452 MINERALOGY 



brown, known as troostite. In all other localities the crystals are 

 very small, as at Altenberg, Saxony, where the crystals are only a 

 few millimeters in length. 



At Franklin it occurs in sufficient quantities to form a valuable 

 ore of zinc, but mostly in cleavable yellow or light green masses, 

 associated with zincite, franklinite, and tephroite. At times it 

 forms long needle-like crystals, apple-green or nearly white in 

 color, or in radiated masses; this last form is rare. The light 

 colored varieties phosphoresce under the influence of magne- 

 sium light or the radiations of radium, especially the radiated 

 variety. 



Willemite has often been observed in furnace slags, where a lead 

 ore containing zinc is being smelted. 



PHENACITE 



Phenacite. Beryllium orthosilicate ; Be 2 SiO 4 ; BeO = 45.55, 

 SiO 2 = 54.45; Hexagonal; Type, Hexagonal Alternating; c = 

 .6611 ; 0001 A lOll = 37 21' ; r A r' = 63 24' ; Common forms, 

 a (1120), m (1010), r(1011), x (1232) ; Cleavage, a indistinct, 

 r imperfect ; Brittle ; Fracture, conchoidal ; H. = 5-8 ; G. = 2.97- 

 3 ; Color, white, pale yellow, brown, or red ; Streak, white ; Luster, 

 vitreous; Transparent to translucent; o> = 1.6542; = 1.6700; 

 -a= .0158; Optically (+). 



B.B. Infusible, with borax in fine powder fuses to a clear 

 glass. With cobalt solution yields a dull blue. Yields reactions 

 for beryllium. 



General description. Crystals are rhombohedral in habit or 

 short prismatic, terminated by rhombohedrons, at times, of all 

 three orders. In its occurrences it is associated with beryl and 

 topaz. 



The largest crystals, some nearly four inches across, are obtained 

 near Ekaterinberg in the Urals, Russia. At Pike's Peak it is found 

 implanted on crystals of microcline. 



Phenacite when clear and free of flaws is polished as a gem 

 stone. 



DIOPTASE 



Dioptase. An acid copper orthosilicate, H 2 CuSi0 4 ; CuO = 

 50.4, SiO 2 = 38.2, H 2 O = 11.4; Hexagonal; Type, Hexagonal 

 Alternating; c =.5341; 0001 A 1011 = 31 40'; r A r' = 545'; 



