374 



DESCRIPTIVE MINERALOGY. 



similar crystals are found, but they do not exhibit this cleavage in so marked a manner. Black 

 mica also occurs in small plates in the gneiss near Yonkers. 



In the immediate yicinity of the village of Rye, specimens occur similar to those above 

 described. 



ORDER III. GLUCINA. 



This includes two species : 



1. Emerald. 



2. Chrysoberyl. 



EMERALD. 



Emerald. Cleaveland and TKomson. — Emeraude. Baiiy and Beudant. — Beryl and Emerald. Phillips and 

 Shepard. — Rhombohedral Emerald. Ja-meson. — Rhomboedrischer Smaragd. Mohs. 



Fig. 397. 



Description. Colour green, blue, yellow and grey. Streak white. 

 It occurs regularly crystallized, and in thin prismatic concretions. 

 Primary form a regular six-sided prism, Fig. 397. Cleavage parallel 

 to all the planes of the primary, but not distinct. Fracture conchoidal 

 and uneven. Lustre vitreous. From transparent to translucent on the 

 edges. Hardness from 7.5 to 8.0. Specific gravity 2 . 58 to 2.73. 

 Before the blowpipe, the transparent varieties become clouded, and on 

 increasing the heat, assume the appearance of mother of pearl ; with 

 borax, it fuses into a transparent colourless glass. 



Varieties. 'Emerald and Beryl differ from each other chiefly in their colours. The former 

 is transparent or translucent, and of an emerald green colour, which it derives from oxide of 

 chrome ; the latter, is either green, yellow, blue, grey, or entirely colourless. Between the 

 two varieties there is a regular series, and hence no line of specific distinction can be drawn 

 between them. The variety which is transparent, and has a sky-blue colour, is denominated 

 by lapidaries Aqua marine. 



Composition. Emerald, from Peru — Silica 68.50, alumina 15.75, glucina 12.50, lime 

 0,25, oxide of chromium 0.30, oxide of iron 1 .00 {Klaproth). 



Beryl, from Broddho — Silica 68.35, alumina 17.60, glucina 13.13, oxide of iron 0.72, 

 oxide of columbium 0.27 {Berzelius). This mineral is a silicate of alumina and glucina. 



