245 



birefringent mineral ; although the eueryptite is here generated from 

 spodume. with deposition of albite, the properties of the inclusions 

 did not agree with those of the two histnamed minerals. The 

 speeitic gravity was pyenometrioallj determined to be c/40 = 2,667 

 at 25° C. ; the available data show the composition not to be the 

 proper one. the iSVOa-amount being 0,6 "/„ too high and that of the 



Fig. 2. 



Kig. 3. 



LiO^ about 'J 7ü l^^s than tiie theoretical value. The fig. 2 and 3 

 represent two microscopical preparations of the thin section between 

 crossed nicols ; in the fig. 3 the preparation is turned over 30° with 

 respect to that in fig. 2; — this fact pointing to a trigonal twinfor- 

 mation. Also the very peculiar structure of the crystals is shown 

 in fig. 3. 



The refracti\'e index w^as microscopically determined on : ^i/> = 1,545 

 ± 0,002. A definite mellingpoint could not be fixed by the usual, 

 dynamical method; at about 1120° C. the mineral gradually changes 

 into a viscous mass, which, on cooling, becomes £i glass. The refrac- 

 tive index of this glass a[)peared to be: 7Z/)=r 1,506 dt 0,001 ; it is 

 thus evidently lower than that for the glass of pseudo-eucry [jtite. 

 On being heated it is devitrified only slightly; there seems to l^e no 

 doubt, that the natural mineral and its glass are other than tiie 

 corresponding phases .of the artificial product. As also never any 

 indication of an occurring inversion could be found, it is highly 

 probable that eueryptite and pseudo-eucryptite are in relation of 

 monotropic modifications to each other. 



§ 8. Synthesis and properties of /^-Spodiunene. 



The compound, whose composition is: LiAlSi.,0^ was prepared 



17 



Proceedings Royal Acad. Amsterdam. Vol. XVll. 



