177 



Finally the case <>t <//<///,/-. \te, as another very curious one, may 

 >< dix-ii^sed here more in detail. 



k. (loco cit.) showed that chabazite : (Ca, Afa,)/!/ 2 (SiO,) + 

 >fl ,O, although completely rhombohedral in its ext. l>ect, 



j in n-.ility only triclinic. On a cleavage-form exhibiting the tf 

 pinacoides, the interfacial angles were found to be: (100): (010) = 

 *42'; (010): (001) = 855', and (100): (001) = 8534'. From thi- it 

 y be seen, that the triclinic crystal is approximately a rhombohedron 

 nth a polar angle of about S4\. As this rhombohedron in its 

 turn closely approaches the form of a cube, the chabazite is tridinic- 



Pseudo-rhombohedron 



of 



six individuals, 

 parallel to (0001). 



b. 

 Penetration- twin 



of 

 two pseudo-rhombohedra. 



Fig. 140. Chabazite. 



Final apparent 

 rhombic-dodecahedron . 



pseudo-trigonal and also pseudo-cubic. In accordance herewith the 

 compound individuals are formed by repeated twinning, as follows. 

 Six or more individuals combine into double twins according to 

 two different twinning laws, the twinning-planes being (1 10) or (1 10). 

 The exterior of the pseudo-rhombohedral crystal may be bordered 

 either by the faces of the pinacoids : {100}, or by {010}, or by {001}, 

 and a basal section of the different pseudo-rhombohedra thus ob- 

 tained, will show six sectors, with an arrangement of their extinction- 

 angles which is in agreement with one of these three types of rhom- 



A. Ben Saude, Neues Jahrb. f. Min. 1. 41. (1882); E. Mallard Bull, de la Soc. 

 Mm. 5. 255. (1882); C. Klein, Zeits. f. Kryst. 9. 54. (1884); Neues Jahrb. f. Miner. 

 1. 240. (1884); 2. 101. (1885); F. Rinne, ibid. 2. 25. (1887); Sitz. Akad. d. Wiss. 

 Berlin, (1890), p. 1175, 1183, 1188, 1190, 1192; W. C. Brogger, Zeits. f. Kryst. 

 16. 565. (1890); C. Klein, Neues Jahrb. f. Min. Beil. Bnd. 1. 93, 96. (1891); 

 R. Brauns, Die Optischen Anomalien, (1891). 



12 



