Apophyllite, Stilbite and other Zeolites

We need to understand the physical make up to explain what unique features Zeolites have. Zeolites are micro porous crystalline solids with well-defined structures. They will normally contain silicon, aluminum and oxygen in their solid structure and cations*, water and other molecules in their open pores. *(An ion is an atom or group of atoms in which the number of electrons is not equal to the number of protons, giving it a net positive or negative electrical charge. An anion is an ion that is negatively charged, and is attracted to the anode or the positive electrode in electrolysis. A cation has a net positive charge, and is attracted to the cathode or the negative electrode during electrolysis.) Many occur naturally as minerals, and are extensively mined in many parts of the world. Others are man-made synthetics, and are made commercially for specific uses.

Because of their unique porous properties, zeolites are used in a variety of applications and are often referred to as molecular sieves. Some of the major uses of Zeolites are in petrochemical cracking (the process whereby complex organic molecules such as kerogens or heavy hydrocarbons are broken down into simpler molecules), ion-exchange (water softening and purification), and in the separation and removal of gases and solvents. Zeolites are stable solids that resist environmental conditions that degrade many other materials. They are not affected by high pressures or temperatures, they don't burn, they will not dissolve in water or inorganic solvents, and they will not oxidize in the air. They're not believed to cause any health problems or harm the environment. Zeolites stable and unreactive nature is not what makes them so useful.

The unique thing about zeolites is their ability to trap other molecules inside their micro porous crystalline structure which is a cage-like or framed type of structure. This is how water molecules and alkali or alkaline-Earth metal ions (sometimes called cations) become a part of zeolite crystals. Zeolites can trade other positively charged ions for the metal ions originally trapped inside them. This is known as cation exchange and as Cronstedt found over 250 years ago; they can gain or lose their water molecules very easily too (this is known as reversible dehydration). Zeolites have fixed sizes of openings in them which let small molecules pass through but trap larger ones, acting as molecular sieves. Natural Zeolites occur in random forms and mixed sizes. Man-made Zeolites are manufactured to a specific size to trap molecules of a certain (smaller) size inside them.

Zeolites help to provide a cleaner, safer environment. Most uses of Zeolites are the result of efforts to meet environmental concerns by reducing toxic waste and energy consumption. Zeolites have replaced harmful phosphate builders in laundry detergent. Phosphate builders have been banned in many parts of the world because of water pollution potential risks. Zeolites are also used as catalysts which make chemical processes more efficient, saving energy and reducing pollution, waste and by-products. Zeolites can also be used as solid acids which reduce the need for corrosive liquid acids. Zeolites can also be used as redox (reduction and oxidation electron exchange processes, reduction is the gain of electrons, oxidation is the loss of electrons) catalysts and sorbents that can remove atmospheric pollutants, such as engine exhaust gases and ozone-depleting CFCs. Zeolites can also be used to remove harmful organics and heavy metal ions such as those produced during nuclear fission from water.

Zeolites are a beautiful group of rare and complex crystal minerals for collectors. These are naturally formed in the cavities (vesicles) of volcanic rocks. Zeolites are Metamorphic Rocks. Some Zeolites form with subtle amounts of heat and pressure, marginally being called metamorphic, while others are found in classic metamorphic formations. Some of the most popular collectible Zeolites come from the Deccan Traps in India. There are 7 families of Zeolites as shown here. Some minerals such as Apophyllite and Okenite, are not Zeolites but are often found in combination with Zeolites, and sometimes erroneously referred to as Zeolites