Calcite crystals that cement the dolostone clasts in a karst breccia from Grand Cayman Island have been substantially altered as a result of infestation by algae and (or) fungi as well as etching that may or may not be related to organic activity. The features produced include (1) borings, (2) constructive micrite envelopes, (3) spiky calcite, (4) almond-shaped etch pits, (5) crystal surfaces with a blocky topography and (6) etched crystal surfaces that suggest some of the crystals may have grown by spiral growth. Spar calcite crystals in a vadose setting can be.subjected to complex chemical and physical processes that are commonly mediated by algae and (or) fungi. These microorganisms are capable of removing CaCO3 from substrates via dissolution, transporting that CaCO3 elsewhere and then directly or indirectly causing its reprecipitation during the calcification of their filaments and (or) the formation of the constructive micrite envelopes.The extensive alteration of calcite cements in the karst breccia from Grand Cayman Island demonstrates that organically mediated processes can be extremely important in vadose diagenesis. Recognition of the features produced by these complex processes may prove useful in the identification of carbonates that have been subjected to vadose diagenesis.