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.

Solid 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.

CLEAVAGEFRACTURE  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.