Download Chapter 4: Minerals

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Chapter 4: Minerals
4.1 What is a Mineral?
Main Idea: Minerals are naturally occurring, inorganic solids, with a specific chemical composition and a definite
crystalline structure.
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Earth’s crust is composed of about 3000 different minerals that play important roles in forming rocks and in
shaping Earth’s surface.
Mineral Characteristics (must have all five):
1) Naturally occurring, meaning that they are formed by natural processes.
2) All minerals are inorganic: they are not made from living organisms.
3) Minerals are solids and therefore have definite shapes and volumes.
Discussion Topic:
Is Ice a Mineral ?
Video to Watch:
Intro to Minerals
4) Each type of mineral has a chemical composition unique to that mineral.
5) The atoms in minerals are arranged in regular, repeated, geometric patterns that result in the formation of a
crystal. A crystal is a solid in which the atoms are arranged in repeating patterns.
 A few minerals are composed of single elements; however, the majority are made from compounds.
 In some minerals, such as feldspar, chemical composition can vary within a certain range, depending on the
temperature at which the mineral crystallizes.
Rock-Forming Minerals
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Eight to ten of the 30 most-common minerals are
referred to as rock-forming minerals because they
make up most of the rocks in Earth’s crust.
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Rock-forming minerals are primarily composed of
the eight most common elements in Earth’s crust.
Two Ways Minerals Form
1. From Cooling Magma
a. Magma (molten material below Earth’s surface) is less dense than the surrounding solid rock, so it can
rise upward into the cooler layers of Earth’s interior, where it cools and crystallizes.
b. The type and number of elements in the magma determine which minerals will form.
c. The rate at which the magma cools determines the size of the mineral crystals. Small crystals form
from rapidly cooling magma, and large crystals form from slowly cooling magma.
2. From a Supersaturated Solution
a. A liquid is saturated when it becomes full of a dissolved substance and cannot dissolve any more of it.
b. If more of the substance is added to a saturated solution, it
is called supersaturated, and the dissolved mineral crystals
from the solution form into solids called evaporites.
Identifying Minerals
Geologists identify minerals using tests based on a mineral’s physical and chemical properties, such as crystal form,
luster, hardness, cleavage, fracture, streak, color, specific gravity, texture, density, and special properties.
Crystal Form: Some minerals form such distinct crystal shapes that they are immediately recognizable.
However, perfect crystals are not always formed, so identification based only on crystal form is rare.
Luster: The way a mineral reflects light from its surface; there are two types—metallic and nonmetallic.
Luster should be used in combination with other physical characteristics to identify a mineral.
Hardness: Measure of how easily a mineral can be scratched.
German geologist Friedrich Mohs developed a scale (Moh’s
Scale) by which an unknown mineral’s hardness can be
compared to the known hardness of ten easily recognized
minerals that are readily found in nature (except diamond).
Cleavage: A mineral has cleavage if it splits relatively easily and
evenly along one or more planes of weak atomic bonds.
To identify a mineral based on its cleavage, geologists count
the number of cleaved planes and study the angle or angles between them.
Fracture: Minerals that break with rough, arclike, or jagged edges because of their tightly bonded atoms.
Streak: Color of a mineral when it is broken up and powdered.
The streak test, which typically involves rubbing the mineral against an unglazed
porcelain plate (streak plate), is most useful in identifying metallic minerals. It
can be used only on minerals that are softer than the porcelain plate.
Color: Eye’s perception of light wavelengths reflected by a mineral, caused by the presence of trace elements.
It is one of the most noticeable, but least reliable properties used to identify a mineral because it is subjective,
many minerals have the same color, and some can be more than one color to due trace elements (e.g. quartz).
Texture: How a mineral feels to the touch, and, like luster, it is subjective.
It is often used in combination with other tests to identify a mineral.
Density: Expressed as D = M / V where D = density, M = mass, and V = volume.
Because density is not dependent on the size or shape of a mineral, it is a useful identification tool.
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Specific Gravity: Ratio of the mass of a substance to the mass of an equal volume of water at 4°C.
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Water conveniently has a density of 1.0 g/mL (or 1.0 g/cm3), so a mineral with a density of 5.2 g/mL
would simply have a specific gravity of 5.2 (since 5.2 g/mL divided by 1g/mL = 5.2).
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Specific gravity is the most common measure of density used by geologists.
 Several special properties of minerals can also be used for identification purposes: magnetism, striations,
double refraction, effervescence with hydrochloric acid, and fluorescence.
 All the physical properties of a mineral result from the mineral’s internal arrangement of atoms.
4.2 Types of Minerals
Main Idea: Minerals are classified based on their chemical properties and characteristics.
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In order to study the many minerals on Earth and understand their properties, geologists have classified
them into groups.
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Each group has a distinct chemical nature and specific characteristics.
Groups of Minerals
Silicates: contain silicon and oxygen, and usually one or more other elements
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Makes up approximately 96% of minerals present in Earth’s crust.
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The basic building block of the silicates is the silica
tetrahedron. A tetrahedron (plural, tetrahedra) is a
three-dimensional shape that resembles a pyramid.
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Because silicon atoms have four valence electrons,
they can bond with four oxygen atoms.
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Individual tetrahedron ions are strong and can bond together to form sheets, chains, and complex threedimensional structures.
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The bonds between the atoms help determine several mineral properties, including cleavage or fracture.
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The tetrahedron formed by silicates contains four oxygen ions bonded to a central silicon atom. Chains,
sheets, and complex structures form as the tetrahedra bond with other tetrahedra. These structures
become the numerous silicate minerals that are present on Earth.
Carbonates: composed of one or more metallic elements and the carbonate ion CO32–
Oxides: are compounds of oxygen and a metal, such as hematite (Fe2O3).
Sulfides: are compounds of sulfur and one or more elements, such as pyrite (FeS2).
Sulfates: are compounds of elements with the sulfate ion (SO42–), such as anhydrite (CaSO4).
Halides: are made up of chloride or fluoride along with calcium, sodium, or potassium, such as halite (NaCl).
Native elements: are made up of one element only, such as silver (Ag).
Uses of Minerals
 Make computers, cars, televisions, desks, roads, buildings, jewelry, beds, paints, sports equipment,
medicines, and many other things.
Ore: A mineral is an ore if it contains a useful substance that can be mined at a profit.
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The classification of a mineral as an ore can change if the supply of or demand for that mineral changes.
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Ores that are located deep within Earth’s crust are removed by underground mining. Those near Earth’s
surface are obtained from large, open-pit mines.
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The removal of unwanted rock and dirt from mining can be expensive and harmful to the environment.
Gems: valuable minerals that are prized for their rarity and beauty
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Gems such as rubies, emeralds, and diamonds are cut, polished, and used for jewelry.