What is a mineral? A homogenous, naturally occurring, solid inorganic substance
with a definable chemical composition and an internal structure characterized by
an orderly arrangement of atoms, ions, or molecules in a lattice.
Mineral class : Groups of minerals distinguished from each other on the basis of
chemical composition.
crystal lattice: The orderly framework within which the atoms or ions of a mineral
are fixed.
Glass has no atomic order, so it is not a mineral. While the lower figure displays
what an ordered atomic arrangement might look like.
Crystals : A single, continuous piece of a mineral bounded by flat surfaces that
formed naturally as the mineral grew.
Crystal faces: flat surfaces that grew flat as the crystal grew.
Stenos law: The angle between crystal faces of a given mineral will always be
constant (see above figure).
crystal form: the geometric shape of a crystal, defined by the arrangement of
crystal faces.
crystal habit : the general shape of a crystal or cluster of crystals that grew
unimpeded.
cube
octahedron
bipyramidal
rhombohedron
pyramid
A crystal structure is defined by the arrangement of the packing of atoms. This is
dependent upon the chemical composition of a mineral. For example, the figure
above is for halite (NaCl), which has a cubic crystal structure.
Polymorphs : Two minerals that have the same chemical composition but a
different crystal lattice structure.
A good example of polymorphs are diamonds and graphite, which are both made
out of carbon (C).
diamonds are hard because of the order of its crystal structure.
graphite is very soft (it is what we write with in pencils) also because of its crystal
structure. It too, just like diamonds, is made of carbon (C).
Mineral growth: Minerals can form in one of five ways:
•
Solidification of a melt (freezing of a liquid like ice from water)
•
precipitation from a solution (ions, atoms or molecules dissolved in water
come out of solution like salt or sugar out of water [rock candy])
•
solid-state diffusion (movement of atoms through a solid to rearrange them
into a new mineral)
•
biomineralization (biological organisms can “make” minerals such as shells
that are made out of the mineral calcite. The shell is the mineral, not the
organism, so it is inorganic)
•
fumerolic mineralization (mineral crystallize from a vapor, like the sulfur
minerals that form around vents in Yellowstone national park.)
M inerals grow outward from a central seed to fill the
available space; their shape is controlled by the
shape of their surroundings. After the animation is
complete, click and drag each crystal to reveal its
individual shape.
8PC version
Minerals can be anhedral (no well developed crystal faces), sudhedral (poorly
developed crystal faces, or euhedral (well developed crystal faces.
euhedral crystal of olivine
sudhedral crystal of olivine
anhedral crystal of olivine
This animation shows the progressive growth and
interlocking of mineral crystals as they cool from a
melt. It illustrates how some minerals interfere with
the growth of other crystals.
8PC version
Minerals have different physical properties that lead to their identification.
1) The color of a mineral can sometimes help in mineral identification, but
commonly the same mineral can be many different colors.
2) The streak of a mineral (the color of the mineral in powered form) can be a
very good tool in the process of mineral identification.
3) The luster of a mineral (the way a mineral surface scatters light) can be
useful.
4) The hardness of a mineral (measure of a minerals bonds to resist breaking,
the stronger the bonds in a crystal structure the harder the mineral) is very
useful in mineral identification and lead to the development of Mohs hardness
scale.
5) Specific gravity (a number representing the density of a mineral, as specified
by the ratio between the weight of a volume of the mineral and the weight of an
equal volume of water) is very helpful in mineral identification.
6) Crystal habit (the shape of a minerals crystal faces or lack there of)
7) Fracture (when a mineral breaks in an irregular pattern) and cleavage (the
tendency of a mineral to break along preferred planes along atomic bonds) is
very helpful in identifying a mineral.
8) Other properties of minerals such as reactions with acids or magnetism are
good identification caricaturists.
Quartz can be many different colors. Color is the poorest tool used in
identifying minerals
Streak
This sample of pyrite has metallic luster
These feldspar samples have a non-metallic luster.
Mohs hardness scale.
This kyanite sample shows a bladed crystal habit.
The quartz crystal to the left is displaying a prismatic habit, while the sample to
the right has a need-like habit.
Mica has a good cleavage in one direction that helps in identifying it from
other minerals.
Halite (to the left) and calcite (to the right) also has distinctive cleavage that
helps in the identification of these minerals.
Cleavage of a mineral can be differentiated from mineral faces by cleavage planes
being repeated while mineral faces tend to be a single surface.
Magnetite is a naturally occurring magnetic mineral.
Minerals are broken up into groups called mineral classes (groups of minerals
distinguished from each other on the basis of chemical composition). The
mineral classes are:
Oxides: Metal cations bonded to oxygen anions (magnetite Fe3O4 is an example)
Sulfides: Metal cation bonded to a sulfide anion (am example is pyrite AKA fools
gold FeS2)
Sulfates: Metal cation bonded to a SO4 anionic group (gypsum is an example
CaSO2*2H2O)
Halides: A cation bonded to a halogen ion like Cl or F (halite AKA salt is an
example NaCl)
Carbonates: a cation bonded to a CO3 anionic group (calcite CaCO3 is an
example)
Native elements : pure masses of a single element such as gold (Au).
Silicates: a cation bonded to the SiO4 anionic group (Quartz SiO2 is an example)
cation: an atom or complex of atoms with an overall positive charge due to the
loss of an electron
anion : an atom or complex of atoms with an overall negative charge due to the
gain of an electron
Magnetite oxide
pyrite sulfide
Gypsum sulfate
Halite halide
Calcite carbonate
Gold native element
Quartz silicate
This is the silicon-oxygen tetrahedron which is the building block of all silicate
minerals.
The silicon-oxygen tetrahedron can be arranged in may different ways to form
different silicate minerals. Above are (A) independent tetrahedra (olivine is an
example) (B) single chains of tetrahedron (pyroxene is an example) (C) double
chain tetrahedra (amphibole is an example) (D) sheets of tetrahedra (mica is an
example). (E) displays how silicon-oxygen tetrahedrons can be linked in 3D by
oxygen sharing.
independent tetrahedra (olivine is an example)
single chains of tetrahedron (pyroxene is an example)
double chain tetrahedra (amphibole is an example
sheets of tetrahedra (mica is an example)