Experiments on zeolitization were conducted on four synthetic monocationic
glasses (Na, K, Ca, or Mg-rich glass) with Si/Al molar ratios of 2.67, simi
lar in acidity to many volcanic glasses of partially zeolitized Italian tuf
fs. The products of the hydrothermal treatment at 100, 150, and 200 degrees
C of single glasses or glass mixtures with deionized H2O or monosaline sol
utions (NaCl, KCI, CaCl2) were characterized by X-ray diffraction, thermal,
microscopic and chemical analyses. Chemical analyses of mother liquors wer
e also performed. Mineral assemblages, containing chabazite, phillipsite, a
nalcime, and K-feldspar, very similar to those found in altered, volcanicla
stic alkali-trachytic or trachytic glass deposits were produced. Potassium
was essential to chabazite and phillipsite crystallization, although philli
psite was obtained also in Ca-Na mixed systems. Sodium was necessary for an
alcime formation. Calcium plays only a secondary role in zeolitization, and
magnesium does not favor zeolite crystallization but promotes the formatio
n of smectite. Glass composition determines the mineral assemblages obtaine
d and hence in those commonly found in nature.