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3.5: Mineral Identification

Difficulty Level: At Grade Created by: CK-12
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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 lesson, 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 Figure above . 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.

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 .

White and purple quartz

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, which often is not the same color as the mineral itself. Many minerals, such as the quartz 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).

Streaking hematite across plate

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 .

Six types of non-metallic luster.
Luster Appearance
Adamantine Sparkly
Earthy Dull, clay-like
Pearly Pearl-like
Resinous Like resins, such as tree sap
Silky Soft-looking with long fibers
Vitreous Glassy

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/cm 3 ; pyrite has a density of about 5 g/cm 3 - that’s another way to tell pyrite from gold. Quartz is even less dense than pyrite and has a density of 2.7 g/cm 3 .

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.

Mohs Hardness Scale: 1 (softest) to 10 (hardest).
Hardness Mineral
1 Talc
2 Gypsum
3 Calcite
4 Fluorite
5 Apatite
6 Feldspar
7 Quartz
8 Topaz
9 Corundum
10 Diamond

(Source: http://en.wikipedia.org/wiki/Mohs_scale , Adapted by: Rebecca Calhoun, License: Public Domain)

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.

Halite has cubic cleavage.

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

Sheets of mica

Sheets of mica.

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

Fluorite has octahedral cleavage

Fluorite has 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

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

Some minerals have unusual properties that can be used for identification.
Property Description Example of Mineral
Fluorescence Mineral glows under ultraviolet light Fluorite
Magnetism Mineral is attracted to a magnet Magnetite
Radioactivity Mineral gives off radiation that can be measured with Geiger counter Uraninite
Reactivity Bubbles form when mineral is exposed to a weak acid Calcite
Smell Some minerals have a distinctive smell Sulfur (smells like rotten eggs)
Taste Some minerals taste salty Halite

(Adapted by: Rebecca Calhoun, License: CC-BY-SA)

A simple lesson on how to identify minerals is seen in this video: http://www.youtube.com/watch?v=JeFVwqBuYl4&feature=channel .

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.

Practice

Use this resource to answer the questions that follow.

1. What color streak will pyrite leave on sandpaper?

2. What color streak would real gold leave on sandpaper?

3. List some examples of the different types of quartz.

4. What characteristic do all types of quartz share?

5. What is unique about mica?

6. What is mica used for?

7. What was azurite used for in ancient times?

8. In what colors can calcite be found?

9. What color does green calcite streak?

10. What is galena?

11. How can you tell that fluorite is not a calcite mineral?

12. How many sides does garnet have?

Review

1. How does color differ from streak and luster?

2. How does cleavage differ from fracture?

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?

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Difficulty Level:

At Grade

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Date Created:

Feb 24, 2012

Last Modified:

Sep 11, 2014
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