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.
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