Optical analysis of thin slides of cave fromations carried out by a number of researchers showed a fine structure consisting of a succession of double layers (thichness less than a fe tens of micron). In each two layer unit, one is thicker, transparent and composed of an assembly of calcite needle shaped crystals oriented perpendicularly to the layer: the other layer is opaque, cryptocrystalline or amorphous. According to some C-14 datings of archaeological remains covered by the formations each couple of layers would correspond to one year. In order to check this hypothesis an experiment was set up in the Novella Cave (near Bologna) under controlled conditions. Cave pearls were chosen for investigation owing to the possibility of studying one of them without introducing any perturbation on the formation of the calcite sinter on the others. The growth of cave pearls was studied in a glass cell composed of three sections and described in the paper. In the first one, some holes of different size and depth were made in order to ascertain the influence of these geometrical parameters in function of the distance from the water drips. Small glass spheres (diameter around 1 mm) were placed in the holes as deposition nuclei. A cover was placed over this section to collect the water splash only when the water flow was measured, in order to avoid any change in the natural evaporation rate. The connection with the second section was obtained by a glass tube designed in order to guarantee a suitable mixing of the water. Both temperature and pH were measured. The third section consisted of a graduated container used to measure the flow rate which varied from 0.4-0.5- to 500-700 drops/min (a range of about three orders of magnitude). Air temperature was also measured during the experiment. The chemical composition of both natural and artificial cave pearls was determined. All the data are reported. No difference (with the exception of Fe concentration) was detected between the natural and the artificial conditions. Therefore after some years of experimentation the growth rate will be determined under controlled conditions.
Forti, P.; R. Casali; and G. Pasini.
Prime osservazioni in margine a un'esperienza di concrezionamento di alabastri calcarei in ambiente ipogeo.
International Journal of Speleology,
Available at: http://scholarcommons.usf.edu/ijs/vol10/iss3/6