176 



way in which the approximation to the higher symmetrical form 

 may take place, we here mention the mineral phillipsite, a zeolithic 

 silicate of the composition: (Ca,K)^4/ 2 (Si0 3 ) 4 + 4^H Z 0. 



This mineral is undoubtedly monoclinic; but its angle (110) : (1 10) 

 is 6042', while (001) : (101) = 90!'. It has therefore an approximate 

 rhombic, as well as a pseudo-hexagonal symmetry. The result of 

 the repeated twinning with (001) and (Oil) as twinning-planes, is 

 a nearly square prism formed by four individuals, the faces of which 

 are finally striated. Now three compound individuals of this kind 

 may interpenetrate, with (110) as twinning-plane, to form a single 

 yet more compound individual, as shown in fig. ijp. 



If the re-entrant angles be now gradually removed by the deve- 



ig. 139. Phillipsite. 



lopment of the faces indicated, this complex of twelve crystals 

 passes finally into the form of an apparent rhombicdodecahedron, as 

 it is found in the cubic system. Each rhombic face of this may then 

 be subdivided into four fields by striations diverging from the 

 centre, and parallel to the position which would be occupied by one 

 of the planes at each end of a ternary axis of the pseudo-dodecahedron 

 thus obtained. 



In general such remarkable mimetic forms are often met with 

 in the group of the zeolithic silicates, a fact which may be connected 

 in some way with the strange behaviour of these minerals with 

 respect to 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 ). 



i) 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) ; 



