22.2 Minerals
Emeralds are a form of the mineral beryl. These
gems form deep beneath Earth’s surface and are
found in relatively few locations.
22.2 Minerals
Minerals and Rocks
What is a mineral?
A mineral is a naturally occurring, inorganic
solid with a crystal structure and a
characteristic chemical composition.
22.2 Minerals
Minerals and Rocks
A rock is a solid combination of minerals or
mineral materials.
• Minerals are inorganic, meaning that living
things did not produce them and they occur
naturally.
• Geologists don’t classify coal as a mineral
because coal was created from plant remains.
• Materials like brick and concrete are not
considered minerals either.
22.2 Minerals
Minerals and Rocks
Each mineral is a unique substance with its
own chemical composition and crystal
structure.
• For most minerals, the proportion of elements
can vary slightly while the mineral still retains a
similar set of characteristics.
• Within each mineral crystal, the chemical
composition is nearly constant.
22.2 Minerals
Minerals and Rocks
Minerals are the building blocks of rocks.
• Only a fraction of the nearly 4000 known
minerals are common. These common minerals
are called the rock-forming minerals.
• Each particle within a rock is a separate mineral
crystal.
• A crystal is a solid in which atoms are arranged
in a regular repeating pattern.
22.2 Minerals
Minerals and Rocks
This portrait of
Abraham
Lincoln on Mt.
Rushmore in
South Dakota
was sculpted in
a granite cliff.
22.2 Minerals
Minerals and Rocks
Granite is made
up of quartz,
feldspar, mica,
and hornblende.
A magnified view
reveals the
individual
crystals of the
minerals that
make up granite.
Mica
Quartz
Hornblende
Feldspar
22.2 Minerals
The Properties of Minerals
What are some important properties of
minerals?
The properties by which minerals can be
identified include their crystal structure,
color, streak, luster, density, hardness,
fracture, and cleavage.
22.2 Minerals
The Properties of Minerals
Crystal Structure
In each type of mineral, the atoms are arranged in
a particular geometric shape, or crystal structure.
• Each mineral always has the same crystal structure.
• The size of a mineral’s crystals can vary.
22.2 Minerals
The Properties of Minerals
Many crystals form long prisms with a
specific number of sides. Other minerals
have crystals shaped like cubes, sheets,
needles, or threads.
Halite, the mineral form of table salt, forms
cubic crystals. Garnet, a red, semiprecious
gem, often forms 12-sided crystals.
22.2 Minerals
The Properties of Minerals
Quartz that is pure silicon
dioxide is clear or white.
Slight impurities produce a
range of colors, including
the violet quartz (amethyst)
specimen shown here.
22.2 Minerals
The Properties of Minerals
Color
Some minerals can be identified by a
characteristic color.
• Pyrite, also known as fool’s gold, is always golden in
color.
• Crystals of pure sulfur are always yellow.
• But a mineral’s color can often be deceptive,
because slight changes in composition can cause
significant changes in a mineral’s color.
22.2 Minerals
The Properties of Minerals
Streak
The color of a mineral’s powder is known as its
streak.
• A mineral’s streak can be found by scraping the
mineral on a piece of unglazed porcelain called a
streak plate.
• The color of a mineral’s streak is not always the
same as the color of the mineral itself.
22.2 Minerals
The Properties of Minerals
Although this piece of hematite is silver-gray,
it can be identified by its red-brown streak.
22.2 Minerals
The Properties of Minerals
Luster
Luster is the way in which a mineral’s surface
reflects light.
• A rough, crumbly surface is said to be an earthy
luster. Other terms that describe luster include silky,
pearly, and vitreous (glassy.)
• Galena and pyrite have a metallic luster.
• Sulfur has a resinous-to-greasy luster.
22.2 Minerals
The Properties of Minerals
Density
The density of a mineral depends on its chemical
composition.
In general, minerals made up of elements with
higher atomic masses have higher densities than
minerals made up of atoms with lower atomic
masses.
22.2 Minerals
The Properties of Minerals
Galena contains much lead, which has a
relatively high atomic mass of 207. Galena’s
density is about 7.5 grams per cubic
centimeter.
Quartz is made up of silicon and oxygen,
which have relatively low atomic masses of
28 and 16 respectively. Quartz’s density is
only about 2.6 grams per cubic centimeter.
22.2 Minerals
The Properties of Minerals
Galena
Pyrite
Sulfur
22.2 Minerals
Density of Minerals
The density of minerals varies, depending on
what elements the minerals contain. Samples
of the minerals in the data table were
analyzed for density, silicon and oxygen
content, and the presence of metals. Study
the data table and then answer the
questions.
22.2 Minerals
Density of Minerals
1. Using Tables Which mineral has the lowest density? The
highest density?
22.2 Minerals
Density of Minerals
1. Using Tables Which mineral has the lowest density? The
highest density?
Answer: quartz; olivine
22.2 Minerals
Density of Minerals
2. Using Tables Which minerals have the lowest percentage
of silicon and oxygen? Which has the highest?
22.2 Minerals
Density of Minerals
2. Using Tables Which minerals have the lowest percentage
of silicon and oxygen? Which has the highest?
Answer: olivine and augite; quartz
22.2 Minerals
Density of Minerals
3. Formulating Hypotheses Olivine and augite are
abundant in oceanic crust and in the mantle. Quartz,
muscovite, and hornblende are abundant in continental crust.
Formulate a hypothesis to explain why continental crust
floats higher on the mantle than oceanic crust.
22.2 Minerals
Density of Minerals
3. Formulating Hypotheses Olivine and augite are
abundant in oceanic crust and in the mantle. Quartz,
muscovite, and hornblende are abundant in continental crust.
Formulate a hypothesis to explain why continental crust
floats higher on the mantle than oceanic crust.
Answer: Continental crust is composed of less dense
minerals than oceanic crust. Therefore, continental crust is
more buoyant and floats higher on the mantle.
22.2 Minerals
Density of Minerals
4. Drawing Conclusions How is a mineral’s density related
to its silicon and oxygen content?
22.2 Minerals
Density of Minerals
4. Drawing Conclusions How is a mineral’s density related
to its silicon and oxygen content?
Answer: In general, the higher the silicon and oxygen content
of a mineral, the lower its density.
22.2 Minerals
The Properties of Minerals
Hardness
The atoms of minerals are held together by
chemical bonds of different kinds and strengths.
Hardness is the resistance of a mineral to
scratching.
22.2 Minerals
The Properties of Minerals
Hardness
A hard mineral can scratch a softer mineral.
The hardness of minerals is ranked on a scale from 1 to
10, called Mohs hardness scale.
• Diamond is the hardest mineral, with a hardness of 10.
• Talc is one of the softest minerals, with a hardness of 1.
22.2 Minerals
The Properties of Minerals
Geologists use scratch tests to determine the
hardness of mineral specimens.
• A glass plate has a hardness of 5.5. A mineral
that scratches the plate has a hardness greater
than 5.5.
• A copper penny, which has a hardness of about
3.5, can scratch talc, but it cannot scratch
quartz.
• A fingernail, with a hardness of 2.5, and a
streak plate, with a hardness of 6.5, can also be
used to test hardness.
22.2 Minerals
The Properties of Minerals
Fracture and Cleavage
The fracture of a mineral is how the mineral
breaks.
Fracture is determined by the crystalline structure
of the mineral and the bonds between the atoms
in the crystals.
22.2 Minerals
The Properties of Minerals
Cleavage is a type of fracture in which the
mineral tends to split along regular, welldefined planes where the bonds are weakest.
• Mica and graphite form sheets. Each sheet
contains chemical bonds that are very strong.
• The sheets are held together with weak bonds,
so they can easily be peeled apart from each
other.
22.2 Minerals
The Properties of Minerals
Mica forms in thin, flat sheets that can be
easily peeled apart. When halite is broken
apart, it forms small cubes that show its
crystal structure.
22.2 Minerals
The Properties of Minerals
Other Properties
Some minerals have unusual properties.
• Acids dissolve calcite easily.
• Magnetite is strongly attracted by a magnet.
• Fluorescent minerals like fluorite give off visible light
when they are held under an ultraviolet light.
22.2 Minerals
The Properties of Minerals
Some minerals have unusual electrical
properties.
Quartz and tourmaline, for example, become
electrically charged when heated and cooled
or subjected to pressure.
Quartz’s electrical properties have
applications in electronics equipment.
22.2 Minerals
Assessment Questions
1. Which of the following properties would not be
used to identify an unknown mineral?
a.
b.
c.
d.
crystal shape
density
hardness
size
22.2 Minerals
Assessment Questions
1. Which of the following properties would not be
used to identify an unknown mineral?
a.
b.
c.
d.
crystal shape
density
hardness
size
ANS: D