Pattern Formation During Crystallization and the Formation of Fine-Scale Layering

Abstract

Fine-scale layering is characterized by the laminar segregation of mineral phases on a millimeter to centimeter scale. Where best developed, it is found in specific associations within the Stillwater Complex. In the Banded series, layer spacing is proportional to crystal grain size, with the best examples of fine-scale layering characterized by coarse rock textures and the development of secondary structures. These latter features include the division of primary banding into two or more layers and the development of “honeycomb” crystal arrangements within the plane of individual layers. Analogy with similar features formed in crystallizing salt solutions suggests that fine-scale layering can develop during textural coarsening of the rock. An assemblage of crystals is inherently unstable to minor perturbations in crystal size: larger grains will grow at the expense of smaller grains in order to diminish the total surface free energy of the system. Such a process can result in an orderly segregation of mineral phases and can produce layering when a gradient of grain size is present across the system. This condition would be expected in an advancing front of nucleation and crystal growth in a solidifying magma. Quantitative modeling can explain many of the features of fine-scale layering, and suggests that textural features of igneous layering in general can become better defined with time.