Monoclinic System

Monoclinic Crystal System

The monoclinic system is the structural category of crystalline solids. Well, crystalline solids can be categorized according to the structure of crystals. In the monoclinic system, crystals are referred to mainly three axes, a, b, and c, where axes a is perpendicular to axes b and c, but simultaneously, a and b are not perpendicular to each other. Suppose atom groups or atoms in crystalline solids are represented by points and lattices when points are connected with each other. The monoclinic unit cell is differentiated by a single axis called two-fold symmetry, where the monoclinic unit cell can be rotated by 180 degrees without disturbing appearance. Some of the solids that belong to the monoclinic crystal system are borax, gypsum, beta-sulfur, orthoclase, muscovite, kaolin, clinoamphibole, azurite, jadeite, and spodumene.


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What is Crystallography and its Types?

Crystallography is the study of the arrangement of bonds of atoms in crystalline solids. In this system, mainly atoms arrangement is studied based on the crystal lattice. In modern days, DNAs and minerals are examined through crystallography. Well, many kinds of crystal systems are used nowadays. All the structure is defined based on three factors: how many axes used, length, and angles of the axis. 

Six different crystal systems are isometric system, tetragonal system, orthorhombic system, monoclinic system, triclinic system, hexagonal system, trigonal subsystem. All these systems have three axes, and the direction of the axis indicates the sides. The longest axis is C, and the shortest axis is A, and axis B is also there; in some systems, you can see axis D.


Monoclinic Crystal Shape And Monoclinic Crystal Angles

In crystallography, the monoclinic crystal system is one of the practical crystal systems. Three vectors describe a crystal system. In the monoclinic system, the quartz is described by vectors of inequitable lengths, as in the orthorhombic system forming a rectangular type prism with a parallelogram base. Hence two combinations of vectors are perpendicular (join at right angles), while the third pair forms an angle other than 90°.

  • Orientation of a crystal has few constraints – where b is the only fixed axis by symmetry.

  • Axis C is generally chosen based on cleavage and habit.

  • α and γ = 90

  • In some cases, the b axis will be 90 degrees that result in pseudo- orthorhombic form.

  • Symmetry operation in a monoclinic system, the unprecedented operation is 2/m – a twofold axis of rotation with a mirror plane.

  • The axis b is the rotation, while c and a lie in the mirror plane

  • Monoclinic crystals have two forms: pinacoids and Monoclinic shape crystals have two shapes: pinacoidal and prisms.

  • Common monoclinic rock-forming crystals include clinopyroxene, orthoclase, mica, and titanite.


Orthorhombic System In Crystallography

As we have discussed, crystallography has many types, and the orthorhombic system is one of them. Orthorhombic lattices are formed by extending cubic lattices with two orthogonal pairs by two different factors. While raising the cubic lattice with the two factors, a rectangular prism is formed, and axis a and b form the rectangular prism base. Axis c determines the height of the prism in the orthorhombic system in crystallography. Here all three-axis a, b, and c are different and intersect each other at the rectangle. Hence, all the three orthorhombic lattice vectors remain mutually orthogonal.

  • In Orthorhombic crystallography, all the three-axis are of a distinct length that is mutually perpendicular to each other.

  • Convention has it that a crystal is oriented so that c is the most significant axis and a minor axis.

  • In such a case, b is taken as unity, and after that, you can calculate ratios.

  • The unique symmetry operation in an orthorhombic system is The special symmetry operation in an orthorhombic system is 2/m 2/m 2/m – Three twofold axes of rotation coinciding with the three crystallographic axes.

  • There are three types of patterns in the class: prisms, pinacoids, and dipyramids.

  • Common orthorhombic rock-forming minerals incorporate andalusite and sillimanite, olivine, orthopyroxene, and topaz.


Forms of Orthorhombic System in Crystallography

The orthorhombic system has two types of forms, unique form, and general form. A possible form has the maximum number of faces of any pattern in its crystal class. Particular forms may appear in any crystal class of the system. In general form, three-axis a, b, and c intersect with each other at a specific angle, and it will never be zero. Different forms are pyramid, prisms, domes, disphenoid, sphenoid, pedion, pinacoids, and dipyramid.

FAQs (Frequently Asked Questions)

1. What is Cleavage?

Answer: Cleavage is referred to the ability of any mineral to break atomic planes of weakness. In the monoclinic crystal system, many crystals have distinct cleavage in one of the three directions. For example, realgar, crocoite, gypsum, orpiment, spodumene, orpiment, diopside, muscovite and orthoclase. But all the minerals are not the same as lazulite. Lazulite is an exception to the cleavage in the monoclinic system, and it is a very known mineral. Lazulite has very poor, or we can say indistinct cleavage in one of the three directions. Cleavage is dependent upon how the atoms are arranged in the mineral. In addition, cleavage is not measured with any particular numbers but it can be good, normal, weak, or excellent. 

2. How Many Orthorhombic Crystal Classes are there?

Answer: There are three crystal classes in the orthorhombic system. The Orthorhombic dipyramidal class is holohedral (exhibits the highest symmetry in the system). The Orthorhombic disphenoidal class is hemihedral and enantiomorphic (lower balance than the holohedral class, exhibiting only half as many faces) and (exhibiting left and right-handedness). The Orthorhombic pyramidal has the lowest symmetry and is hemimorphic (opposite ends of the axis of symmetry exhibit different forms; no transverse mirror plane and no centre of symmetry). There are 32 crystal classes there and among all groups, 2m, 2/mm, and 22 are of orthorhombic crystal classes. The first group has a 2-fold axis and mirror plane.