Inside-out dolomite

Inside-out dolomite crystals are unusual because their cores are younger than their encasing cortices. In the Cayman Formation (middle Miocene) of Grand Cayman they developed diagenetically from zoned dolomite crystals that have cores formed of high-Ca calcian dolomite (HCD; > 55 mol % CaCO3) and outer cortical zones formed of low-Ca calcian dolomite (LCD; < 55 mol % CaCO3) or oscillatory-zoned LCD-HCD. The cores of these crystals, labeled as Type I, have a euhedral, rhombic outline with no embayments or angular reentrants and commonly contain calcite inclusions and/or micropores. Hollow dolomite crystals evolved from these crystals as preferential dissolution removed all or part of the HCD core. Later, calcite or dolomite cement was precipitated inside the hollow crystals. The former produced dolomite crystals with a calcitic core (i.e. dedolomite) whereas the latter produced inside-out dolomite crystals that have a dolomite core (LCD or HCD) encased by a dolomite cortex. The cores of the inside-out dolomite crystals, labeled as Type II, have an irregular outline characterized by angular reentrants and/or embayments, micropores that are usually larger than those in the Type I cores, but few calcite inclusions. The cortical zones have a euhedral rhombic motif irrespective of the type of core. Development of inside-out dolomite crystals may significantly alter the average %Ca of the dolostones as HCD is lost to dissolution and LCD or HCD cement is precipitated in its place. The delta O-18 may also change, inasmuch as it is thought to covary with % Ca in some Tertiary dolostones. The genesis of inside-out dolomite crystal may offer an alternative explanation for the textures (e.g., clotted cathodoluminescence patterns) in some ancient dolomite crystals that have been attributed to replacement and/or recrystallization.