Environmental Engineering Reference
In-Depth Information
Figure 13.9
Weathering sequence and secondary products of
primary rock-forming minerals.
Source: After Selby (1993).
ions. This complex, multiphase reaction is often simplified to the form:
with calcium bicarbonate removed in solution. Each process is reversible, and
reprecipitation of calcium carbonate may occur eventually as
tufa
or a more resistant
crystalline form (
travertine
). The carbonation of orthoclase feldspar creates a new clay
mineral species,
illite
, produces potassium and bicarbonate ions and removes dissolved
silicate:
Hydrolysis
involves H
+
and OH
−
reactions with minerals and is important, for example,
in the decomposition of granite containing plagioclase feldspar, with the clay mineral
kaolinite
the principal product:
Hydration
occurs when minerals absorb water into their crystal lattice and establish
tensile stress in addition to chemical alteration. Hydration of iron oxides to the form
limonite
, an important weathering process in mafic rocks, illustrates the latter and is
reversible:
Two sets of
redox
reactions involve oxygen either by combination (
oxidation
) or
removal (
reduction
), occurring in aerobic or anaerobic environments and changing
valency in a positive or negative direction respectively. Oxidation promotes weathering
in mafic minerals, changing ferrous iron oxide (FeO) to its ferric form (Fe
2
O
3
),
destabilizing the crystal lattice and requiring the compensating loss of another cation.