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:
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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
Metallic
example:
Galena
Non-metallic
example:
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
Galena
SG=7.5
Quartz
SG=2.67
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)
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
Mineral Groups
Element Abundances
SILICATES
Silica
(SiO4)4-
Common cations that
bond with silica anions
All others:
1.5%
Mineral Groups
•
•
Silicates (most abundant)
Non-silicates (~8% of Earth’s crust):
–
–
–
–
–
–
Oxides
Carbonates
Sulfides
Sulfates
Halides
Native elements
O2(CO3)2S2(SO4)2Cl-, F-, Br(single elements; e.g., Au)
Mineral Groups – Silicates
•
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
K-feldspar
light silicates (K-Na-Ca, Al)
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
Mineral Groups
Non-ferromagnesian
Silicates (K, Na, Ca, Al)
Ferromagnesian
Silicates (Fe, Mg)
Oxides
Carbonates
Sulfides/sulfates
Native elements