What are Minerals?

A mineral is a naturally occuring homogenous substance, which has a more or less definite chemical composition and definite atomic structure. The minerals are usually formed by inorganic processes. They possess a set of constant physical properties. Since the determination of atomic structure and chemical composition requires complex laboratory tests, the more easily recognised physical properties are used in the identification of minerals in the field.

The minerals may be divided into two broad groups: (i) rock-forming minerals, and (ii) ore-forming minerals. Rock-forming minerals are those which are found in abundance in the rocks of Earth’s crust. Ore-forming minerals are those which are of economic value and do not occur in abundance in rocks.

Mineral Groups

There are six basic groups of minerals, based on their chemical composition.

1. Oxides: Quartz, Magnetite, Hematite, Limonite, etc.
2. Silicate: Felspars, Mica, Hornblende, Augite, Olivine, etc.
3. Carbonates: Calcite, Dolomite, Sidente, etc.
4. Sulfides: Pyrites, Galena, Sphalerite, etc.
5. Sulfates: Gypsum, etc.
6. Chlorite: Rocksalt, etc.

Over two thousand minerals are known to exist, but most of them are rare. The minerals that occur in common rocks are few in number. They are about 29. It is also interesting to note that only eight elements compose the bulk of these minerals and about 98% of the continental crust. These eight elements are Oxygen, Silicon, Aluminium, Iron, Calcium, Potassium, Sodium, and Magnesium. Out of these, the two most abundant elements are Silicon and Oxygen, which combine to form the mineral group known as the Silicates. The minerals which occur in common rocks can be divided into six groups as shown above.

Physical Properties of Minerals

Physical properties of minerals can be readily determined by inspection or simple tests. Because the physical properties are determined in hand specimens, they are important in the recognition of minerals in the field. The chief physical properties are colour, streak, lustre, hardness, habit, cleavage, fracture, odour, feel, tenacity, fluorescence, phosphorescence, magnetism, specific gravity, flamability, and crystal forms.

The correct identification of minerals is made with the help of a polarising microscope. This involves grinding the minerals or rocks into very thin slices and allowing polarised light to pass through them. In this way, their optical properties are studied, and the minerals are identified. Opaque minerals such as ores are studied under the ore-microscope in the reflected light. For a more detailed study of chemical properties of minerals please read >>>>>

Chemical Properties of Minerals

Each mineral has a crystalline structure and almost a definite chemical composition. The properties, which are related directly to the chemical composition of minerals, are isomorphism, polymorphism, and pseudomorphism. Each of the three properties is directly influenced by the chemical composition of the minerals.

1. Isomorphism. When variation in chemical composition takes place in any one mineral structure, the phenomenon is called “isomorphism”. A group of minerals related in this manner form an “isomorphic series”. These minerals show a continuous variation in their chemical composition, but their crystal structure remains almost the same. The plagioclase feldspars, which are a group of triclinic minerals, provide an example of isomorphism. In these minerals, there is the continuous substitution of (Na +Si 4+) for (Ca2 +Al3) from anorthite, CaAl2Si2O8, to albite, Na Al Si3 O8.
2. Polymorphism. The ability of a specific chemical compound to crystallise with more than one type of structure is known as “polymorphism”. In this case, each crystal form gives rise to a separate mineral species. Such minerals, which have identical chemical composition but different atomic structure, are called “polymorphs”. For example, polymorphs of carbon are graphite and diamond, and of CaCO3 are calcite and aragonite.
3. Pseudomorphism. If a mineral exists with the outward form of another mineral species, the phenomenon is called “pseudomorphism”. Mineral pseudomorphs are formed when one mineral is replaced by another without any change in the outer form of the original mineral. Thus, the chemical composition and structure of a pseudomorph belong to one mineral species, whereas the crystal form corresponds to another. A common example of pseudomorph is a piece of fossil wood where wood fibres have been replaced by silica. Another example is quartz (SiO2) after fluorite (CaF2).

For a more detailed study of the chemical properties of minerals, please read >>>>>

Optical Properties of Minerals

Optical properties of minerals are important for their identification. Optical properties are determined with the help of a “polarising microscope”.

• Ordinary Light. Ordinary light travels in straight lines with a transverse motion. It vibrates in all directions at right angles to the direction of propagation.
• Polarised Light. When the vibrations of the wave motion are confined to a single plane only, the light is called “polarised light. The plane along which such vibrations take place is called “plane of polarisation”. There are three ways by which the polarised light can be obtained: (i) by double refraction, (ii) by absorption, (iii) by reflection. Nicol prisms and polaroids are used in the microscopes to produce polarized light.

Refractive Index of Minerals

When a ray of light passes from air into a denser medium such as glass, it gets refracted. In the glass, the light travels with a lesser velocity than in air and deviates from its previous path. The amount of deviation depends on the angle of incidence and the relative velocity of light in the two media.

The refractive index (n) of a mineral can be expressed as a ratio of the velocity of light in air (V1) and its velocity in the mineral (V2).

n = V1/V2

The velocity of light in air is generally considered equal to 1; therefore, n becomes equal to 1/V2. This shows that the refractive index of a mineral varies inversely with the velocity of light in it.

For a particular mineral, the relationship between the angle of incidence (i) and the angle of refraction (r) is given by Snell’s law. This law states that the ratio of the sine of the incident angle to the sine of the refracted angle is a constant. This constant is called the refractive index (n) of the mineral.

n=sin i/sin r