The morphology, orientation, and abundance of thermal cracks are characterized in unconfined specimens of Sioux Quartzite subjected to (a) slow thermal cycling (= 2°C/min) to 385°, 560°, and 685°C, and (b) slow heating at the same rate followed by quenching from 400°C. In both, all the thermal cracks are microscopic in size and occur primarily along grain boundaries and secondarily as intragranular microfractures. The latter intersect parted grain boundaries. SEM photomicrographs suggest that movement normal to the crack surfaces appears to be much greater than that parallel to them. That is, their abundance increases more between 385° and 560°C than between 560° and 685°C. In the slowly cycled specimens the intragranular cracks are strongly oriented at about 60° to 90° to the axis of the greatest principal elongation (100 x10-6 , elongations counted positive) of the average state of residual elastic strain in the rock prior to thermal treatment, as measured by X-ray diffractometry. This geometry suggests a genetic relation between the cracks and the net stresses resulting from superposition of the residual stresses and the thermo-elastic stresses developed from differential expansions of anisotropic nearest-neighboring grains. That is, upon heating the residual strains are not relaxed (as they might be had their origin been related to the natural thermal history of the rock), but rather they are locally augmented until the elastic limit is reached and intragranular tensile fractures occur preferrentially at high angles to the greatest elongation of the residual strains. In quenched discs, the intragranular microfractures are oriented nearly perpendicular to the circular section of the disc, but they are widely dispersed in azimuth. Accordingly, they seem to reflect the superposition of the "macroscopic" tension developed in the circular section from stresses set up by thermal gradients developed by quenching.

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