Monthly archives: October, 2021

Occurrence of Igneous Rocks

The interior of the earth is very hot. Due to this intense heat, the internal matter of the earth is molten. The molten material of the earth is known as Magma. Magma is produced deep in the earth’s crust where temperatures are very high ranging from 900 degrees Celsius to 1600 degrees celsius. Magma being …

Chemical Composition of Igneous Rocks

Igneous rocks make approximately 90% of the earth’s crust by volume. But their great abundance is hidden on the earth’s surface by a relatively thin layer of sedimentary and metamorphic rocks. Igneous rocks are formed by the cooling and solidification of magma. Magma is a hot viscous, siliceous melt containing water vapour and gases. It …

Introduction to rocks

Rocks: A major part of the earth’s crust is composed of rocks. Rocks may be defined as an aggregate of minerals. Some rocks, such as quartzite (quartz) and marble (calcite), contain grains of one mineral only, but most are composed of a variety of different minerals, The rocks are broadly classified into three groups: (i) …

Common Twinning Laws

Twin Law is a constant crystallographic expression in terms of twin-plane and twin-axis according to which twins of the same substance are found to be related. Following is a brief outline of the most commonly observed twin laws in different crystallographic systems: (1). Twinning Laws in Cubic/Isometric Systems: Spinel Law: It is named after its …

Types of Twinning

There are three basic types of the twinning of crystals; (i) Contact Twins, (ii) Penetration Twins, and (iii) Repeated Twins. (1). Contact Twins: When two halves of a twinned crystal are symmetrical with respect to the twin plane, it is called ”contact twin”. Contact twins have a definite composition plane, which separates the two halves …

Twin Crystals

Twinning: The symmetrical intergrowth of two or more individual crystals are known as the ”twin crystals”. In a twinned crystal the lattice directions of one unit exhibit a definite crystallographic relation to the lattice directions of the other. Twin Plane: This plane exists between two twinned crystals. The twin plane divides the two twinned crystals …

Crystal Groups

Apart from the crystal system, crystals are also found in groups. There are three main crystal groups; (i) Crystal Aggregated, (ii) Parallel Growth, and (iii) Twin Crystals. (1). Crystal Aggregates: The groups of associated crystals are found as ”crystal aggregates”. If the crystals forming the aggregate are made of one mineral only, the aggregate is …

Triclinic Crystal System

Crystallographic Axes: Some minerals like axinite, albite, anorthite, microcline, and kyanite crystallize in the triclinic system. The crystal forms of the triclinic system are referred to as three unequal axes that make oblique angles with each other. The angles between the positive ends of ‘b’ and ‘c’, the ‘c’ and ‘a’, and the ‘a’ and …

Monoclinic Crystal System

Minerals like gypsum, micas, orthoclase, and diopside belong to the monoclinic crystal system. Crystals of this system are referred to three axes (a, b, c) of unequal lengths, in which ‘a’ is perpendicular to b, and c, while b, and c are not perpendicular to each other. The ‘c’ and ‘b’ are right angle to …

Orthorhombic Crystal System

Some common minerals like Baryte, Olivine, Andalusive, and Topaz belong to this type of crystal system. Crystallographic Axes: Crystals belonging to the orthorhombic system are referred to as three unequal, mutually perpendicular axes. The ‘a axis being shorter is called ”brachy axis” and the ‘b’ axis being longer is called ”macro axis”. Classes of Orthorhombic …