Limits on Calcite and Chalk Recrystallization

Abstract

Recrystallization requires dissolution in pore fluids and precipitation on existing particle surfaces in a process known as Ostwald ripening, where the smallest particles feed growth on the larger ones. Recrystallization conditions are optimized in industry when commercial products require larger crystals, and it is the dominant process in burial diagenesis, which turns sediments into rock. In chalk, the original coccolith elements are often still clearly distinguishable, which is a surprise considering the rapid rates of pure calcite recrystallization. We studied the rates of calcite precipitation on the surface of the particles in various chalk samples, using the constant composition method over a wide range of supersaturations. Our results show that dependence of calcite precipitation rate on supersaturation does not obey the parabolic law, which is characteristic for calcite growth. Instead, the dependence was exponential, which indicates surface nucleation-mediated growth. The rates of calcite precipitation on the chalk surfaces at the lowest supersaturation examined in the present work are 3 orders of magnitude lower than those on pure calcite surfaces. This means that, during chalk recrystallization, when supersaturation is extremely low, recrystallization can be significantly suppressed. The presence of organic compounds has an important influence on recrystallization kinetics, so along with other possible causes of inhibition, the mechanism of calcite precipitation is a factor in the extremely low rates of the chalk recrystallization.