Peach-Blossom Coloured Mica. 205 



seems to be the Kerolite of Mr Breithaupt, * and it occurs 

 under the same geognostic relations. It does not melt be- 

 fore the blow-pipe, becomes white, and imparts to the flame 

 a beautiful purple colour. I shall communicate the analysis of 

 these minerals in another paper. 



Art. III. — Observations on the Optical Structure of Lithion- 

 Mica, analysed by Professor Gmelin. By David Brew- 

 ster, LL. D. F. R. S. Lond., and Sec. R. S. Edin. 



As Professor Gmelin had the goodness to transmit to me, 

 along with the MSS. of the preceding paper, some specimens of 

 the Lithion-Mica, with a request that I would examine its opti- 

 cal structure, I lost no time in complying with his wishes. 



In the year 1816, while examining the various Micas, I 

 found that the inclination of the resultant axes of Mica and Le- 

 pidolite was 45° ; and that other Micas had their axes inclined 

 only about 14°, while in talc they formed so small an angle 

 as 7° 24'. I afterwards found two Micas from Greenland, 

 in large masses, which had only one negative axis of double 

 refraction. M. Biot, who also performed many accurate ex- 

 periments on Mica, found specimens in which the inclination 

 of the axes was 30°, 31°, 32°, 34°, and 37°, and some in 

 which it was under 25°. He discovered also some Micas 

 which had a single positive axis, and when these specimens 

 were analysed by M. Vauquelin, the uniaxal crystals were 

 found to contain magnesia, while in the biaxal ones there was 

 not even a trace of that earth ; and the inclination of the axes 

 seemed to diminish as the oxide of iron increased. 



Under these circumstances, the examination of the Lithion- 

 Mica became more than usually interesting. Upon exposing 

 the plates sent me by Professor Gmelin to polarised light, I 

 was very much struck with their compound appearance. In 

 place of being individual crystals, like almost all the specimens 

 of Mica that I had examined, they were obviously composed 

 of several individual crystals, having their axes lying in various 

 directions, and producing most irregular polarised tints. A 

 more particular examination, however, led me to observe the 



• Charaktcristik des Mineral Systems, 2 tc Aufl. p. 145. 



