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Petrology and Geochemistry of Calcite Precipitates and Water from Surface and Spelean Environments, Central Texas: Analogs for Non-marine Carbonate Cements



University of Houston, Houston, TX


Aggregate calcite crystal rafts form quickly (hours to months) at the air-water interface in a surface stream and cave pools in Central Texas. Crystal lengths range from 10 - 100m. Common crystal habits forming in cave pools include equant to subequant rhombohedrons (l:w ~ 1:1) and elongate trigonal prisms (l:w ~ 5:1) in straight bundles and spherulitic hemispheres (10m dia). More elongate crystals precipitate from the more highly saturated waters. Multiple round and angular holes, ~10m diameter, in faces of equant to subequant spelean crystals are interpreted to be of constructional genesis around gas bubbles or particles.

Equant dodecahedrons and rhombohedrons comprise rafts forming in the surface stream. Ubiquitous single round holes appear to penetrate the crystal structure and are probably molds of algal filaments.

The skeletal crystals (hollow cores) of thermal travertine systems have not been found in either environment. In contrast, crystals with well-formed faces and incomplete edges are common. SEM analyses at the micron scale have not revealed any evidence to support microbially induced mineral precipitation (e.g., clusters, chains of spheres or rods).

Geochemical analyses of calcite-water sample pairs provide evidence of temporal and areal variability in the samples. Some calcite-water pairs are close to geochemical equilibrium in contrast oxygen isotopic equilibrium is not observed (dOSMOW: -6 to 1; dCPDB: -15 to -3).

Calcite rafts - analogs for pure non-marine carbonate cements - have received little research attention. Because the crystals are not obscured by sediment, rafts provide an excellent source of material for detailed analyses.