ASTOUNDING RATES OF PRECIPITATION AND RECRYSTALLIZATION IN EXPERIMENTAL CAVE PEARLS IN AN UNDERGROUND LIMESTONE MINE

An underground limestone mine in Quincy, Illinois contains “cave” pools forming calcitic cave pearls where active drips fall directly from the ceiling (~ 3 m) into shallow pools (<3 cm). To test the rate of cave pearl growth, seeds of varying composition were placed in two pools in May 2006. Two experimental cave pearls were recovered after 7 months in Jan. 2007; eight more were recovered after 22 months in March 2008. The pearls from one small 30 cm diameter pool exhibited 0.7 mm of calcite growth in 22 months, giving a growth rate of ~0.4 mm/year. The pearls from a larger 2 m diameter pool had growth rates of 0.8 to 5.5 mm per year with the faster rates from dendrites on the top of the pearls. These rates are much faster than reported rates of 0.1 to 0.2 mm/year for natural cave pearls. This high rate is likely because the mine actively pumps 2800-8500 m³/minute of air through the mine. This prevents build up of CO₂ in the cave atmosphere and promotes continued rapid degassing, which would promote calcite precipitation.

The cave pearls are layered with: 1) finely laminated micrite and microspar, mainly on the base, 2) porous rounded dendrite crystals, mainly on the sides, 3) feather dendrite crystals on the top side, sometimes surrounded by micrite, and 4) equant calcite spar that cross-cuts earlier laminations. All layers are calcite; no aragonite has been detected. The equant calcite spar tends to start within microspar laminae and then extends as a triangular reaction front 0.1 to 0.4 mm into overlying laminae, obliterating the fabric. This recrystallization fabric is mainly, but not only, within the laminated layers. Even the 7-month old cave pearl is partly recrystallized with some reaction fronts extending nearly to the edge of the pearl. Recrystallization has been reported from other cave pearls but these are the first examples where the age of recrystallization can be documented to be months and perhaps as little as weeks. The drive for recrystallization is unknown but may be related to Ostwald ripening of the crystals in the extremely fine micritic layers. This rapid recrystallization could have significant ramifications for the use of speleothems for paleoclimate studies and for early carbonate diagenesis.