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Rocks & Minerals
Geology at its finest
Minerals
• Minerals can be defined by
the 5 properties that they must
possess.
– 1) Inorganic – a mineral
must not be alive or formed
from once living material.
Coal would NOT be a
mineral because it was
once alive (organic)
– 2) Occurs naturally in the
earth. Steel and cement
would not be minerals
because they are man
made.
– 3) Always a solid.
– 4) Has a definite chemical
composition. The same
atoms in the same ratios.
– 5) The atoms form a
repeated pattern called a
crystal
Minerals (Cont.)
• There are more than
2000 minerals.
• Minerals can be made
of a single element.
– Ex. Gold, Copper,
Iron, Sulphur.
• Minerals may be a
compound.
– Ex. Halite (salt)
Mineral Formation
• Most minerals form from
cooling magma or lava.
• Some minerals form from
mineral particles dissolved in a
liquid that evaporates.
– Calcite forms this way.
• The rate of mineral formation
determines the size of the
crystal in the mineral
– When a mineral forms quickly
the mineral crystal will be
small.
– When a mineral forms slowly
the mineral crystal will be
large.
Exceptions to the 5 criteria rules of
minerals
• Diamonds.
– Diamonds ARE
considered minerals
although they form
from coal which is
organic and not
considered a mineral.
• Obsidian
– Obsidian is considered
a mineral even though
it cools so quickly that
crystals don’t form at
all.
Geodes
• A geode is a rock
with a hallow interior
lined with mineral
crystals.
– Most geodes contain
quartz crystals.
Composition of Minerals
• The 2 most common elements
that make up minerals are
silicon and oxygen.
• The next 6 most common
elements are aluminum,
calcium, iron, magnesium,
potassium, sodium.
• Even though these 8 are the
most common elements in
minerals, only 100 are made of
combinations of these
elements and fewer than 20
are found everywhere.
– Many minerals are rare.
Identifying Minerals
• There are 8 ways to help
determine what minerals
are.
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Color
Luster
Hardness
Streak
Density (specific gravity)
Crystal Shape
How it breaks (Cleavage or
Fracture)
– Special Properties
Color
• Obvious, but not always
definitive.
• Sulfur is (almost) always
yellow, and there are a
few others, but not many
minerals have a fixed
color.
• Small amounts of
impurities can drastically
change a mineral's color.
• Many minerals share the
same color. (Black is
common)
Sulfur
Luster
•
•
The quantity and quality of light
reflected from the surface.(How
shiny it is).
Most are relatively obvious, but
some minerals can exhibit a range
of lusters (ex. Hematite).
– Metallic: looks like a metal.
Metallic minerals are commonly
shiny and opaque
– Non-metallic: doesn't look like a
metal. There are many subtle
differences in the non-metallic
lusters, but most are relatively
dull, and are often transparent to
translucent on thin edges. (May
be called pearly, glassy, earthy,
even brilliant)
Galena
Asbestos
Hardness
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Hardness is the resistance of a
mineral to scratching.
It does NOT refer to how easily
the mineral is broken.
Hardness is a measure of the
bonding strength between atoms.
If these bonds are strong, the
mineral is not easily scratched.
Minerals with weaker bonds are
more easily scratched. Pencil
"lead" is softer than paper, so it
writes.
Hardness in minerals can vary
due to impurities, but is usually
definitive. We determine the
relative hardness of minerals
using a scale devised by
mineralogist Friedrich Mohs. The
scale assigns hardness to ten
common index minerals, and is
based on the ability of one mineral
to scratch another.
Hardness may be relatively
determined by the field hardness
scale (1-7)
Moh’s Hardness Scale
Hardness
Index Mineral
1
Talc
2
Gypsum
3
Calcite
4
Fluorite
5
Apatite
6
Orthoclase
7
Quartz
8
Topaz
9
Corundum
10
Diamond
Field Hardness Scale
Hardness
Item
Comments
2.5
Fingernail
We all have 10 of these to
wear out before taking off
our shoes.
3.5
Penny
Sometimes all we have
left.
4.5
Knife
The hardness of a blade
can vary wildly depending
on the quality of your knife.
Check yours before going
into the field so you'll know.
5.5
Glass
There's usually a chunk of
glass available.
7
Quartz
A piece of quartz can be
found almost everywhere.
Often steel files are used to scratch harder minerals if you can’t
identify quartz. This would not work on minerals harder than 7.
Streak
• The color of the powdered
mineral.
• The test is usually
performed by scraping the
mineral across a piece of
unglazed porcelain.
• Streak can be definitive.
Good examples include
hematite (always red-brown
no matter what form it's in)
and chromite (distinguished
from the hundreds of other
black minerals by its
chocolate-brown streak).
• Some minerals don’t streak
or streak is clear. Some
have the same color streak.
Density (Specific Gravity)
• Defined as "the weight of a
specific volume of a mineral
divided by the weight of an
equal volume of water (at
4°C.)"
• Since water is always 1.0, it's
the same number as density
without any units (they cancel).
• This is almost impossible to
measure in the field, but a
rough approximation and be
determined.
Crystal Shape
Determined by the number of faces on the mineral.
The shapes often hard to see and require a microscope
ISOMETRIC
TETRAGONAL
HEXAGONAL
TRIGONAL
ORTHORHOMBIC
MONOCLINIC
TRICLINIC
AMORPHOUS
How the mineral breaks
•
How a mineral breaks is
determined by its internal
structure, and is therefore very
important (and nearly always
diagnostic). Unfortunately, it can
also be the hardest to determine
(sorry). There are two (2) major
subdivisions: fracture and
cleavage.
– Fracture: The mineral just
breaks, leaving an uneven
surface. (jagged) Most are
irregular but there are some
special cases (ex: the
conchoidal fractures common
to quartz and glass)
– Cleavage: The mineral splits
along closely spaces parallel
planes, leaving a mirror
surface which will flash at you
if rotated in the light. Smooth
and even.
Special Properties
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Effervescence (the Fizz test):
Minerals containing calcium
carbonate (CaCO3) will generally
react when exposed to weak acid
(usually hydrochloric acid (HCl), but
even vinegar will work). Carbon
dioxide (CO2) is released and the
mineral or rock literally "fizzes."
Magnetism: Magnetite is naturally
magnetic. Don't put a chunk near
your computer!
Taste: Some minerals have a
distinctive taste. Notable examples
include Halite (rock salt), and
Chalcanthite (a copper sulfate - be
careful with this one!!). I don't
generally recommend the taste test.
Smell: Some minerals have a
distinctive odor. Sulfur is a good
example.
Fluorescence: comes from mineral
fluorite. It glows under a UV light.
Uses of minerals
• Minerals are used for
many everyday uses.
• They are often found in
ores (rock deposits
where economically
useful minerals are mined
for profit)
• They must be mined and
have impurities removed.
• Sometimes impurities are
removed by the process
of smelting. (metals)
• Can mine both metals
and non metals.
Metals
• Many useful minerals are
metals.
– Shiny
– Good conductors
– Malleability (the ability to
be hammered into thin
sheets without breaking)
– Ductility (the ability to be
drawn into wires without
breaking)
• Ex Gold, silver, copper,
iron, lead, aluminum,
ect.
Non Metals
• NOT good conductors,
malleable, ductile or shiny.
– Ex. Sulfur, asbestos, and
halite
• Includes Gems (rare, durable
and beautiful minerals)
– Precious stones – rarest and
most valuable gems,
• Ex Diamonds, emeralds,
rubies and sapphires
– Semiprecious stones- not as
rare or valuable
• Ex. Amethyst, garnet, opal,
turquoise, zircons.
Rocks
• Solids made of one or
more minerals.
• “All minerals are
rocks, but not all
rocks are minerals”
• The study of rocks is
called Petrology
– The word petrology
comes from the
apostle Peter. Matthew
16:18
Types of Rocks
• There are 3 main
types of rock.
– Igneous
– Sedimentary
– Metamorphic
Igneous Rocks
• Igneous rocks are
called fire rocks and
are formed either
underground from
magma or above
ground from lava.
• Most minerals are
also igneous rocks.
Igneous Rock Groups
•
•
A) EXTRUSIVE IGNEOUS ROCKS forms
when molten rock (lava) reaches the earth's
surface and cools.
Air and moisture cool the lava rapidly. The
quick cooling doesn't allow the formation of
large crystals so most extrusive rocks have
small crystals or none at all. Also called
FINE GRAIN
– 1) Basalt (most common fine grain)
– 2) Scoria (holes /trapping of air)
– 3) Pumice (holes /trapping of air)
– 4) Obsidian (no crystal)
•
B) INTRUSIVE IGNEOUS ROCKS
Sometimes the molten rock cools before it
reaches the surface. Molten rock that is still
underground is called magma. Because it
forms deep beneath the earth's surface, it
has more time to cool slowly and develops
large crystals. Also called COARSE GRAIN
– 1) Granite (most common coarse
grain)
– 2) Gabbro
•
C) PORPHYRITIC an igneous rock whose
cooling rate has changed and this gives it 2
or more different crystal sizes.
Sedimentary Rocks
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For thousands, even millions of
years, little pieces of our earth
(Sediments) have been eroded-broken down and worn away
by wind and water. (weathering &
erosion)
These little bits of our earth are
washed downstream where they
settle to the bottom of the rivers,
lakes, and oceans.
Layer after layer of eroded earth is
deposited on top of each.
These layers are pressed down
more and more through time, until
the bottom layers slowly turn into
rock. (compaction)
Most common rock at earth’s
surface. Fossils found in this type
of rock.
Sedimentary Rock Groups
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A) CLASTIC or DETRITAL classified by
size of sediment (mud, sand, or gravel)
– 1)Gravel sized
• a) Conglomerate (round)
• b) Breccia (angular)
– 2) Sand sized
• a) sandstone
– 3) Mud sized
• a) shale
• b) siltstone
B) ORGANIC classified by forming
directly or indirectly from once living
material.
– 1) Limestone
– 2)Chalk
– 3)Coal
C) CHEMICAL formed by chemical
means such as water evaporation and
sediments cementing together.
– 1) rock salt
– 2) gypsum
– 3) limestone
Breccia, sandstone, shale
Organic (fossiliferous) limestone and coal
Rock salt and chemical limestone
Metamorphic Rocks
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Metamorphic rocks are rocks that
have changed ("morphed“) into
another kind of rock.
These rocks were once igneous or
sedimentary rocks.
The rocks are usually buried
deeply underground.
The rocks are under tons and tons
of pressure, which fosters heat
build up, and this causes them to
change. Sometimes chemical
reactions change rocks also.
2 types of metamorphism
Contact Metamorphism rocks
heated by contact with magma or
lava. Found around the edges of
igneous rock formations.
Regional metamorphism occurs
over large areas of rock deep
underground changed by heat and
pressure
Metamorphic Rock Groups
•
2 groups of metamorphic rocks.
•
A) FOLIATED TEXTURE mineral
crystals arranged in parallel
layers. Due to different minerals
having different densities they
often separate into alternating light
and dark bands.
– 1) Slate (used to be shale)
– 2) Schist (used to be granite,
basalt or slate
– 3)Gneiss (used to be granite)
•
B) UNFOLIATED TEXTURE no
bands and do not break in layers.
– 1) Quartzite (used to be
sandstone)
– 2) Marble (used to be
limestone)
The Rock Cycle
• The continuous changing
of one rock type to
another. (Sedimentary to
igneous to metamorphic)
• It involves all processes
between types. (volcanic
activity, melting, erosion,
deposition, compaction,
burying, heat and
pressure)
Rock Cycle Animation