ABSTRACT

Evaluation of calcite crystal habits and fabric and their relation to precipitating fluids in spelean environments indicates that habit variations are controlled primarily by degree of supersaturation. At less than 6 times supersaturation, crystal habits are limited to those with shallow to steep rhombohedral forms (i.e., {1011}, {4041}). With increasing supersaturation, scalenohedrons, extremely steep rhombohedrons, pinacoids and hexagonal prisms are developed. Slight elongation due to differences in crystal dimensions of these forms may arise (i.e., scalenohedrons versus shallow rhombohedrons). At extreme supersaturation (> 12 times), curvature of crystal faces or curved surfaces appear.

The development of fibrous versus equant fabrics is the result of changes or modification of reactant supply rates and the number of crystallites (nuclei) present. Under rapid flow conditions, growth rates are accelerated and greater numbers of nuclei are present; thus, compromise boundaries parallel to c-axis and perpendicular to flow (and substrate) are rapidly attained. In the absence of flow, fewer crystallites are present, and larger, well developed crystals or branching forms dictated by saturation conditions arise. In the absence of flow at low supersaturation, elongate composite crystals, and hence fibrous fabrics, may result from migration of the fluid-crystal interface as a result of changing or oscillating water levels.

A flow control mechanism can be applied to numerous geological conditions in which transitions from fibrous to equant crystal morphologies are observed.