Analysis of Labradorite Felspar. 87 



induced mc to suspect that it was not felspar. From the great 

 difficulty that I experienced in procuring the mineral in a pure 

 state, the analysis was made upon a small scale ; but it was re- 

 peated with nearly the same result. It appears, then, that this 

 mineral is labradorite, with the best analysis of which the fore- 

 going nearly agrees. The large crystals of labradorite that are 

 imbedded in trap-rocks are very much cracked, and so impure 

 that they cannot be employed for analysis. 



About two miles to the west of the village of Milngavie, near 

 the road between Glasgow and Strathblane, there is a very re- 

 markable brown porphyritic trap, that contains large and beau- 

 tiful crystals of yellow labradorite. These crystals are cracked, 

 and when heated, present numerous brown spots, which show 

 that they are not pure. The following is their composition : — 



Silica, 52.341 



Alumina, .... 29.968 



Lime, 12.103 



Soda, 3.974 



Potash, 0.301 



Perox. of Iron, . . . 0.866 



99.553 



All the analyses of labradorite that have hitherto been made 

 differ a little from each other ; some agree with my first analysis, 

 while one of Klaproth's differs but little from the last. It is 

 probable, that, owing to its structure, labradorite has seldom 

 been examined in a pure state. 



The chemical characters of labradorite enable us to distin- 

 guish it from felspar, even when the quantity for examination 

 does not exceed a grain. For this purpose the mineral, in the 

 state of an impalpable powder, is treated with weak muriatic 

 acid in a watch glass, and gently heated for an hour. The so- 

 lution is then evaporated to dryness, and the saline residue is 

 heated until the excess of acid be expelled when it is redissolved 

 in water. To the clear solution, when warm, a few drops of 

 oxalate of ammonia are added, which produces a precipitate ot 

 oxalate of lime if the mineral be labradorite, but does not pro- 

 duce a precipitate if it be pure felspar. 

 Dr MaccuUoch mentions, that the glassy felspar which be found 



