Fehlbau in Quarzkristallen aus Tektoniten

Abstract

Quartz single-crystals from metamorphic tectonites — quartz-lenses in gneisses from the Lepontine Alps (Co. Ticino, Switzerland) and from Madras charnockites — have been examined for structural defects by means of X-rays, polarizing microscope, and electron microscope. Even smallest parts of microscopically uniform quartz crystals proved to consist of many tiny, slightly inclined domains which rotate somewhat freely round the c-axis, the main element of orientation. Some hypothetic conclusions have been drawn with regard to the mechanism of orientation in recrystallization fabrics: In a stress field the subindividuals move into a new position of equilibrium by internal rotation. During continuous small stress-changes the unity of the grain is maintained while strong anisotropies of stress may bring about autonomy of the domains. It is by rotation of the elements, not by translation, that the new thermodynamical state of orientation is attained. Interlocking of grains is a proof of the rotation of the domains. The splitting’ up into domains may result from fissuring or undulation, but in all probability stress inhomogeneities during crystallization are mainly responsible for it. In Fig. 14 the author has attempted to give a model for the exact description of the fabric symmetry that conveys a simple theoretical example not only of point-symmetry but also of space-group properties. The fabric of quartz consists of a system built up by discrete elements repeated periodically in a three-dimensional space in the three orders of magnitude of superindividual, grain and single-crystal domain. In a statistical way — not in the exact sense of modern crystallography — the classical definition of the crystal (e.g. P.Niggli, 1920, p. 6) applies to the quartz fabric as well.