... This study attempts to define the properties of hyaloclastite formations which control their petrophysical characteristics during their progressive alteration. ... Micro porosity in fully altered tuffs ranges from about 7?35%. ... A geochemical comparison of the variably altered tuffs with the compositional field of equivalent fresh rocks indicate the limited mobility of the chemical components, except for CaO, Na2O and Rb which show clear indication of being removed. ...