This study investigated in detail the mineralogical changes within 8 little maturation ponds filled with different clay materials placed in a spa center in southern Italy and kept under the traditional environmental conditions used by the spa itself. Both short- and long-term maturation periods were investigated.

Several changes were observed in all the samples during the first month of maturation and also in the following 2 months. A significant increase in soluble Na occurred in all the samples. No significant variations in the initial mineral assemblage were detectable either in the bulk material and or in the clay fraction except for the crystallization of neoformed Na-minerals. The amount and the type of silicate minerals did not change in time, but the position and the width of the basal reflection of expansible minerals changed due to a gradual incorporation of new ions into the interlayer space. During the first month a variation in the grain-size distribution as well as a decrease in calcareous nannofossils can be observed. A slight increase in Corg was also recorded during the maturation.

Some of the parameters which were more sensitive to short maturation continued to adjust also during a fairly longer time such as 15 months. The spacing and the FWHM of the basal reflection of smectite and mixed layers minerals and the crystallization of soluble salts were good monitors of long-term maturation.

There were two tracks in the maturation period and were related to particular indicators.

Typical indicators of this “first track”, mainly between months 1 and 2 of maturation were the grain size, the calcareous nannofossils and the amount and the type of exchangeable cations. The salt crystallization can be considered a suitable indication of the “second track” of maturation within 6–9 months.