This work presents a quantitative analysis of spatiotemporal patterns that form during the growth of oscillatory zoned plagioclase. These patterns were imaged using Nomarski interference contrast imaging of etched polished surfaces. Analysis of these imaged patterns yields new information and constraints on how patterns form, and consequently on how oscillatory zoned crystals may develop in magmatic systems. Utilizing mathematical techniques recently developed for the analysis of dynamical systems, we show that the irregular nature of oscillatory zoning is not compatible with random or stochastic processes. The analysis of our observations suggests that the plagioclase-liquid system is a deterministic nonlinear system whose attractor has a relatively low dimension. Finally, comparison of our empirical results with computer models of other dynamical systems implies that as few as two variables may be sufficient to describe the dynamics of the plagioclase-liquid system. ¿ American Geophysical Union 1993