32 



MINERALOGY 



as a point in the center of the drawing at the intersection of the 

 lateral axes. All edges parallel to c will also appear as points. 

 Planes parallel to the c axis will appear as straight lines. The 

 angles between these planes will be represented of true size. 



Edges parallel to the plane of projection will also be represented 

 in the drawing by lines equal to the lengths of the edges ; edges 

 inclined to the plane will be proportional to their inclination. 

 Supposing it is wished to represent the crystallization of barite, 

 axial ratio, : b : c = JEt815 + : 1 : 1.313 + , and the following forms: 



a = (100) = a: oob :_ooc, 

 c = (001) = oo a : oob : c, 

 m = (HO) = a:b : ooc, 



b = (010) = ooarb^ooc, 

 d = (102) = 2 a : oob : c, 

 p= (in) = a:b:c, 



m" 



When the indices alone are given it is necessary to transform them 

 to Weiss parameters, as these represent the proportional inter- 

 cepts on each axis. As the two 

 lateral axes appear in the draw- 

 ing in their true length, lay 

 out from c, Fig. 48, the vertical 

 axis, a distance cb and cb' each 

 equal to b, the selected unit of 

 length ; at right angles to cb', 

 as barite is an orthorhombic 

 mineral, lay off ca and ca', 

 each = cb X .815 ; through b,b' 

 draw lines parallel to aa', which 

 will represent the form (oio) ; 

 through a, a' draw lines par- 

 allel to bb' representing the form (100) ; m (no) would be rep- 

 resented by joining the extremities of the axes a and b, but the 

 cross section can be varied to suit the crystal at hand, by moving 

 the line connecting a, b out to a parallel position m, when m, m', 

 m", m'" drawn symmetrically will represent the four faces of the 

 unit prism, m (no) ; these four lines will also represent the inter- 

 section of the unit pyramid, p (in), with m (no). The apex of 

 the pyramid is at c and the ridges will fall on the axes a and b. The 

 base c (ooi) is in the same zone as m (no) and p (in), therefore 

 the intersection of c and p will be parallel to the lines m, m', rep- 

 resented by four lines drawn around c parallel to m, m', m", m'". 

 The size of the base can be varied by the distance from c at which 

 the four lines are drawn. Of the forms to be represented there 



m 



FIG. 48. Orthographic of Barite. 



