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Matter and Minerals (part II)

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Minerals: the building blocks of rocks

Definition of a Mineral:

naturally occurring

inorganic

solid

characteristic crystalline structure

definite chemical composition

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How do we identify minerals?

Physical properties:

Color

Luster

Hardness

Crystal shape

Cleavage

Specific gravity

Other

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Physical Properties of Minerals

Color:

Most obvious, but often misleading

Different colors may result from impurities

Example:

Quartz

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Physical Properties of Minerals

Color:

Streak color of a mineral in powdered form(used for metallicminerals)

Obtained by scratching a

mineral on a piece of

unglazed porcelain.

Example:

Hematite

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Physical Properties of Minerals

Luster:

How a mineral surface reflects light

Two major types:

Metallic luster

Non-metallic luster

Metallic

example:

Galena

Non-metallic

example:

Orthoclase

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Physical Properties of Minerals

Hardness:

How easy it is to scratch a mineral

Mohs Scaleof Hardness

relative scale

consists of 10 minerals, ranked 1 (softest) to 10

(hardest)

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Mohs Scale of Hardness

Hardest (10) Diamond

Softest (1) Talc

Common objects:

- Fingernail (2.5)

- Copper penny (3.5)

- Wire nail (4.5)- Glass (5.5)

- Streak plate (6.5)

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Physical Properties of Minerals

Crystal shape (or form): external expression of a minerals internal atomic

structure

planar surfaces are called crystal faces

angles between crystal faces are constant for any

particular mineral

Quartz Pyrite

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Physical Properties of Minerals

Cleavage vs. Fracture: The way a mineral breaks

Cleavage:tendency of a mineral to break along

planes of weakness Minerals that do not exhibit cleavage are said to

fracture

Do not confuse cleavage planes with crystal faces!

Crystal faces are just on the surface and may not repeat

when the mineral is broken.

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Physical Properties of Minerals

Cleavage is described by: Number of planes

Angles between adjacent planes

These are constant for a particular mineral

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Physical Properties of Minerals

Cleavage (1 direction):

Example: mica

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Physical Properties of Minerals

Cleavage (2 directions):

orthoclase

amphibole

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Physical Properties of Minerals

Cleavage (3 directions):

halite

calcite

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Physical Properties of Minerals

Cleavage (4 directions):

fluorite

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Physical Properties of Minerals

Fracture: minerals that do not exhibit cleavage are said to

fracture

smooth, curved

surfaces when minerals

break in a glass-like

manner: conchoidal

fracture

Quartz

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Physical Properties of Minerals

Specific gravity: weight of a mineral divided by weight of an equal

volume of water

metallicminerals tend to have higherspecific

gravity than non-metallic minerals

Galena

SG=7.5Quartz

SG=2.67

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reaction with hydrochloric acid(calcite fizzes)

Physical Properties of Minerals

Other properties:

taste(halite tastes salty)

feel(talc feels soapy, graphite feels greasy)

magnetism(magnetite attracts a magnet)

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Rock-forming minerals

~30 common minerals make up most rocks in

Earths crust

Composed mainly of the 8 elements that make upover 98% of the crust

Mineral Groups

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Mineral Groups

All others: 1.5%

Element Abundances

Silica

(SiO4)4-SILICATES

Common cations thatbond with silica anions

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Oxides O2-

Carbonates (CO3)2-

Sulfides S2-

Sulfates (SO4)2-

Halides Cl-, F-, Br-

Native elements (single elements; e.g., Au)

Mineral Groups

Silicates (most abundant)

Non-silicates (~8% of Earths crust):

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Mineral Groups Silicates

Silicates

Tetrahedron

fundamental

building block

4 oxygen ions

surrounding a much

smaller silicon ion

Silicon-oxygen

tetrahedron

(SiO4)4-

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Mineral Groups Silicates

Joining Silicate Structures

How tetrahedra may be linked:

independent tetrahedra single chains

double chains

sheets

3-D framework

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Mineral Groups

Silicates

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Olivine Groupdark silicates (Fe-Mg)

Mineral Groups Silicates

No cleavage

ferromagnesian

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Pyroxene GroupFerromagnesian / dark silicates (Fe-Mg)

Mineral Groups Silicates

2-directions

of cleavage

(at nearly 90 degrees)

Augite

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Amphibole GroupFerromagnesian / dark silicates (Ca, Fe-Mg)

Mineral Groups Silicates

2-directions

of cleavage

(not at 90 degrees)

Hornblende

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Mica Group and Clay Mineralslight silicates (K, Al)

Mineral Groups Silicates

1-direction

of cleavage

Muscovite

non-ferromagnesian

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Feldspar Grouplight silicates (K-Na-Ca, Al)

Mineral Groups Silicates

2-directions

of cleavage

(at 90 degrees)

Orthoclase

Plagioclase

K-feldspar

Ca/Na-feldspar

Most common mineral group

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Quartzlight silicates (pure SiO2)

Mineral Groups Silicates

no cleavage

(conchoidal fracture)hard, resistant to weathering

Quartz

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Mineral Groups

Ferromagnesian

Silicates (Fe, Mg)

Non-ferromagnesianSilicates (K, Na, Ca, Al)

Oxides

Carbonates

Sulfides/sulfates

Native elements