The undulatory extinction of quartz has become important in engineering geology because of its supposed indication of an alkali-silica reactive aggregate. In an earlier paper (West 1991) it was suggested that, for granites from a single orogenic region, the undulatory extinction angle of the quartz in the rock may be proportional to the age of the rock. The paper showed that this was true for granitic rocks. The paper showed that this was true for granitic rocks from the Chilean orogenic belt (de Hills & Corvalan 1964) and went on to suggest that it would be worthwhile to undertake a study of the undulatory extinction angle of quartz in British granites. This study has now been carried out by research at Leicester University, working on a project sponsored by TRL, the full results of which have been reported elsewhere (Smith & Dunham 1992). This Technical Note discusses two aspects of these results, namely undulatory extinction angel in relation to geological age and potential reactivity.

Undulatory extinction

There are two methods of measuring undulatory extinction on the ordinary flat stage of a popalarizing microscope. The first is that used by De Hills & Corvalan (1994) and is simply tha angle between the first appearance and last disappearance of the extinction shadow in the quartz grain. The second is that proposed by Dolar&Mantuani (1983) which, in addition, involves measuring the first and the last positions of maximum extinction. The two methods are:

De Hills & Corvalan

Appearance of first extinction shadow A

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