Evidence of the relationship occurring between zeolitization and lithification in the yellow facies of Campanian Ignimbrite (southern Italy)

摘要

An attempt to correlate different facies of an important volcaniclastic formation of southern Italy, the Campanian Ignimbrite, with their zeolite content and physico-mechanical properties was carried out. The following parameters were measured: total zeolite content, solid unit weight (γ_s), porosity, and uniaxial compressive strength. Samples were formerly grouped on the basis of the displayed colour in four categories each of them characterized by a different zeolite grade. An increase of zeolite content determines a decrease of porosity and a good positive correlation with UCS values. The relationship existing between zeolite grade and UCS becomes less pronounced or even lost, when zeolite content reaches about 50 % wt. Here, UCS values scatter in a quite wide range (3-6 MPa). It is hypothesized that the lithification of zeolite-bearing rocks is the result of two combined effects playing a different role during the zeolitization process: the transformation of the amorphous fraction in zeolite and the rock texture. In the first stage of the zeolitization process the role played by the rock texture (clasts/matrix ratio, pumice concentration, fractures, etc.) is totally masked by the overwhelming behaviour of zeolite itself up to that threshold value, above which the influence of these two leading parameters is inverted.