Glassy tuffs

Land processes. Strictly, the majority of diagenic routes are also associated with vulcanism because they are a consequence of alteration processes to wind-blown glassy volcanic ashes or layers of pyroclastic tuffs. 

Zeolites are generally unstable in alkaline environments, as they tend to transform, similarly to glassy systems, into more stable phases, usually into other framework silicates. This behaviour has suggested in some occasions to utilise zeolitic tuffs as raw material for zeolite synthesis [see, e.g., 60]. 

The interaction of zeolite-rich materials with Ca(OH)2 is of special interest, because zeolites, like other reactive aluminosilicate systems, e.g., crushed bricks, give rise to calcium silicates and aluminates, which are able to harden upon hydration in both aerial and aqueous environments. This behaviour, already known in ancient times, is typical of a volcanic, mostly glassy material, called pozzolana, which is the genetic precursor of the mentioned Neapolitan yellow tuff, widely spread in the surroundings of Naples, Italy [61]. That is why every material able to behave as pozzolana is called "pozzolanic material" and the property to react with lime is called "pozzolanic activity". 

O Q Travertin or siliceous tuff Calcedoine, jasper Amorphous Glassy Silica Rocks made from 50 wt.% sihca precipitaed from saturated aqueous solutions or metasomatic reaction. Diatomite are made from accumulation of siliceous skeletons of microorganisms. 

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