SII.K -\TI.S. TITANATKS, ETC. 



Y- = .()">(); Optically (-); Axial plane = 001; Bx* = b ; 

 2 V = 49 50'. 



B.B. Fuses easily in the O. F. and becomes magnetic on coal 

 in II. !'. (lelatini/es with HC1. 



Genera] description. Fayalite occurs in small crystals, flat- 

 tened parallel to the orthopinacoid, in the rhyolites and ob- 

 sidian of the Yellowstone Park. It has also been identified in a 

 granite at Ilockport, Mass. Originally it was described from, 

 Fayal Island, Azores. 



It is also a common constituent of furnace slags. 



1 1 is easily decomposed by weathering, the iron being oxidized 

 to ferric oxide. This takes place along the cracks and on the 

 .-urface. the crystals becoming dark brown or black and at times 

 iridescent. 



Knelx'lite is a massive mineral containing manganese, or may 

 lie considered as a mixture of fayalite and tephroite. 



Tephroite is the manganese orthosilicate, Mn 2 SiO 4 member of 

 the olivine group. It occurs as a brown or red massive mineral 

 at Franklin, New Jersey, where it is associated with zincite, wil- 

 lemite, and franklinite. 



Roepperite is a variety from the same locality containing some 

 zinc. 



WILLEMITE 



Willemite. Zinc orthosilicate ; Zn 2 SiO 4 ; ZnO = 73, SiO 2 

 = 27 ; Hexagonal ; Type, Hexagonal Alternating ; c = .6775 ; 

 0001 A lOfl = 38 2' ; l_0ll A 1011 = 64 30'; Common forms, 

 c(0001), a (1120), r(1011), e (0112) ; Cleavage, basal and pris- 

 matic easy ; Brittle ; Fracture, conchoidal ; H. = 5.5 ; G. = 3.89 

 -4.18; Color, shades of green, yellow, brown, and red; Streak, 

 white or pale; Transparent to translucent; CD = 1.6931; = 

 1.7118; -w = .0187; Optically (+). 



B.B. Whitens and fuses with difficulty. On coal in R.F. 

 with soda and a little coal dust yields a zinc coat. Gelatinizes 

 with HC1. 



General description. Crystals are hexagonal, prismatic in 

 habit, terminated by rhombohedrons. It occurs at Franklin, New 

 .Jersey, in stout prisms an inch across and several inches in length : 

 these crystals contain manganese, and are red or ash color to dark 



