Lesson Objectives

• Explain how minerals are identified.
• Describe how color, luster, and streak are used to identify minerals.
• Summarize specific gravity.
• Explain how the hardness of a mineral is measured.
• Describe the properties of cleavage and fracture.
• Identify additional properties that can be used to identify some minerals.

Vocabulary

• cleavage
• density
• fracture
• hardness
• luster
• mineralogist
• streak

Introduction

Minerals can be identified by their physical characteristics. The physical properties of minerals are related to their chemical composition and bonding. Some characteristics, such as a mineral’s hardness, are more useful for mineral identification. Color is readily observable and certainly obvious, but it is usually less reliable than other physical properties.

How are Minerals Identified?

Mineralogists are scientists who study minerals. One of the things mineralogists must do is identify and categorize minerals. While a mineralogist might use a high-powered microscope to identify some minerals, most are recognizable using physical properties.

Check out the mineral in Figure below. What is the mineral’s color? What is its shape? Are the individual crystals shiny or dull? Are there lines (striations) running across the minerals? In this lesson, the properties used to identify minerals are described in more detail.

This mineral has shiny, gold, cubic crystals with striations, so it is pyrite.

Color, Streak, and Luster

Diamonds are popular gemstones because the way they reflect light makes them very sparkly. Turquoise is prized for its striking greenish-blue color. Notice that specific terms are being used to describe the appearance of minerals.

Color

Color is rarely very useful for identifying a mineral. Different minerals may be the same color. Real gold, as seen in Figure below, is very similar in color to the pyrite in Figure above.

This mineral is shiny, very soft, heavy, and gold in color, and is actually gold.

The same mineral may also be found in different colors. Figure below shows one sample of quartz that is colorless and another quartz that is purple. A tiny amount of iron makes the quartz purple. Many minerals are colored by chemical impurities.

Purple quartz, known as amethyst, and clear quartz are the same mineral despite the different colors.

Streak

Streak is the color of a mineral’s powder. Streak is a more reliable property than color because streak does not vary. Minerals that are the same color may have a different colored streak. Many minerals, such as the quartz in the Figure above, do not have streak.

To check streak, scrape the mineral across an unglazed porcelain plate (Figure below). Yellow-gold pyrite has a blackish streak, another indicator that pyrite is not gold, which has a golden yellow streak.

The streak of hematite across an unglazed porcelain plate is red-brown.

Luster

Luster describes the reflection of light off a mineral’s surface. Mineralogists have special terms to describe luster. One simple way to classify luster is based on whether the mineral is metallic or non-metallic. Minerals that are opaque and shiny, such as pyrite, have a metallic luster. Minerals such as quartz have a non-metallic luster. Different types of non-metallic luster are described in Table below.

Can you match the minerals in Figure below with the correct luster from Table above?

(a) Diamond has an adamantine luster. (b) Quartz is not sparkly and has a vitreous, or glassy, luster. (b) Sulfur reflects less light than quartz, so it has a resinous luster.

Specific Gravity

Density describes how much matter is in a certain amount of space: density = mass/volume.

Mass is a measure of the amount of matter in an object. The amount of space an object takes up is described by its volume. The density of an object depends on its mass and its volume. For example, the water in a drinking glass has the same density as the water in the same volume of a swimming pool.

Gold has a density of about 19 g/cm³; pyrite has a density of about 5 g/cm³ – that’s another way to tell pyrite from gold. Quartz is even less dense than pyrite and has a density of 2.7 g/cm³.

The specific gravity of a substance compares its density to that of water. Substances that are more dense have higher specific gravity.

Hardness

Hardness is a measure of whether a mineral will scratch or be scratched. Mohs Hardness Scale, shown in Table below, is a reference for mineral hardness.

With a Mohs scale, anyone can test an unknown mineral for its hardness. Imagine you have an unknown mineral. You find that it can scratch fluorite or even apatite, but feldspar scratches it. You know then that the mineral’s hardness is between 5 and 6. Note that no other mineral can scratch diamond.

Cleavage and Fracture

Breaking a mineral breaks its chemical bonds. Since some bonds are weaker than other bonds, each type of mineral is likely to break where the bonds between the atoms are weaker. For that reason, minerals break apart in characteristic ways.

Cleavage is the tendency of a mineral to break along certain planes to make smooth surfaces. Halite breaks between layers of sodium and chlorine to form cubes with smooth surfaces (Figure below).

A close-up view of sodium chloride in a water bubble aboard the International Space Station.

Mica has cleavage in one direction and forms sheets (Figure below).

Sheets of mica.

Minerals can cleave into polygons. Fluorite forms octahedrons (Figure below).

This rough diamond shows its octahedral cleavage.

One reason gemstones are beautiful is that the cleavage planes make an attractive crystal shape with smooth faces.

Fracture is a break in a mineral that is not along a cleavage plane. Fracture is not always the same in the same mineral because fracture is not determined by the structure of the mineral.

Minerals may have characteristic fractures (Figure below). Metals usually fracture into jagged edges. If a mineral splinters like wood, it may be fibrous. Some minerals, such as quartz, form smooth curved surfaces when they fracture.

Chrysotile has splintery fracture.

Other Identifying Characteristics

Some minerals have other unique properties, some of which are listed in Table below. Can you name a unique property that would allow you to instantly identify a mineral that’s been described quite a bit in this chapter? (Hint: It is most likely found on your dinner table.)

A simple lesson on how to identify minerals is seen in this video.

Lesson Summary

• Minerals have distinctive properties that can be used to help identify them.
• Color and luster describe the mineral’s outer appearance. Streak is the color of the powder.
• A mineral has a characteristic density.
• Mohs Hardness Scale is used to compare the hardness of minerals.
• Cleavage or the characteristic way a mineral breaks depends on the crystal structure of the mineral.
• Some minerals have special properties that can be used to help identify them.

Review Questions

1. Which properties of a mineral describe the way it breaks apart?
2. A mineral looks dry and chalky. What sort of luster does it have?
3. What causes a mineral to have the properties that it has?
4. Apatite scratches the surface of an unknown mineral. Which mineral would you use next to test the mineral’s hardness — fluorite or feldspar? Explain your reasoning.
5. Why is streak more reliable than color when identifying a mineral?
6. Mineral A has a density of 5 g/cm³. Mineral B is twice as dense as Mineral A. What is the density of Mineral B?
7. Why do some minerals cleave along certain planes?

Further Reading / Supplemental Links

• Mineral Color
• Physical Characteristics of Minerals.
• Mineral Identification.

Points to Consider

• If a mineral is magnetic, do you know for certain what mineral it is?
• Some minerals are colored because they contain chemical impurities. How did the impurities get into the mineral?
• What two properties of a mineral sample would you have to measure to calculate its density?
• How much do minerals reflect the environment in which they formed?