Factors influencing the grain size distribution of authigenic minerals

Abstract

Authigenic minerals in sediments often have grain size distributions that fall in a relatively narrow size range. A simple possible interpretation of this observation is that nucleation of authigenic mineral crystals must be a singular short-term event, and their subsequent growth takes place concurrently under fairly uniform conditions leading to a relatively narrow grain size distribution. Although such circumstances can be readily obtained in batch reactors, in most natural systems this set of conditions is probably rare. In this paper we examine processes occurring for assemblages of crystals whose initial size is greater than few tenths of a micron where contributions of surface free energy to mineral solubility are close to negligible. Model results indicate that for systems in which growth occurs without concurrent nucleation the evolving grain size assemblage differs substantially between transport and surface-controlled growth processes, and that the size ratio of the largest to smallest crystals can decrease by orders of magnitude. In an initially steady-state system, where the solution composition is buffered by a metastable precursor phase and rates of nucleation and surface-controlled growth are constant, the net growth rate increases as the cube of time (reaction progress) leading to "quenching" of the system and a relatively narrow and close to uniform distribution of grain size.