Materials: Mineral Resources

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Earth is a sphere of minerals some 7,920 miles in diameter, molten in the outer core with a plastic solid mantle and thin brittle crust. At present we can only be interested in the crust in terms of mineral extraction, and particularly the surface of that crust where so much of land life is concentrated. This layer of topsoil, vegetation, and animal and microbial life can be seen as the all-important living skin of the planet. It is this vital surface layer that we are destroying at an alarming rate through deforestation, agricultural impoverishment, erosion, desertification, urbanization, and paving for roads. We are also destroying and polluting parts of this layer through the careless extraction of minerals.



Besides this surface scarring, what should our other ecological concerns be in regard to mineral extraction? They are the energy and resources used in the processes of extraction and restoration, the interference with groundwater supplies, the rapid rate of resource depletion, and , finally, the pollution caused during the refining process. This last concern is addressed in the next section, which looks at PROCESSED and SYNTHETIC MATERIALS.

Perhaps the most important of all these concerns, yet one which we continue to ignore, is the over-mining of scarce resources for short-term gain. Although the known reserves of particular minerals are continually being extended, we are extracting first the deposits that are easiest to mine, so there is likely to be greater and greater environmental damage as we continue to extract them from the earth.

CATEGORIES OF MINERALS

There are many different ways in which minerals can be classified; one interesting category from an ecological point of view is that of mineral deposits that are a product of dead organisms. These include the fossil fuels of coal, oil, and natural gas; the calcium deposits of chalk, limestone, and marble; and the uranium deposits of yellowcake.

There are more than 2,500 different types of minerals known to us today. Rocks, which are composed of minerals, are often referred to generically as “minerals” by industry. Industry at present depends on some 80 important types, including combined or impure minerals such as iron ore and bauxite (aluminum ore), which are relatively plentiful.

For our purposes, it is useful to divide minerals into those that can be used in their natural state without further processing, such as building stone, and those that are the raw materials for further industrial processing such as the metallic ores.

STONE

Stone is a generally abundant but nonrenewable material, which is expensive when compared with concrete or brick. One of the main reasons for its expense is the labor that is required for both its extraction and incorporation into a building. Most building stones are strong, durable, and attractive, which has often made stone the first choice as a building material for the wealthy. It is also generally a healthy and nonpolluting material. Its embodied energy costs are variable, depending on the ease of quarrying and transportation, with the latter cost becoming particularly high if the stone is imported from abroad.

Extraction

If extracted in an ecologically sound manner, there is no reason why stone should not be fully exploited. The problem here is how to extract it in a way that does the least damage to the environment. With creative stone extraction in derelict or uninteresting countryside followed by careful landscaping and restoration, an area can even be improved, as opposed to the clumsy and destructive quarrying that is often practiced today. Problems occur when an inappropriate site for extraction is chosen—a national park, for instance— and when the extracting company is allowed to remove too large a quantity from the same place. Experimentation is taking place with new methods, one of which is the “glory hole” method, whereby stone (often granite) is removed from the inside of a giant hole within a large hill or mountain. Access is obtained from the bottom of the hole, and the top is secured for safety.

Different Types of Stone

Geologically, rocks may be classified as igneous, sedimentary, or metamorphic. Igneous rock is cooled from molten magma; it often has a coarse crystalline structure (for example, granite). Sedimentary rock was formed at low temperatures, generally as sediments at the bottom of seas, oceans, and lakes. Typical examples are shales, sandstones, and limestones. Metamorphic rock has been changed through great heat and pressure. Examples of this type of rock are slate (changed from shale) and marble, which began as limestone (a sedimentary rock) and became marble through increased pressure and heat.

There are many kinds of building stones, which mainly fall into five types: granites, sandstones, limestones, flints, and slates.

Granite

Granite is the strongest of building stones found in the British Isles. It is very resistant to weathering and acid rain, and it can be recycled almost indefinitely. It is used as building stone where permanence is required, and in sets and curbstones, where its strength and resistance to wear comes into play. It is used extensively as chippings in the wearing layer of roads and highways. Some granite contains fairly high quantities of uranium and thus emits radon gas at a greater rate than other stone. If you live in a building made of or built on granite, you should have your radon levels checked to see if any protective measures are required (see RADON).


Products from mineral resources

Sandstone

Sandstone is a sedimentary rock that varies widely in its properties and color. Often it contains a proportion of limestone, just as some types of limestone have a proportion of sandstone. Some sandstone is very hard-wearing and is used in paving, while other types are crumbly and need good detailing to prevent them weathering. Sandstone comes in many attractive colors, from red to yellowish brown.

Limestone

Limestone and chalk are the sedimentary remains of countless living organ isms from prehistoric seas. The sheer scale involved in the formation of these materials must surely rank as one of the ecological wonders of the natural world. Limestone is top-quality building stone, partly because it can be carved and cut with greater ease than most other building stones and partly because of its wonderful light honey color. However, it is attacked by acid rain and requires careful detailing to compensate for this. At present it is extracted without due care in many parts of Britain, especially in national parks. If quarried as it used to be, with small-scale extraction as close as possible to the place of use, there would be far less impact, and each area would have its own local supply. It is a stone that needs to be used with much greater care and respect.

Marble, a metamorphosed form of limestone, is often extracted with appalling environmental results, such as has happened in Italy. Britain has no indigenous supplies of marble, though certain decorative limestones, such as the shelly Purbeck Marble, are referred to as marble in the construction industry.

Slate

Slate is an excellent stone for shedding water and has the property of being easily split along the plane of its cleavage. High-quality slates can be recycled several times before becoming weakened through weathering. The different qualities of slate need to be used in different ways to make the most of their characteristics. For instance, the strongest and best-quality slates can be split quite thin and used where strength and lightness are paramount, whereas the weaker slates should be cut thicker or used in smaller sizes. Other slate is more appropriate for walling.

Slate is an excellent roofing material and lasts a long time, depending on the quality of the stone you use. Most of the slate used for roof shingles in the United States will last from 100 to 200 years, if properly maintained, while some high-quality slate from Welsh quarries has stayed in service for more than 1,000 years. Talk about resource efficiency!

The only real drawback to slate is its initial expense, typically between $200 and $1,200 per 100 square feet in the US, depending on the quantity purchased. Slate shingles also break easily and may slide off the roof if their roofing nails rust through. Periodic maintenance and patching will alleviate this problem.

If you have a slate roof and it is in need of renovation, it is worth finding out what you can about the type of slates originally used so that you can match them. It is also worth keeping a small stock of these slates for repairs as they arise. You can often tell where a particular slate has come from by its color: for instance Westmoreland slate is generally green and that of North Wales is dark blue.

Vernacular Character

Stone is one of the most important building materials for giving an area its own unique character. Here in Britain we think of the sandstone used in the Cotswold villages, the limestone of the Dales, and the flints of East Anglia. It is the use of these stones from local quarries that brings a great attractiveness to an area. However, if we are ever to use stone in the ways it was used so successfully in the past, we need to redevelop the skills of quarriers and stonemasons. There are probably few more interesting trades.

If you have a house where the stone has been covered over with cement or stucco, consider removing a portion of the stucco to discover the quality of the stone beneath. If it is of poor quality, then you can re-cover the stonework with an insulating layer. If, however, it is of reasonable quality, it may be worth having it repaired and cleaned. The detailing of less durable stone is important to give it protection from the weather and prevent frost damage. If a building or wall has an obvious weathering defect, it could either be the result of a natural weakness of the stone or evidence of a need to provide the appropriate protection, such as an overhang or coping stones to help shed water away from the face.

Much newly quarried stone is used for hardcore and for building up ground levels. If you require rubble or rough stone hardcore, consider using mine-stone, which is a by-product of mining. Artificial stone is a useful way of using up stone waste and is cheaper than stone; it is often simply concrete with a facing of stone dust mixed in.

SAND and GRAVEL AGGREGATES

There are large deposits of sand and gravel in both Britain and North America; huge quantities of these materials are extracted for the making of concrete. As with any raw material, it is important to draw up carefully thought-out ecological criteria for its extraction. We don’t usually give much thought to this precept unless these materials are being taken from a much- loved local beauty spot. Finding suitable deposits which do not result in environmental damage is becoming increasingly difficult, and is another reason for limiting the use of concrete where possible. However, with proper thought and landscaping, gravel pits can provide inland lakes for both recreation and wildlife. There may also be the potential for extending the proportion of aggregates that are dredged from the sea.

EARTH and CLAY

Earth and clay are among the oldest building materials used by humans, and today there is renewed interest in their use as building materials. Experiments are now taking place around the world to regain the experience necessary for their use in a modern context. There are many different types of subsoil and clay, the nature of which depends on the many variables involved, such as the type of rock from which the material was originally weathered, the local climatic conditions, and the past vegetation. These all have the effect of creating very different colors, textures, structures, and other properties; in addition, lime or clay is sometimes added to rammed earth to improve its cohesion.

As a building material, earth is abundant in most areas. It often requires little “working” if it is dug from the best deposits. When dry, it is strong yet flexible. If built thick enough, it has good thermal and sound-insulating properties. It can't rot and , if protected from rain and detailed properly, can last indefinitely. Its embodied energy is almost negligible.

In Britain wattle and daub was in use as an infill material until the 18th century in parts of the country; many examples still exist, often beneath plaster facing. What is not so well known is that rammed earth was a standard method of building in many villages until the beginning of this century Its vernacular name and method of construction varied from one part of the country to another: cob, witchert, pise, and clay lump were some of the names used. Some of the old earth construction methods are now being updated, as the prospects for using this material in areas that have suitable deposits is good.

Rammed-earth house construction is common to vernacular styles of building throughout the world, and it is also being reintroduced by some environmentally concerned builders today, particularly in parts of Australia and the US. The durability of an earthen wall is very dependent on careful detailing, which is designed to protect it from rain and ground moisture. It requires a good roof overhang and a damp-proof course. On the inside of a building, rooms can be finished with plaster, and on the outside with white wash or wooden siding. This type of construction might be particularly appropriate for a garden shed, garage, or garden wall.

ROCK ASPHALT

Rock asphalt is included here, although it falls between the categories of natural stone and processed minerals. Occurring naturally in various parts of the world, it is the result of oil having seeped to the earth’s surface and the volatile components having evaporated. After heating to make it liquid, It can be laid as a flooring material or applied vertically or horizontally as a waterproof sealant. It is an extremely resilient and hard-wearing material if used away from UV radiation. There are substitutes available for rock asphalt, such as those produced from oil or coal. The advantage of rock asphalt is that it is healthier to use inside the house than other similar products, since all of its volatile compounds have long since evaporated.

RAW-MATERIAL MINERALS

A large proportion of the minerals mined are for processing into substances that often look very different from the raw product. There are three basic categories.

• Minerals such as sand, earth, clay, limestone, and gypsum, which are used for making glass, bricks, lime, cement, and plaster.

• Metallic ores such as iron ore or bauxite, which are processed into steel and aluminum.

• Coal, oil, and natural gas, which are processed into all manner of chemicals, plastics, and paints.

The problems that relate to the extraction of these raw material minerals are largely the same as for those minerals whose problems of overexploitation, pollution, and environmental damage have already been addressed. It Is in the processing of some of these materials that even bigger problems arise; these are addressed in the next section.

PRIORITIES FOR ACTION

+ If you want to use stone in building applications, learn about your local stones and find out where the nearest working quarries are. It is difficult to predict how stone from a quarry will weather, so it is best to locate buildings known to have been supplied from a particular quarry, or, conversely, locate the quarry from which a favored or particularly durable stone came.

+ If you are using aggregates, find out where your local source is and how serious the environmental problems are at the relevant quarries. This in turn might help you decide between using these aggregates or finding an alternative material.

+ If you have a need for rubble or hardcore, avoid using quarried stone, especially limestone. There are many alternatives available, including minestone, the stone waste from the mining of coal and metallic ores. Better still, use recycled crushed concrete (artificial stone), broken bricks, or material from road resurfacing.

+ Consider using earth as a building material if you have an appropriate use, such as for a structure in your garden.

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