187 



Fig. 143. 



Basal section of 



A mmonium-lithiumsulphate. 



number of crystals showing optical rotatory power, also exhibit optical 

 anomalies in the sense indicated above, and that these dextro-, or 

 laevogyratory uniaxial crystals, are in 

 reality all very complicated twins of lower 

 symmetrical material. They are, therefore, 

 true pseudosymmetrical crystals, built 

 up according to definite twinning-laws, 

 by a great number of biaxial lamellae, 

 which in an analogous way to that demon- 

 strated in the cases of phillipsite and chaba- 

 zite, combine into an apparently higher 

 symmetrical, "mimetic" aggregate. The 

 special circumstances of crystallisation 

 seem to have a certain influence on the 

 arrangement of the composing lamellae, 

 so that within certain limits a fluctuation 



of the optical properties of such crystals may evidently occur. The 

 inconstancy of the magnitude of the rotatory power of such Crystals 

 was in many cases confirmed by direct observations. 



9. Now Mallard, basing his theory on the observed fact that 

 the crystals which show optical anomalies are just those whose 

 geometrical properties are approximate to those of higher symmetry, 

 considers the optically anomalous crystals as without exception 

 pseudosymmetrical aggregates of lamellae, 

 the space-lattice of which has a lower degree 

 of symmetry than the crystal as a whole 

 possesses. x ) If for instance (fig. 144) a rhom- 

 bic crystal has a space-lattice, the layers of 

 which, parallel to the plane of drawing, consist 

 of particles arranged in rhomboids of nearly 

 60, then the binary axis of the rhombic 

 individual perpendicular to the plane of this 

 layer is at the same time an axis of apparently 

 threefold symmetry. Thus, if the space-lattice 

 be turned round this axis of apparent symmetry through 120 

 or 240, the space-lattice in its new positions will coincide, not 



x ) E. Mallard, Explication des Phtnomenes Optiques Anomaux dans les 

 Substances Cristallistes, Paris, 1876; Ann. des Mines, (7), 10, 60, (1876); Bull, 

 de la Soc. Miner., 5, 144, 214, (1882); 7, 349, (1884); A. Scacchi, Zeits. der deut- 

 schen Geol. Ges., (1864), p. 35. 



Fig. 144. 



