450 MINERALOGY 



B.B. Fuses on the thin edges. Gelatinizes with HC1 ; this 

 solution freed of silica yields little or no precipitate with ammonia, 

 but a heavy white precipitate with ammonium carbonate. 



General description. In crystalline habit like olivine ; also 

 granular and in cleavable masses. It is a product of metamorphism, 

 and as such occurs in ejected blocks of limestone at Monte Somma, 

 Vesuvius. Large crystals nearly an inch in length occur at Mag- 

 net Cove, Arkansas. In all other respects it agrees with olivine, 

 though much more restricted in its occurrence. 



FORSTERITE 



Forsterite. Magnesium orthosilicate ; Mg 2 Si0 4 ; MgO = 

 57.00, Si0 2 = 42.9; Orthorhombic ; Type, Didigonal Equatorial; 

 a:b:c = .4666:1:. 5868; 100 A 110 = 24 55'; 001 A 101 =51 

 34'; 001 A Oil = 30 21'; Common forms as in olivine; Cleav- 

 age, b distinct, c less so ; H. = 6-7 ; G. = 3.21-3.33 ; Color, 

 white and light shades of yellow to green ; Streak, white ; Luster, 

 vitreous ; Brittle ; Fracture, conchoidal ; Transparent to trans- 

 lucent; = 1.659; -y a high; Plane of the optic axes = 001 ; 

 Bx a = a; 2V = 86. 



B.B. Infusible, gelatinizes with HC1, this solution freed of 

 silica, an excess of ammonia added yields no precipitate with am- 

 monium carbonate (calcium), but yields a white precipitate with 

 ammonium phosphate (magnesium). 



General description. In crystalline habit like olivine, but is 

 largely a product of metamorphism ; as such it occurs in the ejected 

 blocks of limestone at Monte Somma, Vesuvius. A variety, Bol- 

 tonite, occurs as embedded crystals and disseminated grains in a 

 limestone at Bolton, Massachusetts. 



FAYALITE 



Fayalite. Ferrous orthosilicate ; Fe 2 Si0 4 ; FeO = 70.6, Si0 2 

 = 29.4 ; Orthorhombic ; Type, Didigonal Equatorial ; a : b : c 

 = .4584 : 1 : .5793 ; 100 A 110 = 24 38' ; 001 A 101 = 51 39' ; 

 001 A Oil = 30 5'; Forms as in olivine; Cleavage, b distinct, 

 a less so; Brittle; Fracture uneven; H. = 6.5; G. = 4-4.14; 

 Color, yellow to nearly b]ack; Streak, pale when unoxidized; 

 Transparent to opaque; a = 1.824; |3 = 1.864; -y = 1.874; 



