Earth Science Part 2

Rocks and Minerals

Part 1: Identifying Minerals

I will be able to…• Name some rock forming minerals and identify the

group to which most belong.• Discuss the usefulness of colour and crystal shape in

mineral identification.• Describe and give examples of mineral luster, streak,

cleavage, fracture, and hardness.• Define specific gravity, explain how it is determined,

and calculate specific gravity given the necessary data.• Describe other tests for specific mineral identification.

Rock Forming Minerals

• Over 2000 minerals are known.• Many like gold and diamonds are rare.

Rock Forming Minerals

• Others like quartz, feldspar, mica and calcite are common.

• Common minerals that make up most of the Earth’s crust are called rock-forming minerals.– Most of these are – silicate minerals.

Rock Forming Minerals

• Minerals occurring in rock are not always large crystals, but in most cases even small mineral grains can be identified.

• How are they identified?– By their physical properties• Found by inspection (unaided eye), simple physical

tests, and chemical tests.

Rock-Forming Minerals

• The study of minerals and their properties is called mineralogy.

Identification by Inspection

• With inspection of a mineral we can recognize the:– Colour– Luster– Crystal shape

Identification by Inspection

Colour• Most easy to observe mineral property• Some minerals have characteristic colours

that help identify them.

Colour Identification Example

Cinnabar (an Ore of Mercury) • red

Malachite (an ore of copper)• green

Identification by Inspection

• Colour is also the least useful.• Why?– Different minerals have the same colour– Traces of impurities can turn colourless minerals

into coloured ones

Identification by Inspection

Luster• This is the way a mineral shines in reflected

light.• They are either metallic or nonmetallic.

Metallic Luster

• Shines like polished metal

Nonmetallic Luster

• Does not shine like a metal• Several terms are used to describe

nonmetallic luster:– Vitreous luster (like glass)– Pearly luster (like pearl)– Resinous luster (like wax)– Adamantine luster (like diamonds)– And greasy, oily, dull, earthy

Metallic vs. Nonmetallic Luster

Identification by InspectionCrystal Shape• When minerals have enough time and room to

form their atoms arrange themselves into patterns.

• These patterns lead to flat faced, regularly shaped crystals.

• This is very rare because usually there is not room to grow.

• Mineral grains in most rocks are so small or imperfect they are hard to find.

Identification by InspectionThese are some crystal shapes.Page 48 of textbook has another good chart.

Identification by Simple Tests

• Some mineral properties can be determined by simple tests:– Streak– Cleavage– hardness

Identification by Simple Tests

Streak• The colour of it’s powder• Rub the mineral on an unglazed white tile

called a streak plate• For many minerals the streak is not the same

colour as the mineral• The colour of a mineral may vary, but it’s

streak rarely does

Identification by Simple Tests

• Streak rule– The steak of a metallic mineral is at least as dark

as the hand specimen– The streak of a nonmetallic mineral is usually

colourless or white

Identification by Simple Tests

Cleavage• Tendancy to split easily, or separate along flat

surfaces• Example: Mica It splits very easily, and always in the same direction. Mica is said to have one perfect cleavage.

Identification by Simple Tests

• Cleavage surfaces can even be observed on tiny mineral grains therefore cleavage is a useful property for mineral identification.

• Example: Feldspar.It splits readily in two different directions, at or near right angles. It is said to have two good cleavages.

Identification by Simple Tests

• Not all minerals have cleavage, they tend to break along other than cleavage surfaces.

• These minerals are said to have fracture.

Fracture

• Shell-like fracture can be seen in obsidian

Fracture

• Fibrous or splintery fracture is seen in native copper

Fracture

• Uneven or irregular fracture leaves a generally rough surface like in cinnabar

Identification by Simple Tests

Hardness• Resistance to being scratched.• Diamond is the hardest – it will scratch other

minerals• Talc is the softest – all minerals scratch talc.

Identification by Simple Tests

• A hardness scale was developed by the mineralogist Friedrich Mohs

Identification by Simple Tests

• Moh’s scale has 10 known minerals ordered from softest (talc) to hardest (diamond)

Identification by Simple Tests

• Each step in the scale increases in hardness by the same amount, except the last step.– Ten (diamond) is several times harder nine

(corundum).• Hardness should not be confused with

brittleness.– Glass is brittle and breaks easily, but it’s hardness

depends on how scratch resistant it is.

Specific Gravity

• Specific gravity – the ratio of the weight of a mineral to the weight of an equal volume of water.

• That means it tells you how many times denser than water the mineral is.

Specific Gravity

• Nearly all minerals are denser than water so they all have a specific gravity (SG) greater than 1.

• Nonmetallic minerals typically have a SG of less than 3.

• Metallic minerals typically have a SG of 5.• Pure gold has a SG of 19.3

Specific Gravity

• To find the SG:1. Weigh the mineral sample while it is in the air2. Weigh the mineral sample while it is hanging in

water (this gives you the weight of the water of equal volume to the mineral)

3. Take the weight of the mineral (1) and divide it by the difference in the weight of water (1-2)

4.

€

SG =weight mineral in air

weight mineral in air − weight mineral in water

Acid Test

• Some minerals react to acid. • Depending on the mineral you may need a

stronger or weaker acid.• Calcite is the principal mineral in limestone,

and it will bubble and release carbon dioxide when a drop of weak hydrochloric acid is applied.

Special Properties of Minerals

• There are other properties we can use to identify minerals:– Magnetite and iron ore are magnetic– Halite (rock salt) taste salty– Fluorite, calcite glow in ultraviolet light• Called fluorescence

– Willemite, sphalerite continue to glow after ultraviolet light is shut off• Called phosphorescent

Special Properties of Minerals

• There are other properties we can use to identify minerals:

• Uranium minerals are radioactive• Calcite splits light rays into two parts (one straight

through, the other bends)– Called double refraction

• Properties of some common minerals can be found on textbook pages 644 to 645

Class/Homework

• Page 52 Topic question 1 to 6

• If there is time we will watch a video on identifying minerals.