Theory of Tracer Exchange in Single-File Systems

Tracer exchange in single-file systems (one-dimensional diffusional systems where the particles are not able to pass each other) shows peculiar and interesting features which differ considerably from that of transport diffusion or from the behaviour known from ordinary diffusional systems. A formalism relating the tracer exchange curve (as the observable of the tracer exchange experiment) to the residence time distribution (describing the intracrystalline diffusional mechanism) and the boundary condition (describing the situation outside) is introduced. The formalism is guite general and valid for any diffusional regime. Typical examples of the residence time distribution of a single-file system can be inferred from the tracer exchange curves obtained in a previous study by numerical simulations. Based on these examples, the tracer exchange curves for the single-file system subject to different boundary conditions (corresponding to a variety of experimental set-ups) are plotted and discussed.

Non-equilibrium highly anisometric crystals and whiskers of galena

Unusual irregular galena crystals and whiskers were found in close proximity in some druse cavities from the Gradishtc hydrothcrmal lead-zinc deposit in the Madan ore district, Bulgaria. The following crystal forms were observed: (1) straight thin [110] whiskers and thicker needles; (2) kinked whiskers; (3) curvilinear whiskers; (4) complex tortuous whiskers composed of segments with varying directions—[110], [100], [211], (5) thicker irregular elongated crystals. Combinations of these forms occur also. The detailed SEM study shows that all these formations are single crystals of extreme anisometricity, bounded by octahedral and cubic faces as well as by stepped surfaces of these forms. Surface structures such as longitudinal grooves, jagged edges, striations, pits, etc., are abundant.It is assumed that these highly non-equilibrium crystals with large surface areas were formed through rapid directed growth from highly supersaturated solutions under a diffusional regime. Such special environments arose in the ore veins as a result of tectonic shocks leading locally to a drastic volume increase and P and T decrease in the solutions.