BISPHENOIDAL CLASS (Bisphenoidal-tetartohedral.) Here there is only a single dyad axis of symmetry, which coin cides with the principal axis. All the forms, except the prisms and basal pinacoid, are spheroids. The calcium aluminium silicate and meliphane are per haps representatives of this class.
3. ORTHORHOMBIC SYSTEM (Rhombic; Prismatic; Trimetric.) In this system the three crystallographic axes are all at right angles, but they are of different lengths and not interchangeable. The parameters, or axial ratios are a :b :c , these referring to the axes OX, OY and 02 respectively. The choice of a vertical axis, OZ = c, is arbitrary, and it is customary to place the longer of the two horizontal axes from left to right (O Y = b) and take it as unity: this is called the "macro-axis" or "macro-diagonal" (from aaKpos, long), whilst the shorter horizontal axis (OX = a) is called the "brachy-axis" or "brachy diagonal" (from /3paxvs, short). The axial ratios are constant for crystals of any one substance and are characteristic of it ; for example, in barytes a : b :c =0.8152:I:1.3136; in anglesite 1.2894; in cerussite a: b:c=o•6ioo:I :0.7230.
There are three symmetry-classes in this system:—