Petrology and Chemistry of Cave Pearls from Gruta De Las Canicas (Cave of the Marbles), Tabasco, Mexico

Cave pearls are relatively common in caves, but are typically present in very low abundance. Gruta de las Canicas, a cave system developed in Cretaceous carbonates in Tabasco, Mexico, is highly unusual in that it contains a tremendous quantity of pearls (estimated 200 million pearls found on the cave floor to a depth of a meter or more). The mechanism for the formation of this vast quantity of pearls has not been determined. Here we provide the first report of the mineralogy, texture and chemistry of the Canicas pearls. The pearls were studied using standard optical petrographic and geochemical methods (electron microprobe and stable isotopes). The pearls can be subdivided into three distinct zones based upon cement texture, presence of impurities, and porosity. Clay-rich zones are enriched in clay-sized non-carbonate material, and typically contain little to no porosity. Cement-rich, non-porous zones contain mainly radially oriented spar, with lesser amounts of microspar, and also have little to no porosity. Cement-rich porous zones are similar to the non-porous zones, but contain up to 50% porosity, with the pores elongated in a radial manner parallel to the radial spar crystals. The three zone types alternate concentrically in an apparently random manner. Electron microprobe analysis indicates that the carbonate is extremely pure low-Mg calcite (99.4 – 99.8 % CaCO3), with only minor substitution of Mg, and to a lesser extend Mn, Fe, and Sr for Ca in the calcite structure. A microprobe traverse detected no statistically significant variation in elemental composition from pearl center to edge. Microprobe analysis of the non-carbonate clay sized material in the pearls shows that a variety of minerals are present, including quartz and apatite. The abundance and distribution of porosity suggests that a more soluble phase (e.g., aragonite, organic matter) was removed by dissolution.