Geology Reference
In-Depth Information
Nitrogen is an important constituent of all living
matter: the importance of the amino group (-NH 2 ) and
amino acids has been discussed under carbon. In
decaying organic matter, such compounds are decom-
posed bacterially to the gas ammonia (NH 3 ), most of
which is oxidized by soil bacteria to nitrate ( NO ), the
form of nitrogen most readily utilized by plants. A
large amount of nitrate is used as fertilizer which,
owing to its high solubility, is washed into streams, riv-
ers, lakes and groundwater, where it may compromise
water quality.
Phosphorus, unlike nitrogen, does not occur in the
elemental state in nature. In silicate analyses (Box 8.3),
it is reported as the oxide P 2 O 5 . It exists geologically as
the phosphate oxy-anion (PO 4 3 − ), most commonly in
the accessory mineral apatite (Ca 5 (PO 4 ) 3 OH). In basic
magmas, phosphorus behaves as a high field-strength
incompatible element (Box 9.1), but as crystallization
proceeds such melts eventually become saturated with
apatite, whose crystallization then leads to the depl-
etion of phosphorus in subsequent melt fractions.
Nitrogen and phosphorus are both important
biolimiting inorganic nutrients. When confined bod-
ies of water (lakes, estuaries, coastal waters) receive
influxes of nitrate or phosphate - for example, from
sewage outflow or the application of excess fertilizer -
sunlight may promote rapid blooming of bright green
algae at the water surface, preventing normal oxygen-
ation of the water below (hypoxia) that may result in
fish-kill. The phenomenon is known as eutrophication
(from the Greek for 'well nourished').
concentration rarely exceeds 10 ppm, it absorbs solar
ultraviolet radiation strongly, and this 'ozone layer'
protects terrestrial life from the damaging effects of the
Sun's UV rays. At sea-level, however, ozone is an
undesirable pollutant, contributing to photochemical
smog and acid-rain damage (O'Neill, 1998).
Oxygen forms an oxide (oxidation state -II) with
almost every other element. Oxides may have basic or
acidic properties (Box 8.1), or they may be amphoteric ,
exhibiting both aspects. Because these characteristics
depend on the electronegativity of the element bonded
to oxygen, they correlate with its position in the
Periodic Table (Figure 9.4).
The availability of oxygen determines the stability of
many minerals, particularly those containing elements
like iron that have multiple oxidation states. The avail-
ability of oxygen in low-temperature, aqueous env-
ironments is expressed by the oxidation potential Eh
(Figure 4.1b); environments with free access to atmos-
pheric oxygen have high Eh values, whereas anaerobic
conditions are characterized by low Eh values.
In high-temperature systems it is more convenient to
express the availability of oxygen in terms of its partial
pressure (Chapter  4), or the related parameter called
the oxygen fugacity , f O 2 . Partial pressure is appropriate
as a measure of concentration only in low-pressure gas
mixtures, in which molecules are so dispersed that
they behave independently of each other (except when
colliding). This state is called an 'ideal gas'. In high-
pressure mixtures or when a gas is dissolved in another
phase such as an igneous melt, gas molecules interact
more strongly with neighbouring molecules, rather
like ions in non-ideal aqueous solutions (Chapter  4).
Oxygen
Viewed from a terrestrial perspec-
tive, oxygen is the most important of
all chemical elements, being the
most abundant element in the crust
and mantle, and a major life-sup-
porting constituent of the oceans
and atmosphere (Figure 9.3).
Oxygen (1s 2 2s 2 2p 4 ) is the second most electronega-
tive element (3.4; Figure 6.3). It is divalent, having two
vacancies in the valence shell. Diatomic oxygen O 2
makes up 21% of dry air at sea-level. At altitudes
between 10 and 60 km, in the stratosphere, another
type of oxygen molecule plays a significant role: this is
the triatomic molecule ozone , O 3 . Although the O 3
H
BC
N O
S
Se
Te
Al
Si
P
Li
Be
B
CNOF
Ga
In Sn
Ge
As
Sb
Na
Mg
Al
Si
P
S
Cl
Acidic
oxides
K
Ca
Ga
Ge
As
Se
Br
Rb
Sr
ln
Sn
Sb
Te
l
Cs
Ba
Tl
Pb
Bi
Basic oxides
Amphoteric oxides
Figure 9.4 Acidic, basic and amphoteric oxides of the
non-transition elements.
 
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