2.2 Minerals are basic building
blocks of Earth.
A mineral is identified by
its properties.
A mineral’s appearance helps identify it.
• Identify a mineral by it’s
properties (characteristic
features)
– color
• minerals can occur in more than
one color
– how the mineral reflects light,
how shiny or dull it is
– appearance
Color and Streak
• Color
– Some minerals can be almost any color
– Most minerals have a more limited color range
– Three main factors cause minerals to vary in color
• Tiny amounts of an element that is not part of its normal
chemical makeup.
– pure quartz is clear and colorless, but tiny amounts of iron can give
quartz a violet color (amethyst)
• At or near Earth’s surface and is in contact with the
atmosphere or water
• Defects in crystal structures that change color
Color and Streak
• Streak
– May have a different color when they are ground
into a fine powder
– Streak is the color of the powder left behind when
the mineral is scraped across a surface.
– Geologists use a tile of unglazed porcelain (streak
plate), to identify minerals
– Better clue than surface color for identification
– All samples of the same mineral have the same
streak.
Luster
• The way light reflects from the minerals surface
• Metallic and nonmetallic
– Metallic looks as if it was made of metal
– Nonmetallic luster can be shiny, but it does not
appear to be made of metal
• Luster may vary from sample to sample
• Exposure to the atmosphere or to water can
cause the surface luster to become dull
• If broken its characteristic luster can be seen
Color and Streak, Luster
A mineral’s color may change when the mineral is in
contact with the atmosphere or water.
Minerals may get their color from small amounts of
another element.
The crystals of a mineral can have defects that may
change their color.
Color and Streak, Luster
Q. What is streak?
A. Streak is the color of the mineral powder
left behind when a mineral is scraped
across a surface.
Q. What is luster?
A. Luster describes the way in which light
reflects from the surface.
Cleavage
• Each type of mineral, if hit with a hammer, will
break in the same way
• Better clue to its identity than color and luster
• Cleavage is the tendency of a mineral to break
along flat surfaces
– Depends on how the atoms are bonded
• Bonds of the crystal structure are weaker in the directions
in which the mineral breaks
• Cleavage of a mineral is described by the
direction in which the mineral breaks and the
smoothness of the broken
The way a mineral breaks helps
identify it.
Q. Why can the way a mineral breaks help
identify it?
A. Each kind of mineral always breaks in the
same way, making it a better identifier than
luster or color.
Cleavage
Fracture
• Fracture is the tendency of a mineral to break
into irregular pieces
– Some break into pieces with curved surfaces
– Others may break into splinters or rough or jagged
pieces.
• Fracture is displayed when the bonds that join
the atoms are fairly equal in strength in all
directions.
• Does not break along flat surfaces because
there are no particular directions of weakness
in its crystal structure.
Fracture
Q. Why do some minerals break by fracturing?
A. The bonds between the atoms of the
minerals are of fairly equal strength in all
directions, so there is no particular direction
of weakness.
A mineral’s density and hardness help
identify it.
• Density is the amount of mass in a given
volume of the substance
• Very helpful in identifying minerals
– Ex. Gold and pyrite look the same but gold is
much denser.
• Mass of gold is almost four times the mass of a piece of
pyrite of the same size.
• Dense minerals have more mass and are heavier
• Determined by the kinds of atoms that make
up the mineral and how closely they are
joined together.
• Hardness
Hardness
– A mineral’s resistance to being scratched
– Determined by its crystal structure and the
strength of the bonds between its atoms
• Harder minerals have stronger bonds
• A scale known as the Mohs scale is often used
to describe a mineral’s hardness
– A harder mineral will scratch a softer one
– Ten minerals are numbered in the scale from
softest to hardest
• Talc, the softest, has a value of 1
• Diamond, the hardest, has a value of 10
• Can only be scratched by other minerals that
have the same hardness or are harder
• To determine hardness, test whether it
scratches or is scratched by the minerals in the
scale
• Can also use fingernail, a copper penny (1982
or older), and a steel file to test an unknown
mineral.
A mineral’s density and hardness help
identify it.
Q. What are two properties of minerals that you could
sense with your eyes closed?
A. Density (heaviness) and hardness
Q. What is density?
A. The mass of a substance divided by its volume, it
helps to describe how heavy a mineral is compared
to its size.
Q. Which material would scratch the other—quartz or
calcite? How do you know?
A. Quartz; It has a higher value on the Mohs scale
Hardness
a mineral’s resistance
to being scratched
determined in part by the
strength of the bonds
between the atoms
determined in part by
crystal structure
often described using the
Mohs scale of hardness
Some minerals have special properties.
• Carbonate group
– React with acid.
• Chalk
– Tested by putting a drop of a weak solution of
hydrochloric acid on a sample.
– If CO2 gas bubbles out of the acid, then the
mineral is a carbonate.
• Fluorescence
– Minerals glow when they are exposed to
ultraviolet light
– Fluorite, calcite and willemite
– Limited value in mineral identification
• samples of the same mineral may or may not display
fluorescence
• may glow in different colors.
Some minerals have special properties.
• Magnetism
– A magnet is pulled toward some minerals
• Magnetite strongly attracts magnets
• Some other minerals weakly attract magnets
– Magnets are commonly used in laboratories and
industries to separate magnetic minerals from
other minerals.
• Radioactivity
– As unstable elements change into other
elements over time, they release energy
– Geologists can measure this energy and use it to
identify minerals that contain unstable elements.
Some minerals have special properties.
Q. What are four special properties of some
minerals?
• A.
– Carbonate group reacts with acid.
– Some minerals glow under ultraviolet light.
– Some minerals respond to magnets.
– Some minerals give off radiation.