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All rocks are made up of individual substances called minerals. Most minerals are inorganic in origin. The common minerals are mostly composed of silica and oxygen. There are a few rocks composed of one mineral, pure quartz, for example, but many rocks are made up of more than one mineral.
Minerals have a regular crystalline structure of chemical elements and are formed in three ways: molten magma cools and its minerals crystallize at different rates; a gas changes to a solid; a mineral/water solution evaporates to leave crystals.
All crystals are composed of atoms locked together in a certain pattern. The smooth faces of crystals meet in definite angles. Many minerals can have the same chemistry but different crystals because the atoms are linked differently. For example, diamond and graphite are both made of pure carbon, but have different crystal structures. Diamonds are hard while graphite is soft. Scarcely any two crystals seem alike, and there are six types of crystal systems: isometric, tetragonal, hexagonal, orthorhombic, monoclinic and triclinic. The systems differ from one another in the length and arrangement of the imaginary axes that runs through them.
Minerals can be classified, according to their chemical composition, into nine groups (or families): native elements (with alloys, carbides, nitrides, phosphides); sulfides (with selenides, tellurides, arsenides, antimonides and bismuthides); halides; oxides and hydroxides; nitrates, carbonates and borates; Sulfates (with chromates, molybdates and wolframates); phosphates, arsenates and vanadates; silicates; organic substances.
There are seven physical properties used when recognizing minerals: lustre, hardness, colour, streak, cleavage, fracture, and specific gravity.
Light is reflected from the surfaces of minerals in various ways, which produces many different types of lustre. All minerals can be classified in two distinct groups: metallic or non-metallic lustre. Minerals with a metallic lustre are opaque and have a streak darker in colour than the mineral itself. On the other hand, non-metallic minerals become transparent on a thin edge and produce a streak lighter than that of its original colour.
One of the most important properties of a mineral is its hardness, which is a measure of its resistance to abrasion or mechanical damage of any kind. No two minerals have exactly the same resistance, but many are so nearly alike that Friedrich Mohs (a German mineralogist and superintendent of the Imperial Cabinet at Vienna from 1826) recognized the need for a hardness scale. Mohs scale of hardness places minerals of a scale of 1 to 10. The scale does not indicate any exact hardness; so 9 is not three times as hard as 3. It just means that any mineral can scratch all those below it.
Most minerals are coloured by chemical elements that are really minor impurities. They produce a bewildering array of colours without changing the essential composition. Many minerals have a tendency to tarnish, so a freshly broken surface may have to be exposed to reveal the true colour.
Streak is the mark made when a mineral is rubbed across a piece of white unglazed porcelain; this porcelain is also known as a "streak plate." The rough surface turns a part of the mineral into very fine powder, whose colour may be different from that of the mineral itself and much more characteristic. Some hard minerals must be pounded with a hammer to produce a powder.
All crystalline minerals cleave or have cleavage when they break along smooth surfaces in definite directions. The specific pattern of atoms in layers, whose cohesion differs from one to another, produces this feature. Cleavage is described according to the crystal direction and how easily it is obtained.
The manner in which minerals break in irregular directions is described as a fracture. The type of fracture depends on the appearance of the new surface. For example, a series of arcs, like a shell, is called a conchoidal fracture. Other samples of fracture include hackly, even, uneven and earthy.
The weight of a substance in proportion to the weight of its own volume of water is its specific gravity. A few minerals weigh less than water, but others are much heavier; their average weight is about 2.6 times as much as water. That means their specific gravity is 2.6.
As well as these physical properties, there are many other valuable clues for identifying minerals.
Many minerals, for example, belonging to the carbonate class will fizz in a 10% solution of hydrochloric acid, a reaction involving the escape of carbon dioxide gas. Some carbonates are: azurite, calcite, magnesite, malachite and rhodochrosite.
Flexible minerals can be bent by your fingers and will stay that way until otherwise moved. Chlorite is a flexible mineral. In contrast, elastic minerals will snap back into position unless they are actually broken. Typical examples of this property are the mica minerals.
Malleable minerals are those which can be hammered flat without falling to powder. Some native metals, like gold, silver and copper, are malleable. Sectile minerals, on the contrary, will be demolished if hammered, but will not powder if cut with a knife. Gypsum is a sectile mineral.
Minerals that can be attracted by a common horseshoe or bar magnet are magnetic minerals. Hundreds are magnetic after being heated or tested with an electromagnet. Only two common minerals, magnetite and pyrrhotite, can be picked up before such treatment.
Some minerals possess a property that causes them to glow. When fluorescent minerals are exposed to invisible radiations, such as ultraviolet light, X-rays, and cathode rays, they will glow in the dark. A practical source for such radiation is an ultraviolet (long-wave or short-wave) lamp. Even after the lamp is turned off though, some minerals tend to continue glowing. They are described as phosphorescent. Striking or heating other minerals can also cause this luminescence effect.
Substances with radioactive characteristics spontaneously break down at a characteristic rate. They radiate energy in the form of rays or particles as their atoms disintegrate and change to other elements before becoming stable. The majority of highly radioactive minerals are brightly coloured and can be detected with various types of equipment. The most commonly used instrument is the Geiger counter. A tick, a flashing light, or a dial shows the detection of the invisible radiation. The dangers of radioactivity require special handling (lead shielding, decontamination) when large quantities of such materials are being worked with or stored. The most common radioactive minerals contain the radioactive minerals uranium or thorium.
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