Minerals
Geology 115
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
• Olivine and quartz are
examples; in fact, quartz
is made only of silicon and
oxygen, so is the most
common mineral on Earth
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
• Mica minerals, like
biotite and muscovite,
are examples
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
• Feldspar (=“field stone”)
minerals, like plagioclase
and orthoclase, are
examples
The properties of minerals are determined by their
chemical composition and crystal habit; there are
over 7000 unique minerals
Crystal habit
• Crystal shapes are determined by local
(nearest neighbor) intermolecular attractions
Cleavage
• Cleavage, on the other hand, relies on the
alignment of weak areas held together by
only intermolecular forces through the whole
crystal
Color (and luster)
• Mineral color can be
due to its intrinsic
chemical composition
(e.g., copper ores) or
the crystal’s refractive
properties or
“impurities” such as a
low concentration of
metal ions not in the
mineral’s chemical
formula
• Latter reason explains
much of quartz’s color
variability
Specific gravity
• SG is the ratio of the mineral’s density
to water’s density; it can be thought of
as a “unitless” density.
• SG measures how efficiently packed
the atoms are in a crystal, and gives
some indication of composition (e.g.,
the lead in galena).
Hardness
• Hardness is an
indication of
molecule alignment
in crystals (similar to
cleavage) but also
measures the
strengths of the
intermolecular
forces
Strength ≠ Hardness
• Carbon nanowire is
one of the strongest
(tensile) materials
known, but is no
harder than graphite
Sadly, minerals aren’t usually
large enough to identify
• So what
techniques are
available?
• Optical
mineralogy
Optical mineralogy
• Relies on the
behavior of
polarized light
transmitted
through a very
thin cross-section
of a rock sample
X-ray crystallography
• Basic premise:
shoot X-rays at a
crystal, look at the
shadow pattern of
atoms, determine
structure and,
eventually,
identity