Key geological issues - Geotechnical Engineering of Dams - page 24

Figure 2.15. Microscopic view of the structure of slightly weathered granite (Dixon, 1969).

Dearman (1978b) they relate changes in engineering properties to the microfabric
changes.
The chemical reactions involved in chemical weathering include carbonation, hydrolysis,
solution, oxidation and reduction. As most cannot be actually observed, details of the reac-
tions, as published by various workers, are in part speculative. Useful accounts are given by
Selby (1993), Price (1995) and the Geological Society (1995).

2.6.3.1 Susceptibility of common minerals to chemical weathering
As would be expected, the susceptibility to weathering (or the “weatherability”) of minerals
in igneous rocks varies in accordance with the temperatures at which they were formed.
This is illustrated in Table 2.1 . With the exception of quartz, the most stable mineral, all of
the others on Table 2.1 weather eventually to clay minerals. Quartz is slightly soluble in
water. It is almost unaffected by weathering except under tropical conditions when it is
readily dissolved (in a geological time-frame).
The susceptibility of other common minerals to weathering is indicated in Table 2.2 .
The minerals in the carbonate and evaporite groups (shown in this table) may occur in
any of three different ways, namely:

- as rocks, e.g. calcite as limestone, or dolomite as dolomite (rock) or dolostone, or

- as cements in sedimentary rocks, e.g. calcite as cement in sandstone composed mainly of

quartz grains, or

- as veins or joint fillings or coatings in any rock mass.

2.6.3.2 Susceptibility of rock substances to chemical weathering
The susceptibility of a rock substance to weathering depends upon the following:

- the susceptibility to weathering of its component minerals, and

- the nature of its fabric, i.e. the degree of interlock and/or cementation of the mineral

grains, and

- its porosity and permeability.