Minerals and rocks

Minerals and Rocks


Lecture Outline
 What are minerals?
 Common rock-forming minerals
 Physical properties of minerals
 Basic rock types
 The rock cycle


Minerals

A mineral is a naturally occurring, solid
crystalline substance, generally inorganic,
with a specific chemical composition
Natural
Solid
Atoms arranged in orderly repeating 3D array: crystalline
Not part of the tissue of an organism
Composition fixed or varies within defined limits
Minerals are the “building blocks” of rock


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Large individual crystals (rare)

Mass of small grains: each is a crystal,
but grown up against each other


Atomic Structure of Minerals
NaCl - sodium chloride

Halite


Chemical Bonds: Ionic
 Electrical attraction between ions of opposite charge
 Bond strength increases with the electrical charges of
the ions
 Bond strength decreases as the distance between the
ions increases
 Most minerals are this kind of compound


Ionic Bonding example:
halite
Cation

Na+

Anion

Cl-

Covalent Bonds:
Electron sharing
Generally stronger than ionic bonds (e.g., diamond)


Crystallization of Minerals
 Need starting material with atoms that can come

together in the proper proportions
 Growth from a liquid or a gas

 Time and space for crystallization
 Appropriate temperature and pressure
 Examples
 Magma that has cooled below its melting point
 Supersaturated solution --> precipitation


Crystallization of Minerals
 Crystals begin as an initial “seed” - a microscopic

crystal
 Atoms keep being added in a 3D array, repeating the

basic arrangement
 Crystal faces are based on the array structure


Cations and Anions
 Anions are typically large

 Cations are relatively small
 Crystal structure is

determined largely by the
arrangement of the anions


Common cations and anions
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Radii given in angstroms; 10-8 cm


Ions can be compound
 So far, we’ve talked about individual atomic ions
 Many common minerals are silicates

SiO44Complex ions act
as a single ion in
forming crystal
structure


Cation Substitution
 Crystal structure determined by those large anions
 Various cations can substitute for each other in many

minerals

 Same crystal structure
 Different chemical composition


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Polymorphs

Minerals with the same composition, but different
crystal structure.


Common Rock-Forming Minerals
Minerals fall into a small number of related “families” based
mainly on the anion in them
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Silicates
Most abundant minerals in the Earth's crust
Silicate ion (tetrahedron), SiO44-



Quartz (SiO2), K-feldspar (KAlSi3O8), olivine
((Mg, Fe)2SiO4), kaolinite (Al2Si2O5(OH)4)


Quartz (SiO2)
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Silicate structure
 Most of the most common rocks in the crust are silicates
 Silicate tetrahedra can combine in several ways to form

many common minerals

 Typical cations:

K+, Ca+, Na+, Mg2+, Al3+, Fe2+


Different numbers of oxygen ions are shared among tetrahedra

Carbonates
 Cations with carbonate ion (CO32-)
 Calcite (CaCO3), dolomite (CaMg(CO3)2), siderite

(FeCO3), smithsonite (ZnCO3)
 Make up many common rocks including limestone and

marble
 Very important for CCS!


Calcite (CaCO3)

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CaCO3 + 2H+ = Ca2+ + CO2 + H2O

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Smithsonite (ZnCO3)