Key geological issues - Geotechnical Engineering of Dams - page 25

Table 2.1. Susceptibility of igneous rock-forming minerals to weathering.

Temperature of Susceptibility Common igneous

formation to weathering Mineral rock types

Highest Highest Olivine

Basalt, dolerite, gabbro

Calcic
felspar
Augite Andesite, diorite

Hornblende
Sodic felspar
Biotite

Rhyolite, granite

Muscovite

Lowest Lowest Quartz

Table 2.2. Susceptibility of other common minerals to weathering.

Group Mineral Effects of weathering

Carbonates Calcite Readily soluble in acidic waters

Dolomite Soluble in acidic waters

Evaporites Gypsum Highly soluble

Anhydrite Highly soluble

Halite (common salt) Highly soluble

Sulphides Pyrite and various other Weather readily to form sulphates, sulphuric acid and

pyritic minerals limonite

Clay minerals Chlorite Weathers readily to other clay minerals and limonite

Vermiculite Weathers to kaolinite or montmorillonite*

Illite Weathers to kaolinite or montmorillonite*

Montmorillonite Weathers to kaolinite

Kaolinite Stable**

Oxides Haematite Weathers to limonite

Ilmenite Stable

Limonite Stable

* These minerals expand and contract with wetting and drying and this can cause large disruptive
forces and disintegration of some rocks.
** Softens on wetting.

For example, both rhyolite and granite contain sodic felspar, micas and quartz, but in
rhyolite the crystals are much more fine grained and more tightly interlocked than in
granite. Hence granite is much more susceptible to weathering than rhyolite ( Figure 2.16 ).
Also, a dense, non-porous limestone comprising almost 100% calcite is likely to be less
susceptible to weathering (solution) than a porous sandstone comprising 80% quartz
grains which are durable but cemented by calcite.

2.6.4 Weathered rock profiles and their development

The following are the main factors which contribute to the development of weathered
profiles:

- climate and vegetation;

- rock substance types;

- defect types and patterns;