The interfacial energy penalty to crystal growth close to equilibrium

Understanding 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 similar to 45%, and the interface mobility for garnet growth from similar to 535 degrees C, 480 MPa to 565 degrees C, 560 MPa in the phyllosilicate-dominated rock matrix ranged between similar to 10(-1)(9) and 10(-20) m(4) 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.

Automated summary

By studying the compositional growth zoning geometry of garnet porphyroblasts in a mica schist, researchers gained insights into the growth rates of individual crystals, departure from equilibrium, and interface mobility. The study found that garnet growth exhibited size-dependent patterns at macroscopic scales, with smaller crystals experiencing decreased growth rates. Additionally, the presence of abundant graphite in the rock matrix may have hindered the mobility of the garnet-matrix interface.