436 MINERALOGY 



In the formation of amphiboles the rate of cooling, the presence 

 of an excess of magnesium over calcium, as well as the presence of 

 water and pressure, all have an influence in determining whether 

 an amphibole or pyroxene will form. Amphibole is the less stable 

 form of the two, as when fused and cooled a pyroxene will be formed, 

 it being impossible to form amphiboles by a simple fusion in an open 

 crucible. 



The amphiboles artificially are produced with difficulty and only 

 by long-continued heating of their constituents, with water and 

 under pressure. The magnesium metasilicate with optical proper- 

 ties of an amphibole has been formed by simple fusion. 



BERYL 



Beryl. Beryllium aluminium metasilicate, Be 3 Al 2 (Si0 3 )6 ; 

 BeO = 14.11, A1 2 O 3 = 19.05, SiO 2 = 66.84; Hexagonal; Type, 

 Dihexagonal Equatorial; c = .4989; 0001 A 1011 =29 57'; 0001 

 A 2021 =49 2'; Common forms, c (0001), m (10lO), r (lOll), 

 u (2021) ; Twinning not observed ; Cleavage, basal imperfect ; 

 Brittle; Fracture, conchoidal ; H. = 7.5-8; G.= 2.63-2.80; Color, 

 shades of green, yellow, white, or lilac; Streak, white; Luster, 

 vitreous; Transparent to subtranslucent ; co = 1.584; = 1.578; 

 o)_ 6 = .006; Optically (-). 



B.B. Whitens and fuses with difficulty, only on the thinnest 

 edges. When containing alkalies, it may fuse easier. Insoluble 

 in acids. 



General description. Crystals are stout prismatic in habit, 

 usually combinations of the hexagonal prism and pyramid of the 

 first order and the base, often striated or furrowed lengthwise. 

 Various other pyramids and complex combinations have been de- 

 scribed from Alexander County, North Carolina. Hexagonal prisms 

 of enormous size, weighing hundreds of pounds, occur at Grafton 

 and Ac worth, New Hampshire. It is also common in the pegma- 

 tites of Maine, at Albany, Norway, Bethel, and Paris, usually 

 associated with tourmaline, spodumene, and lepidolite. 



The clear green variety is the gem emerald, but owing to the 

 prevalence of cavities it is very difficult to obtain an emerald that 

 is flawless. The emeralds of commerce are in large part mined at 

 Muzo, United States of Colombia, where they are found in a crys- 

 talline limestone and slates. Fine emeralds are also obtained from 



