Geology Reference
In-Depth Information
Sulphur, like oxygen, has a third, outer orbit, which misses two electrons. To arrive at
sulphur, take oxygen, fill its outer, second orbit, and add a third, but with two electrons
missing. Sulphur, like oxygen, thus seeks two electrons for completion, but in a less
frenetic way; it has a mellower (and yellower) character because its two inner orbits are
satisfactorily full. It is essential for life because it is a key component of the amino acid
methionine, without which human biochemistry cannot be sustained. Sulphur shares
some of the passion of oxygen, but being more moderate in its relationships, is able to
form long rings and chains rather like those of carbon, and to bond with other atoms in
a great variety of ways. Strangely, sulphur in its various gaseous forms it is often asso-
ciated with smells that are offensive to the human nose.
Many other elements are crucial for Gaia, and of those calcium, iron and silicon are
of great importance. A calcium atom has four electron orbits, three of which are full,
but its outer orbit has only two electrons, which makes it only too happy to engage in
ionic bonds by giving these electrons away, leaving behind a calcium ion which carries
a double positive charge. It is this that makes calcium so attractive to negatively charged
ions. Calcium has been called the messenger of the cell, because it is somewhat like
those charismatic entrepreneurs of the human world who have a mercurial aptitude for
networking. It is involved in virtually every cellular process, ranging from cell division
to fertilisation to muscle contraction, and without this Hermes-like atom the astounding
coordination of the cell's metabolism would be impossible. But too much calcium can
kill, so cells must expend energy to keep it at a concentration low enough for optimum
functioning.
Iron, like calcium, has three full electron orbits, but has eight outer electrons rather
than two. This means that iron needs to seek ten electrons to find completion, and it does
this in a variety of ways. It has a particular penchant for oxygen: iron sits at the centre
of the haemoglobin molecule, where it binds with oxygen from the lungs, releasing it to
cells where the oxygen tension is low. It also binds to oxygen in two important forms:
as haematite (Fe
2
O
3
), familiar in black volcanic sand beaches, and magnetite (Fe
3
O
4
).
Both of these iron compounds had an important part to play in determining the oxygen
concentration in the Earth's early atmosphere.
Silicon, like carbon, needs four electrons to complete its outer orbit, but unlike car-
bon it has an extra inner orbit with eight electrons. Silicon shares some of the character
of carbon—it is a highly social being which likes to make long chains with other silicon
atoms, and is very fond of doing so as the silicate ion (SiO
4
)
- 4
in which a silicon atom
links up with four oxygen atoms. Silicate ions can link up in an astonishingly large num-
ber of ways, one of which occurs when the four oxygen atoms are shared, giving rise
to silica (SiO
2
), which can arrange itself into the highly ordered, spiral configuration of
quartz crystals. The continents and the sea floor are made out of calcium-silicate rocks
