Undulatory extinction in quartz is a wide spread feature found in many different rock types. The degree of undulosity exhibited is a function of the process of rock formation and deformation. As undulosity suggests a level of instability of the quartz, the degree of undulosity has been used, for some time, to indicate the potential reactivity of the quartz in the alkali-silica reaction. The method of routine measurement of the degree of undulosity, on investigation, has been found to be inaccurate. A new method for this routine measurement is proposed, utilising the universal stage assembly fixed to a petrological microscope. Instability of quartz grains can also be gauged by the assessment of grain textures, and grain and crystallite sizes. Textures and grain size are classified by the visual features exhibited when the grains are viewed using a petrological microscope, whilst crystallite size is measured by the use of X-ray diffraction line broadening techniques. Analysis of a number of quartz-bearing rock types; mainly granites, gneisses, schists, porphyrites, and sandstones, from different parts of Great Britain has been carried out. The results indicate that if all these features are investigated and quantified then it is possible to classify the stability of the quartz, eg a mean tube undulatory extinction value of greater than or equal to 5 degrees would be classed as highly strained, and therefore assess the potential for reaction in the alkali-silica reaction.

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