Rhyolite, dacite and rhyodacite tuff and tuffite layers of Miocene age in the Mecsek Mountains, Hungary, partly or completely altered to containing clay (mostly smectite) and zeolite (clinoptilolite) minerals as dominant alteration product. The (authors') model for this alteration process is: Hydrolysis Z increase in pH Z precipitation of smectite with d value of >14 Å → increase in pH → precipitation of clinoptilolite → decrease in pH → ion exchange between pore fluid and smectite (Ca is replaced by Na, which causes decrease in smectite d value to <14 Å) → precipitation of K-feldspar. The excess silica precipitated as Opal-C at different stages.