250 



crystals are identical with the common /j-form. which represents 

 the stable form at the meltingpoint ; the plates must he a peculiar 

 kind -of crystals of this /3-nioditication. 



However we have till now not succeeded in giving a final explan- 

 ation of the remarkable reddish-lilac colour of many of these pre- 

 parations. It is quite sure, that it does not depend in anj waj^ on 

 the admixture of certain metallic impurities, solved from the crucible- 

 walls; on the contrary it appears to be connected with the macro- 

 crystalline structure of the preparations. The nearer the chemical 

 composition came to the theoretical one, and the slower the crystal- 

 lisation of the mass takes place, by heating during a long time at 

 a temperature just below the meltingpoint, the more the appearance 

 of the violet tinge seems to be probable. 



The same colour appears, if spodumene-powder or the pulverized 

 "glass" of if, are brought into liquids of about the same refractive 

 index (e. g. into orthochlorotoluene, with /i = 1,522); in that case 

 the wellknown phenomenon of the ''monochromes" (Christiansen) 

 will appear. It is not impossible, that in our case the colour is 

 produced in an analogous way by the presence of the tabular, very- 

 thin crystals amidst spodumene-glass, which possesses about the 

 same refractive index (1,519) as the crystal-tables (1,520 till 1,527), 

 or reversely ; these tables would be therefore quite invisible in the 

 surrounding medium. It could be understood in this way also, why 

 in the uncoloured mass in some cases locally smaller or larger 

 pink spherolithes are produced, making the impression, as the molten 

 mass were locally inoculated with germs of the violet substance. 



By means of the ultra-microscope we were able to show, that the 

 preparation was not "optically empty", as a great number ofdiiFer- 

 entl}^ coloured lightspots, which do not move however, could be 

 observed; they are manifesting a structure of some particular kind, 

 without it being possible to ascertain of what kind the imbedded 

 particles are. 



§ 11. As it follows from these investigations, in connection with the 

 meltingpoint determinations of natural a-spodumene later to be described, 

 thai the chemicalbj pure compound LiAlSi^O^ has a meltingpoint 

 considerably lower than the natural spodumene-minerals, — we made 

 a series of investigations to find out, what admixtures of the natural 

 spodumenes might cause the mentioned increase of the meltingpoint. 

 Therefore to an artificial product, whose composition was : 



SiO, 64,7 7„ 



Al^O, 27,1 Vo 



Li,0 8,2 Vo 



