Syntaxial Overgrowths on Dolomite Crystals in the Bluff Formation

ABSTRACT: Complex syntaxial overgrowths are common on many dolomite crystals that line the walls of cavities in dolostones of the Bluff Formation exposed along the coastline of Grand Cayman. Scanning Electron Microscopy shows that the siting and development of the overgrowths, which are initiated on "seed crystals " less than 0.1 #m long, are predetermined by the crystallographic structure of the host crystal. Based on their three-dimensional form, the overgrowths are divided into i) aggregate rigonal prisms, 2) needle, wall, and sheet overgrowths, and 3) needle crystals. The aggregate rigonal prisms with sides up to 12 #m long, are formed of four orders of crystals. The first order crystals, less than 1 um long, are vertically stacked to form 2nd order crystals that are commonly capped by multifacetted crystals. The 2nd order crystals, each maintaining their identity, are arranged in a triangular outline, thereby forming a 3rd order trigonal prism. Filling the spaces in the 3rd order crystals creates a "solid " 4th order crystal. Needle overgrowths are formed of vertically stacked subcrystals (rounded hexagons, 0.5 to 1.0 ~m long and 0.1 #m thick). Lateral merger produces wall overgrowths which in turn merge to form "solid " sheet overgrowths. Such overgrowths, usually 4 to 5 ~zm thick, can occur on all faces of the host dolomite crystal. Needle crystals are non-aggregate, elongate (up to 10 ~m long) crystals that have a round cross section. All the overgrowths occur in near-surface avities that are subjected to daytime temperatures of 27 to 30"C and periodic influxes of rainwater and seawater. Rapid evaporation of the water leaks films of water on the cavity wails from which the syntaxial overgrowths precipitate