Menu

Search results for «minerals»

Extinction Direction of the minerals during optical analysis

When vibration directions of an anisotropic mineral coincide with those of the polarizer and analyzer, the mineral appears dark. This position of an anisotropic mineral is called ”position of extinction”. During a complete rotation of the microscope stage, the position of extinction occurs four times at 90° intervals. In extinction position, the light from the polarizer …

Examination of Minerals in Cross Polars

Examination of Minerals in Crossed Polars: When the analyzer is placed in the position with its plane of polarization at right angles to that of the polarizer, the polars are said to be ”crossed”. If a section of a doubly refracting mineral is placed between crossed polars, the light is doubly refracted and polarized three …

Examination of Minerals under Polarized-Light-Microscope

First of all, we need to prepare a thin section of mineral or rock with the help of a grinder, and then get it polished from its one side. This slide is then cemented to a slip of glass with Canada balsam (n=1.54). Canada balsam is a y-+ellowish resin obtained from the balsam fir and …

Optical Indicatrix of Minerals

In anisotropic minerals, the refractive index varies with the crystal direction. This relationship can be illustrated with the help of a geometrical figure called the ”optical indicatrix”. The optical indicatrix of an anisotropic crystal is an ellipsoid whose semiaxes are proportional to the refractive indices in the respective directions. In the crystals of cubic system …

Biaxial minerals

Crystals belonging to orthorhombic, monoclinic, and triclinic systems have two optic axes and therefore they are called ”biaxial”. These optic axes intersect each other to form acute and obtuse angles. The acute angle between the two optic axes is called the ”optic axial angle” and is commonly designated as 2v. The plane containing the two …

Uniaxial minerals

The crystals of the tetragonal and hexagonal systems possess only one optic axis, which is parallel to the c-axis. For this reason, they are called ”uniaxial”. The optical characters of the uniaxial minerals may be summarized as follows: When a ray of light passes through a uniaxial crystal in any direction other than the optic …

Isotropic and anisotropic minerals

Depending upon the optical properties, the minerals can be divided into two groups: (i) isotropic minerals and (ii) anisotropic minerals. Isotropic Minerals: In crystals belonging to the cubic system, the light travels with the same velocity in all directions and therefore each mineral has only one refractive index. Such minerals are called ”isotropic minerals”. Non-crystalline …

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 …

Optical Properties of Minerals

The optical properties of minerals are important for their identification. Optical properties are determined with the help of a ”polarizing microscope”. The study of optical properties of minerals is known as Optical Mineralogy. With the help of optical mineralogy, we study the minerals and rocks with the help of their optical properties. The rocks and …

Zinc and Lead minerals

Minerals enriched with lead and zinc in their chemical composition are said to be Zinc and Lead Minerals. Some of the important minerals that contains zinc and lead are as follow: (1). Galena: PbS Physical properties of Galena: Crystal Systems: Galena is found in the cubic crystal systems. Cleavage of Galena: perfect cubic cleavage (cubic …

Copper minerals

Copper minerals are a group of minerals, which contain the copper element in their chemical composition. Here are some important copper minerals; (1). Azurite: Cu3(CO3)2(OH)2 Physical Properties: Crystal System: monoclinic Cleavage: Perfect prismatic cleavage Hardness: 3.5 – 4. Specific Gravity: 3.77 Lustre: Vitreous Colour: Azure-blue Streak: pale-blue Habit: Crystalline. Also in radiating groups or massive. …

Manganese Minerals

(1). Braunite: Mn2+Mn3+6[O8|SiO4] Braunite is a silicate mineral, which contains both di- and tri-valent manganese. Common impurities include iron, calcium, boron, barium, titanium, aluminium, and magnesium. Physical Properties of Braunite: Crystal System: Hexagonal Cleavage: Braunite has no cleavage Fracture: Uneaven Hardness: 6 – 6.5 on Moh’s scale Sp.Gr: 4.75 – 4.82 Lustre: submetallic lustre Colour: …

Iron Minerals

(1). Magnetite: Fe3O4 Physical properties of Magnetite: Crystal System: cubic Cleavage: Absent Hardness: 6 in Moh’s scale Sp.Gr: 5.18 Lustre: metallic to dull Colour: Iron Black Special Character: Strongly magnetic. A natural magnet is called ”Iodestone”. Habit: Octahedral crystals, massive or granular. Occurrence of Magnetite: Magnetite is a high-temperature mineral, which occurs as an accessory …

Alluminium Minerals

The minerals which contain aluminium element are known as the Aluminium Minerals. Some of the important aluminium minerals are as under: (1). Bauxite: Al2O3 Bauxite is a mixture of hydrous aluminium oxides. The physical properties of Bauxite are as under; Physical Properties: Crystal System: Bauxite is a mixture of several minerals such as gibbsite,diaspore, and …

Calcium Minerals

Calcium Minerals: Calcium minerals are those minerals, which, somehow, contains calcium in their chemical composition. Some of the calcium minerals are as under: (1). Calcite: CaCo3 Physical properties:  Crystal System: Hexagonal Cleavage: Calcite has perfect cleavage. Hardness: Calcite is a soft mineral, which has a hardness of 2 at Moh’s scale. Specific Gravity: The specific …

Al2SiO5 (Polymorphs) minerals

Al2SiO5/Polymorphs Group of Minerals: The three important minerals of this group are ; Andalusite Sillimanite Kyanite These minerals occur in metamorphosed luminous rocks, such as pelitic schists. (1). Andalusite (Al2SiO5): Physical properties of Andalusite: Crystal System: Andalusite occurs in orthorhombic crystal form. Cleavage: Andalusite has a poor cleavage. Fracture: Uneven fracture Hardness: Andalusite has a …

Minerals of Clay Group

Clays consist mainly of a group of crystalline substances known as the ”clay minerals”. They are all essentially hydrous aluminium silicates. Well known minerals are of as under: (1). Kaolinite: Al2O3 2SiO2 Physical properties of kaolinite are as under; Crystal System: Triclinic crystal system. Cleavage: Perfect basal cleavage. Hardness: 2 at Moh’s Scale Sp.gr: Specific …

Minerals of Serpentine Group

The word serpentine has been derived from ”Serpent”, which means greenish. Serpentines are so called because of their green colors. Serpentine occurs in two distinct forms: (i) ”Antigorite”; a platy variety, and (ii) ”Chrysolite; a fibrous variety. Minerals species of Serpentine Group: Amesite: Chemical composition of Amesite is Mg2Al2SiO5(OH)4., however if an Amesite has Ni …

Chlorite group of minerals

The chlorite group includes a number of minerals, which have similar physical properties. They are sheet silicate (phyllosilicate) minerals, which were formed during early metamorphism. They have an oily/soapy feel and perfect cleavage. They are found in all sedimentary, igneous, and metamorphic rocks. The important mineral of this group is ”Chlorite”. Chlorite Minerals: (Mg,Fe) 3(Si,Al) …

Minerals of Garnet Group

The garnets comprise a group of isomorphous minerals with the general formula X3Y2(SiO4), where X may be Ca, Mg, Mn, or Fe+2 and Y may be Al, Cr3+. The common garnets are: Pyrope: Al2Mg3O12Si3: Red to Purple with a hardness 7-7.5 on Moh’s scale. Almandine: Fe++3Al2(SiO4)3: Red brown in colour, and has a hardness 7-7.5 on …