Orthorhombic System, Crystallographic Axes, Symmetry & Forms

    Crystallographic Axes
    The crystallographic axes of the orthorhombic system are three in number. They make 90° angles with each other and are of unequal lengths. The relative lengths of the axes, or the axial ratio, has to be determined for each orthorhombic mineral. Any one of the three axes may be chosen as the vertical or c axis. The longer of the other two is taken as the b axis and is called the marco-axis and is called the brachy-axis. The decision as to which of the three axes shall be chosen as the vertical or c axis depends usually upon the crystal habit of the mineral being considered. If its crystals commonly show an elongation in one direction, usually this direction is chosen as the c axis, see Figs. 157-159, p.55. If on the other hand the crystals show a prominent pinacoid and therefore are tabular in habit, this pinacoid is usually taken as the horizontal (basal) pinacoid with the c axis normal to it, of course if a substance is well known and the orientation of its crystals given in the literature it is customary to conform to that orientation. The length of the axis which has been chosen as the b axis is taken as unity and the relative lengths of the a and c axes are given in terms of it. Fig. 146 represents the of the a and c axes are given in terms of it. Figs. 146 represents the crystallographic axes for the orthorhombic mineral sulfur, whose axial ratio would be as follows: a: b: c = 0.813 : 1 : 1: 1.903.

    Symmetry and Forms
    They symmetry of the Normal Class, Orthorhombic System, is as follows: The three crystallographic axes are axes of binary symmetry and the three axial planes are planes of symmetry (see Figs. 147 and 148).

    1. Pyramid
    And orthorhombic pyramid has eight triangular faces, each of which intersects all three of the crystallographic axes. There are various different pyramids with varying intercepts on the axes. A unit pyramid (see Fig. 149) would have for its symbol (111).

    2. Prism
    Ant orthorhombic prism has four vertical rectangular faces, each of which intersects the two horizontal axes. There are various prisms, depending upon their differing relations to the horizontal axes. A unit prism (see Fig. 150) would have for its symbol (110).

    3. Macrodome
    A macrodome is a form consisting of four rectangular faces, each of which intersects the a and c axes and is parallel to the b or macro-axis. it is named from the axis to which it is parallel. There are various macrodomes with different axial intercepts. A unit form would have for its symbol (101).

    4. Brachydome
    The brachydome consists of four rectangular faces, each of which intersect the b and c axes and is to the a or brachy-axis. There are various brachydomes with different axial intercepts. A unit form would have for its symbol (011).

    5. Macropinacoid
    The macropinacoid has two parallel faces, each of which intersects the a axis and is parallel to the b and c axes.

    It derives its name from the fact that it is parallel to the b or macro-axis. it is represented in Fig. 153 and its symbol is (100).

    6. Brachypinacoid. This is a form consisting of two paralle faces, each of which intersects the b axis and is parallel to the a (brachy) and the c axes. It is represented in Fig. 153 and its symbol is (010).

    7. Basal Pinacoid
    The basal pinacoid is a form consisting of two horizontal faces. It is represented in Fig. 153 and its symbol is (001).

    Combinations
    Practically all orthorhombic crystals consists of combinations of two or more forms. Characteristic combinations of the various forms are given in Figs. 154-164.

    Hemimorphic Class
    The only orthorbombic mineral of importance belonging to this class is calamine. When its crystals are doubly terminated they show different forms at either end or the vertical axes. Fig. 165 represents a characteristic crystal.

    Characteristics of Orthormbic Crystals
    The most distinguishing characteristic of orthormbic crystals are as follows: The three chief directions at right angles to each other are of different lengths. These three directions are axes of binary symmetry. the crystals are commonly prismatic in their development and show usually cross sections that are either rectangles or truncated rectangles.