Microarchitecture of dolomite crystals as revealed by subtle variations in solubility: Implications for dolomitization

Despite their small size, most dolomite crystals in the dolostones of the Miocene Cayman Formation on Grand Cayman are zoned, with a core formed of high-calcium calcian dolomite (HCD) encased by a cortex formed of low-calcium calcian dolomite (LCD). These two different types of dolomite are separated by a sharp, well-defined cortical boundary. Acid etching shows that the HCD cores are characterized by numerous growth defects including twinning, modulations, and dissolution slots whereas the LCD that forms the cortex contains far fewer and less well-developed growth defects. The fact that the HCD is far more soluble than the LCD is attributed to its higher CaCO3 content and greater density of growth defects.The cortical boundary that separates the HCD core from the LCD cortex is a discontinuity that may have developed as a response to a change in diagenetic regimes or a change in the composition of the dolomitizing fluids. Deciding between these two possibilities, however, is difficult because the small size of the crystals precludes collection of data needed to resolve the issue. Irrespective of the underlying cause, the development of the zoning is critical from a diagenetic perspective because the HCD with its high CaCO3 content and high density of growth defects is far more prone to dissolution than the LCD cortex. Thus, zoned crystals of this type are highly susceptible to diagenetic modifications that will lead to the development of hollow crystals, dedolomite, or inside-out dolomite.