Download Earth Science Chapter 4: Minerals Chapter Overview Section 1

Earth Science
Chapter 4: Minerals
Chapter Overview
Section 1: What is a Mineral
1. Mineral Characteristics
A mineral is a naturally occurring, inorganic solid with a specific chemical composition and a
definite crystalline structure
• Naturally occurring and inorganic – minerals are formed from natural processes--so
materials made by humans in a lab are not naturally occurring--and minerals have never
been alive
• Solids with specific compositions – minerals are always solids, never liquids or gases.
Most minerals are made from compounds, but a few, like gold, silver, and iron, are
composed of single elements.
ƒ Each mineral has a specific and unique chemical composition. However, there can
be some variation in the amounts of atoms of each element in the compound, but the
ratio of the atoms remains the same.
• Definite crystalline structure – the atoms in minerals are arranged in regular geometric
patterns that are repeated again and again. A crystal is a solid in which the atoms are
arranged in repeating patterns.
ƒ There are six major crystal systems: cubic, tetragonal, hexagonal, orthorhombic,
monoclinic, and triclinic. Occasionally a crystal will grow in open space and take the
shape of one of the crystal systems, but this is rare
2. Minerals from Magma
Minerals can form from the cooling of magma. Magma is molten material found beneath
Earth’s surface. If magma cools slowly the atoms in the magma have time to form large
crystals. If the magma cools quickly small crystals, or no crystals, are formed
3. Minerals from Solution
Minerals can form from solution in two ways: If a solution becomes supersaturated solid
precipitates form and drop out of the solution, and if the water in a supersaturated solution
evaporates the elements that remain may begin to arrange into crystals
4. Mineral Groups
3,000 minerals are found in Earth’s crust; however, only about 30 of them are common.
These 30 are referred to as the rock forming minerals. The vast majority of minerals are
made up of eight elements: oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and
magnesium. The minerals can be classified according to composition into seven groups:
native elements, oxides and hydroxides, halides, carbonates, sulfates, silicates, and sulfides
• Minerals that contain oxygen, silicon, and usually one or two other elements are known
as silicates. Silicates make up about 96% of the minerals found in Earth’s crust.
Common silicates include mica and quartz
• Carbonates are minerals composed of one or more metallic elements with the carbonate
compound CO3. Calcite and dolomite are common carbonates and are found in limestone
and marble.
• Oxides are minerals of oxygen and a metal, example: hematite, Fe2O3
• Sulfides are minerals of sulfur and one or more other elements, example: pyrite, FeS2
• Sulfates are minerals containing the sulfate compound, SO4, and some element
• Halides are minerals composed of either fluorine or chlorine combined with sodium,
potassium, or calcium. Ordinary table salt, formula: NaCl, is an example
• Native elements are minerals made up of one element only, copper, silver, and gold are
examples
Section 2: Identifying Minerals
1. Mineral Identification
Geologists rely on several relatively simple tests to identify minerals. These tests are based
on a mineral’s physical and chemical properties
• Color – One of the most noticeable characteristics of a mineral is its color. A mineral’s
color can be determined by the presence of trace elements in the mineral, and one
mineral can be several different colors. Because of this the color of a mineral is one of
the least reliable tests for identification
• Luster – The way that a mineral reflects light from its surface is called luster. Luster is
described as either metallic or nonmetallic. Metallic luster is characterized by shiny
surfaces that reflect light. Nonmetallic luster is not reflective and can be described as dull,
pearly, waxy, or silky
• Streak – a mineral rubbed across an unglazed porcelain plate will sometimes leave a
colored powdered streak on the surface of the plate. Streak is the color of a mineral
when it is broken up and powdered. Sometimes a mineral’s streak does not match its
external color, but regardless of the external color, the color of a mineral’s streak rarely
changes
• Hardness – One of the most useful tests for identifying minerals is hardness. Hardness
is a measure of how easily a mineral can be scratched. Mohs scale of hardness uses the
hardness of ten known minerals to compare with the hardness of unknown minerals.
Talc, the softest mineral, has a hardness of 1 on the Mohs scale while diamond, the
hardest mineral, has a hardness of 10
• Cleavage and Fracture – Atomic arrangement can also determine how a mineral will
break. Minerals break along planes where bonding is weak. A mineral that splits relatively
easily and evenly along one or more flat planes is said to have cleavage. Minerals that
break with rough or jagged edges are said to have fracture
• Density and Specific Gravity – Sometimes two minerals of the same size will feel different
when they are lifted—one is much heavier than the other. Differences in weight are the
result of differences in density. Density is defined as mass per unit volume, or density
equals an object’s mass divided by its volume. Geologists use specific gravity, which is
the ratio of the weight of a substance to the weight of an equal volume of water at 4OC,
as an identification test
2. Mineral Uses
Minerals are used everywhere. They are used to make cars, computers, furniture, jewelry,
road, and houses. They are used in foods, medicines, and toothpaste. Virtually every modern
convenience utilizes one or more minerals somehow
• A mineral is an ore if it contains a useful substance that can be mined at a profit.
Minerals that can be considered ores include hematite (iron), bauxite (aluminum), and
rutile (titanium)
• Ores are extracted from Earth’s crust through mining. There are many different mining
methods. Which method used depends on several factors including the grade (quality) of
the ore, the size of the ore body, the depth of the ore body, environmental conditions, and
regulatory requirements
3. Gems
Gems are valuable minerals that are prized for their rarity and beauty. Gems can be cut,
polished, and used for jewelry. In some cases the presence of a trace element can make one
variety of a mineral more colorful and valuable than other varieties. Amethyst is the gem form
of quartz, which contains trace elements that give it a purple color.