Geology Reference
In-Depth Information
state IV). About 10 8 tonnes per year of SO 2 are released
into the atmosphere by fossil-fuel burning and other
industrial processes. On dissolving in water droplets
(Box  9.3), SO 2 oxidizes to form an aerosol of sulfuric
acid (H 2 SO 4 , oxidation state VI). This was the main
contaminant in acid rain , which caused severe and
widespread damage to lakes, rivers and forests in the
northern hemisphere.
Sulfide minerals are also susceptible to atmospheric
oxidation. The weathering of near-surface sulfide ore
bodies by the downward percolation of oxygenated
water may lead to further enrichment of the ore.
Just below the surface (Figure  9.6), sulfides are oxi-
dized to sulfates, which migrate downward in sol-
ution. Commonly a number of carbonate, sulfate and
oxide ores are precipitated just above the water table,
whereas reducing conditions below the water table
lead to deposition of secondary sulfide minerals. This is
called supergene enrichment.
Sulfate minerals occur in two other environments.
Barite (BaSO 4 ) occurs in low-temperature hydrother-
mal veins (as in the English Pennine orefield -
Chapter  4), commonly in association with sulfides.
Minerals like anhydrite (CaSO 4 ) and gypsum
(CaSO 4 .2H 2 O) are characteristic of evaporites.
Fluorine
Fluorine has the highest
electronegativity of all
elements (Figure 6.3), and
is the most reactive. It
forms strongly ionic com-
pounds. The commonest
fluoride mineral (and the
chief industrial source of
fluorine) is fluorite , CaF 2
(Figure 7.3d), which occurs most commonly in hydro-
thermal veins. The ionic radius of the fluoride anion
F - (1.25 pm, Box 7.2) is similar to those of O 2 - and OH - ,
and fluorine is a common substituent for OH - in
hydrous minerals like amphiboles, micas and apatite.
Being more reactive and electronegative, fluorine
can displace oxygen from most silicates. Dissolving
hydrogen fluoride (HF, a gas) in water produces hydro-
fluoric acid , which is widely used in analytical geo-
chemistry as it is the only acid capable of attacking
silicate rocks (in powdered form) to bring them into
solution for analysis. It is a dangerous reagent that
requires special training and handling: unlike the more
familiar hydrochloric acid, it causes no burning sens-
ation on contact with the skin, but penetrates into
deeper tissue and may cause intense pain after a few
hours. Because HF (as solution or gas) attacks glass, it
must be used only in platinum or plastic containers, in
specially designed fume cupboards.
He
B
C
N
O
F
Ne
Al
Si
P
S
Cl
Ar
Ga
Ge
As
Se
Br
Kr
ln
Sn
Sb
Te
l
Xe
ATMOSPHERE
GOSSAN
(hydrated oxides
and residual silica )
Oxidation to
soluble sulphates
LEACHED
ZONE
Deposition of oxidised ores:
- carbonates (e.g., azurite, malachite)
- sulphate (anglesite, PbSO 4 )
- oxides (e.g., cuprite)
- native metals (e.g., Cu)
ZONE OF
OXIDISED
ENRICHMENT
Water
table
ZONE OF
SECONDARY
(SUPERGENE)
ENRICHMENT
Chlorine and other halogens
Deposition of
secondary
sulphides in
reducing conditions
(e.g., covellite)
Chlorine is the third most electronegative element
(Figure 6.3). In silicate rocks it is a trace element, being
about four times less abundant in the crust than F. The
chloride anion (Cl ) is, however, the most abundant
dissolved species in seawater, and it is the dominant
ligand in brines and hydrothermal fluids. Chlorine gas
(Cl 2 ) has many industrial uses.
Organochlorine compounds are widely used in
industry as solvents, propellants and refrigerants
owing to their chemical inertness. Among the most
inert are chlorofluorocarbons ( CFC s), hydrocarbon
PRIMARY
SULPHIDES
Figure 9.6 Idealized cross-section of a near-surface sulfide
ore body, showing the zonation due to percolating oxygen-
bearing solutions. The mineral assemblages produced are
illustrated by typical copper minerals (stability relations are
indicated in Figure 4.1a), but many other minerals occur in
such environments.
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