Environmental Engineering Reference
In-Depth Information
TABLE 6.10
Causes of Chemical Decomposition of Minerals
Cause
Process
Oxidation
Oxygen from the atmosphere replaces the sulfur element in rock
minerals such as pyrite containing iron and sulfur, to form a new
compound — limonite. The sulfur combines with water to form a
weak solution of sulfuric acid that attacks the other minerals in the rock
Leaching
Chemical components are removed by solution. Calcite the chief
component of limestone dissolves readily in carbonic acid, formed
when carbon dioxide from the air dissolves in water or when
rainwater permeates organic soils. The greater the amount of
dissolved oxygen, the greater is the leaching activity
Hydrolysis
The chemical change that occurs in some minerals, such as the
feldspars, when some of the mineral constituents react with the water
molecule itself Orthoclase feldspar is changed to kaolin by hydrolysis
and appears in the rock mass as a very white, friable material. The
formation of kaolinite causes further disintegration of the rock
mechanically by expansion because the kaolinite has a greater volume
than the feldspar. It is considered, therefore, a cause of exfoliation
Reduction
Humic acids, produced from decaying vegetation in combination with
water, attack the rock mass
When encountered in tunnels, the clay squeezes into the excavation, often under very high
swelling pressures, and control can be very difficult.
6.7.2
Factors Affecting Decomposition
The Factors
Climate, parent rock, time, rock structure, topography, and the depth of the groundwater
table affect the rate of decomposition, the depth of penetration, and the mineral products.
Urbanization may accelerate decomposition in some rock types through percolation of
polluted waters.
Climate
Precipitation and temperature are major factors in decomposition. General relationships
among climate, type of weathering (mechanical vs. chemical), and activity resulting in
rock decay are given in
Table 6.11.
Climatic regional boundaries in terms of mean annual
rainfall and temperature vs. the intensity of types of weathering are given in
Figure 6.68,
and climate vs. weathering profiles in tectonically stable areas are given in
Figure 6.69.
In
general, the higher the temperature and precipitation, the higher is the degree of decom-
position activity.
Parent Rock Type
Significance
The parent rock type influences the rate of decomposition, which depends on the stabil-
ity of the component minerals for a given climatic condition and other environmental
factors.
