Zeolites, which were precipitated on the silica mineralsand phyllosilicates, can most often be identified macroscopically.In the majority of cases, the zeolites occur aseuhedral crystals from 0.1 to 0.5 cm (but single crystals upto 2 cm have been found) in partly filled vugs in the lavasequences (Fig. 4). Zeolites can also be found in tuff layers,where the pore spaces are much smaller (2 mm), andthe secondary minerals totally filled the pore space. Thevesicles and veins generally contain distinct mineralassemblages of more than one zeolite with other secondaryphases.