Download Mineral Groups Silicates

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
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
2.2 Minerals
Mineral Groups
2. Carbonates
• 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
• 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)
Ferromagnesian
Silicates (Fe, Mg)
Oxides
Carbonates
Sulfides/sulfates
Native elements
How do we identify minerals?
•
Physical properties:
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Color
Luster
Hardness
Crystal shape
Cleavage
Specific gravity
Other
2.3 Properties of Minerals
Color
 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
 Streak is the color of a mineral in its
powdered form.
2.3 Properties of Minerals
Luster
 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
 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
 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
 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
 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
 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
 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)