The basalts of the Drakensberg Formation consist of a succession of lava flows which total some 1400m in thickness. Many of the lava flows are characterized by amygdaloidal zones. Many of the primary minerals within the basalts have been subjected to varying degrees of deuteric alteration which has led to the formation of clay, and chlorite and zeolite to a lesser extent. Some interstitial glass has also broken down to form clay. These secondary minerals, together with zeolites which occur notably as amygdaloidal fillings, mean that many of the basalts break down rapidly on exposure. The breakdown results principally from the expansion which occurs when the clay minerals swell on absorption of water. This gives rise to crazing.
The mineral content of the basalts was determined by modal analysis and X-ray diffraction. In addition, methylene blue adsorption was used to estimate the clay content. The secondary mineral rating was determined primarily from the modal analyses. Soak tests and free swell tests using ethylene glycol and water; water absorption, wet and dry, and freeze- thaw tests were carded out to assess the durability of these basalts. The olivine basalts and those types which contained disseminated clay spots were the least durable, some breaking down rapidly on testing. This was caused mainly by the expansion of clay minerals. Water absorption was also influenced by the presence of zeolites, notably laumontite. Another factor which has to be taken into account in relation to break down and durability is texture since this can restrict access of water to swelling minerals.