An Introduction to Environmental Chemistry

Chemical weathering is caused by water—particularly acidic water—and gases,
for example oxygen, which attack minerals. However, many of these reactions are
catalysed by bacteria (biological weathering). Chemoautotrophic bacteria, for
example Thiobacillus, derive their energy directly from the weathering of miner-
als (see Section 5.4.2). Some ions and compounds of the original mineral are
removed in solution, percolating through the mineral residue to feed ground-
water and rivers. Fine-grained solids may be washed from the weathering site,
leaving a chemically modified residue, the basis of soils. We can view weathering
processes—physical, biological and chemical weathering usually occur together
—as the adjustment of rocks and minerals formed at high temperatures and pres-
sures to Earth surface conditions of low temperature and pressure. The miner-
alogical changes occur to regain stability in a new environment.

4.4 Mechanisms of chemical weathering

Different mechanisms of chemical weathering are recognized and various
combinations of these occur together during the breakdown of most rocks
and minerals.

4.4.1 Dissolution

The simplest weathering reaction is the dissolution of soluble minerals. The
water molecule (Box 4.1) is effective in severing ionic bonds (see Section 2.3.2),
such as those that hold sodium (Na+) and chlorine (Cl-) ions together in halite
(rock salt). We can express the dissolution of halite in a simple way, i.e.:

eqn. 4.3

This reaction shows the dissociation (breaking of an entity into parts) of halite
into free ions, forming an electrolyte solution. The reaction does not contain
hydrogen ions (H+), showing that the process is independent of pH.

4.4.2 Oxidation

Free oxygen is important in the decomposition of reduced materials (Box 4.3),
typically iron, sulphur and organic matter. For example, the oxidation of reduced
iron (Fe^2 +) and sulphur (S) in the common sulphide, pyrite (FeS 2 ), results in the
formation of sulphuric acid (H 2 SO 4 ), a strong acid:

eqn. 4.4

Where mining exposes large amounts of pyrite to weathering, the generation of
this acid creates significant environmental problems (see Section 5.4.2).
Reduced iron-bearing silicate minerals, for example some olivines, pyroxenes
and amphiboles, may also undergo oxidation, as depicted for the iron-rich olivine,
fayalite:

27 7 2FeS 2 ()sgl++Æ^12 O 22 () H O() Fe OH() 3 ()s + 4 H SO 24 ()aq

NaCl

halite

s Na Cl

HO

() ()aq aq

+ ()-

()

ª +

2

The Chemistry of Continental Solids 77