Rocks
What is a rock?
A rock is a naturally formed
aggregate composed of one
or more minerals
rock, an aggregate of more than one minerals
What is a Mineral?
A mineral is a naturally formed inorganic crystalline solid
with a definite chemical composition and identifying
physical properties
naturally formed
formed by geologic processes in nature, not by humans
inorganic
was never alive
crystalline solid
a solid composed of atoms arranged in a repeating orderly
framework
definite chemical composition
a homogeneous chemical compound with a chemical formula
distinctive, identifying physical properties
The stuff that makes up all
matter
• The make-up of solid matter on Earth:
Atoms Elements Compounds Minerals Rocks
(smallest)
• Elements:
– fundamental building blocks
– smallest matter that can’t be broken down
(largest)
Physical Properties of Minerals
1. Crystal form
Is a set of crystalline faces having a definite
geometric relationship to one another
Garnet
Quartz
Physical Properties of Minerals
2. Color
Is due to visible wavelengths of light not absorbed
Is the most obvious but least reliable property to use
for identification
Milky quartz
Citrine
Colors of ruby and sapphire,
varieties of corundum (Al2O3)
Amethyst
Smoky quartz
Colors of Varieties of Quartz
Physical Properties of Minerals
3. Streak
The color of a mineral in powder form
Figure 1.8
Physical Properties of Minerals
4. Luster
Is the intensity of light reflected from a surface
Pyrite
Potassium feldspar
Galena
Have a metallic luster
Has a nonmetallic luster
Physical Properties of Minerals
5. Cleavage
Is breakage along planes of weakness
Is due to weak bonding between those planes
Various Types of Cleavage
Pyramidal, Cubic, and Rhombohedral Cleavage Displayed
by Fluorite, Halite, and Calcite
Fluorite
Halite
Figure 1.11
Calcite
Distinguishing between Cleavage Planes and Crystal Faces
Physical Properties of Minerals
6. Fracture
Is breakage in random directions
Is due to the absence of weak bonding between planes
Conchoidal fracture yields scalloped edges like in broken glass
Physical Properties of Minerals
7. Hardness
Is the resistance to scratching
Is based on the Moh’s Scale
1 talc
2 gypsum
3 calcite
4 fluorite
5 apatite
6 potassium feldspar (orthoclase)
7 quartz
8 topaz
9 corundum
10 diamond
Moh’s
Hardness
Scale
Physical Properties of Minerals
8. Tenacity
resistance to breaking or bending
9. Specific gravity: an expression of heaviness
density of a substance
SG = ----------------------------------density of water
Physical Properties of Minerals
12. Reaction with acid
minerals containing a CO3 ion fizz (release CO2) when
in contact with hydrochloric acid
2HCl + CaCO3 → Ca2+ + 2Cl- + H2O + CO2
13. Striations
Are saw-tooth lines present on crystal planes due to
lamellar twinning (repetition) of crystals on that plane
Striations are characteristic of plagioclase feldspar,
calcite, dolomite, galena, and sphalerite
Elemental Abundances in Continental Crust
Mineral Classes
Silicate Minerals
Non-silicate Minerals
Silicate Minerals
Silicate minerals (silicates) are composed of
silica tetrahedra (SiO4 4-)
All the common rock-forming minerals are
silicate minerals
For silica tetrahedra to be stable, they must
either:
be balanced by positive ions,
share oxygens with adjacent silica tetrahedra,
or
substitute one or more Al 3+ for Si 4+
Silicate Minerals
Compositions of the silicates
Mafic composition
Is rich in magnesium, iron, and/or
calcium
Intermediate composition
Is compositionally between mafic and
felsic
Felsic composition
Is rich in feldspar and/or silica (quartz)
The Silica Tetrahedron
(composed of 4 oxygen atoms surrounding 1 silicon atom)
Two
Illustrations
of the
Si–O4
Tetrahedron
Single Island Silicates
(ex.: olivine)
Single Chain Silicates
(ex.: augite pyroxene)
Single Chain Silicates: The Pyroxenes
Double Chain Silicates: The Amphiboles
(e.g., hornblende)
Sheet Silicates: The Micas
(biotite, muscovite, and the clays)
Framework Silicates:
(potassium feldspar, sodium and calcium plagioclase feldspar, quartz)
The Common Rock-Forming Silicate Minerals
NON-SILICATE MINERAL CLASSES
Native Elements: consist of only one element.
Au (gold), Ag (silver), Cu (copper), S (sulfur), C
(graphite, diamond)
Halides: contain F 1-, Cl 1-, Br 1-, or I 1 NaCl (halite), KCl (sylvite), CaF2 (fluorite)
Non-silicate Mineral Groups
Native Copper
How do minerals form?
By crystallization from magma (molten rock material)
a saturation response
By crystallization (precipitation) from aqueous fluids
a saturation response
By chemical reaction with
magmatic fluids
hydrothermal fluids
water during weathering
By solid state transformations (metamorphism)
changes crystal form
moves ions to new locations
promotes growth along the edges of mineral grains (crystals) at
the expense of their neighboring mineral grains
Crystallization of Minerals in Cavities:
Geodes
The Effect of Crowding on Crystal Growth
Polymorphs
Polymorphs are minerals that have the same chemical
composition but a different crystal form
Graphite and diamond polymorphs of carbon
Graphite forms at low temperature and pressure
Diamond forms at high temperature and pressure
Quartz, stishovite, and coesite are polymorphs of SiO2
Quartz forms at low to medium temperature and pressure
stishovite and coesite form at high pressure, such as that
associated with meteor impacts
Andelusite, kyanite, and sillimanite are polymorphs of
Al2SiO5
Andelusite is the low temperature low pressure polymorph
Kyanite is the low temperature high pressure polymorph
Sillimanite is the high temperature high pressure polymorph
Pseudomorphs
Pseudomorphs are minerals that have
the same crystal form but a different
chemical composition
Limonite forms cubic pseudomorphs
after pyrite
Quartz forms pseudomorphs after
fluorite
MINERAL GROUPS
Non-ferromagnesian
Silicates (K, Na, Ca,
Al)
Ferromagnesian
Silicates (Fe, Mg)
Oxides
Carbonates
Sulfides/sulfates
Native elements