Rock-Mass Characteristics - Geotechnical Engineering Investigation - page 468

FIGURE 6.69
Formation of weathering mantle in tectonically stable areas. (From Morin, W. J. and Tudor, P. C., AID/Csd
3682, U.S. Agency for International Development, Washington, DC, 1976. With permission.)
Igneous and Metamorphic Rocks
Sialic crystalline rocks (light-colored rocks such as granite, granite gneiss, rhyolite, etc.),
composed of quartz, potash and soda feldspars, and accessory micas and amphiboles, rep-
resent the more resistant rocks.
Mafic crystalline rocks (dark-colored rocks such as basalt, gabbro, dolerite, etc.) and
metamorphic rocks of low silica content contain the minerals least resistant to decomposi-
tion, i.e., the ferromagnesians. The relative susceptibility of rocks composed of silicate
minerals to decomposition is given in Table 6.13 . Clays are the dominant products of
decomposition of igneous rocks and are summarized in terms of rock type and original
mineral constituents in Figure 6.70.
Sedimentary Rocks
Argillaceous and arenaceous rock types are already composed of altered minerals (clay and
sand particles) and normally do not undergo further decomposition, except for impurities
such as feldspar and other weatherable minerals.
Carbonates and sulfates go into solution faster than they decompose, except for mineral
impurities, which decompose to form a residue in the normal manner.
Final Product and Thickness
The final product is usually a mixture of quartz particles (which are relatively stable), iron
oxides, and clay minerals (Section 5.3.3). Vermiculite and chlorite are uncommon since
they alter readily to montmorillonite, illite, and kaolinite. The clay minerals result from the