Chapter 3: Matter and Minerals (part II)

Minerals: the building blocks of rocks

• Definition of a Mineral: naturally occurring inorganic solid characteristic crystalline structure definite chemical composition

How do we identify minerals?

• Physical properties: Color Luster Hardness Crystal shape Cleavage Specific gravity Other

Physical Properties of Minerals

• Color:– Most obvious, but often misleading

– Different colors may result from impurities

Example:Quartz

Physical Properties of Minerals

• Color: Streak – color of a mineral in powdered form

(used for metallic minerals)

Obtained by scratching a mineral on a piece of unglazed porcelain.

Example:Hematite

Physical Properties of Minerals

• Luster:– How a mineral surface reflects light

– Two major types:

• Metallic luster

• Non-metallic luster

Metallicexample:Galena

Non-metallicexample:

Orthoclase

Physical Properties of Minerals

• Hardness:– How easy it is to scratch a mineral– Mohs Scale of Hardness

• relative scale

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

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)

Physical Properties of Minerals

• Crystal shape (or form):– external expression of a mineral’s internal

atomic structure

– planar surfaces are called crystal faces

– angles between crystal faces are constant for any particular mineral

Quartz Pyrite

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.

Physical Properties of Minerals

• Cleavage is described by:– Number of planes– Angles between adjacent planes

– These are constant for a particular mineral

Physical Properties of Minerals

• Cleavage (1 direction):

Example: mica

Physical Properties of Minerals

• Cleavage (2 directions):

orthoclase

amphibole

Physical Properties of Minerals

• Cleavage (3 directions):

halite

calcite

Physical Properties of Minerals

• Cleavage (4 directions):

fluorite

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

Physical Properties of Minerals

• Specific gravity:– weight of a mineral divided by weight of an

equal volume of water

– metallic minerals tend to have higher specific gravity than non-metallic minerals

GalenaSG=7.5

QuartzSG=2.67

– 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)

• Rock-forming minerals– ~30 common minerals make up most rocks in

Earth’s crust

– Composed mainly of the 8 elements that make up over 98% of the crust

Mineral Groups

Mineral Groups

All others: 1.5%

Element Abundances

SilicaSilica(SiO(SiO44))4-4-SILICATES

Common cations thatbond with silica anions

– 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 Earth’s crust):

Mineral Groups – Silicates

• Silicates– Tetrahedron

• fundamental building block

• 4 oxygen ions surrounding a much smaller silicon ion

Silicon-oxygentetrahedron

(SiO4)4-

Mineral Groups – Silicates

• Joining Silicate Structures

– How tetrahedra may be linked:

• independent tetrahedra

• single chains

• double chains

• sheets

• 3-D framework

Mineral Groups– Silicates –

Olivine Groupdark silicates (Fe-Mg)

Mineral Groups – Silicates

No cleavage

ferromagnesian

Pyroxene GroupFerromagnesian / dark silicates (Fe-Mg)

Mineral Groups – Silicates

2-directionsof cleavage(at nearly 90 degrees)

Augite

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

Mineral Groups – Silicates

2-directionsof cleavage(not at 90 degrees)

Hornblende

Mica Group and Clay Mineralslight silicates (K, Al)

Mineral Groups – Silicates

1-directionof cleavage

Muscovite

non-ferromagnesian

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

Mineral Groups – Silicates

2-directionsof cleavage

(at 90 degrees)

Orthoclase

Plagioclase

K-feldspar

Ca/Na-feldspar

Most common mineral group

Quartzlight silicates (pure SiO2)

Mineral Groups – Silicates

no cleavage(conchoidal fracture)

hard, resistant to weatheringQuartz

Mineral Groups

FerromagnesianSilicates (Fe, Mg)

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

OxidesCarbonatesSulfides/sulfatesNative elements