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G eo fac t s an d ac t i v i t i es f o r t h e c l ass room
Iron is the second
most abundant
metal on Earth
Aluminium is the most abundant metallic
element but iron has played a much
greater role in human history and is an
essential part of everybody, literally!
Iron is one of 94 naturally occurring
elements. It is a metallic material with a
silver-grey colour and metallic lustre. It’s
Atomic Number is 26 and its symbol is
Fe. Iron is found in all parts of the planet.
It comprises about 5% of the Earth’s
crust but is thought to make up perhaps
as much as 80% of the planets core.
There are four naturally occurring stable
isotopes of iron with at least four unstable
isotopes also known to exist.
A rusty red world
Iron rarely occurs as a pure metal in
nature because it rapidly reacts with
oxygen to form iron oxide. Rust is an
iron oxide most people would be familiar
with. In fact, most iron ore mined today
is a natural accumulation of rust! The
four most common iron ore minerals
are magnetite [Fe3O4], haematite
[Fe2O3], goethite [FeO.OH] and limonite
[FeO(OH)·nH2O ].
Iron oxides are very stable minerals that
are formed through the oxidation of other
iron rich minerals weathering out of rocks.
Places such as Australia (and probably
Mars) have a characteristic red hue to
their landscapes due to the accumulation
of strongly coloured iron(III) oxides in
the soils and on weathering surfaces.
However, in order to form iron(III) rich
minerals free oxygen must be available
to oxidise iron(II). When the Earth first
formed there was no free oxygen and iron
in the oceans was probably mostly iron(II)
in solution.
It is thought the evolution of
photosynthetic bacteria may have
changed the way the world worked. They
probably produced enough oxygen to
start oxidising iron(II) in the oceans and
begin precipitating iron(III) oxides onto the
ocean floor. This resulted, over millions
of years, in the formation of huge layers
of iron oxide deposits on the floors of the
worlds oceans. These layers, often called
Banded Iron Formations – or BIF’s – now
outcrop as massive iron ore deposits and
are the basis of the large iron ore mining
industry in Western Australia. Eventually,
enough oxygen was produced to oxidise
all the free iron(II) in the oceans and as
a result oxygen began to accumulate in
the atmosphere. Atmospheric oxygen is
now a key component in the weathering
and erosion of rocks in the terrestrial
landscape too, resulting in iron(III)
oxides forming and accumulating in the
landscape.
Properties and uses of iron
Iron, aluminium, copper, zinc and lead
are the most commonly used metals
today. Iron is the metal of choice for
many items because of its strength and
abundance. However, most uses of iron
do not use the pure metal.
Steel is an alloy of iron with small
amounts of carbon, less than 2%. Iron
with more than 2% carbon develops a
lower melting point. The melting point of
pure iron is 1538oC while iron with more
than 2% carbon melts at around 1370oC
and is called cast iron. This type of iron
was probably the first type of iron used by
people because the temperatures and low
oxygen environments required could be
achieved in simple furnaces. It is usually
cast into shapes, hence the name, and
although strong it is very brittle.
Low carbon steels are more ductile and
malleable and more easily welded. By
varying the carbon content between 0.2
and 1.5% steels with varying degrees
or hardness, strength and workability
can be produced, making iron one of the
most important materials in use today.
Everything from buildings to bridges, car
bodies to springs, watches and wire rope
contains iron in the form of steel.
Stainless steel is an iron based material
most Australians would be familiar with
because of its use in sinks, troughs and
many other household appliances. It is
an iron-carbon alloy that contains at least
10.5% chromium. The chromium forms a
layer of chromium(III) oxide (Cr2O3) when
exposed to oxygen and even though too
thin to see this layer protects the steel
beneath it from oxidation and quickly
reforms if scratched off.
Iron and some iron oxides are
magnetic. Magnetite, [Fe3O4], is the
most magnetic natural mineral known.
Lodestone, a type of magnetite, was
used to build the earliest magnetic
compasses.
Iron is also the most essential trace
element in humans and most other
animals because it is central to the
structure and functioning of blood in
transporting oxygen around the body.
Acknowledgement: Greg McNamara, Education and Outreach, Geological Society of Australia
http://www.gsa.org.au
n
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G eo fac t s an d ac t i v i t i es f o r t h e c l ass room
Iron colours worlds red
1. Iron atoms are present on Earth as four naturally
occurring isotopes; 54Fe, 56Fe, 57Fe and 58Fe.
Given that iron has an Atomic Number of 26
how many protons, neutrons and electrons
does each isotope have?
2. Iron has two common oxidation states: Fe 2+
and Fe 3+ The mineral magnetite has the formula
Fe3O4 but it is also sometimes written as FeO.Fe2O3.
.What are the oxidation states of each of the iron
atoms in FeO.Fe2O3. given oxygen is O2-?
Explain your answer
3. The mineral limonite is a hydrated iron oxidehydroxide. It has the general formula FeO(OH)·nH2O.
What three elements are found in this mineral?
4. Why is the mineral limonite called
a hydrated iron oxide?
5. The mineral haematite [also spelled hematite]
usually forms in water or in the presence of water.
All minerals, including haematite, have a distinctive
spectra of light they reflect. The spectral signature
of haematite has been detected at several places
on the surface of Mars.
What does this suggest about past
environments on Mars?
6. Many minerals have a distinctive property called
streak. Streak is the powder colour of a mineral
and is usually observed by scratching a mineral on
a white unglazed tile to make a powder that sits on
a white background. The fine powder in the streak
can be an important diagnostic tool in mineral
identification and is often very different to the colour
of the larger mineral.
Mineral
Streak
Magnetite
Black
Haematite
Bright to dark red
Goethite
Brown to yellow-brown to orange-yellow
Limonite
Yellow-brown
In Australian desert landscapes the soils have a
distinctive reddish hue. Which iron oxide is most
likely to be the cause of this hue and what size are
the oxide grains likely to be? Gravel, sand or clay
sized? Explain your reasoning.
7. The massive iron ore deposits of Western Australia
are found in Banded Iron Formations (BIF’s). The
iron rich bands are interbedded with iron poor
shales and cherts. Assuming the iron rich bands
are formed from iron(II) in the oceans being oxidised
and precipitating out as iron(III) oxides what natural
phenomena could explain the alternating iron-rich
and iron poor bands? Note that the oxygen at the
time was being supplied by photosynthetic bacteria.
Think about natural processes that control rates of
photosynthesis and explain your answer accordingly.
8. When rocks weather the iron rich minerals within
them are converted to clays and the iron oxidised
to form strongly coloured iron oxide minerals such
as goethite and haematite. Granite forms very pale
yellow clay rich soils. Basalt forms chocolate brown
soils. Which rock type is likely to have the highest
percentage of iron-rich minerals in its composition?
Justify your answer.
Acknowledgement: Greg McNamara, Education and Outreach, Geological Society of Australia
http://www.gsa.org.au