Vitric

Incongruent mass transfer in the case of parabolic kinetics based on diffusion control can be readily explained by differences in the diffusion coefficients of chemical species. However, the proposed mechanisms invoked to explain linear kinetics involve the dissolution of the bulk silicate phase, which, over extended periods, should result in the chemical components being transferred to solution from the silicate in their stoichiometric ratios (25). However, many field studies have shown that silicates weather incongruently in nature (27). This is supported by our studies of ground waters associated with vitric rhyolite tuffs. 

Experiments on the vitric tuff showed that the rate of mass transfer of an ion from the glass was inhibited specifically by... 

The location in the geologic section from which the B and T tunnel-complex water samples were obtained, comparison of their chemical composition with data obtained from laboratory experiments on both vitric- and crystalline-tuff, and hydrologic data from the Rainier Mesa system, combine to imply that the vitric material (Rainier Mesa Member and Paintbrush Tuff) of Rainier Mesa is responsible for the water quality observed above the zeolitized zone. 

Table III.—Mass Transferred From Rainier Mesa Vitric Material Per Liter of Solution for Each pH Interval...

If it is assumed that precipitation of montmorillonite of this composition is occurring contemporaneously with dissolution of vitric material in the Paintbrush Tuff, the ratios of maior cations (in solution) consumed is approximately Mg Ca Na K = 1 0.40 0.14 0.14. 

Turin H. J., Groffman A. R., Wolfsberg L. E., Roach J. L., and Strietelmeier B. A. (2002) Tracer and radionuclide sorption to vitric tuffs of Busted Butte. Nevada. Appl. Geochem. 17(6), 825-836. 

Ever since the discovery of analcime in tuffaceous rocks, most workers have assumed that the analcime formed directly from vitric material. The presence of vitroclastic texture and pyrogenic crystals in some anal-cimic tuffs seemed sufficient evidence however, these criteria do not necessarily prove that the glass altered directly to analcime. Hay (48) and Sheppard and Gude (123) concluded from a study of tuffs in saline-lake deposits that analcime commonly formed from alkalic, silicic zeolite precursors. Formation of analcime from clinoptilolite and phillipsite was documented in tuffs of the Miocene Barstow Formation. Relict fresh... 

In Japan, clinoptilolite is the commonest zeolite formed from altered pyroclastics. Four modes of occurrence have been found, replacement of vitric materials and precipitation in interstitial voids being predominant. By chemical analyses, clinoptilolite is classified into 3 types, Ca-, Na-, and K-type. In a ternary diagram of Ca(Mg), Na, and K, the field of clinoptilolite is not overlapped by that of heulandite. The x-ray powder profiles of clinoptilolites resemble that of heulandite, but their thermal behavior differs. When heated to 250°C, heulandite changes to heulandite-B this transition is not observed in clinoptilolite. Furthermore, the thermal behavior of Ca-clinoptilolite differs from alkali-clinoptilolite. This may be attributed to the difference of dehydration between Ca-clinoptilolite and alkali-clinoptilolite, which seems to depend on the atomic ratio of Ca and alkalies. 

Bulk of zeolitized rhyolite vitric tuff of the Nakanosawa tuff the same sample as listed in Table I. 

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