Mineral Identification
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Printed: October 2, 2012
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C ONCEPT
Concept 1. Mineral Identification
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Mineral Identification
Lesson Objectives
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•
•
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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
The tendency of a mineral to break along certain planes to make smooth surfaces.
density
The amount of matter in a certain amount of space; mass divided by volume.
fracture
(minerology) The way a mineral breaks when it is not broken along a cleavage plane.
hardness
The ability of a mineral to resist scratching.
luster
The way light reflects off of the surface of the mineral.
mineralogist
A scientist who studies minerals.
streak
The color of the powder of a mineral.
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.
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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 1.1. 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.
FIGURE 1.1
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 1.2, is very similar in color to the pyrite in Figure 1.1.
The same mineral may also be found in different colors. Figure 1.3 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.
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 above, do
not have streak.
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Concept 1. Mineral Identification
FIGURE 1.2
This mineral is shiny, very soft, heavy, and
gold in color, and is actually gold.
FIGURE 1.3
Purple quartz, known as amethyst, and
clear quartz are the same mineral despite
the different colors.
To check streak, scrape the mineral across an unglazed porcelain plate (Figure 1.4). Yellow-gold pyrite has a
blackish streak, another indicator that pyrite is not gold, which has a golden yellow streak.
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 1.1.
TABLE 1.1: Six types of non-metallic luster.
Luster
Adamantine
Earthy
Pearly
Resinous
Silky
Vitreous
Appearance
Sparkly
Dull, clay-like
Pearl-like
Like resins, such as tree sap
Soft-looking with long fibers
Glassy
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FIGURE 1.4
The
streak
of
hematite
across
an
unglazed porcelain plate is red-brown.
Can you match the minerals in Figure 1.5 with the correct luster from Table 1.1?
FIGURE 1.5
(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
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Concept 1. Mineral Identification
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
1.2, is a reference for mineral hardness.
TABLE 1.2: Mohs Hardness Scale: 1 (softest) to 10 (hardest).
Hardness
1
2
3
4
5
6
7
8
9
10
Mineral
Talc
Gypsum
Calcite
Fluorite
Apatite
Feldspar
Quartz
Topaz
Corundum
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 breaks between
layers of sodium and chlorine to form cubes with smooth surfaces (Figure 1.6).
Mica has cleavage in one direction and forms sheets (Figure 1.7).
Minerals can cleave into polygons. Fluorite forms octahedrons (Figure 1.8).
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 1.9). Metals usually fracture into jagged edges. If a mineral
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FIGURE 1.6
A close-up view of sodium chloride in a water bubble aboard the International Space Station.
FIGURE 1.7
Sheets of mica.
FIGURE 1.8
Fluorite has octahedral cleavage.
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 1.3. 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.)
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Concept 1. Mineral Identification
FIGURE 1.9
Chrysotile has splintery fracture.
TABLE 1.3: Some minerals have unusual properties that can be used for identification.
Property
Fluorescence
Magnetism
Radioactivity
Reactivity
Smell
Taste
Description
Mineral glows under ultraviolet
light
Mineral is attracted to a magnet
Mineral gives off radiation that can
be measured with Geiger counter
Bubbles form when mineral is exposed to a weak acid
Some minerals have a distinctive
smell
Some minerals taste salty
Example of Mineral
Fluorite
Magnetite
Uraninite
Calcite
Sulfur (smells like rotten eggs)
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=JeFVwqBuY
l4&feature=channel.
Lesson Summary
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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?
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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?
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