596 MINERALS/Zeolites Zeolites in Sedimentary Rocks The most widespread and economically important of all zeolite occurrences are in sediments and sedimentary rocks composed partially or wholly of volcanic clasts During diagenesis (see Diagenesis, Overview) the volcanic component of these rocks reacts with pore fluids to form clay and zeolite minerals This alteration can take place in various environments, including shallow alkaline lakes, the deep seafloor, thick terrestrial pyroclastic deposits, and sandstone beds in shallow to deep marine basins Diagenetic Figure Pressure temperature ranges of environments of zeolite formation The boundary between diagenesis and meta morphism overlaps considerably more than shown here Numbered curves are experimentally determined or calculated stability limits of selected zeolites: 1, analcime ỵ quartz albite ỵ water; 2, heulandite laumontite ỵ quartz ỵ water; 3, laumontite þ prehnite epidote þ quartz þ water at low pressure end and laumontite ỵ pumpellyite epidote ỵ chlorite ỵ quartz þ water at high pressure end; and 4, laumontite wairakite þ water The depth scale is based on a lithostatic gradient of 25 MPa km Reproduced with permission from Deer WA, Howie RA, Wise WS, and Zussman J (2004) Rock Forming Minerals, Volume 4B Framework Silicates: Silica Minerals, Feldspathoids and the Zeolites London: The Geological Society reactions merge with those of burial metamorphism (Figure 7), and the distinction is a matter of the terminology used by researchers in this field In this review, diagenesis refers to all changes of the initial sediment between deposition and the completion of lithification or cementation, and metamorphism refers to changes occurring after diagenesis, generally involving the breakdown of the first diagenetic minerals in response to increases in temperature and pressure Shallow alkaline lakes Beds of zeolite-rich rock a few centimetres to several metres thick occur in several areas of the world, such as the western USA, Serbia, and western Turkey Clinoptilolite is the most abundant zeolite in these beds, but it may be associated with chabazite, phillipsite, erionite, and analcime The zeolitic beds are interlayered with silt and were deposited in shallow playa lakes in closed basins With no outlets, these lakes become saline and alkaline through extended periods of evaporation Nearby explosive rhyolitic volcanism produced vitric ash that reacted with the alkaline water of the lake to produce the beds of zeolite-rich rock Figure shows the relationships between mineral zoning and hydrology in this type of environment Glass persists unaltered where pore solutions have a pH of less than about 8.5 The clinoptilolite zone may contain other zeolites, such as erionite, chabazite, and phillipsite, all of which pseudomorph glass shards Analcime, which is abundant in the next zone inwards, replaces both pre-existing zeolites of the clinoptilolite zone and, less commonly, glass shards Potassium feldspar, dominant in the innermost zone, replaces earlier-formed zeolites This horizontal zonation of the zeolites and potassium feldspar along a single tuff bed is characteristic of this type of alteration, and puzzling to understand A kinetic model has been proposed to explain the formation of zeolites in tuffaceous rocks The model Figure Relationships among hydrology, geology, and mineralogy in a playa saline lake TDS, total dissolved solids Reproduced with permission from Deer WA, Howie RA, Wise WS, and Zussman J (2004) Rock Forming Minerals, Volume 4B Framework Silicates: Silica Minerals, Feldspathoids and the Zeolites London: The Geological Society