chapter 3: matter and minerals (part ii). minerals: the building blocks of rocks definition of a...
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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