Minerals 2
talc
What is a Mineral ?
Defining characteristics of minerals:
• Naturally occurring
• Inorganic (or at least never alive)
• Solid
• Ordered, repetitive internal structure (crystalline)
• Definite chemical composition (allowing for
small variation within set limits)
Can ice be considered a mineral ?
Mineral Families, continued
So far, we have looked at :
Silicates (containing silica ion SiO4 4- )
Carbonates (containing carbonate ion CO3 2- )
Sulphates (containing sulphate ion SO4 2- )
We will continue looking at the remaining important mineral
families:
Phosphates (containing phosphate ion PO4 3- )
Sulphides (containing phosphate ion PO4 3- )
Halides (containing halide ions, e.g. Cl – and F- )
Oxides/Hydroxides (containing oxide O 2– / hydroxyl OH -)
Don’t be intimidated by this – the main point is that each mineral
group is named after its constituent anion (no big whoop).
Phosphates
The form of phosphate called hydroxyapatite is a very
important constituent of bones and teeth in vertebrate
animals. Hydroxyapatite gives bones and teeth their
great strength and provides animals phosphate for other
purposes (we will discuss this later).
Hydroxyapatite (from a rock sample)
Hydroxyapatite crystals in tooth enamel
Sulphides
Sulphides are not only formed by
inorganic processes – they are also
readily formed by organisms through
their metabolic activities.
For example, in a complex series of
reactions, sulphur-reducing bacteria
use dissolved sulphate (SO42- ) in
water for energy and produce the
poisonous, smelly, gas called
hydrogen sulphide (H2S).
This hydrogen sulphide can then
react with dissolved iron to form the
mineral pyrite (FeS2). Pyrite is an
important mineral in fossil
preservation (as we will see later
on).
Pyrite (FeS2)
Pyritized ammonite fossil
Halides
Halides contain members of the halide
family of ions (e.g. Cl-, F-)
Halides can be important indicator
minerals for climatic conditions. For
example most halite (NaCl- salt) is
deposited as a result of seawater
becoming too salty to hold all its salt in
solution. Thus salt deposits are good
indicators of seas located in hot, dry
climates.
Fluorite, is an important halide in that it
is a good source of fluoride. Fluoride
ions are incorporated in minor amount
in bones and teeth, providing extra
strength (yes, fluoride in toothpaste is
commonly derived from this mineral)
Halite (NaCl)
(sodium chloride)
Fluorite (CaF2)
(calcium fluoride)
Oxides/hydroxides
Hematite (Fe2O3) is a major component of
rust (this is what gives rust its red colour)
and is a common weathering product of
other iron minerals and is a good indicator
of abundant oxygen.
Interestingly, bacteria are important
mediators of rust formation.
Magnetite (Fe3O4) has the unique property
of magnetism.
Some bacteria use this property to their
advantage. They secrete chains of
magnetite crystals – these are thought to
help in a crude sort of navigation.
Oxides/hydroxides (examples)
The yellowish orange-coloured mineral goethite is
an iron oxide with a hydroxyl ion (OH-) attached to it.
It is commonly found in association with hematite.
Not surprisingly, this mineral is the other important
component of rust and is a common weathering
product of iron-bearing minerals.
Goethite (FeO(OH))
Mineral properties
The appearance and physical behaviour of minerals are directly related
to their chemical composition and bonding characteristics of their
components.
So…the physical properties of minerals are very handy in the accurate
identification of minerals. A few examples of important properties
include:
Hardness
Crystal form
Manner of Breakage (Cleavage/Fracture)
Lustre
Colour
Reaction to Acid
Note: for this particular course, I will not expect you to remember
chemical formulae, but anything else is fair game unless otherwise noted
(including the basic components of minerals where given).
Hardness
One of the most useful diagnostic properties is hardness (a measure of
the resistance of minerals to abrasion or scratching).
Hardness is measured in units of Mohs scale of hardness (a relative
scale developed by geologist Frederick Mohs based on the ability of
harder minerals to scratch softer minerals). Hardness reflects, to some
extent, the strength of bonds within a mineral.
So, for example if a mineral
can be scratched by apatite
but not fluorite, we can say
that the mineral must have a
hardness between 4 and 5.
No, I don’t want you to
memorize this table – but at
least know the concept of the
scale.
Hardness: examples
For some minerals, it is hardness that makes them useful
to organisms. For example, our bones are made of
hydroxyapatite (similar to apatite), which has a hardness of
about 5.
Thus, hydroxyapatite is a good material to make bones and
teeth out of !
Arrangement patterns of atoms within minerals are revealed in their
external crystal form.
The packing arrangement of atoms within any given mineral results in a
characteristic geometry of crystal faces. Each mineral has its own set of
crystal shapes that relate to the stacking patterns of unit cells.
Packing arrangements
of differently sized
atoms (smallest possible
unit is called a unit cell)
crystals of halite
crystals of quartz
Crystal Form
Some minerals can have the same composition, but have different
crystal structures. For example, both diamond and graphite are pure
carbon. But they are obviously very different minerals ! Minerals with
the same composition but different crystal stucture are called
polymorphs.
Diamond: a rigid
framework of covalently
bonded carbon atoms
Graphite: covalently bonded
carbon atoms forms sheets that
are weakly bonded
Cleavage
Arrangement patterns of atoms within minerals also result in characteristic
patterns of breakage.
In some minerals, bonds between some atoms are weaker than others.
The weaker bonds are often follow the boundaries of unit cells.
When a mineral breaks along well-defined planes, it is said to possess
cleavage.
A cleavage plane in halite
Broken sample of halite showing 3
cleavages at 90o
Common Minerals with Distinctive Cleavage
1 cleavage
Biotite Mica
2 cleavages
3 cleavages
Potassium
Feldspar
(at 90O)
Halite
(at 90O)
Amphibole
(not at 90O)
Calcite
(not at 90)
4 cleavages
Fluorite
6 cleavages
Sphalerite
Some minerals can be identified based on the numbers and angles of
cleavage (note, this doesn’t have to be memorized either).
Cleavage
Even in closely-related minerals, slight differences in the
shapes and packing arrangements of unit cells can result
in differences in cleavage.
90o
90o
Pyroxene
(Mg,Fe)SiO3
2 cleavages at 90o
Stacking of single-chain units
120o 60o
Amphibole
Ca2(FeMg)5Si8O22 (OH)2
2 cleavages (60o and 120o)
Stacking of double-chain units
Fracture
Some minerals do not have a distinct cleavage (i.e do not
break along nice flat planes). This is due to more uniform
bond strengths throughout the crystal structure (so no
preferred direction of breakage).
Quartz has a distinctive type of fracture called conchoidal
fraction (names referring to the shape of a seashell). Glass
breaks in the same manner.
Conchoidal
fracture in
quartz
Scoop-shaped
fracture
surfaces
Lustre
Lustre is the appearance or quality of light reflected from the surface of a
mineral. For some forms of minerals, lustre can be very diagnostic and
can be directly attributed to the form and arrangement of its crystals.
For example, the nacre made by molluscs (pearls and mother of pearl)
has a soft, pearly lustre that results from aragonite crystals acting as little
prisms within the material.
Large pyrite crystals have
a metallic lustre
Quartz has a vitreous
(glass-like) lustre
In the form of nacre,
aragonite has a pearly lustre
Colour
Colour primarily manifests the chemical content of a mineral.
In a few minerals, colour can be a very diagnostic property.
For example, the common sulphide mineral pyrite (iron
sulphide- “fools gold”) has a characteristic brassy colour,
whereas galena (lead sulphide) has a silvery grey colour.
Pyrite
FeS2
Galena
PbS
Colour
In other minerals, colour can be
very misleading.
Minor impurities or crystal
defects can impart different
colours
Fluorite
CaF2
Reaction with acid: some minerals will effervesce (fizz)
when reacted with acid. Calcite (CaCO3) and a similarlooking carbonate mineral, dolomite (CaMg(CO3)2) react
with acid, but only calcite fizzes violently (dolomite has to
be powdered before it fizzes)
calcite (CaCO3)
dolomite (CaMg(CO3)2)
Reaction of limestone
(made of calcite) with
dilute hydrochloric acid.
Optical properties: some minerals, such as calcite
(CaCO3), will produce a double image when an object is
viewed through its crystals.
This is due to the splitting of light rays as they pass through the
calcite crystal.
Note: extinct creepy crawlies called trilobites
actually made their eye lenses out of calcite
(not exactly a choice material for this
purpose, but trilobites modifies lens shape to
correct for this optical effect) !
Various other unique properties, alone, or in combination,
can also be useful in the identification of minerals.
For example,
When rubbed, sphalerite (ZnS) has the smell
of rotten eggs. It also has a resinous lustre.
Graphite (pure carbon) has a greasy lustre, is
opaque, and is very soft (this is what makes it
a good drawing tool)
Halite (NaCl) tastes salty.
Now that we have looked at minerals, we can move on
to rocks…
END OF LECTURE