RECRYSTALLIZATION OF CAVE PEARLS FROM UNDERGROUND LIMESTONE MINE, QUINCY, ILLINOIS

Fourteen calcite cave pearls, concentrically laminated coated grains that grow in cave pools were taken from both splash cup and rimstone damn pools within an underground, limestone mine in Quincy, Illinois. These pearls were collected between 2006 and 2012 and can be shown to be <5 years old. The purpose of this study is to examine recrystallization in these very young pearls. In order to observe the process of recrystallization within the cave pearls of the mine, petrographic and scanning electron microscope analysis was performed. SEM samples were lightly etched with dilute HCl. Four fabrics were found in the pearl cortex: (1) black to brown to tan micritic laminations, (2) microspar to fine bladed spar with discontinuous laminations, (3) coarse bladed to radiating spar, and (4) grey to clear dendritic layers. The first three types show evidence of recrystallization.

(1) Micritic laminations range in thickness from 1-2 µm up to 20 µm. The finest laminations are darker in color, light to dark brown, where thicker laminations are lighter in color, light brown to pale tan. In the SEM, pale layers etched with HCl show etched lines that correspond to the original 1 µm layering. Intervals of micritic laminations can be traced around the pearl with 1 µm dark brown laminae gradually coarsening and lightening in color. Therefore, laminated layers made up of micritic calcite recrystallized to form microspar. (2) The microspar to fine bladed layers exhibit “ghost laminations”; discontinuous laminations present within the microspar layers. These layers are regularly light grey in color with discontinuous laminations dark gray in color. These discontinuous laminations are 1-5 µm. In the SEM, the fine bladed spar often shows This fabric is interpreted as a laminated layer that has been almost completely recrystallized into microspar. (3) Bladed to radially bladed calcite layers have crystals which range from 20-100+ µm in length. These layers also contain discontinuous, dark gray micritic laminae.. The larger crystals may be original while the discontinuous laminae appear to be cross-cut by the larger crystals, suggesting recrystallization. This extremely rapid recrystallization of calcite to calcite while retaining fine laminae suggests recrystallization may be more widespread than previously recognized.