6th Grade Earth Science
 A mineral is a naturally occurring inorganic solid
that has a crystal structure and a definite
chemical composition.
 For a substance to be a mineral it must have all
five of these characteristics.
 To be classified as a mineral a substance must
be formed by a process that occurs in the
natural world.
 The mineral quartz forms naturally as magma
cools and hardens deep in the Earth.
 Materials made by people such as plastic, brick,
glass, and steel are not minerals.
 A mineral must also be inorganic.
 This means that the mineral cannot form from
materials that were once part of a living thing.
 Coal forms naturally in the crust.
 Coal is not a mineral because it comes from the
remains of living things.
 A mineral is always a solid, with definite volume
and shape.
 The particles that make up a solid are packed
together tightly, so they cannot move like the
particles that make up a liquid.
 The particles in a mineral line up in a pattern
that repeats over and over.
 The repeating pattern of the particles form a
solid called a crystal.
 A crystal has flat sides called faces, that meet at
sharp edges and corners.
 A mineral has a definite chemical composition.
 This means that the mineral always contains
certain elements in specific portions.
 Almost all minerals are compounds.
 Quartz- has one atom of silicon for every two
atoms of oxygen.
 Some like copper, silver, and gold are minerals
in a pure form. They are elements and are not
part of a compound.
 Almost all pure solid elements are metals.
 Geologists have identified about 3,000
minerals.
 Each mineral has characteristic properties that
can be used to identify it.
 The color of a mineral is an easily observed
physical property.
 The color alone provides too little information to
identify a mineral.
 Color can be used to identify only a few minerals
that always have their own characteristic color.
 Malachite is always green.
 Azurite is always blue.
 The luster of a mineral describes the way a
mineral reflects light from its surface. Certain
minerals have a metallic luster, such as silver,
copper and gold. Minerals that do not reflect
light have a nonmetallic luster, and are
described by terms like glassy, pearly, dull and
silky.
 Color cannot be used exclusively to identify minerals.
For example the color of turquoise can vary from blue
to green
 Minerals may have varying degrees of transparency,
which is the ability of light to pass through a
substance—quartz can be transparent, but can
contain flaws that make it translucent. This is related
to Luster.
 Luster is definitely an identifying physical
characteristic
 It describes how light reflects from the surface of the
mineral. Describers might include:
 Metallic, waxy, pearly, earthy, dull,
glassy (vitreous), silky
 The ability of a mineral to resist being scratched
is known as its hardness. Hardness is one of
the most useful properties for identifying
minerals. Friedrich Mohs, a German
mineralogist, worked out a scale of hardness for
minerals ranging from 1 to 10. The number one
is assigned to the softest mineral, talc and 10 is
assigned to the mineral, diamond.
 This is how easily a mineral can be scratched and is
definitely an identifying characteristics. In order to be
scratched by an object, the mineral must be softer than the
object doing the scratching.
 The Mohs scale is a system of comparing the hardness of a
list of 10 minerals (The Giant Cat Found a Foolish Quail that
Couldn’t Dance)
 Developed by Frederich Mohs
 It lists them in order from 1 (softest) to 10
 It gives a list of common objects and their hardness
 If the mineral scratches the common object but the object won’t scratch
the mineral, the mineral is the hardest
 Moh's Scale
 Streak is the color of the powder form of the
mineral

A streak plate is used to do this test. It is a
piece of unglazed porcelain—which has a
hardness of 7 on the Moh’s scale. A streak of the
mineral is left on the porcelain when it is rubbed
across with the mineral. The mineral must be
softer than the streak plate if this is to work.

Some minerals leave a certain color streak,
which is an identifying characteristic.
 An example of this is hematite, which although it is a
black mineral, leaves a red streak
 Minerals have a characteristic crystal shape that
results from the way the atoms or molecules
come together as the mineral is forming. There
are six basic shapes of crystal structures: cubic,
hexagonal, orthorhombic, monoclinic, tetragonal
and triclinic.
 The terms cleavage and fracture are used to
describe the way a mineral breaks. Cleavage is
the tendency of a mineral to split along smooth,
definite surfaces. Some minerals, like halite,
break into small cubes. Micas cleave along one
surface, making layers of thin sheets. Most
minerals do not break along smooth lines.
 When a mineral breaks it does so either by
fracturing or by cleaving. Crystal cleavage is a
smooth break producing what appears to be a
flat crystal face. Here are a few rules about
cleavage. First cleavage is reproducible,
meaning that a crystal can be broken along the
same parallel plane over and over again. All
cleavage must parallel a possible crystal face.
 Fracture describes the way a mineral breaks
and is different from cleavage. A fracture might
be splintery, conchoidal (like glass) , jagged or
earthy (like a ball of clay)
 Some minerals can be identified by special
properties. Magnetite is naturally magnetic.
Fluorite glows under ultraviolet light. Halite
tastes salty. Sulfur smells like rotten eggs.
Calcite fizzes when hydrochloric acid is added to
. Uraninite is radioactive.
 The term ores is used to describe minerals or
combinations of minerals from which metals and
nonmetals can be removed in usable amounts.
 Metals are elements that have shiny surfaces
and are able to conduct electricity and heat.
Metals can be pressed or hammered into thin
sheets and other shapes without breaking.
Metals cans also be pulled into thin strands.
Iron, lead, aluminum, copper, silver and gold are
examples of metals.
 Most metals are found combined with other
substances in ores. After the ores are removed
from the Earth by mining, the metals must be
removed from the ores. During a process, called
smelting, an ore is heated in such a way that the
metal can be separated from it. Metals are
useful. Copper is used in pipes and electrical
wire.
 Nonmetals are elements that have dull surfaces
and are poor conductors of electricity and heat.
Nonmetals are not easily shaped. Some are
removed from the Earth in usable form. Others
must be processed. Sulfur,a nonmetal, is used
to make matches, fertilizers and medicines.
 Gemstones are minerals that are hard, beautiful
and durable and can be cut and polished for
jewelry and decoration. Once a gemstone is cut
and polished, it is called a gem. The rarest and
most valuable gemstone- diamonds, rubies,
sapphires and emeralds, are known as precious
stones. All other gemstones, amethysts,
zircons, garnets, are known as semiprecious
stones.
 Density is the amount of matter in a given space.
The density of a mineral is always the same, no
matter what the size of the mineral sample.
 The color of the powder scraped off a mineral
when it is rubbed against a hard , rough surface
is called its streak. The streak may be different
from the color of the mineral. Streak can be
observed by rubbing the mineral sample across
a piece of unglazed porcelain, which is called
the streak plate. A streak plate has a hardness
slightly less than 7.
 A rock is a hard substance composed of one or
more minerals. A rock can also be made of or
contain naturally occurring substances that do
not perfectly fit the definition of a mineral. Rocks
can be composed of volcanic glass or of opal.
Both of these substances lack a crystalline
structure.
 Geologists place rocks into three groups
according to how they form: igneous,
sedimentary and metamorphic.
 Igneous rocks were originally hot, fluid magma
within the Earth. Igneous get their name from
the Latin word, ignis, which means “fire”.
 Igneous rocks were originally hot, fluid magma
within the Earth. Igneous get their name from
the Latin word, ignis, which means “fire”.
 Most sedimentary rocks are formed from
particles that have been carried along and
deposited by wind and water. These particles,
or sediments, include bits of rock in the form of
med, sand or pebbles. Sediments also include
shells, bones, leaves, stems and other remains
of living things. Over time they are pressed
together to form rocks.
 Metamorphic rocks are formed when chemical
reactions, tremendous heat and great pressure
change existing rocks into new kinds of rocks.
These new rocks have chemical and physical
properties usually quite different from the original
rocks.
 The continuous changing of rocks from one kind
to another over long periods of time is called the
rock cycle. The rock cycle has no definite
sequence. It can follow many different
pathways.
 Because granite is made of hard materials it is
resistant to nature’s forces. It can be slowly
worn down until bits of granite flake off and fall in
streams and are eventually reduced to sand.
The sand from granite, along with other
sediments is carried to the sea and is deposited
on the floor. The weight of layers piling on puts
pressure on lower layers and with calcite the
granite becomes part of a sedimentary rock.
After many years, under great pressure and
temperature the sedimentary rock will change to
a metamorphic rock, quartzite
 Igneous rocks are classified according to their
composition and texture.
 Composition refers to the minerals of which
rocks are formed.
 Texture means the shape, size, arrangement
and distribution of the minerals that make up
rocks.
 Both are evident in a rock’s appearance.
 Igneous rocks have four basic types of textures:
glassy, fine-grained, coarse-grained and
porphyritic.
 Glassy igneous rocks are shiny and look like
glass. The minerals that make up a glassy
igneous rock are not organized into crystals.
Obsidian has a glassy texture.
 Fine-grained rocks, unlike glassy rocks, are
made of interlocking mineral crystals. These
crystals are too small to be seen without the help
of a microscope. The dark gray rock known as
basalt has a fine-grained texture.
 Coarse-grained rocks, such as granite, consist
of interlocking mineral crystals, which are all
roughly the same size and visible to the unaided
eye.
 Porphyritic rocks consist of large crystals
scattered on a background of much smaller
crystals. Sometimes these small background
crystals are too tiny to be seen with a
microscope. Porphyritic rocks have a texture
that resembles rocky road ice cream.
 Where and how magma cools determines the
size of mineral crystals. The longer it takes
magma to cool, the larger are the crystals that
form. Glassy and fine-grained rocks form from
lava that erupts from volcanoes and hardens on
the Earth’s surface. Coarse-grained rocks form
from molten rock that cools and hardens within
the Earth.
 Rocks formed from lava are called extrusive
rocks. Because lava is brought to the surface by
volcanoes, extrusive rocks are also known as
volcanic rocks. Basalt and obsidian are two
kinds of extrusive rocks that are quite solid.
Pumice, another extrusive rock, is filled with
bubbles.
 Igneous rocks formed deep within the Earth are
called intrusive. They form when magma forces
its way upward into preexisting rocks and then
hardens. Intrusive rocks include granite and
pegmatite. Intrusive rocks are also known as
plutonic rocks. A mass of intrusive rocks are
known as a pluton. Plutons may produce
landforms by pushing up layers of rock above
them, such as domes.
 The most widely used classification system for
sedimentary rocks places them into three main
categories according to the origin of the
materials from which they are made. These
three categories are: clastic rocks, organic
rocks and chemical rocks.
 Sedimentary rocks that are made of the
fragments of previously existing rocks are known
as clastic rocks. Clastic rocks are further
classified according to the size and shape of the
fragments in them.
 Some clastic rocks are made of rounded
pebbles cemented together by clay, mud or
sand. If over a third of the rock is made of
pebbles, the rock is called a conglomerate. The
pebbles in conglomerates are smooth and
rounded because they have been worn down by
the action of water. They are also called
puddingstones.
 Clastic rocks made of small, and-sized grains
are called sandstones. At least half the particles
in a clastic rock must be sand sized in order for it
to be considered a sandstone. Sandstones are
very common rocks. They are formed from the
sand on beaches, in riverbeds and in sand
dunes. In a sandstone, the grains are cemented
together by minerals that harden.
 Many geologists use term shale to describe all
the clastic rocks that are made of particles
smaller than sand. Shale forms from small
particles of mud and clay that settle to the
bottom of quiet bodies of water such as swamps.
Most shale can be split into flat pieces.
 Organic rocks come from organisms.Limestone
are often but not always organic rocks. Deposits
of limestone may be formed from the shells of
creatures when they die. Creatures may also
cement their shells together and over time form
reefs. Coal is also made from the remains of
living things. It is made from plants that lived
millions of years ago.
 Some sedimentary rocks are formed when a sea
or lake dries up, leaving large amounts of
minerals that were dissolved in water. Examples
of chemical rocks formed this way include rock
salt and gypsum. Some limestone rocks are
formed by inorganic processes in caves. As
water evaporates, a thin deposit of limestone is
left behind.
 When already existing rocks are buried deep
within the Earth, tremendous heat, great
pressure and chemical reactions may cause
them to change into different rocks with different
textures and structures. The changing of one
type rock into another as a result of heat,
pressure and /or chemical reactions is called
metamorphism.
 Metamorphic rocks may be formed from
igneous, sedimentary or metamorphic rocks.
Heat and pressure are great enough to make
tock undergo change. Temperatures of 100
degrees to 800 degrees cause some minerals to
break down, allowing their atoms to form other
more heat-tolerant minerals.Texture, mineral and
chemical composition may change.
 The amount of heat, pressure and chemical
reactions varies during metamorphism. Thus
the degree of metamorphism also varies. The
characteristics of the original rock also affect the
degree of metamorphism. Many metamorphic
rocks can be produced from more than one kind
of rock.
 Like igneous and sedimentary rocks,
metamorphic rocks can be classified according
to texture. The classification for metamorphic
rocks are based on the arrangement of the
grains that make up the rocks.
 In the first group, the mineral crystals are
arranged in parallel layers, or bands. The word
foliated comes from the Latin word for leaf. It
describes the layers in such metamorphic rocks,
which are thin and flat. Most metamorphic rocks
are foliated, like schist, slate and gneiss.
 In the second, smaller group of metamorphic
rocks, the rocks are not banded and do not
break into layers. These rocks are said to be
unfoliated. Marble and quartzite are examples
of unfoliated rocks.