Melt diffusion-moderated crystal growth and its effect on euhedral crystal shapes

Abstract Crystal growth is often described as either interface-controlled or diffusion-controlled. Here, we study crystal in an intermediate scenario where reaction rates at the crystal-melt interface are similar to of diffusive transport ions through melt advancing surface. To this end, experimentally investigated euhedral plagioclase shapes dry mafic (basaltic) and hydrous silicic (haplodacitic) melts. Aspect ratios inferred relative 3D short (S) (I) dimensions vary significantly between melts, with δS:δI = 1:6–1:20 basalt 1:2.5–1:8 haplodacite. The lower aspect grown coincide 10 100× diffusion than melt. Using anisotropic model, show that such differences diffusivity can explain discrepancy ratios: if magnitude, then a facet high interfacial may be limited by diffusion, while another same grow uninhibited diffusivity. This selective control on results progressively more equant decreases, consistent our experimental observations. Importantly, crystals formed diffusion-moderated, regime not any classical diffusion-controlled features. proposed model was developed for microlites but should generalisable all microlite volcanic rocks.