We present the effects of solfataric alteration on the mineralogy of rocks from the main degassing areas of Campi Flegrei.

Acidic, sulfuric fluids intensely alter the rocks to residues of amorphous silica and alunite.

Solfataric alteration increases porosity and permeability and decreases rock mechanical properties.

Fragmentation of altered rocks produces a high proportion of fines causing a severe respiratory health hazard.

We present a conceptual model for the formation of near surface high-sulfidation alteration.