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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 …

Hexagonal Crystal System

The hexagonal system, one of the principal categories of crystal systems, in which the components of crystals are located by reference to four axes; three of these axes are of equal length and set at 120° to one another and a fourth axis is perpendicular to the plane of the other three axes. If the …

Tetragonal Crystal System

In our previous post, we discussed in detail the classes and forms of Cubic/ Isometric Crystal System, which is a very common type of crystal and includes hundreds of minerals in this system. This post is about the ”tetragonal crystal system”. The tetragonal crystal system is another important system. Zircon, Cassiterite, and Rutile are the …