Minerals

What are minerals?7th Grade Science

Minerals

A mineral is defined as a naturally-occuring, inorganic solid with a definite chemical composition and an orderly arrangement of atoms.

Minerals are the building blocks of rocks. Most rocks are typically made with crystals from several different minerals.

About 4000 minerals are found on Earth, but they all share four characteristics.

Characteristics of Minerals

Minerals are natrually occuring: they form from processes that occur on or in the Earth with no input from humans.

Minerals are inorganic: They are not made from life processes.

Every mineral is an element or a compound with a definite composition. For example Halite, which is ALWAYS NaCl.

Minerals are crystalline solids.

Crystalline Solids

Crystal: a solid in which the atoms are arranged in an orderly pattern that is repeated over and over again.

This crystalline quality is one thing that separates minerals from other elements.

For example, Graphite has atoms that are arranged in a repeated set of layers. Opal, on the other hand, does not have a definate, repeating pattern of atoms. So, although both are natrually

occuring and inorganic, only graphite can be a mineral.

Crystal Systems

Not all mineral crystals are smooth and regularly-shaped. The look of the crystal of a mineral is based upon where and how the crystal forms (in a tight space, in an open space, etc.)

A crystal system is a group of crystals that have similar atomic arrangements and, therefore, similar external crystal shapes.

There are six major crystal systems: hexagonal, cubic,

tetragonal, orthorhombic, monoclinic, and triclinic.

Crystal Formation

Crystals form in many ways, but two important ones are crystals formed from magma, and

crystals from solutions of salts.

Crystals from Magma

Magma: Hot, melted rock under the Earth's surface. It cools when it reaches Earth's surface.

When magma cools slowly, large crystals form. You can see them with the naked eye. If it cools rapidly, the crystals will be small and unable to be seen individually.

Crystals From Solutions

Crystals can also form from minerals dissolved in water.

When water evaporates, ions that are left behind can come together to form crystals.

Also, if too much of a substance (saturation) is dissolved in water, ions of that substance can come together in the solution to form crystals. In this way, crystals can be formed without evaporation.

Mineral Compositions

Most common rock-forming minerals belong to a group called silicates. Silicates are minerals that contain silicon and oxygen, and usually one or two other elements.

Silicon and oxygen combine to form the basic building blocks of most of the minerals in Earth's crust.

One common silicate is quartz.

Other minerals are grouped according to their compositions.

Physical Properties of Minerals

There are seven ways that we can use physical properties to indentify minerals.

Appearance

Color and Appearance are one way to identify minerals.

These clues alone, however, are not usually enough to completely identify a mineral.

An example of this are the mineral pyrite and gold. Both look alike and have the same golden color. Pyrite is often referred to as fool's gold. Other methods of identifying the minerals must be used to truly distinguish them.

Hardness

Hardness is a measure of how easily a mineral can be scratched. The mineral talc is so soft that your fingernail can scratch it, while

diamonds, on the other hand, are the hardest mineral. In 1824, the Austrian scientist Frederich Mohs developed a list of

common minerals to compare their hardness. This list is called the Mohs scale of hardness.

The scale lists the hardness of ten minerals. Talc, the softest mineral, has a hardness of one, while diamond has a hardness of ten.

You use the scale by taking an unknown mineral and trying to scratch is with either another mineral or something such as your fingernail or an iron nail. Because other minerals have a hardness assigned to them, and so do your fingernail or the nail, you can see whether or not these items scratch the mineral. This lets you estimate a hardness number and determine which mineral you have.

Mohs Hardness Scale

Luster

Luster is the way a mineral reflects light. Luster can be metallic or nonmetallic. Minerals with a metallic luster shine like

metal, and can, therefore, be compared to other metal objects.

Minerals that do not shine like metal are nonmetallic. Some names for this are dull, pearly, and silky.

Specific Gravity

Minerals can also be distinguished by their specific gravity

Specific gravity: the ratio of a mineral's weight compared with the weight of an equal volume of water.

This is expressed as a number. Going back to our gold and pyrite, gold's specific

gravity is about 17, while pyrite's is about 5. This means that an equal volume of gold is about 17-times heavier than water, and an equal volume of pyrite is about 5-times heavier than water. The pyrite would feel much lighter than the gold.

Streak

When a mineral is rubbed across a piece of unglazed porcelain tile, it produced a streak of powder.

Streak is the color of a mineral when it is in powder form.

The streak test works only for minerals that are softer than the streak plate.

Gold would have a yellow streak, while pyrite's would be greenish-black.

Cleavage and Fracture

The way a mineral breaks can also tell you what the mineral is.

Minerals that break along smooth, flat surfaces have cleavage. Cleavage is determined by the arrangement of the mineral's atoms.

Cleavage is like a layer cake that can be undone into its nice, even layers.

Minerals that break unevenly or have jagged surfaces have fracture. This would be like grabbing a chunk out of the side of the cake.