Minerals
Geology 101, Fall 2012
Mineral
• Definition: “A naturally-occurring
homogenous inorganic solid substance
with a definite chemical composition
and a characteristic crystalline
structure.”
• What about ice (frozen H2O)?
Mineral
• Definition: “A naturally-occurring
homogenous inorganic solid substance
with a definite chemical composition
and a characteristic crystalline
structure.”
• Ice does not have a “characteristic
crystalline structure” -- thus it is a
mineraloid.
Atomic theory
• John Dalton (1800): “Matter is made out of
atoms -- the smallest units with
distinguishable chemical properties.”
Molecules
• Atoms form associations called molecules;
atoms in molecules are held together by
chemical bonds
Crystals
• Molecules can attract
each other due to
intermolecular forces
-- much weaker than
chemical bonds
• If the molecules are
held together in a
regularly spaced lattice,
a crystal is formed
Ice’s structure
• Note large-scale irregularities in the
symmetry of the lattice -- ice is a mineraloid
Mineral intermolecular force
• Molecules in minerals
are held together by
ionic bonds -- the atoms
lose or gain electrons
and are thus called
ions
• Positively charged ions
are cations; negatively
charged ions are
anions
Geologists and minerals
• Minerals are the components of rocks
• Unlike minerals, rocks do not have a
single definite chemical composition,
and minerals retain their integrity when
incorporated in a rock
Granite = rock
Mineral classification
• Basic classification is by chemical
composition, typically by the anion it
contains:
• Carbonates (CO32–)
• Halides (e.g., Cl–)
• Phosphates (PO43–)
• Sulfates (SO42–)
• Oxides (O2–)
• Sulfides (S2–)
Silicates
• But no mineral class
is as prevalent as
those made with the
silicate anion
(SiO44–) -- 95% by
volume of the crust
• Tetrahedral shape
Silicate structure
• Since silicates are all
the same shape, it is
the arrangement of
the silicates, and the
cations that electrically
balance the silicates
that determine the
structure of the
mineral
Silicate structure
• Knowing the way the
ions stack can tell you
some mineral properties
• Sheet silicates
(phyllosilicates) form
thin sheets because
there are not many ionic
bonds between different
levels of molecules
Silicate structure
• Tectosilicates
(framework silicates)
have a much more
complex structure but
have regular planes of
weaker forces -- leads
to characteristic
60°/120° cleavage of
these minerals