Ministry of Lands and Mineral Resources Fiji Mineral Resources Department MINERALS

WHAT ARE MINERALS?

The rocks which form the Earth, the Moon and the planets are made up of minerals. Minerals are chemical elements or compounds which occur naturally within the crust of the Earth. They are the discrete crystalline particles of which nearly all rocks are made. Minerals are solid substances made up of atoms with an orderly and regular arrangement, which is the basis of their crystalline state. Because of their orderly atomic arrangement it is also possible to express the composition of a mineral as a chemical formula.

Minerals are inorganic substances (not derived from living things) composed of elements such as silica, oxygen, aluminium, iron, etc. Fossil fuels such as coal, oil and natural gas are organic (formed from once-living matter) and hence are not minerals in the strict sense.

Minerals provide the elements essential to life, the metals of industry and the materials for building (Tables 2-7). The calcium and phosphorus in bones and the iron in blood are made available to the body through plants which extract these elements from minerals in the soil.

Mineralogists (geologists who study minerals) have identified nearly 3000 minerals. About 100 of these are regarded as common minerals and less than 20 can be found everywhere.

Nearly all rocks are composed of minerals (some contain glass or organic matter) but it is rare to find good mineral specimens. These tend to occur in gaps, cavities and fractures within the rocks where the crystals have been unobstructed (have much free space) during their growth. Rocks can be formed from a combination of several different minerals or a single mineral can make up the bulk of a rock, for example, limestone or marble is mainly composed of the mineral calcite (CaCO3).

GENERAL CHARACTERISTICS

Many minerals are found as beautiful crystals. Crystals are formed when minerals are free to grow without constraint. Crystal faces reflect the regular atomic arrangement of the mineral. The incredibly regular arrangement of crystals has been recognised for about 200 years but the internal structure of minerals has only been determined this century by the use of X-rays. The regularity of crystals is due to the angle between the same two faces of all crystals of the same mineral species being constant, even though the faces may look variable in size and shape. This crystal structure determines the physical properties of minerals. Crystals are symmetrical and can be grouped according to their symmetry into seven crystal systems. The study of crystals and crystal systems is called crystallography.

Minerals vary in colour and in the way they look and feel. Some minerals are hard and sparkle and shine like glass. Others are slippery and soft or hard and scratchy. The way minerals look depends on their interaction with light, and these optical properties can be used to classify and identify minerals. These include properties such as transparency, reflection and refraction, lustre, colour, streak (colour of the powdered mineral) and fluorescence.

Minerals can be classified according to their hardness as described by Moh's Hardness Scale (Table 1). In 1812, F. Mohs arranged ten minerals in order of hardness so that each mineral can be scratched by the one following it on the scale. Thus talc is the softest and diamond is the hardest.

Table 1. Moh's Scale of Hardness

Minerals also occur as aggregates of crystals that rarely show perfect crystal shapes. The form of the aggregate can be useful for identification, i.e. whether they are fibrous, dendritic, lamellar or foliated, etc.

The density or specific gravity of a mineral depends on several factors including the kind of atoms in the structure and how closely they are packed. For example, the minerals tridymite and quartz are both silica (SiO2) but quartz, the closely packed form, has a specific gravity of 2.65 at room temperature whereas tridymite, with a more open structure, has a specific gravity of 2.26 at room temperature.

Other properties that mineralogists use when identifying and classifying minerals are related to how they break i.e. cleavage (breaks along a planar surface related to the structure and parallel to a possible crystal face) and fracture (broken surface is irregular), magnetic, electrical and radioactive properties.

The composition of a mineral can be expressed as a chemical formula which is a simple way of expressing mineral chemistry. The mineral chemistry can be used to further classify minerals. Silicate minerals (combinations of silicon and oxygen with other elements) are the most abundant rock-forming minerals. Silicate minerals that commonly occur in the rocks of Fiji include feldspar, mica, olivine, pyroxene and amphibole. Quartz is simply a combination of oxygen and silicon, without other elements.

TYPES OF MINERALS

There are two types of minerals: metals and non-metals, and they have different properties as outlined below.

Metals

• Strong, can support heavy loads without breaking.

• Conduct heat and electricity.

• Opaque - do not allow light to pass through.

• Malleable - can be bent without breaking.

• Ductile - can be stretched into wire.
  

Non-Metals

• Weak, shatter when hit with a hard object.

• Most do not conduct heat and electricity.

• Can be transparent, translucent or opaque.

• Not malleable.

• Not ductile.
  
  

ORIGIN OF MINERALS AND ROCKS

Most minerals crystallise from some sort of solution. Some of the ways minerals can form are from :

(i) cooling of magma (complex solutions of molten rock materials within the earth) or lava (complex solutions of molten rock material that reach the Earth's surface);

(ii) cooling of subsurface solutions involving hot water or hot gases, including steam, as in many mineral veins and metallic ore deposits;

(iii) hot vapours condensing to form minerals, such as sublimates of sulphur near volcanic vents;

(iv) chemical reactions with previous minerals, such as the hydrothermal alteration of feldspar to mica, or the oxidation of iron-bearing minerals in the zone of chemical weathering at the earth's surface;

(v) replacement or substitution of an earlier-formed mineral;

(vi) recrystallisation of earlier-formed minerals to form new compounds under changed temperature and pressure conditions; or

(vii) evaporation of water solutions.

ORE MINERALS

Many minerals are useful to us. A useful mineral from which a valuable element can be extracted is called an ore mineral. Ore is a rock that has enough concentration of one or more such minerals to make it profitable to mine it. Mineral ores are the source of all the metals and many other substances that we use every day (Table 5). To obtain these Earth resources we have to take them out of the Earth by mining and then change them into different substances. Ore also contains economically useless gangue minerals which have to be removed before the ore minerals can be concentrated.

USES OF MINERALS

Minerals have always been used by humans in almost every facet of daily living. Primitive peoples who used rocks to form weapons and other useful tools in the Stone Age. Then, as people discovered the methods of isolating metals from their mineral ores, "technology" and "civilisation" advanced successively through the Copper, Bronze, Iron, Steel and Atomic Ages. At each step, minerals assumed progressively greater importance.

The tables below (Tables 2-7) represent some of the common minerals and their many uses in our everyday lives.

Table 2. Uses of Some Common Metallic Minerals

Table 3. Uses of Some Non-Metallic Minerals

Table 4. Minerals Used in a School Room

Table 5. Minerals Used in a Bathroom

Table 6. Minerals used in Sporting Equipment

Table 7. Minerals Used in Furniture, Fittings and other Equipment

NECESSITY OF MINERALS

Today, mineral resources play a role second in importance only to that of agriculture, and there are thousands of uses for the various materials of the Earth. There isn't much that one can think of that doesn't come from some sort of mining process and it can honestly be said that:

"If it's not grown, it's mined"!

Virtually no aspect of our lifestyle could be sustained without minerals. Minerals are used in every facet of our daily living from food production and processing, building, providing shelter/housing to transport, communications, medicine and leisure activities.

EXPLORATION AND MINING

Exploration is the search for mineral deposits containing sufficient ore to be economically mined. Geologists use all available geological and geophysical tools to assist exploration. Ore minerals are extracted from the Earth's surface and from underground by mining. The ore is then milled, (crushed), processed and refined to produce the metal or non-metallic product.

FIJI'S MINERAL WEALTH

Fiji's mineral wealth includes gold which is mined at Vatukoula and provides the fourth largest source of foreign exchange. There are plans to open gold mines at Mt Kasi in south-west Vanua Levu and at Tuvatu near Nadi. Advanced exploration is also currently underway at Namosi for copper-gold, at Wainivesi for marble and in other parts of Fiji for gold. In addition, there is quarrying of rock and sand for road metal, concrete aggregate and landfill and of coral sand for cement manufacture. Clay is used by many villagers to make pottery.

SUGGESTIONS FOR FURTHER READING

Allison, I. S. & Palmer, D. F. 1980. Geology, the science of a changing Earth. VII Edition. McGraw-Hill Inc.

Cox, K. G., Price, N. B. & Harte, B. 1974. An Introduction to the Practical Study of Crystals, Minerals and Rocks. Rev. 1st ed., John Wiley & Sons Inc., New York.

Hamilton, W. R., Woolley, A. R. & Bishop, A. C. 1984. The Hamlyn Guide to Minerals, Rocks and Fossils. The Hamlyn Publishing Group Ltd, London.

Lye, K. 1979. Minerals and Rocks, (Kingfisher Guides), Ward Lock Ltd., London.

ISSN 1016-2135