175 



Fig- '37- 



Scolezitc; section parallel 

 to (001) (Lacroix). 



Scolezite : CaAl z Si 3 O lo + 3 H 2 is monoclinii . with an axial 

 ratio a: b: c = 0,9764 : i : o 3434, and /3 = 

 98*18'. The prism-angle (110): (110) is 

 8837'. The space-lattice is therefore pseu- 

 do-tetragonal. Twins occur with (100) as 

 t winning-plane; a section parallel to (001) 

 shows the division in fields and strips, as 

 drawn in fig. 137. (Lacroix). 



How perfectly such crystals may approxi- 

 mate to real hexagonal or trigonal symmetry 

 may also been seen from the stereographical 

 projection of a Ront gen-pattern of cerus- 

 site, parallel to (001). and obtained by us 

 in our series of investigations of the symmetry of the Rontgen- 



patterns of isomor- 



phously related 

 substances l ) in ge- 

 nrrul. Although it 

 is obvious, that 

 the symmetry is 

 only rhombic, it 

 is remarkable how 

 closely the image 

 obtained approxi- 

 mates to that of 

 a true hexagonal 

 crystal (fig. /jS). 



5. If rep< 

 twinning occurs in 

 several directions 

 at the same time, 

 compound pene- 

 tration-twins of a 

 very complicated 

 structure may be 

 formed, and the approximation to higher symmetrical individuals 

 obtained in this way may go remarkably far. As an instance of the 



Fig 138. 



Stereographical Projection of the Rontgen-pattern 

 of Cerussite; plate parallel to (001). 



i) F. M. Jaeger and H. Haga, Proceed. Kon. Akad. van Wet. Amsterdam. 

 18. 1357. (1915). 



