Minerals6th Grade Earth Science

What is a Mineral? 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.

Naturally Occurring 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.

Inorganic 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.

Solid 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.

Crystal Structure 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.

Chemical Composition 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.

Identifying Minerals Geologists have identified about 3,000

minerals. Each mineral has characteristic properties that

can be used to identify it.

Color 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.

Luster 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.

Physical Appearance 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

Hardness 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.

Hardness 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 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

Crystal Shape 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.

Cleavage and Fracture 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.

Cleavage and Fracture 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)

Special Properties 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.

Ores The term ores is used to describe minerals or

combinations of minerals from which metals and nonmetals can be removed in usable amounts.

Metals 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.

Smelting 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 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 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 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.

Streak 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.

Rocks 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.

Types of Rocks Geologists place rocks into three groups

according to how they form: igneous, sedimentary and metamorphic.

Igneous Rocks Igneous rocks were originally hot, fluid magma

within the Earth. Igneous get their name from the Latin word, ignis, which means “fire”.

Igneous Rocks Igneous rocks were originally hot, fluid magma

within the Earth. Igneous get their name from the Latin word, ignis, which means “fire”.

Sedimentary Rock 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 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.

Rock Cycle 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.

Granite and the Rock Cycle 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

Fluid and Fire: Igneous Rocks 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 Rock Textures Igneous rocks have four basic types of textures:

glassy, fine-grained, coarse-grained and porphyritic.

Glassy Igneous 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 Igneous 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 Rock 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 Igneous Rocks 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.

Igneous Variety 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.

Extrusive Rocks 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.

Intrusive Rocks 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.

Slowly Built Layers: Sedimentary Rocks 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.

Clastic 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.

Conglomerates 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.

Sandstones 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.

Shale 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 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.

Chemical Rocks 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.

Changes in Form: Metamorphic Rocks

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 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.

Metamorphic Rocks 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.

Metamorphic Classification 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.

Foliated 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.

Unfoliated Rocks 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.