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 Crystal System:

Baryte type is the normal class of the orthorhombic crystal system.

Symmetry elements of Baryte Type: 

The symmetry elements of this class are as follows.

• Planes: There are three axial planes in the crystal of baryte-type minerals.
• Axes: There are three crystallographic axes of 2-fold symmetry
• A center of symmetry

Different Forms of Baryte Type:

The typical forms of this class are as follows.

1. Pinacoids: A pinacoid is composed of two parallel faces each of which intersects one crystallographic axis and is parallel to the other two. Pinacoids are of three types.
   • Basal Pinacoid (001): Its faces intersect c-axis and lie parallel to ‘a’ and ‘b’.
   • Macro Pinacoid (001): It is also called ”a-pinacoid” as its faces intersect ‘a’ axis and lie parallel to ‘b’ and ‘c’.
   • Brachy Pinacoid (010): It is also called ”b-pinacoid” as its faces intersect b-axis and lie parallel to ‘a’ and ‘c’.
2. Prisms (hko): A prism is composed of four vertical faces. Each face intersects both the horizontal axs and is parallel to the vertical axis. Unit prism has the symbol (110). Various other prisms are possible depending on the variation in the axial intercepts, e.g., (210), (120), etc.
3. Macrodomes (hol): This form is composed of four faces. Each face is parallel to the ‘b’ axis (macro-axis) and intersects ‘a’ and ‘c’. The symbol of the unit macrodome is (101). Other microdomes are (103), (102), (203), etc.
4. Brachydomes (okl): This form is composed of four faces. Each face is parallel to the ‘a’ axis (brachy-axis) and intersects ‘b’ and ‘c’. The symbol of the unit is (011). Other Brachydomes are (013), (012), and (023), etc.
5. Dipyramid (hkl): An orthorhombic dipyramid has eight triangular faces, each of which intersects all three crystallographic axes. The symbol of the unit dipyramid is (111).

Related posts:

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