Autocatalytic model of oscillatory zoning in experimentally grown (Ba,Sr)SO4 solid solution.

Abstract

Oscillatory zoning (OZ) is a phenomenon common to many natural minerals whereby the mineral composition varies more or less regularly from the core of the crystal to its rim. Oscillatory zoned barite-celestite (Ba,Sr)SO4 crystals are one of the very few examples of the OZ phenomenon that were obtained under controlled laboratory conditions. It is known that such crystals can be synthesized by precipitation from an aqueous solution during counterdiffusion in a gel column connecting two reservoirs. We present here a model of oscillatory zoning in such a binary solid solution grown from an aqueous solution. By expanding on a previously suggested model, we obtain oscillatory dynamical solutions for two limit cases: the growth of a flat crystal face and the growth of a spherical crystallite. We consider an autocatalytic dependence between the crystal growth rate and the crystal surface composition. The oscillatory patterns then arise as a kinetic effect due to the coupling between the diffusion field around the crystal and the fast crystal growth under far-from-equilibrium conditions. The effects of fluctuations in the aqueous solution concentrations are also considered. It is shown that they may lead to noisy oscillatory patterns.