Hampden-Wilbraham Regional District

The Wonders of Rocks and Minerals
MineralsScript

How are Minerals and Rocks Different?

The Earth’s crust is the source for a wide variety of useful minerals.

Most people know that copper is used for electrical wiring…
…and gold is used for jewelry.

But most people are not aware that baby powder comes from the mineral talc…
…or that the mineral quartz is used to make glass.

So, what is a mineral? Minerals are defined as naturally occurring inorganic (nonliving) solids…
…that have an orderly internal structure. In other words they are made of elements that are arranged in a regular order.

Click on the group of spheres that are arranged in a regular order.

Click on the group of spheres that are arranged in a regular order.

When two or more elements combine chemically they form a “compound”. Most minerals are compounds. (The mineral halite is a compound of the elements sodium and chlorine (NaCl, sodium chloride). Click on the photo of halite to see another name for sodium chloride. – CLICK – Sodium chloride is also known as common salt.)

Based on our definition of a mineral, which of the above is a mineral. (Click on the correct answer.) Quartz or Petroleum

Based on our definition of a mineral, which of the above is a mineral. (Click on the correct answer.) Ice or Animal bone

Based on our definition of a mineral, which of the above is a mineral. (Click on the correct answer.) Steel or gold

How are rocks different from minerals? (Focuses in on magnified view of granite.)

A rock can be defined simply as a combination of one or more minerals.

For example, this granite contains the three minerals quartz, hornblende, and feldspar.

Although most rocks are composed of more than one mineral…
…certain minerals are found by themselves in large impure quantities where they are considered rock. (Picture of limestone outcrop composed of the mineral calcite.)

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Important Mineral Properties

 Every mineral has an orderly arrangement of elements and a definite chemical composition which gives it a unique set of physical properties. Quartz, feldspar, malachite, pyrite

A mineral’s crystal form (shape) depends on how its elements are arranged.
( pyrite = cubic crystals and quartz = hexagonal crystals with pyramidal-shaped ends)

Another mineral property, luster, is how a mineral shines in the light. Galena, fluorite, quartz, muscovite (mica)

Minerals that have the appearance of metals are said to have a metallic luster, while minerals that do not shine like metal have a nonmetallic luster. Click on the mineral that has a metallic luster. Galena or gypsum

Nonmetallic lusters are described by other terms, such as glassy, pearly, and earthy (dull). Click on the mineral that looks glassy. Quartz or Bauxite

Click on the mineral that appears to have an earthy luster. Pyrite or Limonite

Click on the mineral that looks pearly. Talc or Quartz

Color is undoubtedly the most obvious characteristic of a mineral. Sulfur, copper, malachite, pyrite

Howevere, color is not always useful when trying to identify a mineral sample. Crystal quartz (colorless)

For example, slight impurities in the mineral quartz give it a variety of colors. Crystal quartz (colorless), citrine (brownish yellow quartz, amethyst (purple quartz), smoky quartz (gray to black)

Streak is the color of a mineral in its powdered form. Mineral color=brassy yellow, streak=gray-black

The streak is obtained by rubbing a mineral across a piece of unglazed porcelain called a streak plate.

Although the color of a mineral may vary from sample to sample, the streak usually does not, as shown above. Hematite (variety: oolitic) hematitie (variety: specular)

 Hardness is a measure of how easily a mineral can be scratched. Diamond is the hardest mineral while talc is the softest. Mohs Scale of Hardness

Any mineral of unknown hardness can be compared to other minerals or objects of known hardness. This mineral scratches glass so it is harder than glass (hardness 5.5).

This mineral can be scratched by a fingernail so it is softer than a fingernail (hardness 2.5).

This mineral is unable to be scratched by a fingernail.

The same mineral is unable to scratch glass. Glass hardness 5.5

Which of the above best describes the hardness of this sample? A) greater than 5.5, b) less than 5.5, c) between 2.5 and 5.5 and d) less than 2.5

Which of the above best describes the hardness of this sample?

Sample scratched by fingernail A) greater than 5.5, b) less than 5.5, c) between 2.5 and 5.5 and d) less than 2.5

 Which of the above best describes the hardness of this sample?

Sample scratched glass A) greater than 5.5, b) less than 5.5, c) between 2.5 and 5.5 and d) less than 2.5

Cleavage describes the way a mineral can break.

Minerals can break or cleave in one direction or several directions. Micas cleave easily into thin, flat sheets in only one direction. Biotite (mica)

The mineral feldspar has two cleavage directions that meet at nearly 90 º angles. Two planes of cleavage (diagram) Click and drag on the mineral to rotate it. Click “Hand sample” to reveal cut sample

The mineral halite breaks evenly in three directions, also at nearly right angles. Three planes of cleavage (diagram) Click and drag on the mineral to rotate it. Click “Hand sample” to reveal cut sample

How many planes of cleavage does this sample have? A) 1 plane of cleavage, b) 2 planes of cleavage, c) 3 planes of cleavage, d) 4 planes of cleavage Click and drag on the mineral to rotate it

Do these planes meet at right angles (90 º )? A) yes b) No

How many planes of cleavage does this sample have? A) 1 plane of cleavage, b) 2 planes of cleavage, c) 3 planes of cleavage, d) 4 planes of cleavage Click and drag on the mineral to rotate it

Do these planes meet at right angles (90 º )? A) yes b) No

How many planes of cleavage does this sample have? Sample = muscovite mica A) 1 plane of cleavage, b) 2 planes of cleavage, c) 3 planes of cleavage, d) 4 planes of cleavage

 How many planes of cleavage does this sample have? Sample = halite A) 1 plane of cleavage, b) 2 planes of cleavage, c) 3 planes of cleavage, d) 4 planes of cleavage

It is important not to confuse cleavage with crystal form. Fluorite-crytal form and fluorite –cleavage sample

Remember, crystal form is the external appearance of a mineral’s orderly arrangement of elements.

Whereas, cleavage describes the way a mineral can break.

All minerals have crystal form, but some minerals do not have cleavage. Crystal form of quartz and broken quartz (no cleavage)

Minerals that do not cleave smoothly when broken are said to fracture. Malachite

Minerals that break into smooth curved surfaces resembling broken galss have conchoidal fracture. Quartz

Other minerals break into splinters and are said to have fibrous fracture.Serpentine asbestos

 While still others fracture irregularly. Bornite

What is the fracture style of this sample? A) irregular fracture, b) conchoidal fracture (resembles broken glass) c) fibrous fracture sample =Serpentine (asbestos)

 What is the fracture style of this sample? A) irregular fracture, b) conchoidal fracture (resembles broken glass) c) fibrous fracture sample =quartz (with conchoidal fracture)

What is the fracture style of this sample? A) irregular fracture, b) conchoidal fracture (resembles broken glass) c) fibrous fracture sample =gypsum (variety:alabaster)

Specific gravity is the number representing the ration of the weight of a mineral to the weight of an equal volume of water. Equal volume of water weighs 100 grams (pic. of beaker of water) specific gravity = 7.5, Mineral sample weighs 750 grams (pic); sample is 71/2 times heavier than an equal volume of water; Mineral sample weighs 250 grams (new pic); Sample is 2 ½ times heavier than an equal volume of water.

Most common minerals have a specific gravity of between 2 and 3. Note: Because minerals are not pure substances, the specific gravity may vary somewhat from sample to sample. Quartz has a specific gravity of 2.65; Feldspar has a specific gravity of 2.6; halite has a specific gravity of 2.1

 Other special properties can be used to help identify some minerals. Taste, magnetism, feel, acid test

For example, Halite is ordinary table salt, so it can be identified with your tongue…CAUTION Do not taste any samples without your instrucor’s permission. Taste

Graphite feels greasy… Feel

…some varieties of magnetite are natural magnets… Magnetism

… and certain minerals will effervesce (fizz) when dilute (weak) hydrochloric acid is applied. Pic. of Calcite reacting Reactive to acids

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Minerals
How to Identify Minerals

MINERAL IDENTIFICATION KEY
Hardness	Cleavage	Luster Other	Name
Harder than glass (>5.5)	yes	Nonmetallic (light colored)	Feldspar
	no	Nonmetallic (light colored)	Quartz
	yes	Nonmetallic (dark colored)	Hornblende
	no	Metallic	Pyrite
Softer than glass but harder than a fingernail (2.5-5.5)	yes	Nonmetallic (light colored)	Calcite
	yes	Nonmetallic (light colored)	Halite
	yes	Nonmetallic (dark colored)	Biotite
	yes	Metallic	Galena
Softer than a fingernail (<2.5)	yes	Nonmetallic (light colored)	Muscovite
	no	Metallic	Graphite

In this exercise you are going to use the mineral identification key shown above to identify some common minerals.