GUIDE TO THE MIXERALOGIC COLLECTIONS 



31 



Trigonal pyramid. The trigonal pyramid (fig. 118) consists of 

 six triangular faces, each of which cuts two basal axes (a) at 

 equal distances and the third at a distance 

 which is relatively half as great, each face 

 also interests the vertical axis. 



Orthovhombic system 



Crystal forms included in the ortho- 

 rhombic system are referred to three un- Fig us 

 equal uninterchangeable axes at right angles to one another. 

 These axes are shown in fig. 121 ; the shorter horizontal 

 one, called the brachyaxis, is designated by a, the longer 

 horizontal axis, called the macroaxis, by b and the vertical 

 axis by c. The relative position of the macro and brachy 



Fig. 121 Fig. 122 



axes in a crystal of any orthorhombic species is determined by 

 the intercepts of a face occurring in that species, called a unit 

 plane. The unit plane is selected from among those which cut 

 both a and b axes and is preferably a plane which intersects all 

 three axes. The intercepts of the a and c axes in terms of b 

 constitute the axial ratio, which is a constant for each ortho- 

 rhombic species. Difficulty is sometimes experienced in prop- 

 erly orienting an orthorhomlm- crystal owing to the fact that 

 the crystal is often flattened in the direction of the macroaxis; 

 thus in fig. 122, which shows a crystal of cerussite in plan, the 

 brachyaxis appears to be longer than the macro because the 

 crystal is elongated in the direction of a. 



Normal gi-oup 



Forms of the normal group are symmetric to three planes 

 of symmetry intersecting in the crystallographic axes, which are 

 axes of binary symmetry (fig. 123). 



