Anomalous water absorption in low-grade serpentinites: more water than space?
Bralia, Andrea; Ceccherini, Sandra; Fratini, Fabio; Del Fa, Carlo Manganelli; Mellini, Marcello; Sabatini, Giuseppe
European Journal of Mineralogy Volume 7 Number 1 (1995), p. 205 - 216
published: Feb 8, 1995
manuscript accepted: Sep 2, 1994
manuscript received: Feb 3, 1994
ArtNo. ESP147050701018, Price: 29.00 €
Abstract Serpentinites used as decorative stones in the Siena Cathedral, as well as in several historical buildings in Central Italy, show water saturation indices greater than the maximal theoretical value of 100 percent. Namely, the volume of absorbed water is larger than the volume of absorbed helium. Whereas anomalous saturation indices are common in low-grade lizardite + chrysotile serpentinites, normal saturation indices occur in metamorphosed antigorite serpentinites. Based upon chemical and physical tests, we conclude that the mechanism of excess water uptake in serpentinites may be explained by chemically induced permeability through pre-existing barriers. These barriers normally prevent the entrance of gases into pre-existing, but closed-off pore spaces; the barriers are located along the numerous grain boundaries and are thought to have composition close to MgSi2O5, that is the serpentine residue left after acidic treatment. The pore-filling barrier material is formed by weathering processes on a short time scale (from a few centuries for the historical buildings to perhaps one century for old quarry outcrops). It does not occur in very fresh specimens, which show normal water absorption properties (i.e., saturation indices less than 100 percent).