The Zeolite Group of Minerals
The zeolites are a popular group of minerals for collectors and an important
group of minerals for industrial and other purposes. They combine rarity,
beauty, complexity and unique crystal habits. Typically forming in the
cavities, or vesicles, of volcanic rocks, zeolites are the result of very
low grade metamorphism. Some form from just subtle amounts of heat and
pressure and can just barely be called metamorphic while others are found
in obviously metamorphic regimes. Zeolite crystals have been grown on board
the space shuttle and are undergoing extensive research into their formation
and unique properties.
The zeolites are framework silicates consisting of interlocking tetrahedrons
of SiO4 and AlO4. In order to be a zeolite the ratio (Si +Al)/O must equal
1/2. The alumino-silicate structure is negatively charged and attracts the
positive cations that reside within. Unlike most other tectosilicates,
zeolites have large vacant spaces or cages in their structures that allow
space for large cations such as sodium, potassium, barium and calcium and
even relatively large molecules and cation groups such as water, ammonia,
carbonate ions and nitrate ions. In the more useful zeolites, the spaces are
interconnected and form long wide channels of varying sizes depending on the
mineral. These channels allow the easy movement of the resident ions and
molecules into and out of the structure. Zeolites are characterized by their
ability to lose and adsorb water without damage to their crystal structures.
The large channels explain the consistent low specific gravity of these
minerals.
Zeolites have many useful purposes. They can perform ion exchange, filtering,
odor removal, chemical sieve and gas absorption tasks. The most well known
use for zeolites 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
reversable. In a similar way zeolites can adsorb ions and molecules and thus
act as a filter for odor control, toxin removal and as a chemical sieve.
Zeolites can have the water in their structures driven off by heat with the
basic structure left intact. Then other solutions can be pushed through the
structure. The zeolites can then act as a delivery system for the new fluid.
This process has applications in medicine, livestock feeds and other types
of research. Zeolites added to livestock feed have been shown to adsorb
toxins that are damaging and even fatal to the growth of the animals, while
the basic structure of the zeolite is biologically neutral. Aquarium
hobbyists are seeing more zeolite products in pet stores as zeolites make
excellent removers of ammonia and other toxins. Most municipal water
supplies are processed through zeolites before public consumption. These
uses of zeolites are extremely important for industry, although synthetic
zeolites are now doing the bulk of the work.
Zeolites have basically three different structural variations.
- There are chain-like structures whose minerals form acicular or
needle-like prismatic crystals, ie natrolite.
- Sheet-like structures where the crystals are flattened platy or tabular
with usually good basal cleavages, ie heulandite.
- And framework structures where the crystals are more equant in
dimensions, ie Chabazite.
A zeolite can be thought of in terms of a house, where the structure of the
house (the doors, windows, walls and roof) is really the zeolite while the
furniture and people are the water, ammonia and other molecules and ions
that can pass in and out of the structure. The chain-like structures can be
thought of like towers or high wire pylons. The sheet-like structures can be
thought of like large office buildings with the sheets analogous to the
floors and very few walls between the floors. And the framework structures
like houses with equally solid walls and floors. All these structures are
still frameworks (like the true tectosilicates that zeolites are).
These variations make the zeolite group very diverse, crystal habit-wise.
Otherwise zeolites are typically soft to moderately hard, light in density,
transparent to translucent and have similar origins. There are about 45
natural minerals that are recognized members of the Zeolite Group.
Industrially speaking, the term zeolite includes natural silicate zeolites,
synthetic materials and phosphate minerals that have a zeolite like
structure. The complexity of this combined group is extensive with over
120 structural variations and more are being discovered or made every year.
Collecting zeolites can be very enjoyable and fulfilling.
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