The Zeolites are a popular group of
minerals which are not only Pleasing
to collectors but also have a use in our
day to day life. Zeolites typically form
in the cavities, or vesicles, of volcanic
rocks, zeolites are the result of very
low grade metamorphism.
More than 150 Zeolite types have been
synthesized and Nearly 50 naturally
occurring zeolites are known. Occurring
as hydrates. All members of the family
contain at least one silicon atom per aluminum atom.
Natural Zeolites form in nature as a result of the chemical reaction
between volcanic glass
and saline water. The natural reaction temprature range is from 27°C
to 55°C, and the pH
is typically between 9 and 10. Nature requires 50 to 50,000 years to
complete the reaction. Naturally occurring zeolites are rarely phase-pure
and are contaminated to varying degrees
by other minerals. For this reason, naturally occurring zeolites are
not used in many important commercial applications where uniformity
and purity are essential.
Synthetic Zeolites have advantages over natural Zeolites.
First of all synthetics can be manufactured in a uniform, phase-pure
state. It is also possible to manufacture desirable
zeolite structures which do not appear in nature. Of course because
silica and alumina, which are among the most abundant mineral components
on earth, are used to manufacture Zeolites. The potential to supply
zeolites is quite unlimited. Zeolites are undergoing extensive research
into their formation and unique properties and have even been grown
on board the space shuttle.
The term zeolite was originally used in the 18th century by a Swedish
mineralogist named Cronstedt who observed, while rapidly heating a natural
zeolite, that the stones began to
move around as the water boiled off. Using the Greek words which mean
"stone that boils,"
he called this material zeolite. A commonly used description of a zeolite
is a crystalline aluminosilicate with a cage structure. Technically,
we speak of a zeolite as a crystalline hydrated aluminosilicate.
Zeolites are framework silicates consisting of interlocking tetrahedrons
of SiO4 and AlO4. Zeolites have large open spaces or cages in their
structures that form channels. These channels allow the easy movement
of ions and molecules into and out of the structure. This ability puts
Zeolite in the class of materials known as "molecular sieves."
Zeolites have basically three different structural variations. There
are chain-like structures whose minerals form acicular or needle-like
prismatic crystals, as found in natrolite. there are sheet like structures
where the crystals are flattened platy or tabular as found in heulandite.
There are also framework structures where the crystals are more equal
in dimensions, as found in Chabazite. These variations give the Zeolite
group, many pleasing and varying
crystal forms.
Commercially useful zeolites owe their value to one or more of three
properties: adsorption, ion exchange, and catalysis. Zeolites are characterized
by their ability to lose and absorb
water without damage to their crystal structures and the large channels
explain the consistent low specific gravity of these minerals.
Adsorption is the most fundamental use of Zeolite. The adsorption
of minerals or other particles by Zeolites is called molecular sieving.
Minerals or other particles who's diameter is too large to pass through
a zeolite pores are effectively "sieved." This "sieve" effect can be
utilized to produce sharp separations of molecules by size and shape.
Used in many different applications, the water in Zeolite structures
is driven off by heat with the basic structure left intact. Then other
solutions can be pushed through the structure. The Zeolites absorb ions
and molecules and thus act as a filter or chemical sieve. For odor control,
toxin removal and or other unwanted elements. Zeolites are also more
effective than previously used filtration compounds. The pores in some
dehydrated zeolites are as small as
6 Ångstroms in size, while those of a typical silica gel average about
50 Å, and activated carbon averages 105 Å.
The most well known form of ion exchange is in water softeners. Calcium
in water can cause
it to be "hard" and capable of forming scum and other problems. Zeolites
charged with the much less damaging sodium ions can allow the hard water
to pass through its structure and exchange the calcium for the sodium
ions. This process is reversible so the Zeolite can be reused several
times.
Steric phenomena are very important in zeolite catalysis, and a new
term, "shape selective catalysis," was coined to describe these effects.
Catalysis Zeolites make extremely active catalysts. Very selective reactions
can be made to occur when certain products, reactants or transition
states are kept from forming within the pores because of size or shape.
Zeolite's use in the global laundry detergent market. Is by far it's
most prominent use. Estimated world wide consumption of detergent zeolite
at the end of 1992 was thought to be 1.44 million metric tons per year.
Unlike phosphates, zeolite detergents are not detrimental to lakes,
streams and ocean bays.
Other applications include refrigeration as the heat
of water adsorption for zeolites is high. They also possess high adsorption
capacity, undergo reversible adsorption/desorption, and
are structurally stable. These properties enable zeolite to be used
in solar-powered refrigerators and to store energy during off-peak periods
and release it during peak periods. Zeolites can also be used in refrigeration
and air cooling systems to reduce water in the air to very low concentrations,
allowing very effective evaporative cooling to occur.
Many of the uses for zeolites are environmentally related. Such as
radioactive waste treatment. Natural zeolites are being used to treat
low and intermediate aqueous waste and municipal waste water treatmentNatural
zeolites are uniquely effective in adsorbing ammonia and also adsorb
hydrogen sulfide. These properties make natural zeolites ideal for use
in pet litter to prevent irritating odors. For similar reasons, natural
zeolites can be used for effective control of irritating gases in stables,
barns, kennels, etc
Many other minerals have similar cage-like framework structures or similar
properties to Zeolite. Or may be associated with zeolites. They include
the phosphates: kehoeite, pahasapaite and tiptopite; and the silicates:
hsianghualite, lovdarite, viseite, partheite, prehnite, roggianite,
apophyllite, gyrolite, maricopaite, okenite, tacharanite and tobermorite.
Interesting comparisons can be made between these minerals and zeolites.
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