Experimentally produced oscillatory zoning in the (Ba, Sr)SO₄ solid solution

Abstract

WHEN crystals grow from a multicomponent fluid phase under conditions where ionic diffusion in the solid is negligible compared to that in the liquid, any compositional gradients in the crystal record the evolution of the solid/liquid interface composition during growth. For the particular case of oscillatory zoning^1–3, a relatively common feature of natural crystal growth⁴, there has been considerable theoretical interest^4–6, but the specific question of whether high or low supersaturations are required to explain the development of the zoning remains unanswered. Experimentally produced compositional oscillations have been observed^7,8, but the role of supersaturation had to be inferred, rather than measured directly. Here we describe major-element oscillatory zoning in (Ba, Sr)SO₄ solid solutions grown by the counter-diffusion of (Ba²⁺, Sr²⁺) and SO²⁻₄ ions through a porous silica-gel transport medium. We demonstrate how the different solubilities of the two pure phases determine the threshold supersaturation for nucleation, and show how coupling between the compositional gradients in the solid and the liquid results in the observed oscilla-tory behaviour.