869 



diinng 27 hours in a steelbomb lined wilii [)la(innni. Of lliis preparation, 

 looking also like radial bundles of needles with trigonal symmetry, 

 we obtained some sections, normal lo the axis of the needles. In 

 convergent polarized light it was in reality possible to observe an inter- 

 ference-image, closely similar to that of an optically uniaxial crystal of 

 positive character; however, on moving the section and rotating the 

 table of the microscope, the image did not remain unaltered, but the 

 black cross opened its branches somewhat, and it was immediately 

 evident, that a huLn'ml crystal, however with a 

 very small axial angle, was [)resent. Of yet higher 

 importance however seemed the fact, that the 

 crystalplate showed itself in parallel polarized light, 

 to he coin/)Osed of a nuiaher of sectors (tig. 3) ; in 

 every sector the black cross between crossed nicols 

 seems to open itself somewhat. It is hardly to be 

 f^'B- ^- doubted, that a pseudo-symmetric aggregation is 



present here, with noticeable optical anomalies. 



The repeated experiments of Mr. Friedel gave, as he wrote us, 

 no other crystals whatever, than biiuial ones, with very small axial 

 angles, amounting occasionally to about 27°. The experiments were 

 made in these cases, by heating 2,68 grams of dry SiO^ and 0.885 grams 

 of Li^O (as hydroxyde) in solution, with a known quantity of tiucly 

 divided muscovite, at 545° oi' 550° C. during 30 hours. Also without 

 addition of mica, and at lower as well as at higher temperatures, the 

 same results were (obtained. {A and 7i, experiment XI, 1913). 

 Although all conditions of the earlier experiment I, 1898) were as 

 much as possible fulfilled, the obtained needles appeared however, 

 always to he hiaxial : a form oï comph'te trigonal symmetry was 

 never produced in any of these cases. 



The birefringence of the pseudo-trigonal needles ot preparation 

 (I, 1898) was about -. 0.021 ; that of the crystals from molten LlCl 

 was: 0,023 values do not differ noticeably from that, obtained by 

 us formerly with the biaxial silicate. 



The preparations A and B, and XI, J 913, were investigated in 

 convergent and in parallel polarized light. Some sections, perpendicular 

 to the axis of the needles, are reproduced in lig. 4. Doubtless it must 

 result from this, that very complicated individuals and polysynthetic 

 twins of lamellae, crossing under 60°, are present ; the pseudo- 

 trigonal habitus is caused thus by i)olysyjithetic twinning. 



The composing lamellae are all biaxial, the axial angle is very 

 small, and rarely exceeds, as said already, 27^ By means 

 of the immersion-method we were able to determine with some of 



