CRYSTALLOGRAPHY 



39 



axial cross of rho.lonite, & : b : c = 3.072 ( : 1:.6212 + ; =193 18'; 

 ft = mx 1 1' ; y = 81 39' ; and the angle 100,010 = 94 26'. 



Thf cnii>t ruction of the c and ft axes is the same as in the mono- 

 clinic >vsteiii, hen- the plane containing the b and 6 axes is not at 

 90 from that containing the a 

 and c axes, hut as in this case is. 

 '. 1 26'. Having constructed the 

 project ions of a and c, lay off the 

 angle a"ob' = lOOvOlO = 94 26', 

 make bob' = a - 90 = 13 18'j 

 make ob the true length of b, draw bb' at 90 to ob'. The 

 extremity of the b axis will lie below the horizontal plane a dis- 

 tance bb', due to the angle being 13 18' larger than a right angle ; 

 draw bV at 90 to xy ; the projection of b will lie on the line 



FIG. 59. The Unit Pyramid of Am- 

 phibole. 



FIG. GO. Axial Cross of Rhodonite. 



Fiu. 61. 



b'b", extended, if necessary, a distance b"b = bb' -f J b'b", 

 b b will then be the projection of the axis b. Figure 61 

 represents the combination of (100); (010), (001), (110) of 

 rhodonite. 



Example I of clinographic projection. After the axial cross is 

 projected, the clinographic projection of any crystal is a problem 

 in the intersections of planes ; the inclination of the faces is given 

 by the parameters. Let it be required to project clinographieally 

 the same forms used to illustrate the orthographic method on page 

 32. Construct the axial cross and connect the extremities of the 

 axes, Fig. 62, which will represent the unit pyramid (111) ; the base, 

 c = (001), will truncate the pyramid above and below o, and will 

 be parallel to the plane containing the axes & and b. Let it cut 

 the c axis above at c, below at d. This distance will depend upon 



