2.2 Minerals

Big Idea: Minerals are basic building blocks of Earth.

Key Concept: A mineral is identified by its properties.

Remember: Minerals are all around us.

VOCABULARY:

1. streak- the color of the powder left behind when the mineral is scraped across a surface

2. luster- the way in which light reflects from a mineral’s surface

3. cleavage- the tendency of a mineral to break along flat surfaces

4. fracture- the tendency of a mineral to break into irregular pieces

5. density- the amount of mass in a given amount of a substance

6. hardness- a mineral’s resistance to being scratched

A mineral’s appearance helps identify it. (p 50)

To identify a mineral, you need to observe its properties. You may observe its color, the way it reflects light, or how hard it is.

Color and streak (p 51)

· Most minerals have a limited color range (like from brown to black).

· Some minerals can be almost any color.

· 3 main factors cause minerals to vary in color:

1) Tiny amounts of an element that is not part of its normal chemical makeup.

Pure quartz is clear and colorless.

Tiny amounts of iron give quartz a violet color.

2) A mineral’s color can change when it is at or near Earth’s surface and in contact with air or water.

3) Mineral crystals may have defects that change color.

· Streak is the color of the powder left behind when the mineral is scraped across a surface.

· Geologists use an unglazed porcelain tile (streak plate) as the surface to scrape.

· Streak is a better identifier than color. Two samples of the same mineral may be different colors, but they will leave the same color streak.

Luster (p 52)

· The way in which light reflects from a mineral’s surface.

· 2 major types:

1) metallic (pyrite)

2) nonmetallic (garnet)

*Minerals can still be shiny, but not like metal.

· One sample of a mineral may have more luster than another sample of the same mineral. Exposure to air and water causes this. If you break the mineral that is more dull, though, it’s true luster will be seen.

The way a mineral breaks helps to identify it. (p 52)

The way a mineral breaks is a better clue to its identity than its color or luster.

Cleavage (p 53)

· Cleavage is a mineral’s tendency to break along flat surfaces.

· Depends on how the mineral’s atoms are bonded.

Carbon bonds in graphite.

· Geologists look at the direction and the smoothness of the breaks.

Fracture

· Fracture is the tendency of a mineral to break into irregular pieces.

· Minerals that fracture have bonds of fairly equal strength. They break in any direction because there are no real weaknesses in the crystal structure.

A mineral’s density and hardness help identify it. (p 54)

Density

· The amount of mass in a given volume of a substance.

· Gold and pyrite (fool’s gold) look very similar. You can tell them apart by their densities. Gold is much denser, almost four times as dense as pyrite. It will feel much heavier.

Hardness

· Hardness is a mineral’s resistance to being scratched.

· Determined by the mineral’s crystal structure.

· Mohs scale is used to describe hardness.

· This scale is based on the fact that a harder mineral will scratch a softer one.

· Diamond is the hardest of all minerals, with a hardness of 10.

· To test the hardness, you scratch the mineral with one of the minerals from the scale.

· Or you could use: fingernail (2.5), steel file (6.5), pennies from 1982 or earlier (3)

Some minerals have special properties. (p 56)

· Carbonates such as calcite react with acid.

· Fluorescent minerals glow.

· Some minerals are magnetic.

· Some are radioactive.