There are about 40 minerals known as zeolites. Often white or colourless, they are commonly found in volcanic rocks, much like grains in a loaf of bread. But they can also occur in massive form as bedded deposits. They are often associated with volcanic rocks that have been buried and subjected to pressure or hydrothermal alteration.
Zeolites have a unique open crystal structure with many surfaces. This provides a huge surface area for chemical exchange and absorption. Zeolites can be used to soak up oil and chemicals spills, and for cat litter.
The only site that has been mined in New Zealand is at Ngākuru near Rotorua. Ngākuru zeolites, mixed with sand, have been used to improve the growth of the turf at Westpac Stadium in Wellington.
After the Second World War, serpentine was added to the fertiliser superphosphate. Its free-running properties when crushed allow superphosphate to be sprayed from aircraft. It is also added to other fertilisers and applied to magnesium-deficient soils.
By 1993, 4.2 million tonnes of serpentine had been quarried in New Zealand. At that time Greenhills near Bluff, and Piopio near Te Kuiti were the main producers. But by the 2000s, many accessible New Zealand deposits had been worked out. Production also fell because of the development of granulated, free-running superphosphate. At some locations, serpentine was found to contain asbestos fibres and quarries were closed for health reasons.
Mica is a glittering flat mineral that sometimes occurs as large overlapping flakes termed mica books. The Inuit people of the Arctic are said to have used large transparent flakes of muscovite mica as window panes in their igloos. In New Zealand, individual sheets of mica rarely reached this size, but during the Second World War a small amount was mined briefly in the Mataketake Range in South Westland. In 1942 mica was considered an important war mineral, as the Allies used it for spark-plug washers in aircraft engines. In New Zealand it was also used for radio condensers. Wartime shortages led to a scramble for a new supply, but the deposits proved too small to be economic.
Significant asbestos deposits have been identified in two areas – the Cobb River valley of north-west Nelson, and the Red Hills in South Westland. From about 1913 until the 1950s a reclusive couple, Henry and Annie Chaffey, lived near the isolated Cobb River, where Henry Chaffey prospected for asbestos and promoted the development of the region’s deposits. After a road was built to a hydroelectric dam at the headwaters in the 1930s, the Cobb River deposits were mined on a small scale from 1941 to 1963. About 5,000 tonnes were produced.
Road to nowhere
In the early 1970s the isolated Red Hills in Fiordland were considered a likely asbestos prospect. A mining company tried to blaze a trail with a bulldozer from Arawata to the remote claim. The motley convoy included tractors, sheds on sleds and other gear. To make a path they had to use explosives in the Monkey Puzzle Gorge, and drill and blast enormous boulders. Swamps, forests, and rough ground added to the obstacles. They never reached their destination, and returned from the 200-kilometre trip without doing a single day’s prospecting.
During the 1970s several mining companies spent considerable time and effort exploring the potential of the asbestos deposits in South Westland’s Red Hills. Three horizontal tunnels, or adits, were driven into one hillside and samples were taken. While a large deposit with good-quality fibre was found, it did not yield enough asbestos and the isolated location meant that mining would not be economic.
Asbestos was used to manufacture pipes and corrugated roofing from the late 1930s. Thousands of tonnes were applied to boilers and pipes and used as a fire retardant in homes and commercial buildings. In the 1970s the health risks of inhaling asbestos fibres were exposed, and since 1987 no asbestos has been used in New Zealand products. Costly techniques are used to remove asbestos from old buildings. Care must be taken with cement sheeting used in older buildings. It should not be sanded as fibres may enter the air.