MORPHOLOGICAL INSTABILITIES IN PATTERN-FORMATION BY PRECIPITATION ANDCRYSTALLIZATION PROCESSES

Morphological instabilities in periodic patterns occurring both in pre cipitation and crystallization processes (Liesegang rings and crystal zoning) are investigated and compared with similar patterns in geologi cal samples (zebra rocks and mud bands in snow sediments). In classica l Liesegang systems, undisturbed parallel or concentric precipitation bands are emanated from even or concentric diffusion sources in homoge neous diffusion matrices of gelatine or other gels, In the case of sup erposing diffusion sources, sources with undulatory curvatures or loca l diffusion barriers there may occur several types of instabilities wi thin the sequence of regular patterns: (a) gaps within the bands formi ng radial alleys free of precipitate, (b) transition from broken bands to speckled patterns and (c) apparent branching of bands linked toget her by so-called anastomoses, Calculations with a competitive particle growth (CPG) model show that lateral instabilities in Liesegang bands (gaps and radial alleys of gaps) are the result of Ostwald ripening e ffects taking place after precipitation, Apparent branching of bands o r formation of anastomoses can be simulated with a prenucleation model according to Ostwald's supersaturation theory, Similar irregularities can be observed in zebra rocks (e.g. banded siderite) whose bandings are commonly explained by sequential sedimentation processes, A very d ifferent mechanism is assumed to be responsible for the origin of mud bands in snow sediments. An initially homogeneous distribution of intr insic mud in snow sediments can be arranged into parallel bands accord ing to a crystal zoning mechanism which is based on repeated thawing a nd freezing of the snow sediment due to the daily alternation of sun a nd darkness.