All of us like sparkle of gold jewelry studded with colorful stones like emerald, garnet, ruby, sapphire, etc. But what is emerald, gold, ruby or garnet? They are compounds of natural chemical elements and called minerals.

A mineral, generally crystalline, is an inorganic and natural substance that has a definite chemical composition. Most of the minerals are solid but some are found in gaseous (natural gas) and liquid (mercury) forms.

Properties

Physical characters / properties of minerals depend on arrangement of atoms and chemical components. Properties of minerals can be studied under the following sub-headings:

Sense based properties of minerals include feel, odor, and taste. Odor is typical smell of a mineral on breathing, heating, and rubbing. To define odor of minerals different terms are used such as alliaceous (smell of garlic), argillaceous (smell of clay), fetid (smell of rotten eggs), and sulphurous (smell of burning sulphur). Feel of a mineral on touching it can be soapy (serpentine) smooth, greasy (chlorite), harsh, rough and so on. Taste of minerals may be alkaline, astringent, bitter, cooling, saline and sour.

Heat (based) affects conductivity, optical properties and fusibility. Conductivity is ability to conduct heat and electricity. A blow-pipe flame is used to determine fusibility of a mineral. Fusibility is temperature at which minerals convert into liquid form. But fusibility is different from melting point of a mineral. For instance quartz fuses at 1600 oC. Von Kobell's following fusibility scale of six minerals is used to identify the minerals.

1. Stibnite (525 oC)
2. Chalcopyrite (965oC)
3. Almandine garnet ( 1, 200 oC)
4. Actinolite (1,296 oC)
5. Orthoclase (1,300 oC)
6. Bronzite ( 1,400+ oC)

Specific gravity refers to ratio of the mass of the unit volume (density) of a mineral to that of water at 4 oC.

Electricity, Magnetism and Radioactivity based features also vary from mineral to mineral. Magnetic minerals are generally rich in iron. However, all iron containing minerals may not be magnetic. Magnetite and pyrrhotite are highly magnetic minerals Almandine and chromite are moderately magnetic. Feldspars and quartz are non-magnetic minerals. If minerals develop electrical charges due to temperature changes, they are called pyroelectric minerals. The minerals developing electrical charges because of variation in stress are called piezoelectric. Radioactive minerals contain elements that decay over a period of time. These rare minerals occur as salts of thorium and uranium.

Color, degree of transparency, fluorescence, luster, phosphorescence, streak and optical properties depend on light. Minerals' color depends on absorption and reflection of rays of white light. Black minerals absorb all light rays and reflection is zero. White minerals reflect all light rays. Red minerals reflect red vibrations and absorb all other rays. Color variation in minerals of same group is common. For instance, minerals of quartz group may have pink (rose quartz) and purple (amethyst) varieties. Similarly, beryl comes in emerald (green) and aquamarine (blue) colors. Sometimes minerals show bands of different colors e.g. fluorite and tourmaline. Minerals containing aluminum (Al), barium (Ba), calcium (Ca), potassium (K), and zirconium (Zr) are generally light colored. Minerals having iron (Fe), manganese (Mn), chromium (Cr), cobalt (Co), nickel (Ni), vanadium (V) and copper (Cu) are dark colored. Play of colors can be seen in some minerals like quartz and diamond. Play of colors means ability to see different colors from different directions. Streak is color of mineral powder. It may vary from that of the solid mineral. To obtain it, scratch or rub mineral on a streak plate. Luster could be metallic (gold), vitreous (broken glass), resinous (resin), pearly (pearl), silky (silk, satin spar), and adamantine (diamond). If mineral surface reflects objects clearly like mirror, it is splendent. If surface is not very brilliant and reflection of objects is not clear, it is shining. Dull surface means no luster at all.

If outline of an object can be seen clearly in a mineral, it is transparent. In sub transparent minerals, objects are indistinct. Translucent minerals can transmit light but objects can not be seen through them. No light is transmitted through opaque minerals.

When minerals are heated, rubbed or exposed to UV light / radiation, they emit light. This process is called phosphorescence. Rubbing of quartz in dark room exhibits phosphorescence. Diamond and ruby display phosphorescence if exposed to X-rays.

When minerals exposed to electrical radiation, they emit light. This process is called fluorescence. For instance, fluorite emits light while exposed to this type of radiation.

Cohesion and elasticity define cleavage, fracture, hardness and tenacity of a mineral. Cohesion is a force that binds molecules and prevents separation of molecules. The force which brings back the molecules to their original state after a disturbance is called elasticity. Cohesion and elasticity vary from mineral to mineral. Cleavage is ability of crystalline mineral to split in a specific direction. As a result, almost smooth surfaces of minerals are exposed. Non-crystallized or amorphous minerals do not have cleavage. Fracture surface is irregular unlike cleavage plane. Tenacity refers to deformation of mineral on bending and crushing.

Atomic structure controls hardness, pseudomorphism and surface tension. Hardness is defined by Moh's scale of hardness. Talc is the softest (1) and diamond is the hardest mineral (10) on the scale. Minerals that occur in crystal forms of other minerals are called pseudomorph.

Rock Forming Minerals

Rocks are made up of various mineral groups. Silicate minerals are found in igneous rocks. SiO2 is the most common oxide in igneous rocks. Mineral formation during crystallization of magma depends on its composition and temperature. Silica, alumina and alkali rich magma crystallizes in alumino silicates. Magma rich in iron and magnesium produces ferromagnesian minerals.

Quartz and feldspar are commonly found minerals in sedimentary rocks. Quartz is compound of silicon and oxygen. Rock granite is composed of a number of minerals including quartz. Common minerals in metamorphic rocks are chlorite, epidote, staurolite, and garnet.

Importance of Minerals

Minerals, found in the earth's crust, are of great economic value and play important role in our daily lives. Rocks are repositories of many of commonly used minerals such as gold, silver, copper, iron ore, coal, oil, and natural gas. Gold and silver are extensively used for jewelry and various industrial processes. Oil and natural gas are called full minerals because they are essential for transport industry and cooking. Without them life would come to a halt.