—exist in “shells”

– Mostly empty space

Visualizing the size of an atom

• The nucleus is very small compared to the overall size of an atom

• Imagine 5 ½ football fields (minus end zones) placed end to end (100 yrds long x 160’ wide)

• Nucleus is like a grain of sand in the middle of them

It’s All About the Electrons, Baby…

• A stable atom is one in which the outermost shell of electrons is completely filled

• Atoms can fill their outer shells by bonding to other atoms– Ionic bonding– Covalent bonding

Bonding in a [Nut] Shell…

• Ionic Bonding– Cations (atoms that have

lost an electron) are attracted to anions (atoms that have gained an extra electron)

– Neutral compounds – Weaker but more

common than covalent bonds

Bonding in a [Nut] Shell…

• Covalent bonding– Two or more atoms

share their electrons, producing the effect of filled outer electron shells

– Very strong bond

Crystalline Structure• Arrangement of atoms

are– Orderly– Regular– 3-D– Repeating pattern

Halite

Biotite

Fluorite

Can You Spot the Mineral Wannabes?

PyriteFeS2

oil

ChalcanthiteCuSO4-5H2O

Silicon CarbideSiC

Growing a Mineral

• Unit cell—smallest unit structure of a mineral

• Brick Wall example

Growing a Mineral: Halite

Halite unit cell

Halite crystal

What halite looks like to us

Identifying a Mineral

• All minerals have physical and chemical properties that can be easily recognized

• Direct result of structure and composition

• Crystal habit• Cleavage• Fracture• Hardness• Color • Streak• Luster

Cleavage

• Not the kind you’re thinking of…

• The tendency of a mineral to break along flat surfaces

• Governed by bond strength and crystal structure

• Stronger bonds = poor or no cleavage→

Oooo…shiny!• Luster

– The way a mineral reflects light

• A metallic mineral is said to have a metallic luster

• Those that do not have a metallic luster (non-metallic) are assigned names that are self-explanatory – Pearly, resinous, earthy,

glassy, etc…

Pyrite has a metallic luster

Quartz has a glassy luster

School of Hard Knocks…• Hardness

– Resistance of a mineral to scratching

• Hardness can be determined by scratching mineral with an object of known hardness

• Scale—Mohs– Based on 10 minerals: 1

being talc and 10 being diamond