Minerals introduction to earth science

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.
cation

Chlorine picks up an electron
becoming a negatively charged
anion.
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


Asbesto
s
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?