CAVE PEARLS FROM THE QUINCY LIMESTONE MINE, QUINCY, ILLINOIS

Cave pearls from the Quincy limestone mine are rounded or oval shaped sedimentary concretions consisting of carbonate layers around a central detrital nucleus. These pearls form in shallow pools with active dripping water.

Pool 05-01 was sampled on two occasions, two months apart. The first collection yielded three pearls, each consisting of four continuous layers around a detrital core: 1) crystalline microspar, 2) light and dark colored bands (ranging from 7-9 pairs), 3) sparsely packed dendritic calcite, and 4) another light and dark banded layer. The second sampling yielded two pearls, each consisting of three layers: 1) crystalline microspar, 2) alternating light and dark colored bands (up to 16 pairs), and 3) an outer densely packed dendritic layer. Differences among the pearls include the variation in the number of bands in layer two, the character of layer three, and the presence/absence of layer four.

The single sample from Pool 06-02a has four distinct layers, three top layers and one bottom. The thin banded bottom layer terminates as it curves upward and contacts the three upper layers. The upper layers consist of an inner alternating dark and light banded layer (over 30 pairs) intermixed with crystalline microspar. Layer two is a dendritic layer that contains undulating and linear lines. Layer three is dendritic but is darker and lacks undulating lines.

Pool 06-02b yielded two pearls, both consisting of a single complex banded light/dark layer with >40 pairs. Many layers terminate laterally or pinch out.

All pool samples contain a layer of distinct light and dark banding but the number of bands is highly variable. This variation even occurs within single pools as every pearl has a unique number of bands. Additionally, the composition and total number of layers is different for pearls from each pool and collection date. Compositional differences even exist among the pearls collected from pool 05-01 on different dates.

The difference between the pearls indicates that the banding around the nucleus is not a result of annual deposition. Rather, other geochemical processes (e.g. path of groundwater flow, pool depth, and chemical composition of the pool water) or non-geochemical processes (mining activities) are likely contributing to the formation of the pearls.