An Introduction to Environmental Chemistry

(Rick Simeone) #1
speeds up chemical reactions by supplying energy (Box 4.8). For most reactions
a 10°C rise in temperature causes at least a doubling of reaction rate. This sug-
gests that weathering in the tropics, where mean annual temperatures are around
20°C, will be about double the rate of weathering in temperate regions where
mean annual temperatures are around 12°C.
The effect of temperature is linked to the availability of water. The dry air of
hot, arid environments is an ineffective weathering agent. Vegetation and hence
soil organic matter are sparse, and this reduces the concentration of organic acids.
Moreover, close contact between rock particles and acid is prevented by the lack
of water. Short-lived rainfall events may move surface salts into the soil, but the

96 Chapter Four


Ferromagnesian
series

Olivine
(monomer silicate)

Pyroxene
(chain silicate)

Amphibole
(double chain silicate)

Biotite mica
(sheet silicate)

Felsic
series

Calcium feldspar
(framework silicate)

Muscovite mica
(sheet silicate)

Sodium feldspar
(framework silicate)

Potassium feldspar
(framework silicate)

Quartz
(framework silicate)

Increasing
temperature
of mineral
crystallization
in magma

Increasing
susceptibility
to weathering

Fig. 4.14Common silicate minerals ranked in Bowen’s reaction series. Note that minerals
formed under high temperatures with more ionic bonding are more susceptible to weathering.
Ferromagnesian—minerals which contain essential iron and magnesium. Felsic—a rock
containing feldspars and quartz.
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