Minerals occur naturally in the rocks of the earth’s surface. They may be either elements or compounds. An element is a single, pure substance. Gold is sometimes found as nuggets of the element. A compound is two or more substances bound together in such a way that chemical processes are necessary to separate them. Iron is usually found as a compound. Most rocks are mixtures of several minerals.
Native elements and ores
Arsenic, copper, iron, gold, silver and sulphur are all elements that may occur by themselves. Geologists call them native elements. Most minerals are compounds of several elements. Geologists have discovered about 20000 minerals, of which only about 100 are common.
The two main groups of minerals are metallic and non-metallic. Metallic minerals contain native elements and ores from which minerals can be extracted. Examples of extracted; bauxite (aluminum); galena (lead and silver); malachite (copper); pitchblende (uranium) and pyrites (iron and sulphur). Non-metallic minerals include graphite, gypsum, halite (rock salt), quartz, talc and diamond.
Identifying minerals
The best way to identify minerals is by chemical analysis in a laboratory, but geologists have devised some simple tests which can be carried out in the field to provide the first clues to the presence of minerals. Laboratory tests are then carried out later on samples. Field tests to identify minerals use colour and streak, lustre, hardness, crystal forms and cleavage.
Colour can sometimes be misleading, but experienced geologists can often make a good guess based on colour. Azurite, for example, is one mineral that is always blue.
The steak of a mineral is the colour produced by scratching the surface to get a small amount of powder, which is often a different colour from the solid mineral.
Lustre is the way the mineral reflects light and with experience it is possible to identify, for example, diamonds, which have a brilliant lustre; quartz, which has a glassy or vitreous lustre; and some other minerals which have a metallic lustre.
Hardness is tested in the field by trying to scratch one mineral with another or with a metal instrument like a knife of file. Geologists use a scale of hardness called Mohs scale to compare and measure hardness in the field. Mohs’ scale lists ten common minerals, from talc, which is very soft, up to diamond, which is extremely hard. A mineral high on the scale will scratch all those below it but not the one above it, so it is possible to grade the hardness of mineral samples in this simple way.
Different minerals have different crystals, and large crystals are easy to identify because each mineral has its own distinctive form. Some minerals, like galena and halite, form a cube; some, like zircon, form twelve-sided crystals; some, like quartz, form long hexagonal crystals.
Cleavage is the way a mineral splits or flakes. Some minerals split easily, others do not. Cleavage runs parallel to the faces of the mineral crystal and is therefore not the same as fracture, or breaking, which may run in any direction in any mineral.
Searching for minerals
Many geologists work as prospectors for mining companies, or governments. They search for water, metals, coal, natural gas, petroleum, uranium and other valuable minerals such as lead, zinc and copper.
The early prospectors lead lonely lives and suffered many hardships in their search for gold and other valuable minerals. Their simple equipment consisted of picks, shovels, and large, flat, sieve-like pans for ‘washing’ earth and minerals.
The modern prospector is usually a geologist trained in chemistry and physics. In the search for valuable metals, minerals, fuels and even water, he will use a wide range of scientific tools. Aerial photographs and pictures transmitted back from satellites are used to show up features of the terrain, or earth’s surface, which cannot easily be seen from the ground. By studying the fossils, prospectors can tell the age of rocks; by detonating explosives in the ground and measuring the shock waves with instruments called seismographs, they can tell the density of the rock. For locating radioactive substances, such as uranium, they use Geiger counters, which respond to the amount of radioactivity in the area by clicking.
Modern prospecting methods are particularly important today in the search for more oil and one of the under-ground structures of particular interest is an anticline. This is an upfold caused by pressure in subterranean layers of rock. Layers of rock are pushed into shapes like arches or domes and caverns can form inside them. Natural gas and petroleum are often found under these domes.
Native elements and ores
Arsenic, copper, iron, gold, silver and sulphur are all elements that may occur by themselves. Geologists call them native elements. Most minerals are compounds of several elements. Geologists have discovered about 20000 minerals, of which only about 100 are common.
The two main groups of minerals are metallic and non-metallic. Metallic minerals contain native elements and ores from which minerals can be extracted. Examples of extracted; bauxite (aluminum); galena (lead and silver); malachite (copper); pitchblende (uranium) and pyrites (iron and sulphur). Non-metallic minerals include graphite, gypsum, halite (rock salt), quartz, talc and diamond.
Identifying minerals
The best way to identify minerals is by chemical analysis in a laboratory, but geologists have devised some simple tests which can be carried out in the field to provide the first clues to the presence of minerals. Laboratory tests are then carried out later on samples. Field tests to identify minerals use colour and streak, lustre, hardness, crystal forms and cleavage.
Colour can sometimes be misleading, but experienced geologists can often make a good guess based on colour. Azurite, for example, is one mineral that is always blue.
The steak of a mineral is the colour produced by scratching the surface to get a small amount of powder, which is often a different colour from the solid mineral.
Lustre is the way the mineral reflects light and with experience it is possible to identify, for example, diamonds, which have a brilliant lustre; quartz, which has a glassy or vitreous lustre; and some other minerals which have a metallic lustre.
Hardness is tested in the field by trying to scratch one mineral with another or with a metal instrument like a knife of file. Geologists use a scale of hardness called Mohs scale to compare and measure hardness in the field. Mohs’ scale lists ten common minerals, from talc, which is very soft, up to diamond, which is extremely hard. A mineral high on the scale will scratch all those below it but not the one above it, so it is possible to grade the hardness of mineral samples in this simple way.
Different minerals have different crystals, and large crystals are easy to identify because each mineral has its own distinctive form. Some minerals, like galena and halite, form a cube; some, like zircon, form twelve-sided crystals; some, like quartz, form long hexagonal crystals.
Cleavage is the way a mineral splits or flakes. Some minerals split easily, others do not. Cleavage runs parallel to the faces of the mineral crystal and is therefore not the same as fracture, or breaking, which may run in any direction in any mineral.
Searching for minerals
Many geologists work as prospectors for mining companies, or governments. They search for water, metals, coal, natural gas, petroleum, uranium and other valuable minerals such as lead, zinc and copper.
The early prospectors lead lonely lives and suffered many hardships in their search for gold and other valuable minerals. Their simple equipment consisted of picks, shovels, and large, flat, sieve-like pans for ‘washing’ earth and minerals.
The modern prospector is usually a geologist trained in chemistry and physics. In the search for valuable metals, minerals, fuels and even water, he will use a wide range of scientific tools. Aerial photographs and pictures transmitted back from satellites are used to show up features of the terrain, or earth’s surface, which cannot easily be seen from the ground. By studying the fossils, prospectors can tell the age of rocks; by detonating explosives in the ground and measuring the shock waves with instruments called seismographs, they can tell the density of the rock. For locating radioactive substances, such as uranium, they use Geiger counters, which respond to the amount of radioactivity in the area by clicking.
Modern prospecting methods are particularly important today in the search for more oil and one of the under-ground structures of particular interest is an anticline. This is an upfold caused by pressure in subterranean layers of rock. Layers of rock are pushed into shapes like arches or domes and caverns can form inside them. Natural gas and petroleum are often found under these domes.
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