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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,
verplanckite and
wairakite.
Environments:
Pegmatites
Metamorphic environments
Hydrothermal environments
Basaltic cavities
Zeolites are minerals of the
zeolite facies, generally formed at low pressure. At high pressure
heulandite or
laumontite is replaced
by lawsonite, and analcime by
albite or jadeite
(ZoW p10).
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.
Land surfaces
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.
Burial Metamorphism
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.
Contact Metamorphism
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.
Primary Zeolites
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.
Localities
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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