182 



in some way or other with the strange behaviour of these minerals 

 regarding their loss and absorption of water. Analogous phenomena 

 as discussed here in the case of phillipsite, are found with harmo- 

 tome, stilbite, etc., while the connection between the content of water 

 and the occurrence of optical anomalies has been established beyond 

 doubt in the case of heulandite, chabazite, analcite, etc., by the in- 

 vestigations of Mallard, Klein, Rinne, and others 1 ). 



Finally, the case of chabazite may be discussed here more in 

 detail, as another very curious example of this kind. 



Becke (loco cit.) showed that chabazite: (Ca,Na 2 )Al 2 (Si0 3 ) 4 + 

 6H 2 0, although completely rhombohedral in its external aspect, 

 is in reality only triclinic. On a cleavage-form exhibiting the three 

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

 8342'; (010): (001) = 855', and (100): (001) = 8531 J'. From this it 

 may be seen, that the triclinic crystal is approximately a rhombo- 

 hedron with a polar angle of about 84 1. In accordance here with, 

 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), of {010}, or of {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 the three types of rhom- 

 bohedra just mentioned (fig. 1420). The angles which these extinction- 

 directions make with the diagonal of each rhombohedron-face will, in 

 these three cases, be respectively: 46, 11, or 24. 



The mimetic form thus obtained is, therefore, now an apparent 

 rhombohedron. Two such rhombohedral forms may, moreover, com- 

 bine into penetration-twins, with either their pseudo- trigonal axis 

 c as twin-axis, or, rarely, with the face (1011) as twinning-plane. 



!) Cf. also: E. Mallard, Ann. des Mines, 10, 111, (1876); F. Becke, 

 Tscherm. Min. Mith., 2, 391, (1879); A. v. Lassaulx, Zeits. f. Kryst.,5,330, (1881); 

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

 Min., 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. End., I, 93, 96, (1891) ; 

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



