ABSTRACT

Calcite precipitation at the air-water interface in cave pools in central Texas (U.S.A) produces floating rafts of interconnected, low-Mg calcite crystals with individual crystal habits that range from equant (length to width = 1:1) to prismatic (4:1). Saturation state for the waters ranges from -0.19 to +1.35 (SI_c = log IAP/K). Equant crystals precipitate from waters with a lower saturation index (SI_c) and form rafts with a fused fabric whereas prismatic crystals precipitate from higher-SI_c waters and form rafts with an interlaced fabric. The pools contain low-magnesium (molal Mg/Ca < 0.6), calcium-bicarbonate, and calcium-bicarbonate-nitrate type waters with temperatures that range from 13.8°C to 20.7°C and pH readings from 6.7 to 8.4.

Equant euhedral crystals precipitate from water with saturation states close to equilibrium. Visible defects in the equant crystals include incomplete edges between faces and nearly circular (mean diameter 9 µm) to angular holes in faces. The holes apparently result from crystallite growth around foreign objects on the crystal surface such as gas bubbles. Prismatic crystals precipitate from supersaturated water. Faces of these subhedral crystals are smaller in area than the faces of the equant crystals. No intrafacial circular holes are seen, but trigonal intercrystallite pores are common in the prismatic crystals. The variety in crystal habits, from equant to prismatic, is due to preferential growth along one crystallographic axis. Thus, in these cave pools crystal habit is related to the saturation state of the water.