ABSTRACT

A very unusual calcite cement was observed in some beach-chips found on an unconsolidated beach surface of Chandeleur Island which lies offshore some 35 nautical miles south of Mississippi in the Gulf of Mexico. The beach-chips are irregularly shaped and are well cemented by this unusual calcite. This calcite crystal structure has not previously been reported as existing in a marine environment. A similar cement has been found in freshwater lake beachrock and in some travertine samples. The calcite crystals are elongated parallel to the c-optic axis, and are composed of bunches of crystallite blades. The crystallite blades of each crystal bunch are pointed and are more bladed than freshwater cement crystals. The intercrystallite pore-space contains no fine calcite silt as was observed in the lake samples. Fresh water, provided by rainfall, may be held in the pore-spaces and bounded to the quartz grain-surfaces by ionic attraction. Marine spray atop and saline water concentrated underneath form a sandwich effect at the micropore level, allowing rapid growth and precipitation of these very unusual calcite crystals in a single-phase low-salinity fluid.