Geoscience Reference
In-Depth Information
190
C under 10
in water solutions at 180
12 atm pressure. There are thousands
of reports on various aspects of both natural and synthetic zeolites.
All the natural zeolites can be included in the following genetic types
[30]
:
1. Crystals resulting from hydrothermal or hot-spring activity involving reaction between
solutions and basaltic lava flows.
2. Deposits formed from volcanic sediments in closed alkaline and saline lake systems.
3. Similar formations from open freshwater lakes or groundwater systems acting on volcanic
sediments.
4. Deposits formed from volcanic materials in alkaline solids.
5. Deposits resulting from hydrothermal or low-temperature alteration of marine sediments.
6. Formations which are the result of low-grade burial metamorphism.
The above reactions occur in open systems and may be as vast in scale as in the
range of variables such as pressure, temperature, and time. Zeolites' formation,
either in nature or in the laboratory, takes place under conditions in which water is
present in considerable amounts, often at elevated temperatures and hence, under
hydrothermal conditions. Thus, zeolites have been made in the laboratory only by
the hydrothermal method because the open aluminosilicate host framework must be
stabilized during growth by being filled with guest molecules. In particular, zeolites
never form under acid hydrothermal conditions. The sodalites and cancrinites, like
zeolites, have open frameworks, but their synthesis can be achieved pyrolytically
as well as hydrothermally because nonvolatile salts can act like zeolitic water and
as stabilizers by occupying the pore spaces within the aluminosilicate framework.
In hydrothermal synthesis using alkaline salt solutions, water and salts may compete
as stabilizers so that each may be present within the intracrystalline pores
[31]
.
However, the laboratory synthesis of zeolites differs much from that of the natural
process, as the laboratory synthesis involves a closed system. Early attempts to
synthesize zeolites centered around imitating geological conditions and it was
mainly the recrystallization of zeolites taking place until the first-synthesized anal-
cime. In 1948, the first synthesis of a zeolite that did not have a natural counterpart
was carried out by Barrer.
Laboratory synthesis has evolved by duplicating the conditions under which
natural zeolites were produced. However, the one condition that we can never dupli-
cate is crystallization of time, which covers 1000 years or more. The laboratory
systems operate at high pH (
7for
aluminophosphates zeolites) and high temperature and, thereby, produce smaller, less-
perfect, crystals. So, the majority of the synthetic zeolites are formed under nonequi-
librium conditions and are metastable phases produced at supersaturated conditions.
Experimental results indicate that, the less stable the phase, the greater the
chance that, initially, it will nucleate and grow fast, but its chance of subsequent
survival is less.
Synthetic zeolites represent metastable structures that may, under given condi-
tions, be transformed into other thermodynamically more stable types of zeolites.
Also, the synthetic zeolites contain both inorganic and organic cations. Although
more than 150 synthetic zeolites have been reported, many important types have a
12, usually
14 for aluminosilicates, and
.
.
.
