4.3 Mineral Identification

Can you identify this mineral?

Check out the mineral above. How would you figure out what kind of mineral it is? By color? Shape? Whether it's shiny or dull? Are there lines (striations) running across the minerals? This mineral has shiny, gold, cubic crystals with striations, and smells like sulfur. What is it? In this concept, we will discuss how to identify a mineral as one would "in the field," that is, without using fancy lab equipment.

How Are Minerals Identified?

There are a multitude of laboratory and field techniques for identifying minerals. While a mineralogist might use a high-powered microscope to identify some minerals, or even techniques like x-ray diffraction, most are recognizable using physical properties.

The most common field techniques put the observer in the shoes of a detective, whose goal it is to determine, by process of elimination, what the mineral in question is. The process of elimination usually includes observing things like color, hardness, smell, solubility in acid, streak, striations and/or cleavage.

Check out the mineral in the opening image. 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 concept, the properties used to identify minerals are described in more detail.

Color, Streak, and Luster

Color

Color may be the first feature you notice about a mineral, but color is not often important for mineral identification. For example, quartz can be colorless, purple (amethyst), or a variety of other colors depending on chemical impurities Figure below.

Streak

Streak is the color of a mineral's powder, which often is not the same color as the mineral itself. Many minerals, such as the quartz in the Figure above, do not have streak.

Hematite is an example of a mineral that displays a certain color in hand sample (typically black to steel gray, sometimes reddish), and a different streak color (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.

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 (Figure below) breaks between layers of sodium and chlorine to form cubes with smooth surfaces.

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

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

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.

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 concept? (Hint: It is most likely found on your dinner table.)

Summary

• Some minerals have a unique property that makes them fairly easy to identify, such as high specific gravity or salty taste.
• Color is not a reliable indicator of mineral type for most minerals, but streak is for certain minerals.
• Cleavage can be a unique and beautiful indicator of mineral type.

Review

1. How does the color of a mineral differ from it's streak and luster?
2. Does diamond exhibit cleavage or fracture? Why is this important?
3. What's the first thing you should do when trying to identify a mineral? What do you do if you still can't identify it?

Explore More

Use this resource to answer the questions that follow.

1. What are two reasons that color isn't a good way to identify minerals?
2. What is luster? List are some types of luster with one to a few words to describe each.
3. What is streak? Why is streak color a better indicator of a mineral's color?
4. What is hardness? What are the least and most hard minerals on the Moh's scale?
5. How do you use reference minerals to tell the hardness of an unknown mineral?
6. What is the difference between cleavage and fracture?
7. Would a mineral that exhibits fracture but not cleavage be a good gemstone? Why or why not?

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