1824.] Albite, Labrador, and Anorthite. 57 



great analogy of composition. However, an analysis undertaken 

 by my brother, gave, excepting a greater quantity of alumina, 

 almost the same results as that of Klaproth. This chemist has 

 already demonstrated that the iridescent felspar from Friedrich- 

 warn, in Norway, cannot be ranked in this class ; it is also dis- 

 tinguished from it by the incidence of the two faces of cleavage 

 which is equal to 90°. The acids act upon this mineral in a 

 different manner than upon felspar and albite ; for concentrated 

 hydrochloric acid, according to Fuchs, entirely decomposes 

 labrador, and has no action upon felspar or albite. 



Fourth Species. — Anorthite. 

 The primitive form of anorthite is a doubly oblique prism, 

 fig. 5, 6, in which the planes M and T are inclined at an angle 

 of 117° 28' ; the planes M and P at an angle of 94° 12', and the 

 planes T and P at an angle of 1 11° 57'. The section perpendi- 

 cular to the planes M and T is an oblique angled parallelogram, 

 the obtuse angle of which of 117° 28' is divided by the plane 

 produced by two rows in breadth on the edge G into two angles, 

 the one of 59 o 30' and the other of 57° 58' ; the first of which 

 has one of its sides in the plane T, and the other one of its sides 

 in the plane M. The section perpendicular to the planes M and 

 P is an oblique angled parallelogram, the obtuse angle of which 

 equal to 94° 12' is divided by a plane produced by a decrement 

 by one row on the edge B of the primitive into two angles, the 

 one of 46° 47', and the other of 47° 25' ; the first of which has 

 one of its sides in the plane P, and the other one of its sides in 

 the plane M. The planes I have observed are : 



PMT *G-.GH 2 TBCAA AOA*A ,0 4AE 2 (figs.7 8 9). 



I ft t n i> y x <j t o u m Tv> > i J 



Incidences. 



TonM 117° 28' 



Ton/ 120 30 



M' on I 122 2 



M on z 14' 1 



Ton; \6 



M'on/ 15, 



P on y' 98 29 



P on x' 128 27 



Ponr/ 145 12 



Pon* 138 46 



Pono' 121 50 



P on »' 94 53 



P on m 134 46 



/on/ J51 28 



P on M 85 48* 



P on n 133 13* 



