Monoclinic System,
Crystallographic Axes

Crystallographic Axes
The crystallographic axes of the Monoclinic System are there in number. They are of unequal lengths. The axes a and b, and b and c, make 90° angles with each other, but a and c make some oblique angles with each other. The relative lengths of the axes and the angle between the a and c axes vary for each monoclinic mineral and have to be determined in each case from appropriate measurements. The a axis is known as the clino-axis, while the b axis is known as the ortho-axis. The length of the be axis is taken as unity and the lengths of the a and c axes are expressed in terms of it. When properly orientated the c and the a axis is inclined downward toward him. The smaller of the two supplementary angles that a and c make with each other is designated the crystallographic axes of the monoclinic mineral orthoclase, the axial constants of which are expressed as follows: a: b: c=0.658; 1:0.555; B= 63° 57’

In any monoclinic crystal the position of the b axis and that of the plane in which the a and c axes lie is fixed by the symmetry. The directions which shall serve as the a and c axes, however, are matters of choice and will depend customarily upon the crystal habit. If the crystals of the substance show and elongated development (prismatic habit) parallel to some direction in the a -c plane, that direction serves as the c axis. Further if there is a prominent sloping plane or planes, these may be taken as parallel to the inclined axis a it is quite possible that there may be two, or even more, different choices as to the directions of the a and c axes in a monoclinic crystal that are equally good. Naturally once established, the orientation of the crystals of a given substances is followed in subsequent descriptions.