Abstract: 

An underground limestone mine in Quincy, Illinois, contains “cave” pearls forming in splash pools. To better examine cave-pearl growth, seeds of different composition were placed in two pools and left for either 8 or 22 months. The resulting cave pearls accumulated ∼ 1 to 6 mm of low-Mg calcite with growth rates of 0.4 to 5.5 mm/yr. These pearls are composed of: (1) thinly laminated alternations of micrite and microspar, mainly on the bases, (2) dendrite crystals, mainly on the top and sides, and (3) equant calcite spar that crosscuts earlier-formed laminations. X-ray diffraction and microprobe analysis shows that all three fabrics are low-Mg calcite with no detectable compositional differences. Scanning electron microscopy reveals fossil exopolymeric substances (EPS) and fossil microbes, but they are widely scattered.

The micrite–microspar laminae range from < 5 µm to > 50 µm thick. The 8-month cave pearls contain approximately 100 couplets, and the 22-month pearls contain 50–150 couplets. Therefore, individual couplets are forming in days to weeks. This does not correlate with any known climate signal, and their origin remains a mystery. The dendrite layers are thicker than the micrite–microspar laminated areas with growth rates up to 5.5 mm/year, attributable to rapid CO2 degassing assisted by the mine ventilation.

The equant calcite spar tends to start within microspar laminae and then extends as a triangular reaction front up to 1.5 mm into overlying laminae, obliterating the original fabric. This recrystallization fabric is mainly, but not only, within the laminated layers and starts in as little as 1 to 2 months after laminae precipitation. Ostwald ripening (aggrading neomorphism) is the likely drive for this recrystallization as the outer layers of the pearl continue to grow during recrystallization of the interior. This rapid recrystallization from calcite to calcite has significant ramifications for the use of speleothems for paleoclimate studies and for the rate of early carbonate diagenesis.

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