Hydrothermal conversion of trachytic glass to zeolite. 3. Monocationic model glasses

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.