FLOATING RAFTS OF CALCITE CRYSTALS IN CAVE POOLS, CENTRAL TEXAS: CRYSTAL HABIT VS. SATURATION STATE

Calcite precipitation at the air-water interface in spelean pools produces floating rafts of interconnected, low-Mg calcite crystals with habits that range from equant (length to width=1:1) to tabular (1:5). Data from sample pairs (rafts and water) provide evidence that individual crystal habit is related to saturation state of the water at the site of precipitation. Saturation state ranges from 1 (equilibrium) for waters precipitating equant crystals to 22X saturation for tabular precipitates. Crystal growth is limited to the water surface, the most dynamic microenvironment of pools with stagnant (or imperceptible) flow conditions.

Temporal changes in stable isotopic ratios and increasing concentrations of ions within individual pools indicate that the processes driving calcite precipitation are evaporation and CO2-degassing. During a 3-month sampling period (regional drought conditions), the water level in one pool near the cave entrance dropped (~5 cm), P_CO2 decreased (0.01 to 0.001 atm), oxygen and carbon isotopic values became higher (2 and 6 ‰, respectively), and dissolved ions became concentrated (Na, Mg, K, SiO₂, Cl, SO₄, NO₃). In another pool, far from the entrance, during a 2-month sampling period, P_CO2 decreased (0.23 to 0.06 atm), delta¹⁸O and delta¹³C values became slightly lower (0.2 and 0.4‰, respectively), and ion concentrations changed little.

Equant crystals, with no crystallographic defects, precipitate from water with saturation states close to equilibrium. Tabular crystals are disequilibrium precipitates from supersaturated water. Crystal defects in the tabular crystals include inter-crystalline pores, rounded faces, and nearly circular holes (diameter=5-10 µm) on faces. The holes apparently result from rapid crystallite growth around foreign objects on the water surface (e.g., biotic or abiotic particles, or escaping gas bubbles). Crystal habits range due to preferential growth along one axis, a phenomenon related to the saturation state of the water.