Doleritic intrusions in Natal and adjoining areas of South Africa are very numerous, and are associated with the break‐up of Gondwana which took place some 187 to 155 million years ago. These dolerites are either tholeiitic or olivine dolerites. Plagioclase and pyroxene represent their two major mineral components. A series of tests was carried out to determine the physical and mechanical properties of dolerites from a number of localities in and around Natal, in addition to the determination of their chemical and mineralogical compositions. Some dolerites in Natal have been found to deteriorate within a short space of time when exposed and when used as construction materials. These have been designated rapidly weathering dolerites. Such disintegration is due to the presence of chlorite and clay minerals. Frequently, the clay material is smectitic, in which case breakdown results from the expansion that occurs when such clay minerals swell on absorption of water. Unfortunately, however, such dolerites cannot be distinguished readily from sound dolerite. As a consequence, several durability tests were undertaken. The results from the latter were compared with those of the physical and mechanical properties, and certain petrological indices derived from the mineralogical character of the dolerites to see if there were any significant relationships that were diagnostic in terms of recognizing rapidly weathering dolerite. Rapidly weathering dolerites can possess high strengths and ultrasonic velocities. Generally, they can be detected by ethylene glycol soak testing and perhaps may be recognized by their petrological weathering index.

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