Minerals

There are about 3,000 known minerals, only about 30 are common. The most common are quartz, feldspar, mica, and calcite.

Most of the Earth’s crust is made up of feldspar, followed by quartz and mica.

So what is a mineral?What are the characteristics of all minerals?

Mineral Characteristics

occurs naturally is solid has a definite chemical

composition atoms are arranged in an

orderly pattern is inorganic (was never

alive)

1. What is a mineral?A mineral is a naturally formed, inorganic solid with a crystalline structure. Rocks are actually made of many minerals but minerals are not made of rocks.

To be able to identify these and other minerals, we need to look at the properties used to separate and distinguish these minerals.

Rarely is a mineral identified by a single property.

These properties need to be considered together to correctly identify a mineral.

Identifying Minerals

Mineral propertiesO Color

O LusterO StreakO Cleavage & FractureO HardnessO Other

Define the

Property

Possible Qualitativ

e Observati

ons

Color of mineral. Color is the most easily observed mineral property and the least useful!

Define the Property

Many minerals have a similar color.

For example, pure quartz is colorless or white. Yet, impurities can make the mineral rose, purple or pink!

Many minerals can turn colors due to impurities. They may also have a mottled or wavy appearance.

Possible Qualitative Observations

Luster: the way a mineral shines in reflected light.

Notice the difference between these two minerals?

Define the Property

Certain minerals have a metallic luster, such as silver, copper and gold. Minerals that do not reflect light have a nonmetallic luster, and are described by terms like glassy, pearly, dull and silky.

Possible Qualitative Observations• metallic luster (looks like a

metal)• nonmetallic luster (does not look

like a metal)• glassy• pearly• dull• silky• greasy • earthy

Streak: the color of the minerals powder when rubbed on an unglazed white tile.

Define the Property

• The streak is often not the same color as the mineral. • A minerals color may vary, but the streak rarely will!

Possible Qualitative Observations

• color of streak• a mineral may not leave a streak

Cleavage: how it breaks.

Define the Property

• Cleavage: if it looks like it has been cut, has flat surfaces• Fracture/Irregular: does not have clean breaks

Possible Qualitative Observations

Mica is probably the best example as it splits into thin sheets. It is said to have one perfect cleavage.

Feldspar splits readily in two directions, always at or near right angles.

Cleavage is the tendency of a mineral to break along smooth planes parallel to zones of weak bonding.

Fracture is the tendency of a mineral to break along curved surfaces without a definite shape. These minerals do not have planes of weakness and break irregularly. (Ex: quartz)

Hardness: how resistant the mineral is to being scratched. We use the SCRATCH test!

Define the Property

Diamond is the hardest of all minerals, and talc is the softest.

Friedrich Mohs devised a hardness scale. In this scale, ten well known minerals are given numbers from one to ten.

Talc (left) is the softest and has a hardness of 1. A soft pencil lead will scratch talc.

Gypsum (right) is a bit harder and has a hardness of 2. A fingernail scratches gypsum.

Calcite has a hardness of 3 and a copper penny just scratches it.

Diamond, with its hardness of 10, can easily scratch the rest of the minerals.

Possible Qualitative Observations Can the mineral be

scratched with a finger nail? With a penny? With a nail?

Would a fingernail scratch calcite?NO

NOWould a steel nail scratch orthoclase?

YESWould a copper penny scratch talc?

NOWould a copper penny scratch fluorite?

YESWould a fingernail scratch gypsum?

Magnetic

Radioactive

Fluorescence

Taste

Crystal ShapesDensi

ty

Crystal shape can be a useful property to identify minerals if the minerals have had the time and space to form crystals. Most mineral grains that are found in rocks lack the room to grow.

Halite (rock salt) can be identified by its taste.

This practice is not recommended!

Knowing the density of a mineral is very useful, because the density of minerals is very consistent, meaning it does not change from one sample to the next.

For example, every piece of pure GOLD has a density close to 19.3 g/cm3. The mineral PYRITE (also called fool’s gold), looks similar to gold, but has a density of around 5.1 g/cm3.

This is the state of glowing while under a ultraviolet light.

Some minerals even glow once the light is turned off!

Some minerals, such as this uraninite, are radioactive.

They give off subatomic particles that will activate a Geiger counter.

Some minerals that contain Iron, are magnetic and can be picked up by a magnet.

Other Properties crystal

shapes taste density fluorescenc

e radioactive magnetic