470 



MINERALOGY 



axinite. Striations on the prism zone parallel to the vertical axis 

 are characteristic. The color is usually clove-brown, but varies 

 with the replacement of calcium with iron or manganese. 



Like most other borates, axinite is formed by pneumatolytic 

 action, and therefore usually appears in the cracks and veins of 

 granites and diabases, or in metamorphic contact zones. 



Fine crystals implanted on the walls of veins in diabase occur 

 at Bourg d'Oisans, Dauphine', France; at St. Gothard, Switzer- 

 land. In the United States it occurs a Franklin, New Jersey, in 

 yellow crystals associated with garnets and rhodonite ; at Bethle- 

 hem, Pennsylvania, and at Phippsburg, Maine. 



PREHNITE 



Prehnite. An orthosilicate of calcium and aluminium ; H 2 Ca 2 - 

 Al 2 (Si0 4 ) 3 ; CaO = 27.1, A1 2 O 3 = 24.8, SiO 2 = 43.7^ H 2 O = 

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

 .8401 : 1 : .5549 ; 100 A 110 = 40 2'; 001 A 101 = 33 27'; 001 A 

 Oil = 29 2'; Common forms, c (001), a (100), b (010), m(110), 

 o (061) ; Cleavage, basal distinct ; Brittle ; Fracture, uneven ; 

 H. = 6-6.5 ; G. = 2.8-2.95 ; Color, light green, oil-green, yellow, 

 white; Streak, white; Luster, vitreous; Nearly transparent to 

 translucent; a = 1.616; p = 1.626; -y = 1.649; -y - a = .033; 

 Optically ( + ) ; Axial plane = 010 ; Bx a = c ; 2 V = 69 22'. 



B.B. Fuses easily with intumescence to a blebby glass and 

 gelatinizes after fusion with HC1. After the separation of silica 



and aluminium with 

 ammonia, yields a 

 heavy white pre- 

 cipitate with am- 

 monium carbonate 

 (calcium). Yields 

 water in the closed 

 tube. 



General descrip- 

 tion. Crystals are 

 small and rarely 

 simple, but in par- 

 allel position or 



FIG. 496. Prehnite. Bergen Hill, New Jersey. 



joined in ridged groups with a rough surface on which the indi- 

 vidual crystals may be seen to be joined by the base, with the 



