The zeolite group is a large group of tectosilicates (framework silicates). They have a porous structure that can accommodate a wide variety of cations, such as Na+, K+, Ca2+ and Mg2+. These positive ions are rather loosely held and can readily be exchanged. Due to their very regular pore structure, which is of molecular dimensions, zeolites can sort molecules selectively by size. Zeolite minerals include analcime, bellbergite, brewsterite, chabazite, clinoptilolite, dachiardite, epistilbite, erionite, faujasite, ferrierite, gismondine, gonnardite, heulandite, laumontite, lévyne, merlinoite, mordenite, natrolite, phillipsite, pollucite, scolecite, stilbite, thomsonite and wairakite.
Zeolites are minerals of the
zeolite facies, generally formed at low pressure. At high pressure
laumontite is replaced
by lawsonite, and analcime by
albite or jadeite
Low Temperature Origins (4o to 40oC)
Explosive volcanic eruptions produce large amounts of glassy ash that can fall on land or in water, with different ensuing alteration processes.
Zeolites can form as a result of ground water moving downward through volcanic ash, precipitating zeolites then flowing out of the rock unit. The zeolite species formed are determined by the composition of the ash.
In silica-rich tuff zeolite precipitation occurs only for ash deposits that are more than 500 metres thick. Clinoptilolite and mordenite are the major minerals, with minor phillipsite, analcime and the non-zeolite K-feldspar at deeper levels.
In low silica tuff precipitation of zeolites can occur for much thinner layers, down to as little as 10 metres thick, producing phillipsite, natrolite, gonnardite, analcime, chabazite and rarely faujasite and gismondine (ZoW p11).
Saline Alkaline Lakes
Volcanic ash that falls into saline alkaline lakes can change into zeolites over a period of thousands of years. The water in the lakes is close to neutral (pH = 7) near the edges, but becomes more alkaline (higher pH) towards the centre.
At pH about 8.5 phillipsite, clinoptilolite, erionite and chabazite form.
Near the centre of the lake, at pH 9 to 10, analcime forms, together with the non-zeolites K-feldspar, trona and borosilicates such as searlesite.
Saline alkaline lakes are rich in sodium and potassium. The most common zeolites found there are analcime, chabazite, clinoptilolite and mordenite; ferrierite, merlinoite and harmotome are rare. Non-zeolite minerals found near the centre of the lakes include K-feldspar, carbonates, quartz, fluorite, dawsonite, trona, nahcolite, pirssonite, gaylussite, searlesite and halite.
Deep Sea Zeolites
Some of the deep-sea zeolites result from the alteration of volcanic ash, but most of them form from a reaction between fossil-rich oozes and seawater. phillipsite forms before clinoptilolite, both of which are common in deep-sea sediments. Crystallisation of phillipsite can take up to ten million years. It forms as a meta-stable phase a few centimetres below the sediment/water interface at a pressure under 0.7 kbar and temperature 4o to 34oC. It may take another hundred million years for these two minerals to alter first to analcime and then to K-feldspar. Erionite, mordenite and laumontite are only rarely found.
On the South Indian Ridge in the Indian Ocean merlinoite is found with plagioclase, stilbite, chabazite and quartz in manganese nodules.
Elevated-Temperature Origins (40o to 250oC and rarely above)
As the temperature rises chemical reactions proceed more quickly. The heat is provided by magma bodies near the surface or by molten volcanic rock on the surface. The water needed for zeolite formation is usually surface water that has been heated by hot surrounding rocks as it percolates down through them.
Cooling of a Volcanic Flow
When molten volcanic lava comes into contact with water the base of the flow is quenched to form glass, gas bubbles form and are frozen into vesicles. The lava cools over a period of several years, and if water is present clay minerals line the cavities, followed by zeolites, including phillipsite, chabazite, lévyne, offretite, mesolite, thomsonite, heulandite and stilbite.
Pegmatites and Miarolytic Cavities
When granitic magma containing some water cools and finally forms pegmatites, the first minerals to form are anhydrous, leaving the remaining magma richer in water. This water is a necessary component of zeolites, which are some of the last minerals to form. Pollucite crystallises at 400 to 300oC, then analcime at 250 to 150oC. Below 250oC the low temperature zeolites natrolite, analcime, stilbite and laumontite start to form.
Regional and Localised Hydrothermal Solutions
When hydrothermal solutions reach volcanic rocks they flow along the inter-connected cavities and vesicles, where they may deposit zeolites and other minerals. Which zeolites are formed is largely determined by the temperature. In Western Iceland it has been shown that chabazite, lévyne and phillipsite formed at 55 to 70oC, mesolite, scolecite, thomsonite and gismondine at 60 to 90oC, stilbite, heulandite, epistilbite and mordenite at 90 to 110oC, laumontite, mordenite and analcime at 110 to 230oC and wairakite at up to 300oC.
As sediments become buried temperature and pressure increase, causing minerals in the original rocks to alter to other minerals that are more stable in the changed conditions. An increase in depth (and temperature) causes a decrease in the hydration of minerals. Zeolites become unstable and alter into the non-zeolites feldspar, prehnite, pumpellyite and actinolite that are more stable at higher temperatures. Further heating produces epidote and hornblende.
When molten granite intrudes into the surrounding rocks conditions in the area surrounding the intrusion change, with the lowest water content and highest temperature closest to the intrusion, and higher water content and lower temperature further away.
Fibrous zeolites such as natrolite have a comparatively low water content, and they are deposited close to the intrusion, whereas platy zeolites such as stilbite and heulandite that have a high water content form further from the intrusion.
Analcime is the only zeolite that crystallises directly from molten rock. It forms in deep basaltic magma chambers at 5 to 13 kbar pressure and temperature of 600o to 640oC. It must be transported rapidly to the surface if it is to survive the change of environment. A slower change would result in the analcime reacting with the basaltic magma to form albite.
The reference for all of the above section on the formation of zeolites is "Zeolites of the World" (1992) by Rudy Tschernich.
Minerals often found associated with zeolites include apophyllite, calcite, cavansite, prehnite, epidote, quartz, pyrite, and clay minerals.
Almost all known zeolites and their associated minerals occur in the Deccan Traps of India. This is an enormous area, more than 200,000 square miles, of basaltic layers with a thickness of over 6,500 feet. Much of it is in the state of Maharashtra (R&M 85.3.220-229).
In the vicinity of Meshkinshahr, Ardabil Province, Iran, the order of crystallisation has been reported as analcime, mesolite, thomsonite, phillipsite, stilbite and heulandite. Associated minerals are apophyllite and calcite.
Analcime crystals are found embedded in potassium-rich basalt associated with chabazite, mesolite and/or thomsonite.
Mesolite is the most common zeolite here, and is almost always associated with thomsonite and analcime (R&M 92.6.541-542).
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