NZMIA > resources for schools > industrial minerals

The earth's crust is made of rock of three main types: sedimentary, igneous and metamorphic.

Sedimentary rock is formed as layers of material are deposited under water, usually the sea. Often sedimentary rock contains broken or weathered pieces of other rock. Over millions of years the layers harden and form solid rock. Sometimes fossils are trapped between the layers. Limestone is a good example of a sedimentary rock.

Igneous rock is formed when molten material cools and solidifies. The basalt that forms the volcanoes around Auckland is an igneous rock. Obsidian - volcanic glass - is also an igneous rock, so is pumice. New Zealand's volcanic past means we have lots of igneous rock types.

Metamorphic rocks are changed from one rock type to another by very high temperatures or tremendous pressure. Metamorphic rocks rocks can be formed from igneous or sedimentary rock, or even other metamorphic rock. When limestone is heated it alters to form marble. Slate is a metamorphic rock. It is often used for floor tiles. Stronger metamorphism changes it to schist which is used as a building stone.

Minerals are the building blocks of all rocks. There are over 10,000 different minerals, although only 25 are common in the earth's crust. We use many of these minerals in our everyday lives - usually without knowing it. The products of these minerals are on your breakfast table, in your lounge, at work and school, and in every building. We rely on the mining and quarrying industries to produce the raw materials needed to manufacture these everyday products.

The minerals used by all types of industry can be divided into two main categories:

More than 100 minerals are used by industry in New Zealand. Most of these have to be imported from other countries. We do, however, produce a range of industrial minerals sourced from various locations around the country.

Some industrial mineral deposits are known to exist but are not being worked for a variety of reasons. These include the quality of the material, the size of the market in New Zealand, the size of the deposit, or its distance from the market. For example, New Zealand's largest known deposit of talc and magnesite occurs in the northwest Nelson region and has been worked in the past. The area now lies within the Kahurangi National Park and further production here is unlikely.

New Zealand's industrial mineral resources are constantly being studied to see if new deposits can be worked for local use and export. Wollastonite deposits near Nelson could be used as a substitute for asbestos. Silica in Southland could be used to make ferrosilicon which is used in metal alloys.

More than half of New Zealand's manufacturing industry is located in the Auckland region. Often it is cheaper to import raw materials than to develop distant deposits within New Zealand and transport the material to the manufacturing location.

The sand, gravel and rock used for building and road making are called aggregate. They are the most widely used mineral products in the country. About nine tonnes are used each year for every person in New Zealand. Most aggregates are mined from quarries, but sand and gravel is extracted from river deposits and by dredging the bed of the sea. There are about 600 aggregate mining operations throughout the country. Most are used locally because of the high cost of transport. Most quarries are located near towns or cities and are quite small, producing the equivalent of about three truckloads per day. Many small quarries operate only when material is needed for a local project. Nearly all large quarries producing more than 100,000 tonnes per year are located near main centres.

Each use requires different material. Most quarries produce several types, the largest making up to thirty types. For more detail on aggregates click here.

Aggregate has a wide range of uses and laboratory and field tests are used to ensure it is suitable. Examples of these include measuring the skid resistance of the rock used for sealing roads, the strength and chemical stability of rock used for making concrete, and load bearing tests for the base layers that support roads.

Bentonite Clay …

The country's largest bentonite quarry is in the Harper Hills near Christchurch. The quarry is worked in the dry summer months and the clay is processed at the nearby township of Coalgate. Steam and soda ash are introduced to the clay. This produces sodium bentonite by exchanging sodium for the calcium and magnesium which occurs naturally in the clay. Sodium bentonite is unusual in that it swells and forms a jelly-like mass when water is added to it. This gives bentonite a wide range of uses: making paint, cosmetics, lotions children's crayons and detergents. The processing plant operated by Transform Minerals produces several rades of bentonite for use by industry in New Zealand and overseas. The bentonite produced in New Zealand is used for paper making, for stabilising drill holes during drilling, in agriculture as a growing medium and stock food additive, and a range of geotechnical and environmental applications including sealing ponds and dams to prevent leaks. It is being increasingly used in water treatment where it helps to remove suspended silt which discolours water, and for treating waste effluents.

Clay for brickmaking has been quarried in many places throughout the country. In recent years most smaller brickworks have closed, and now the only major brickworks is that of CSR Building Materials Ltd in Auckland. Clay obtained from quarries in the Auckland and waikato regions is blended with sand, pumice, scoria and other minerals which are added to produce a range of colours. Water is added to give the right consistency. The clay mixture is extruded through a die then wires slice the clay into brick sized pieces. The bricks are dried then loaded onto kiln cars and fired in a gas-powered kiln at a temperature of more than 1000 degrees C.

Cement

Most of the cement used in New Zealand is produced at two plants, by Golden Bay Cement Ltd (a subsidiary of Fletcher Building) near Whangarei in the North Island, and Holcim NZ at Westport on the West Coast of the South Island. The plant at Tarakohe in Golden Bay closed in 1988.

The two plants produce about one million tonnes of cement each year, and production has increased in recent years as demand for building has increased.

Cement is made from clay and limestone that is crushed finely and heated in a rotary kiln (a horizontal cylinder that rotates). Coal is used as the heat source. It takes approximately a tonne of coal to calcine a tonne of limestone to turn it into cement. The material reaches a temperature of about 1,500 degrees celsius to produce clinker. When it is cooled it is finely ground with gypsum (calcium sulphate) added. The resulting fine powder is cement.

Concrete is made in plants that are located around the country. Cement is mixed with water and larger quantities of aggregate and sand, to make concrete which is transported in concrete trucks.

A type of kaolin clay. This high quality clay is found at Matauri Bay in Northland and exported to more than 20 countries where it is used to make white translucent china and porcelain. A very pure clay with a low mineral content and very small mineral grains (less than 0.002mm). Halloysite is the only industrial mineral produced in New Zealand mainly for export. Kaolin clays are used in the manufacture of bricks, tiles, pipes and pottery, as well as being used as a filter medium in the manufacture of paper, paint, pharmaceutical and animal health products. For a detailed case study of halloysite clay, click here.

The most widely used industrial mineral. Limestone is mainly calcium carbonate and forms on the bed of the sea from the remains of sea shells and other marine organisms. Beds of limestone are common throughout New Zealand. Limestone is used as a base for fertilisers with phosphate and other minerals such as potassium, salt, sulphur and serpentine being added. High quality lime is used in industry. Limestone is the basic material used to make cement. It is heated to remove carbon dioxide and water, and then shale or clay are added. When sand and gravel and water are added concrete is produced. Limestone has a wide range of other uses, many of them far removed from its origins. For a detailed case study of limestone click here.

A glassy volcanic rock which contains up to 6% water. When heated quickly to about 900C the water vaporises creating tiny bubbles in the softened rock which expands like popcorn. Perlite can expand up to 20 times its original volume, forming a white, fluffy powder. In this form it is a good insulator of heat and sound and is used mainly in building materials. It is also used in potting mix. Perlite is produced from a quarry at Atiamuri in the central North Island.

A lightweight pale coloured rock which can float on water when it is dry. It is a volcanic rock with many small holes caused by gas escaping after it was erupted. Pumice eruptions have occurred in the central North Island, particularly around Lake Taupo. Large quantities of pumice have been carried down the Waikato River and also to the Bay of Plenty. Pumice is mined as a lightweight aggregate. It is used for horticulture as a soil additive and in hydroponics as a growing medium for plants. One of its more interesting uses is to stone wash denim.

Produced by the natural or artificial evaporation of seawater from salt lakes or by mining buried salt deposits which formed millions of years ago. All of the salt produced in New Zealand is made by Dominion Salt Ltd either by evaporating seawater at Lake Grassmere near Blenheim or by the vacuum pan evaporation method. Lake Grassmere is a suitable evaporation site as the area had low rainfall, high sunshine hours, and is windy. Salt is produced between October and March. Seawater is pumped into the main lake and evaporated. This increases its salt concentration from 2.5% to 20%. This saturated brine is stored in deep ponds over the following winter and then pumped into crystallisation ponds where a salt crust up to 75mm thick forms on the bottom of the ponds. The crust is removed each year in late summer by special harvesters. The amount of salt produced depends on the weather. An average of 60,000 tonnes is made each year. Dominion Salt Ltd refine salt at Lake Grassmere, and also at Mount Maunganui. The latter site was chosen because it is close to a port and to the paper industry which is a major user of salt.

A dense rock, grey, green, or black in colour. Rich in magnesium and trace elements. Found in many places throughout New Zealand and quarried for use as a fertiliser additive.

A non-metallic element used mainly as a fertiliser, either directly or when used to manufacture fertilisers such as superphosphate. Sulphur is imported in large quantities. A large deposit exists at Lake Rotokawa north of Lake Taupo. The deposit lies beneath the bed of the shallow lake in an active thermal area. A small amount has been mined from deposits on the surface, but as yet no satisfactory method has been found to mine the large buried deposit.

A class of minerals which have an open framework structure which allows them to be used as molecular sieves and to absorb gases and metals dissolved in water. They are also used as catalysts in chemical reactions used in industrial processes. Although synthetic zeolites can be produced they are expensive to manufacture. Zeolite is produced by the action of hot geothermal water on fine grained volcanic rock which is rich in silica. The first commercial production in New Zealand was in 1993 from a quarry at Ngakuru south of Rotorua.

Glass is an everyday product of mining that we just take for granted. It is used for windows, bottles and containers, television and computer screens, lenses for spectacles, and many other everyday items.

Glass consists mainly of silica (SiO2) which contains silicon and oxygen - the two most abundant elements in the crust of the earth. The mineral quartz is a crystalline form of silica. It is abundant, although it usually occurs along with other minerals. Quartz is hard and is concentrated along the coast as deposits of white sand. Most other minerals break down and are carried away by the sea. Almost all glass is made from sand deposits that are located or were formed close to the coast.

Pure silica is needed to make clear glass. Coloured glass has iron or other minerals added to it.

Glass bottles and containers are manufactured by ACI NZ Glass Manufacturers Ltd at Penrose in Auckland.

Dredging silica sand

The main raw materials for making glass are recycled glass, silica sand, limestone and soda ash. Imported sand is currently being used. Silica sand has been pumped from the seabed at Parengarenga near the northern tip of the North Island and transported by barge to Auckland. Limestone is produced at many locations. The glassworks obtains its supplies from the Waikato. The soda ash (sodium carbonate) is imported. This is added to lower the melting point of the raw materials to reduce the energy needed and to make the molten mixture flow more easily.

All of the raw materials are loaded into a furnace which is fuelled by natural gas and heated to a temperature of 1540C. At this temperature the raw materials melt. The molten glass flows to a bottle forming machine where compressed air forces the molten glass against the inside of the mould. The bottles are then annealed by reheating and slow cooling to remove stress points in the glass and prevent cracking. When cool the bottles are inspected and then packed.

Any material used to build your house which isn't grown has to be mined. Industrial minerals are a major part of our everyday life.