Along grain boundaries of quartz from metamorphic and igneous rocks complex interfingering (sutures) may occur. Under the light microscope the lengths of the sutures range from approximately 10-1000 lxm. The sutured grain boundaries are statistically self-similar over one to two orders of magnitude. They represent fractals. Their mathematical counterpart are Koch curves which are developed after two to four iterations. The fractal (Hausdorff-Besicovitch) dimensions D of sutured quartz grain boundaries from different grades of m e t a m o r p h i s m are between ca. 1.05 and 1.30. The D-value decreases with increasing temperature during formation of the sutures. On a statistical basis, D can be used as a measure of this temperature and, therefore, as a deformation-related thermometer. Key words G e o t h e r m o m e t e r • Q u a r t z . Grain boundary migration • Grain boundary suture • Fractal • Koch curve atoms in the crystal structure. On the atomic scale, along grain boundaries, steps are formed which are controlled by the relative orientation of the neighbouring crystals. Larger sutures which are visible under the light microscope may represent a greater number of smaller steps. Specifically, the deformation of neigh-J. H. Kruhl (~1) • M. Nega
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