PITS, OUTGROWTHS, AND INCLUSIONS AS COATED GRAIN KINETIC INSTABILITIES

Abstract

The development of outgrowths or pits of various shapes on coated grains is explained via a quantitative model of grain growth/dissolution kinetics coupled to evolving grain geometry (morphological dynamics). Grain-coating thinning or fracturing occurring due to nonplanar growth (and consequent grain surface area increase) is shown to underlie an instability to the formation of bumps, or in the case of undersaturated systems, to pitting. Examples of diagenetic outgrowth phenomena on clay-coated quartz are presented. A quantitative model of coupled quartz growth and coating dynamics is shown to imply many features observed in natural systems. Crystal growth anisotropy is shown to strongly influence the morphology of the outgrowths. The creation of inclusions is shown to be closely related to the present morphological instability. These morphological instability phenomena are interesting examples of geochemical self-organization.A steady-state model of the diffusion of solutes across the grain coating is shown to yield a novel nonlinear equation to be solved for the rate of growth of coated grains. This equation leads to a complex dependence of the growth/dissolution rate on saturation (or more generally on the composition of the fluid) in the medium surrounding the coated grain. The feedback between the dynamics of the coating thickness and morphology changes makes the phenomenon of interest here distinct from that arising from the coupling of grain growth and diffusion in the surrounding medium. This makes pits and needles possible even in a well-stirred surrounding medium, a fact of interest in interpreting the geologic record. For example, the present model can explain the development of spike, mushroom, and other outgrowths on clay-coated quartz grains in a sedimentary rock, whereas the classic Mullins and Sekerke diffusion model cannot, i.e., the time scale for eliminating concentration gradients in a pore is much shorter than that for grain growth. Predictions of the model are consistent with observations on quartz when typical values of diffusion, growth rate coefficients, and other parameters are used. In this paper, we emphasize that pitting can be closely related to outgrowth instabilities. If this be the case, then pitted dissolution of feldspars is a likely example. As feldspars dissolve, they commonly surround themselves with a clay coating, often leaving only a ghost remnant of the original growth.