Mineral samples from two Miocene tuff deposits from Romania (Goleşti and Slănic) were studied in order to understand which parameters may control the mineral assemblage. Both acidic tuffs were formed of kaolinite, minerals of the illite/smectite series (R0 illite/smectite and illite), clinoptilolite, alkali feldspars and SiO₂ phases. The characterization of clay and zeolite minerals and the percentage of smectite layers in R0 illite/smectite suggest that the formation of R0 illite/smectite at Goleşti occurred directly from volcanic glass, while at Slănic it could be the result of the smectite illitization. The percentage of smectite layers and the comparative clay minerals/clinoptilolite ratio would indicate that the two deposits might have had different evolutions. Phase transformation at Goleşti potentially proceeded at a relatively low depth, suffering an important chemical control, while at Slănic the authigenesis suggests a greater formation depth and a weaker chemical control. Therefore, the two deposits may depict different alteration degrees of the same volcanic glass.