CRYSTALLOGRAPHY 



37 



xircon .640. The unit pyramid of xircon will be projected by 

 connecting tin- three a\i:il units, Fig. 52a. 



Hexagonal. Hen- the problem of the ;i\i:il croflfl differ-, as the 

 lateral a\e> are thn-c and not at right angles. Draw xy, Fig. 53, 

 and pmject c and a-> as before. 

 I )raw a/oas' = 00, making a 3 'o 

 = a 2 'o, then draw a' 3 a3 par- 

 allel to cc, placing as, I the 

 distance from xy as a' s , when 

 - a 3 a 3 will be the projection of 

 the axis required. The third 

 lateral axis is found by laying 

 off the angle as'oa/' = 60 and 

 following the same construction 

 as before; by connecting the ex- 



FIG. 54. The Unit Pyramid of 

 Apatite. 



tremities of all the axes the unit pyramid will result, Fig. 54. 



Orthorhombic. Here the three axes are at right angles, but 

 all of a different length. Taking barite as an example, where 

 & : b : c = .81 + : 1 : 1.313, to project the axial 'cross. Draw xy, 



Fig. 55; lay out ob' at 

 18 26' from y and some 

 selected unit in length 

 as 50 mm. ; find the pro- 

 jection of b as before. 

 The c axis is found as in 

 the tetragonal system, 

 oc' = 50mm. X 1.313+ = 

 65. 65 mm. The a axis 

 is found by laying off oa' 

 at 90 to ob', and = 

 50 mm. X. 81 =40.5 mm., 

 when the projection aa 

 is obtained as before; 

 connecting the extremi- 

 ties and Fig. 56 will rep- 

 resent the unit pyramid 

 of barite. 



-c 



FIG. 55. Axial Cross of Barite. 



Monoclinic. Here the problem differs in that the clinoaxis & 

 is not at 90 to the vertical axis c. Let it be required to draw the 

 axial cross of amphibole, a : b : c = .55+ : 1 : .29 + , ft = 73 58'. 



Project b and c as in the orthorhombic system ; to find the pro- 



