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?

R. Density (heaviness) and hardness

Q. What is density?R. 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 crystal structure

determined in part by the strength of the bonds between the atoms

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.