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What are Zeolites? |  |
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What are Zeolites? Zeolites are crystalline, hydrated aluminosilicates that contain alkali and alkaline-earth metals. Their structure is based on a three dimensional framework structure of silica-oxygen tetrahedra. The crystal framework is in a honeycomb network with pores that negative charges which are balanced with cations such as calcium, magnesium, potassium and sodium. These cations can be exchanged. How were they formed in NZ? About 250,000 years ago, in the Rotorua/Taupo area, intense volcanic activity generated huge ash showers which were washed and eroded into lakes forming sediment beds up to 80 metres deep. Subsequent sub-surface thermal activity forced hot water (120 C) up through these beds transforming the clays into soft rocks with ordered internal structural sequences which are now called zeolites. Also at that time severe block faulting drained the lakes exposing many deposits. Subsequent ash showers covered some of the deposits to depths of 30 meters. What type of zeolite? There are about 40 different zeolite types and their occurrence depends on conditions during formation. The Ngakuru zeolites are predominantly mordenite and clinoptilolite. The location, duration and intensity of the thermal water flows during formation determines the degree of thermal alteration. Deposits close to thermal fissures are fully altered and often mechanically strong whilst those further away are often poorly altered and can breakdown into constituent clays. How do they work? Ionic adsorbing power During the thermal alteration phase amorphous material was washed from the clays leaving a 3D skeleton of aluminium and silica oxides which due to a unique configuration have a high negative charge (cation exchange capacity, typically greater than 100 meq/100g). Positively charged cations in solution (or molecules suspended in air) can be absorbed onto the latticework and depending on pH, cation concentration and charge characteristics can later be released. Physical absorbing power The 3D-lattice work also gives zeolite another property of large internal surface area (up 145m2/g). Geologically young zeolite like those found at Ngakuru are able to absorb more liquids than other, older, glassy zeolites. When dried some of these zeolites are able to absorb up to 70% of their own weight as liquid. Each characteristic supports specific uses however the dual capacity enhances certain uses. For instance in a sports turf situation  zeolite will:
ii. absorb water, increasing water-holding capacity without detrimentally influencing pore space and infiltration rates.
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