Environmental Engineering Reference
In-Depth Information
TABLE 6.12
Relative Stability of Common Rock-Forming Minerals
a
Group
Minerals
Relative Stability
Silicates
Feldspars
Orthoclase (potash)
Most persistent of feldspars
Plagioclase (soda lime)
Weathers to kaolinite
Micas
Muscovite (white mica)
Persistent, weathers to illite
Biotite (dark mica)
Easily altered to vermiculite, iron element causes
staining
Amphiboles (hornblende)
Persistent
Pyroxines (augite)
Less persistent than hornblende
Decomposes to montmorillonite
Olivine
Readily decomposes to montmorillonite
Oxides
Quartz
Most stable: slightly soluble
Iron oxides
Hematite
Relatively unstable
Limonite
Stable; product of alteration of other oxides of iron
Carbonates
Calcite
Readily soluble
Dolomite
Less soluble than calcite
Sulfates
Gypsum
More soluble than calcite
Anhydrite
Like gypsum
Hydrous aluminum
Kaolinite
The most stable clay mineral
silicates (clay minerals)
Illite
Alters to kaolinite or montmorillonite
Vermiculite
Alteration product of chlorite and biotite
Alters to kaolinite or montmorillonite
Montmorillonite
Alters to kaolinite
Chlorite
Least stable of clay minerals
Alters readily to any or all of the others
a
After Hunt, C.B.
Geology of Soils
, W.H. Freeman and Company, San Francisco, 1972. Reprinted with permis-
sion of W. H. Freeman and Company.
most common groups of silicates (feldspars and ferromagnesians). The ferromagnesians
usually contain iron, which decomposes to form iron oxides that impart a reddish color
typical of many residual soils.
The thickness of the decomposed zone is related directly to rock type in a given climate
as well as to topography (see “Other Factors” below). Froelich (1973) describes the depth
of decomposition in the Baltimore-Washington, DC, area of the eastern United States.
Formerly a peneplain, the typical topographic expression in the area is given in
Figure
6.71.
The quartz veins, dikes, and indurated quartzites have little or no overburden, and
the massive ultramafic bodies (serpentinite) generally have less than 5 ft of overburden in
either valley bottoms or ridgetops. Foliated mafic rocks (greenstones) commonly have 5 to
20 ft of cover; gneisses and granitic rocks are mantled by as much as 60 ft of saprolite but
may contain fresh corestones (see
Section 6.7.3)
;
and schists and phyllites commonly have
from 80 to 120 ft of overburden. Saprolite is thickest beneath interstream ridges and thins
toward valley bottoms. The larger streams are incised in, and may flow directly on, hard
fresh rock.
Other Factors
Time
The length of geologic time during which the rock is exposed to weathering is a significant
factor. Decomposition proceeds very slowly, except in the case of water flowing through
