OttDEK vi. PRISMATIC FELD-SPAR. 857 



ii. Labradorite. 



The varieties of the present species differ but little from 

 the preceding one, in regard to their geometrical proper. 

 ties, at least their combinations likewise possess a tetar- 

 to-prismatic character, and the angle at which the two 

 more distinct faces of cleavage meet is also nearly 93 

 30'. They present very frequently the same kinds of 

 twin-crystals, or regular compositions. Their cleavage, 

 particularly in the direction of Pr -f co, is less perfect 

 than in most varieties of the two preceding species. The 

 hardness is almost exactly identical, but the specific gravity 

 of a variety from the coast of Labrador was found = 2*750. 

 The limits are 2-69 ... 2-76. The remarkable opalescent 

 and iridescent tints of colour appear most bright upon 



Pr + os, but they are also observed upon ?. The va- 



riety from Norway, commonly called Labradorite, in which 

 the opalescence takes place in the direction of Pr + co, does 

 not belong to the present species, but to that of prismatic 

 Feld-spar. 



iii. Common Felspar from the Saualpe In Carinthia and from 

 St Gotlwrd. 



Some of the varieties found in these localities seem to be- 

 long likewise to a particular species. Their forms are te- 

 tarto-prismatic, but differing as it seems in regard to their 

 angles from Albite. Cleavage is much more easily observ- 



ed in the direction of 5 and I (|!LifjlI. than in the 



2 2 



direction of Pr + co, which generally appears only in very 

 slight indications. The specific gravity is equal to that of 

 prismatic Feld-spar, the limits being 2-54 ... 2-56. This 

 species occurs in very remarkable twin-crystals, some of 

 them like Fig. 80 and 81, or Fig. 87, some like Fig. 88. 



In the latter the face of composition is parallel to ?5, 



2 

 while the axis of revolution is parallel to the edges of com- 



VOL II. R 



