Calcite fabrics, growth mechanisms, and environments of formation in speleothems from the Italian Alps and southwestern Ireland

Five fabrics were identified in Alpine and Irish caves on the basis of morp hological and microstructural characteristics, and related to growth mechan isms and growth environment. Columnar and fibrous fabrics grow when speleot hems are continuously wet, and from fluids at near equilibrium conditions ( low supersaturation; SIcc < 0.35), through the screw dislocation mechanism. The highly defective microcrystalline fabrics form at the same supersatura tion range as columnar fabric but under variable discharge and the presence of growth inhibitors. Dendritic fabrics, which have the highest density of crystal defects, develop in disequilibrium conditions (high supersaturatio n) under periodic very low-flow-regime periods that result in prolonged out gassing. Cave calcareous tufa forms in disequilibrium conditions. Only the calcite crystals of fabrics formed at low supersaturation seem to precipita te near-isotopic-equilibrium conditions.