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Chapter 30
Minerals and Their
Formation
Background

Rocks are made up of minerals like how
atoms make up molecules

Most minerals are compounds of different
elements

Most minerals are not perfect like we will
see in class…they are just blobs in rocks
because of where they are formed
Physical Properties of
Minerals (KNOW!)

Mineral appearance

Crystal form and color
Hardness
 Luster
 Specific gravity
 Streak
 Cleavage and fracture

Crystal Form

Crystal Form: orderly arrangement of atoms in a
crystal (what a crystal looks like is its
arrangement)

Example: Quartz = six-sided prisms and
asbestos = narrow fibers

When two or more minerals contain the same
elements in the same amounts but they are
arranged different it is a POLYMORPH

Ex. Graphite and diamond
Quick Question

We can identify minerals by their physical
characteristics but why is it hard to identify
minerals solely by their crystal form?
Hardness

A measure of how easily a mineral can be scratched or
how resistant it it to being scratched

Ex. Diamond can scratch glass
Hardness Cont.
Why is a diamond so much harder than
another mineral?
 The bonds that hold the atoms together
are basis for the minerals strength.

Gold has larger atoms and is loosely packed
so it has weaker bonds (24K gold is VERY
soft)
 Diamonds are made of little carbon atoms so
it is tightly packed (a diamond is very hard)

Another look
Mohs Hardness Scale
Mineral
Rating
Testing Method
Talc
1
Softest known mineral. It flakes easily when
scratched by a fingernail.
Gypsum
Calcite
2
A fingernail can easily scratch it.
3
A fingernail cannot scratch it, but a copper
penny can.
Fluorite
Apatite
Feldspar
4
A steel knife can easily scratch it.
5
A steel knife can scratch it.
6
Cannot be scratched by a steel knife, but it
can scratch window glass.
Quartz
Topaz
Corundum
Diamond
7
Can scratch steel and hard glass easily.
8
Can scratch quartz.
9
Can scratch topaz.
10
Hardest known mineral. Diamond can
scratch all other substances.
Cleavage
 Cleavage is the way that mineral breaks, its
planes of weakness.
 Minerals that break along smooth, flat
surfaces have cleavage.

Ex. Mica has cleavage
and Fracture!...
Mineral that breaks uneven, rough, or
jagged surfaces have fracture.
 Quartz has fracture

quartz
Luster
The way a mineral reflects a light luster.
 Does not depend on color!


Either metallic or nonmetallic
Streak



While colors helps us identify minerals it isn’t
super reliable
When a mineral is rubbed across a piece of
porcelain tile a streak of powdered mineral is left
behind.
Streak: color given off in its powered form
(important for minerals with a metallic or semimetallic luster
Specific Gravity

The specific gravity of a mineral is the ratio
of its weight compared with the weight
of an equal volume of water.

Gold has specific gravity of 19

It means gold is 19 times heavier than
water.
19 times heavier
Quick Question
 Why
is panning for gold possible?
Chemical Properties

Taste test and Acid Test
Taste test helps identify salts
 Never taste minerals without being an expert
some minerals are actually poisonous
 Some minerals react with acid and “fizz”

 Carbonate
minerals react
30.2 Minerals into Rocks
Minerals make up rocks and minerals are
made of elements that are in the earth
 1000’s of minerals and only about 24
common minerals
 The 24 minerals are made up of mainly 8
elements (out of all 88 found on earth)
 8 elements are 98% of the crust
 About ½ or 50% of the crust is oxygen

5 groups of rock-forming minerals

Silicates


90% of earths crust (oxygen and silicon bond
to make quartz)
Oxides

Oxygen with one or more metals (great
economic importance)
5 groups cont.
•Carbonates
•Limestone, carbon and oxygen
bonded
•Sulfides and Sulfates
•Sulfur as main element combined with
metallic elements
•Fools gold
30.3 On the Way to Rocks

Minerals are formed by crystallization


Growth of a solid from atoms that form
together in a certain arrangement
Overall minerals are formed from the
crystallization of magma and from the
precipitation or evaporation of water
REVIEW SLIDES
An illustration
appearance:luster,color and streak
An illustration
cleavage and fracture