Section 1:Properties of Minerals

After completing the lesson, students will be able to . . .

Identify the characteristics of a mineral; Identify the properties of minerals and

explain how minerals are identified.

Mineral—A naturally occurring, inorganic solid that has a crystal structure and a definite chemical composition.

There are more than 3,000 minerals that geologists have identified, with 300 being the only ones that are common.

About 20 minerals make up most of the rocks of Earth’s crust—these are known as rock-forming minerals.

To be classified as a mineral, a substance must occur naturally.

Cement, brick, steel, and glass all come from substances found in Earth’s crust, but since they are created and manufactured by people, they are not considered to be minerals.

Inorganic—Not formed from living things or the remains of living things. Example:

Even though coal is formed naturally in the crust, it is not considered a mineral because it is formed from the remains of living things.

A mineral is always solid, with a definite volume and shape.

The particles that make up a solid are packed together very tightly, so they cannot move like the particles that make up a liquid.

The solid keeps its shape because its particles can’t flow freely.

The particles in a mineral line up in a pattern that repeats over and over again.

Crystal—A solid in which the atoms are arranged in a pattern that repeats over and over again.

A crystal has flat sides, called faces, that meet at sharp edges and corners.

Sometimes, the crystal structure is obvious, but many minerals have to be viewed through a microscope to see the crystal patterns.

A few minerals, such as opal, are considered minerals even though their particles are not arranged in a crystal structure.

A mineral has a definite chemical structure, which means that a mineral always contains certain elements in definite proportions.

Element—A substance composed of a single kind of atom.

All the atoms of the same element have the same chemical and physical properties.

Almost all minerals are compounds.

Compound—A substance in which two or more elements are chemically joined.

Each compound has its own properties, which usually differ greatly from the properties of the elements that form it.

Some elements occur in nature in a pure form, not as part of a compound. Examples:

Gold, Copper, and Silver.

Because there are so many different kinds of minerals, telling them apart can be a challenge.

The color of a mineral alone often provides too little information to make an identification.

Each mineral has its own specific properties that can be used to identify it.

You can identify some properties by looking at minerals, but you may need to do more tests to properly identify it.

When you identify a mineral, one of the best clues you can use is the mineral’s hardness.

Mohs hardness scale—A scale ranking ten minerals from softest to hardest.

Look at figure 5 on page 122.

Read this section on page 122.

Streak—The color of a mineral’s powder.

You can observe a streak by rubbing a mineral against a piece of unglazed tile called a streak plate.

Even though the color of the mineral may vary, its streak does not.

Surprisingly, the streak color and the mineral color are often different.

Luster—The way a mineral reflects light from its surface.

Minerals containing metals are often shiny. Example:

Galena (Figure 7: Picture A; pg 123) Other minerals, such as quartz, have a

glassy luster. Some of the other terms used to describe

luster may include: Earthy Waxy Pearly

Read this section on page 123.

The crystals of each mineral grow atom by atom to form the mineral’s particular crystal structure.

Geologists classify these structures into six groups based on the number and angle of the crystal faces, which are called the crystal systems. Cubic Hexagonal Tetragonal Orthorhombic Monoclinic Triclinic

Crystals that grow in open space can be almost perfectly formed.

Crystals that grow in a tight space are often incompletely formed.

Look at figure 8 on page 24.

CLEAVAGE FRACTURE

The way a mineral breaks apart can help to identify it.

Cleavage—A mineral’s ability to split easily along flat surfaces.

Whether a mineral has cleavage depends on how the atoms in its crystals are arranged.

Depending on the arrangement of the atom in the mineral, it will break apart more easily in one direction than another.

Most minerals do not split apart evenly, but instead have a type of fracture.

Fracture—The way a mineral looks when it breaks apart in an irregular way.

Read the remainder of this section on page 125.

Fluorescence—The property of a mineral in which the mineral glows under ultraviolet light.

Magnetism is another special property that occurs naturally in a few minerals.

Some minerals react chemically to acid, and some produce a small electric current when pressure is applied.