1

M a t t e r &

M i n e r a l s

Chapter 3

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Minerals

• Study of minerals = mineralogy

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Minerals

Basic building blocks of the crust

By definition, a mineral must - 1. Occur naturally

2. Be inorganic

3. Be a solid

4. Have an orderly internal (crystal) structure, &

5. Have a definite chemical composition

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Examples of Minerals

• Quartz SiO2

• Gold Au

• Gypsum CaSO4.2H2O

• Diamond C

• Ice H20

About 4,400 minerals have been named

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Examples of Non-Minerals

• Coal

• Petroleum

• Synthetic Diamonds, Rubies,

Sapphires, etc.

• Opal (a mineraloid)

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Rocks

• A quick view• A mass of mineral or mineral-like matter

occurring naturally as part of a planet• May consist of only one mineral; usually more

than one• More details in later chapters

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Composition of Minerals

• Minerals are made of Elements– Elements are the basic building blocks of minerals

– 112 are known / 92 occur naturally

• Elements are made of Atoms

• Atoms consist of electrons, neutrons, protons

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Atomic Structure (Fig. 3.4)• Nucleus

–Protons–Positive electrical charge (+)

–Neutrons–Electrically neutral

• Shells–Electrons

–Negative electrical charge (-)

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Atomic Structure• Atomic Number

– Number of protons in the nucleus • Defines a specific Element

• Atoms are electrically neutral– No. of Protons = No. of Electrons

• Electrons in the outermost shell are called “valence electrons”

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Chemical Bonding• 8 valence electrons (VE) bring stability

– Noble gases (Neon, Argon, etc.)

• Octet Rule

– Most atoms bond with others to have 8 VE

• Atoms bond chemically by either– Gaining electrons

– Loosing electrons, or

– Sharing electrons

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Ions & Types of Bonds• Ions (electrically charged atoms)

– Anion

• Negatively charged by gaining electrons

– Cation

• Positively charged by loosing electrons

• Ionic Bond (Fig. 3.5)– Ions held together by electrostatic forces

– Ex: Sodium chloride

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• Covalent Bond– Atoms bond by sharing electrons

– 2 Chlorine atoms share two electrons

– Silicon & Oxygen form covalent bond

– Carbon atoms bond covalently to form diamond & graphite

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• Hybrid Bond– Combination of Ionic and Covalent bonds

• Metallic Bond

– Free floating electrons are shared by all

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Isotopes & Radioactive Decay

• Atomic number– No. of protons in the nucleus

• Mass number – No. of neutrons + no. of protons

• Isotope– Same atomic number but different mass

number

– For example, Carbon 12, 13 & 14

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Radioactive Decay

• Some isotopes are unstable and disintegrate

naturally (they decay)

• Decay gives off radiation (more on this later)

• Rate of decay is constant with time

• Radioactive decay can be used as a

geological clock (again, more on this later)

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Structures of Minerals• Internal structure depends on

– Size of the ions

– Electrical charges on the ions

– Shape of multi-atom ions

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• Polymorphs

– Minerals of same chemical composition but with different physical properties

– Diamond & Graphite

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Physical Properties of MineralsPrimary Properties• Crystal Form

• Luster

• Color

• Streak

• Hardness

• Cleavage

• Specific Gravity

Secondary Properties• Magnetism

• Taste

• Feel

• Smell

• Elasticity

• Malleability

• Double refraction

• Reaction to HCl

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Physical Properties of MineralsCrystal Form• The external expression of the internal arrangement

of atoms – we’re lucky to see this

Luster• Appearance in reflected light

– Metallic

– Nonmetallic: vitreous, pearly, silky, earthy, etc.

Color• Varies in most minerals, constant in some

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Pyrite (fool’s gold)

Metallic

Brassy color

Cubic crystals

Quartz

Nonmetallic

Colorless (can be many other colors)

Hexagonal crystals

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Physical Properties of Minerals

Streak• Color of the powder on a streak plate

(unglazed porcelain tile), or of the powdered mineral

Hardness• Resistance to abrasion• Mohs scale of hardness (relative)

• Absolute hardness

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Mohs Scale of Hardness

1. Talc

2. Gypsum

3. Calcite

4. Fluorite

5. Apatite

6. Orthoclase

7. Quartz

8. Topaz

9. Corundum

10. Diamond

Made up of 10 relatively common minerals

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penny – pre-1982 (why?)

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Physical Properties

• Cleavage

– A plane of weakness in the crystal structure

along which it easily breaks. Ex: Mica

• Fracture

– Minerals that do not exhibit cleavage when

broken have fracture. Ex: Quartz

32Perfect cleavage - mica

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• Specific gravity– Essentially, the mineral’s density– Ratio between the mass of a volume of

mineral to the mass of an equal volume of water

– Most common minerals have a specific gravity of 2.5 to 3

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Some other properties, for fun

• Fluorescence– Discovered by accident by George Stokes– The reaction of certain minerals to ultraviolet radiation– High energy light (UV – the stuff that can cause

sunburns) is converted to low energy light (the kind we can see) (we’ll leave the physics alone…)

– This property has everyday applications (look above your head)

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The minerals calcite, willemite, & franklinite. Under short-wave UV, the calcite appears red, the willemite green, and the franklinite black.

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• Pleochroism– Some minerals show a different color

depending on which direction you look at it– Dichroism – you will see two colors– Trichroism – you will see three colors

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A crystal of the gem Tanzanite (a variety of the mineral zoisite), viewed from three directions.

“front”

“side”

“top”

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Incandescent lightTransmitted light

sunlight

The gem alexandrite (June birthstone) under different light sources

39http://socrates.berkeley.edu/~eps2/wisc/jpeg/2cryso.jpeg

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Mineral Groups

• About 4,000 minerals have been named

• 8 elements make up over 98% (by weight) of the continental crust

Oxygen (46.6) Silicon (27.7)

Aluminum (8.1) Iron (5)

Calcium (3.6) Sodium (2.8)

Potassium (2.6) Magnesium (2.1)

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Mineral Groups• Chemical groups based on the anions

• Silicates & Nonsilicates

• Si & O combine to form the building blocks of

silicates

• Silicates account for >90% of the crust

• Igneous rocks are made mostly of silicates

• Many nonsilicates are economically important

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Silicates

• All silicates contain Silicon (not silicone)

& Oxygen

• The fundamental building-block of all

silicates is the Si - O tetrahedron

• 4 O ions surround one Si ion (Fig. 3.19)

• SiO4-4 (electrical charge on SiO4

tetrahedron is -4)

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SiliconOxygen

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Silicate Structures

• SiO4 tetrahedra can join together

– Oxygen ions are shared between tetrahedra

• “polymers”

• Affects certain physical and internal properties of silicate minerals

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Silicate Structures

• Single chains (Pyroxenes)

• Double chains (Amphiboles)

• Sheet structures (Mica)

• Framework structures (Quartz)

• Rings (Beryl [emerald])

• Cations (Fe, Mg, K, Na, Al, Ca) neutralize

charges on tetrahedra

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Common Silicate Minerals

• Most silicates form as molten rock cools

• Minerals indicate formation history

• Common silicates in the earth’s crust

– >50% feldspars

– 12% Quartz

• The “Light” & “Dark” Silicates (kinda like

chicken…)

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The “Light” Silicates

• Generally light in color

• Contain Al, K, Ca, Na (except quartz)

• No iron or magnesium minerals

• Specific gravity is about 2.7

• Feldspars, Quartz, Muscovite (mica)

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• Feldspars (“aluminosilicates”)

– Potassium feldspar group (K ion)

• Orthoclase & Microcline

• Light cream to salmon pink

• No striations

– Plagioclase feldspar group (Na & Ca ions)

• White to medium gray

• Striations

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www.uwm.edu/Course/422-100/Mineral_Rocks/plagioclase.feldspar1.jpg

Parallel to this line

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• Quartz– Hardness 7

– Resistant to weathering

– No cleavage

– Conchoidal fracture

– Colorless when pure

– Impurities make it white, gray, yellow, pink, purple, etc.

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• Muscovite mica– Clear with excellent cleavage

– Insulator, capacitors, window/oven glass

– Sparkles in nail polish

• Kaolinite (a clay; not a primary mineral)– Fine chinaware

– Paper coating

– Thicken milkshakes

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The “Dark” Silicates• Iron- and magnesium-bearing

(ferromagnesian) minerals• Dark color (green, black, brown, red)• Specific gravity 3.2 - 3.6

– Olivine group– Pyroxene group (Augite)– Amphibole group (Hornblende)– Biotite mica– (Garnet group)

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Nonsilicate Minerals• Make up about 8% of the earth’s crust• Divided into classes based on the anion

– Carbonates (CO32-)

– Halides (Cl-, F-, Br-)

– Oxides (O2-)

– Sulfides (S2-) / Sulfates (SO42-)

– Native elements– And others

• Economically important

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Nonsilicate Minerals• Calcite / Dolomite• Halite (also a rock)/ Fluorite• Hematite / Magnetite / Ice (also a rock)• Galena / Pyrite / Chalcopyrite• Gypsum / Barite• Gold / Copper / Silver / Diamond /

Sulfur / Platinum• to cite but a few

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Gemstones

“Precious”:• Diamond

• Emerald (Beryl)

• Opal

• Ruby (Corundum)

• Sapphire (Corundum)

“Semi-precious”:• Alexandrite

(Chrysoberyl)

• Amethyst (Quartz)

• Garnet

• Jade (Jadeite or Nephrite)

• Topaz

• Turquoise

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• A gemstone/jewelry aside: four homophones (words that sound the same, but spelled differently, with different meanings)– Carrot– Caret – Carat– Karat

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• Carrot– A veggie– Bugs Bunny’s

favorite food– They do come from

the ground…..

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• Caret• ^ (look at the “6” key on your computer

keyboard)

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• Carat– A gemstone weight

– Based on the carob seed (supposedly all have the same mass)

– 200 milligrams (0.2 grams)

– Carat weight & size of a given gem will depend on the density of the mineral Rhodolite – 3.91 ct

10.9 x 8.7 mm

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• Karat– Measure of the

purity of gold, scale of 0 to 24

– 24 K = 100% – Why 24? I don’t

know– Being replaced by

millesimal fineness – parts per thousand of the metal in the alloy Natural gold crystals

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• A last bit of mineral fun …

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Crystals of the mineral rutile – intergrown to form letters of the alphabet (use your imagination)

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End of Chapter