Mineralogy: It is a subject of geology that the scientific study of minerals and their structure, physical (including optical) properties, and minerals artifacts.
It has some sub-division –
1) Chemical mineralogy
2) Physical mineralogy
3) Optical mineralogy
4) Descriptive mineralogy
An
Crystallography.
Mineral: Minerals are naturally occurring inorganic homogenous solid that has a definite chemical composition and definite internal atomic structure.
Over 2000 minerals exist on the earth, but most of them are rare. The interesting fact is that only eight elements compose the bulk of the minerals and about 98% of the earth's crust. These elements are- Oxygen, aluminum, magnesium, Sodium, Potassium, iron, calcite, and especially silica. The two most abundant elements, Oxygen, and silica when combined together, then a new type of minerals are formed. These minerals are known as silicate minerals. The structure of silicate minerals contain SiO4 tetrahedral, either isolated or joined through one or more oxygen atom to form group, chain, sheet and three-dimensional structures with metallic elements. On the basis of the different ways the silicon tetrahedron are grouped, the silicate structures can be classified into six types-
1) Nesosilicate: These are independent or isolated SiO4 tetrahedral that bounded to each other by ionic bonds through interstitial cations. These structures depend chiefly on the size and shape of the interstitial cation.
In Neso silicate structure, the unit composition is (SiO4) ^4-. Here net charge on SiO4 is -4 (Si has four positive and each oxygen has two negative valances) that can be balanced by two divalent cations. Such as Mg2+, Ca2+, ba2+, etc. Here the ratio of silica and oxygen is 1:4 these structures are also known as Orthosilicates structures.
2) Sorosilicate: This is a double tetrahedral group of silicate that forms when two SiO4 groups of tetrahedra join together by sharing a single oxygen atom. Here the unit composition is (Si2O7) ^6-. The net charge on (Si2O7) is -6 that can be balanced by either two trivalent (Fe3+, Al3+, etc.) cations or three divalent (Mg2+, Ba2+, Ca2+, etc.). Here the ratio of silica and oxygen is 2:7. These silicate structures are also known as Pyrosilicate.
3) Cyclosilicate: these types of silicates are formed when each SiO4 tetrahedra shares its two oxygen atoms with neighboring SiO4 tetrahedra. They may be linked into rings and hence they are also known as ring structures. Here the unit composition is (SiO3) ^-2. The ratio of silica and oxygen is 1:3. The net charge on (SiO3) is -2 that can be balanced by two divalent cations like Mg2+, Ca2+, Ba2+, etc. In Cyclosilicate, there are three possibilities to exist-
a) Each of the three tetrahedra shares one oxygen atom.
b) Each of the four tetrahedra shares one oxygen atom.
c) Each of the six tetrahedra shares one oxygen atom.
4) Chain structure: Chain structure forms when SiO4 tetrahedra groups are joined together vertically. Chain structures extend indefinitely. These structures are also known as ‘Ino-silicate’. There are two types of chain structure-
a) Single-chain structure: Single-chain structure forms when SiO4 tetrahedra groups are joined together vertically by sharing two oxygen of each SiO4 group. Here the unit composition of it is n(Si2O6). The net charge on it is -4 which can be balanced by two divalent cations, like Mg2+, Ca2+, Ba2+, etc. The Ratio of silica and oxygen is 2:6.
b) Double chain structure: Double chain structure forms when Alternate SiO4 tetrahedra groups are arranged in two parallel ways and these chains are extended indefinitely in the c-crystallography axis direction.
5) Sheet structure: Sheet structures are also known as Phyllosilicate. When each SiO4 tetrahedra group shares three of its oxygen atom with the neighboring SiO4 tetrahedra group, then the sheet structure is formed. These structures are extended indefinitely. The ratio of silica and oxygen is 4:10. This is the fundamental unit of all mica and clay structures.
6) Tectosilicate: The Tectosilicate structures are also known as 3-dimensional structures. These structures are formed when four oxygen atoms of each SiO4 tetrahedra group shares with another SiO4 tetrahedra group. Here the ratio of silica and Oxygen is 1:2. Members of feldspar, Feldspathoid, quartz, Zeolite, etc. show this type of silicate structure.
The physical property of minerals: Physical properties of minerals are the unique properties of minerals through which minerals can be classified and characterized. Color, Streak, Luster, Hardness, Fracture, Cleavage, Transparency, Specific gravity are examples of it.
Color: Color of the minerals due to absorption of certain wavelength by atoms making up of crystal. Some minerals possess characteristic and fairly constant color. Such as brass-yellow of pyrite, green of calcite, lead-grey of galena, etc. But the other cases, the color of the minerals is variable, such as quartz and it cannot be relied on as a guide to determine the color of the minerals. The presence of impurities can change the color of the mineral. Hematite is the most coloring impurities and it imparts red color by other minerals like feldspar, jasper, calcite, etc. Some minerals when viewed from a different angle then it shows an irregular change in color tint, which is known as the play of color. The term ‘opalescence’ is used for the minerals, which show a milky appearance.
Streak: Streak is nothing but the powder colors of the minerals that form when a mineral rubs against unglazed porcelain plates known as a streak plate. Sometimes minerals are not present in their original color due to the presence of impurities then the study of streak helps us to determine it. For example, hematite appears almost black in color but it gives a red color streak.
Luster: It is important physical properties of minerals that may be defined as how lights are reflected on the surface of minerals. There are various types of luster-
1) Metallic luster: If a mineral reflects light like metal or shine like metal then the mineral is called to have a metallic luster. For example- gold, iron.
2) Sub-metallic luster: if a mineral imperfectly reflects light or shine then the mineral is called to have sub-metallic luster. For example-columbine, psilomelane.
3) Vitreous luster: if a mineral reflects light or shines like broken glass then the mineral is called to have vitreous luster. For example-Quartz
4) Pearly luster: if a mineral reflects light like pearl or shine like pearl then the mineral is called to have pearl luster. For example-Talc, muscovite
5) Dull or earthy luster: if minerals do not show any kind of lusters then the minerals are called to have dull or earthy luster. For example-Kaolinite.
Hardness: Hardness may be defined as the resistance of minerals to scratching. It can be determined by rubbing a mineral of unknown hardness against one of the known hardness minerals, which is present in a specific hardness scale known as ‘Mohr’s hardness scale’.
Habit: habit may be defined as the size, shape and structure, or form shown by crystal aggregates and cryptocrystalline mass.
1) Acicular: Minerals show needle-like crystals. For example- natrolite.
2) Bladed: minerals show blade-shaped crystals. For example- kyanite.
3) Granular: minerals show aggregates of crystals. For example- chromite.
4) Cubic: minerals show cube-like crystals. For example-
5) Columnar: minerals show columnar crystals. For example- calcite.
Cleavage: cleavage may define as the tendency of a mineral to break more easily with a smooth surface along the planes of weak bonding.
Fracture: Minerals, which do not exhibit cleavage, break with irregular surfaces. The nature of these broken surfaces is called a fracture.
Specific gravity: Specific gravity may be defined as the ratio of the weight of a mineral to the weight of an equal volume of water at room temperature. Specific gravity depends on its atom constituents and their packing in the unit cell.
There are various methods to the determination of specific gravity-
1) Walker’s Steel Yard: It is an instrument that uses to determine the specific gravity of comparatively large mineral specimens.
The formula for determining the specific gravity of minerals specimen through walker’s steelyard is- w1/w1-w2. Here w1 is the weight of the mineral in air and w2 is the weight of the water.
2) Jolly spring balance: It is an instrument that uses to determine the specific gravity of small fragments.
The formula for determining the specific gravity of minerals specimens through jolly spring balance is – b-a/b-c. Here ‘a’ is the initial reading.
Comments
Post a Comment