Z

◊ zeolites ◙ zeolite facies ◊

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zeolite facies

The zeolite facies results from low grade metamorphism due to low temperature/low pressure (T ~50 - 150 °C, burial at 1 - 5 km depth). Metamorphosis proceeds by dehydration of clays during compaction, and from mild temperature elevation due to blanketing by continued deposition of sediments. Compaction causes foliation parallel with the bedding plane of the rocks due to alignment of platy clay minerals in a horizontal orientation. The zeolite facies is considered to be transitional between the processes of diagenetic alteration that convert sediments into sedimentary rocks, and the prehnite-pumpellyite facies (a hallmark of subseafloor alteration of the oceanic crust around mid-ocean ridge spreading centres). Rocks of the zeolite and prehnite-pumpellyite facies can be found in the Valley and Ridge Province, Great Smokey (Blue Ridge) Thrust. These rocks are folded Precambrian to Lower Cambrian shales and carbonates affected by the Alleghenian Orogeny. The pelitic rocks show recrystallization of clay minerals, whereas carbonate rocks have an assemblage of calcite or dolomite ± quartz, and rare basic rocks have chlorite, calcite, and rare pumpellyite [Ca4(Mg,Fe)(Al,Fe+3)5Si4O23(OH)3.2H2O]. Although zeolite metamorphism does not affect plutonic or volcanic rocks, vesicular basalts can experience vesicular infilling by zeolites minerals, producng an amygdaloidal texture The zeolite facies results from metamorphosis of pelitic sediments rich in aluminium, silica, potassium and sodium, but typically low in iron, magnesium and calcium. Mineralis of the zeolite series include zeolites, albite, and quartz, and the assemblage quartz + laumontite + chlorites is diagnostic. Zeolite facies metamorphism usually converts low temperature clay minerals into higher temperature polymorphs such as kaolinite and vermiculite. Mineral assemblages also include montmorillonite with laumontite, wairakite, prehnite, chlorites and chlorite. Potassium rich sediments yield phengite and adularia.

links: images: crystals: zeolite crystal, 2, 3, 4, 5, 6, clinoptilolite; hand-sample: zeolite facies, 1; formation: the shallowest level of oceanic crust is only weakly metamorposed to zeolite facies, pillow basalts, Oman; Cyprus pillows, and zeolite mineralization that have been formed within the pillows by the movement of the heated seawater vertical view of the latticing, side view, and closeup of a spherical crystallizing zeolite mineral; zeolite strata, Grasshopper Creek; webpages: zeolite veins and faults; North America's Zeolite

zeolites

mineral / chemical formula		properties / significance	occurrence
zeolites alumino-silicates for example, Na2Al2Si3O10-2H2O is the molecular formula for natrolite	zeolite samples superimposed on the "molecular sieve" structure of ZSM-5, hi-res (click to enlarge)	zeolites are alumino-silicate minerals with a micro-porous structure that can accommodate a wide variety of exchangeable cations, such as Na+, K+, Ca2+, Mg2+ (molecular sieves)	naturally occurring zeolites are contaminated by other minerals, metals, quartz or other zeolites; zeolites slowly crystallize in post-depositional environments (shallow marine basins), or form where volcanic rocks and ash layers react with alkaline groundwater

hand specimen image courtesy of USGS

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