The interfacial energy penalty to crystal growth close to equilibrium

Fred Gaidies,Freya R George

doi:10.1130/g48715.1

Abstract

AbstractUnderstanding the origin of rock microstructure is critical for refining models of the geodynamics of the Earth. We use the geometry of compositional growth zoning of a population of garnet porphyroblasts in a mica schist to gain quantitative insight into (1) the relative growth rates of individual crystals, (2) the departure from equilibrium during their growth, and (3) the mobility of the porphyroblast-matrix interface. The driving force for garnet growth in the studied sample was exceedingly small and is comparable in magnitude to the interfacial energy associated with the garnet-matrix interface. This resulted in size-dependent garnet growth at macroscopic length scales, with a decrease in radial growth rates for smaller crystals caused by the penalty effect of the interfacial energy. The difference in growth rate between the largest and the smallest crystal is ~45%, and the interface mobility for garnet growth from ~535°C, 480 MPa to 565°C, 560 MPa in the phyllosilicate-dominated rock matrix ranged between ~10−19 and 10−20 m4 J−1 s−1. This is the first estimation of interface mobility in natural rock samples. In addition to the complex structural and chemical reorganization associated with the formation of dodecahedral coordination polyhedra in garnet, the presence of abundant graphite may have exerted drag on the garnet-matrix interface, further decreasing its mobility.

Highlights

The microstructure of a rock is defined by the three-dimensional distribution of the sizes, shapes, and abundances of its grains and the spatial arrangement of the grain-boundary network separating them
We propose a method for estimating the departure from equilibrium as well as the interface mobility associated with the growth of porphyroblasts in metamorphic rocks
Even though the overall steepening of the compositional gradients with decreasing crystal size can be observed in the zoning of all dr dt where dr/dt is the radial growth rate, M is the interface mobility, σ is the interfacial energy, v is the molar volume of garnet, and r is the radius of its surface curvature assuming a spherical garnet geometry

Summary

Introduction

The microstructure of a rock is defined by the three-dimensional distribution of the sizes, shapes, and abundances of its grains and the spatial arrangement of the grain-boundary network separating them. Significant chemical disequilibrium of these components developed only across the garnet crystals, resulting in their compositional growth zoning (Fig. 2; George and Gaidies, 2017, their figure 4).

Results

Conclusion