The influence of mineralogy and texture in the water content of rock salt formations. Its implication in radioactive waste disposal

The consideration of the use of salt formations as possible radioactive waste disposal sites led us to attempt to determine the extent to which their brine content could influence the performance of the disposal system. Bedded rock salt from the Cardona, Zaragoza and Guendulain Fms., as well as diapiric rock salt from northern and southeastern Spain, have been selected and their water contents characterized by thermogravimetry. Free water content (intergranular water and water in fluid inclusions) in the studied formations ranges from 0.01 to 1.24% weight. In addition, the presence of hydrated minerals increases the amount of total water in the rock (up to 3.50%). Clear differences between the studied formations are observed in the total amount of water and in the form of water entrapment in rock salt. The results obtained have allowed the classification of the studied rock salt formations with respect to their free-brine content as water-poor (around 0.1% in average), intermediate water-rich (around 0.2%) and water-rich (higher than 0.3%). The petrographical features of the rock salt, such as mineralogical content and halitic textures, play an important role in its water content. Whilst brine in fluid inclusions is related to the halite texture, intergranular brine depends mainly on the content of clay and sulphate minerals.