EVIDENCE FOR EXTREMELY RAPID AND VARIED STYLES OF RECRYSTALLIZATION IN MODERN CAVE PEARLS FROM AN UNDERGROUND LIMESTONE MINE, QUINCY, ILLINOIS

Cave pearls are coated grains that form in caves and mines. Cave pearls forming in an underground limestone mine, Quincy, Illinois were monitored from 2005-2014. A classic splash pool (“pearl nest”) accumulated >20 cm of pearls and pool spar during this period, while a new area filled shallow rimstone dam pools with 1-3 cm cave pearls between 2008 and 2014.

Regardless of pool, cave pearls are layered with four main fabrics: 1) dendrite layers of porous branching crystals; 2) v. dark brown to pale tan laminated layers; 3) microspar to fine bladed spar layers with poorly defined lamina; and 4) coarse radiating bladed spar. XRD of pearl components (N = 25) show only low-magnesium calcite (LMC). Stable isotope values (N = 18) of these components show no differences (δ13C = -9.7± 0.6; δ18O = -5.7 ± 0.2).

Petrography combined with SEM examination of etched thin sections and chips reveals multiple recrystallization styles. Previous work showed recrystallization of laminated layers by cross-cutting microspar advancing as reaction fronts starting in dendrite layers. New work has shown that the brown to tan laminated layers are also internally recrystallized. The darkest laminae are composed of 1 µm crystals defining 1-3 µm laminae. While some dark laminae go all the way around a pearl, more commonly they transition laterally to progressively lighter layers with darkest laminae preferentially on outside corners. In SEM, the petrographic transition to lighter tan is matched by a gradual increase in crystal size from ≤1 µm to >10 µm. The 1-3 µm laminae are destroyed in the process, but often can be traced in SEM as etch lines within coarser spar. The microspar to fine bladed spar layers may also be recrystallized but conclusive proof is elusive. Discontinuous laminae may be remnants of better defined layers, but the transition is not preserved.

While recrystallization is well known from ancient carbonates, it is rarely identified in modern carbonates. Here cave pearls only a few years old are recrystallizing without changing mineralogy (LMC to LMC) or apparently water chemistry (isotopic values are constant). The only obvious change is crystal size, suggesting Ostwald ripening. Similar processes are likely operating in modern carbonate muds.