Plate 163

Primary chemical structures: limestone encrustations

Plate 162 A and B document two details of speleothems,  as cave deposits are called. A stalactite has been cut, and the section shows two distinct generations of chemical precipitates, both consisting of calcium carbonate. The spongy inner zone (dripstone) is the result of dripping from the cave ceiling; the banded outer zone (flowstone) reflects, instead, the slow growth of crystals from solutions flowing along  the solid surface. Flowstone deposition seals and drapes the dripstone, which implies a change in water circulation in the karst system. All cave "formations" (the term is used by speleologists for morphological varieties of precipitates, which geologists would call "structures": stalactites, stalagmites, basins, ponds, etc.) owe their origin to the release of carbon dioxide from water percolating through the karst system. Carbonate ions are present in solution, and the escape of CO force them to separate as a solid phase by decreasing their solubility.

In plate 162 B it can be more clearly seen that: 1) flowstone bands are sets of depositional laminae, recognizable by differences in color; 2) the laminae are made of elongated crystals, which are oriented normally to the lamina surface; 3) the parallelism and continuity of the laminae is remarkable; a small displacement is observable, however, along a fracture on the right (a micro fault). Flowstone laminae are possibly organized in seasonal rhythms (varves ) and longer-term cycles, as indicated by minor and major color changes between individual laminae and laminasets, respectively. The color of pure carbonate is white; darker hues are due to variable concentrations of impurities (iron oxides and hydroxides). The major changes in impurity content should reflect multiannual hydrologic-climatic changes.

Calcite crystals formed the laminae by growing side by side and disturbing each other; maximum growth then occurred perpendicularly in the free space of the cavity, which explains their elongated shape. This banded cave deposit is used as ornamental stone with the name of calcareous alabaster. 

In plate 162 C, the carbonate encrusts vegetal remains. Precipitation can be restricted to the vicinities of springs or falls, or be more extensive and form continuos layers. Travertine  deposits, or tufa, characterized by a spongy or vuggy texture, can thus accumulate. Their thickness can reach tens of meters; several travertine quarries are open in central Italy. The stone is used as a cheaper substitute for marble.

Plate 162 D shows an aggregate of calcareous piso-liths,  spheroidal particles made of concentric shells of calcium carbonate. The internal structure is magnified in plate 163. Pisoliths grow in both external and subterranean ponds containing slightly turbulent water saturated in carbonate. Similar but smaller particles, called ooliths  (or, better, ooids ) are found in agitated waters of the marine realm, more precisely in marginal areas of carbonate platforms. They derive from stepwise coating of tiny suspended particles in CO2-rich water currents.

All samples belong to the collection of the Department of Geological Sciences, University of Bologna.

Photos: G. Piacentini 1970.