249 



tinged with u nice, reddish Hiac hue; they were in most cases 

 obtained by longer heating, somewhat beloin the meltingtemperaturc. 

 Tlie meltingpointdetermination witli sncii an intensively coloured 

 product, prepared from LlAlL\ and SiO^ and showing by analysis 

 the following composition : 



I. II. Calculated : 



Si 0, 64,92 7„ 64,77 7„ 64,67„ , 

 ^40, 28,10 7„ 27,94 7.. 27,4 7„ 

 LiJ) 6,98 7„ 7.29 7'o 8,0"/,, 

 and thus evidently containing about 0,63 7« h)o much ahimiiia, gave 

 the following results (statical method): 



After heating at 14660 M. V. : alt glass. 

 „ 14640 M. V. : all glass. 

 „ 14600 M. V. : all glass. 

 „ 14500 M. V. : all crystallized. 

 ,, ,, ,, 14560 M. V. : all crystallized. 



The meltingpoint is thus situated at 14580 M.V., corresponding 

 with 1420° C. (G.Th.). 



Such lilac coloured preparations present in most cases rather 

 larger individuals of the /^-modification, which possess a tabular shape 

 with appreciably stronger birefringence than the common crypto- 

 crystalline masses, although the mean refractive index is the same. 

 While commonly this birefringence varies between 0,001 and 0,003, 

 it amounts in these preparations to about 0,007 ; the principal refrac- 

 tive indices are about: 1,520 and 1,527. In convergent polarized 

 light, at the border of the field an interference-image is partially 

 visible, giving the impression of that of an uniaxial crystal. However 

 on moving the table of the microscope, one can easily observe the 

 curvature and even tiie hyperbolic form of the dark beams; undoubtedly 

 an optically biaxial crystal with a very small axial angle, is present 

 here ; while the position of the first bisectrix and the character 

 of the dispersion, point to monodinic symmetry, with a strong 

 tendency to tetragonal forms. This last pecularity can be deduced, 

 — besides from the apparent uniaxity — from the fact, that rectanguhir 

 plates are not rare, which possess an extinction under right angles 

 or parallel to a diagonal, and a system of cleavage-directions under 

 45° with the optical sections. The specific gravity, like the refractive 

 indices, does not differ appreciably from that of the common /J-form 

 and was determined at 25° C. : (/,o := 2,401 ± 0,008, measured with 

 several preparations. We obtained these same apparently uniaxial 

 plates, also from natural spodumenes by melting and slowly crystal- 

 lizing; there is no doubt whatever about the fact, that these tabular 



