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 each other, but a-axis is inclined to the plane containing the other two axes. The ‘a’ axis is called ”clino-axis” and the ‘b’ axis “ortho-axis”. The angle between +a and +c is greater than 90° and is designated as β.

Class of Monoclinic Crystal System: 

Gypsum type is the normal symmetry class of the monoclinic system.

Gypsum Type Monoclinic Crystals:

Symmetry elements of Gypsum Type: The symmetry elements of Gypsum type  are as following.

  • Planes: They have 1 plane ‘a’ and ‘c’ axes.
  • Axes: They have 1 axis (b-axis) of 2-fold symmetry.
  • A center of symmetry.

Different Forms of Gypsum Type Monoclinic Crystals: The chief forms of gypsum type are as follows.

  1. Pinacoids: The pinacoids are of three types: (i) Basal Pinacoid, (ii) Orthopinacoid, and (iii) Clinopinacoid.
    1. Basal Pinacoid (001): This form includes two faces. Each face intersects c-axis and lies parallel to the plane containing ‘a’ and ‘b’ axes.
    2. Orthopinacoid (100): The orthopinacoid is also called ”a-pinacoid”. It is composed of two faces. Each face intersects ‘a’ axis and lies parallel to the plane containing ‘b’ and ‘c’ axes.
    3. Clinopinacoid (010): This form is also called ”b-pinacoid”. It is composed of two faces. Each face intersects ‘b’ axis and lies parallel to the plane containing ‘a’ and ‘c’ axes.
  2. Prism (hko): A monoclinic prism consists of four faces. Each face is parallel to the vertical axis and intersects both the horizontal axes. The symbol of the unit prism is (110).
  3. Hemiorthodome (hol): There are two hemiorthodomes: (i) positive (hol), and (ii) negative (-hol). This form is composed of two faces. Each face lies parallel to the b-axis and intersects ‘a’ and ‘c’ axes. The positive unit hemiorthodome (101) has the faces 101 and (-1 0-1), and the negative unit hemiorthodome (-1 0 1) has the faces (-1 0 1) and (1 0 -1).
  4. Clinodomes (okl): A clinodome has four faces. Each face is parallel to the ‘a’ axis and intersects the ‘b’ and ‘c’ axes. The symbol of the unit clinodome is (011).
  5. Hemipyramids (hkl): There are two hemipyramids (i) positive (hkl) and (ii) negative (-h k l). This form has four faces only. Each face intersects all three crystallographic axes. Examples of positive and negative hemiphyramids are (321), and (-3 2 1) respectively  with the faces 321, 3 -2 1, -3-2-1, and -3 2 1, 3 2-1, 3 -2-1.

Related Articles: 

  1. Introduction to Crystallography
  2. Unit Cells of the Crystal Lattice 
  3. Crystallographic Axes
  4. Interfacial Angle of Crystals
  5. Parameters of Crystal faces
  6. Crystallographic Notation
  7. Forms of Crystal faces
  8. Symmetry Elements of Crystals
  9. Six Types of Crystal Systems
  10. Cubic/ Isometric Crystal System
  11. Tetragonal Crystal System
  12. Hexagonal Crystal System
  13. Orthorhombic Crystal System

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