Chapter 3: Matter and Minerals (part II)

2.2 Minerals Definition of a Mineral 1. Naturally occurring 2. Solid substance 3. Orderly crystalline structure 4. Definite chemical composition 5. Generally considered inorganic Minerals: Building blocks of rocks

2.2 Minerals How Minerals Form 1. Crystallization from magma 2. Precipitation 3. Pressure and temperature 4. Hydrothermal solutions

Minerals Formed as a Result of Crystallization of Magma

Mineral Groups 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

2.2 Minerals Mineral Groups  Can be classified based on their composition 1. Silicates Silicon and oxygen combine to form a structure called the silicon-oxygen tetrahedron. This silicon-oxygen tetrahedron provides the framework of every silicate mineral.

The Silicon-Oxygen Tetrahedron

Silicon-Oxygen Chains, Sheets, and Three-Dimensional Networks

SILICATES Mineral Groups Common cations that bond with silica anions Element Abundances Silica (SiO4)4- SILICATES Common cations that bond with silica anions All others: 1.5%

Mineral Groups Silicates (most abundant) Non-silicates (~8% of Earth’s crust): Oxides O2- Carbonates (CO3)2- Sulfides S2- Sulfates (SO4)2- Halides Cl-, F-, Br- Native elements (single elements; e.g., Au)

Mineral Groups – Silicates Tetrahedron fundamental building block 4 oxygen ions surrounding a much smaller silicon ion Silicon-oxygen tetrahedron (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 –

Mineral Groups – Silicates Olivine Group dark silicates (Fe-Mg)  ferromagnesian No cleavage

Mineral Groups – Silicates Pyroxene Group Ferromagnesian / dark silicates (Fe-Mg) Augite 2-directions of cleavage (at nearly 90 degrees)

Mineral Groups – Silicates Amphibole Group Ferromagnesian / dark silicates (Ca, Fe-Mg) Hornblende 2-directions of cleavage (not at 90 degrees)

Mineral Groups – Silicates Mica Group and Clay Minerals light silicates (K, Al)  non-ferromagnesian Muscovite 1-direction of cleavage

Mineral Groups – Silicates Feldspar Group light silicates (K-Na-Ca, Al) K-feldspar Most common mineral group Orthoclase Plagioclase 2-directions of cleavage (at 90 degrees) Ca/Na-feldspar

Mineral Groups – Silicates Quartz light silicates (pure SiO2) no cleavage (conchoidal fracture) hard, resistant to weathering Quartz

2.2 Minerals Mineral Groups 2. Carbonates 3. Oxides Minerals that contain the elements carbon, oxygen, and one or more other metallic elements 3. Oxides Minerals that contain oxygen and one or more other elements, which are usually metals

2.2 Minerals Mineral Groups 4. Sulfates and Sulfides 5. Halides Minerals that contain the element sulfur 5. Halides Minerals that contain a halogen ion plus one or more other elements 6. Native elements Minerals that exist in relatively pure form

Sulfides

Native Copper

Mineral Groups Non-ferromagnesian Silicates (K, Na, Ca, Al) Silicates (Fe, Mg) Oxides Carbonates Sulfides/sulfates Native elements

How do we identify minerals? Physical properties: Color Luster Hardness Crystal shape Cleavage Specific gravity Other

2.3 Properties of Minerals Color 2.3 Properties of Minerals  Small amounts of different elements can give the same mineral different colors.

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

2.3 Properties of Minerals Streak 2.3 Properties of Minerals  Streak is the color of a mineral in its powdered form.

2.3 Properties of Minerals Luster 2.3 Properties of Minerals  Luster is used to describe how light is reflected from the surface of a mineral.

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

Pyrite (Fool’s Gold) Displays Metallic Luster.

2.3 Properties of Minerals Crystal Form 2.3 Properties of Minerals  Crystal form is the visible expression of a mineral’s internal arrangement of atoms.

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

Quartz Often Exhibits Good Crystal Form.

2.3 Properties of Minerals Hardness 2.3 Properties of Minerals  Hardness is a measure of the resistance of a mineral to being scratched.  Mohs scale consists of 10 minerals arranged from 10 (hardest) to 1 (softest).

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

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)

2.3 Properties of Minerals Cleavage 2.3 Properties of Minerals  Cleavage is the tendency of a mineral to cleave, or break, along flat, even surfaces.

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

Mica Has Cleavage in One Direction

2.3 Properties of Minerals Fracture 2.3 Properties of Minerals  Minerals that do not show cleavage when broken are said to fracture.  Fracture—the uneven breakage of a mineral

Conchoidal Fracture

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

2.3 Properties of Minerals Density 2.3 Properties of Minerals  Density is a property of all matter that is the ratio of an object’s mass to its volume.

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 Galena SG=7.5 Quartz SG=2.67

2.3 Properties of Minerals Distinctive Properties of Minerals 2.3 Properties of Minerals  Some minerals can be recognized by other distinctive properties.

Physical Properties of Minerals Other properties: reaction with hydrochloric acid (calcite fizzes) taste (halite tastes salty) feel (talc feels soapy, graphite feels greasy) magnetism (magnetite attracts a magnet)