SILICATES, TITANATES, ETC. 455 



The scapolites are easily decomposed by weathering, particularly 

 those containing sodium, the ultimate products being kaolin, talc, 

 or micas, with calcite and quartz. 



Tiit- artificial production of the scapolites is in doubt. 



VESUVIANITE 



Vesuvianite. An orthosilicate of calcium and aluminium, 

 Ca(Al . OH)Al 2 (SiO 4 ) B , in which other isomorphous elements 

 may enter; CaO = 42.3, A1 2 = 19.1, SiO 2 = 37.5, H 2 O = 1.1; 

 Tetragonal; Type, Didigonal Equatorial; c = .5372; 001 A 101 

 = 28 15'; 001 A 111 = 37 14'; Common forms, c (001), a (100), 

 m(110), p(lll); Cleavage, prismatic imperfect, a and c less so; 

 Brittle ; Fracture, uneven ; H. = 6.5 ; G. = 3.35-3.45 ; Color, 

 shades of brown and green; Streak, white; Translucent; <o = 

 1.705; c = 1.701 ; <o - = .004; Optically (-), sometimes (+) 



B.B. Fuses with intumescence at three, to a brown slag, which 

 when powdered gelatinizes with HCl. Some varieties will yield 

 reactions for manganese or copper. 



General description. Crystals are well developed, stout pris- 

 matic combinations of the prisms of the first and second orders 

 terminated by the base and the unit pyramid. Striations on the 

 prism zone lengthwise. Beautiful crystals, combinations of 

 these forms, are found on the Vilui River, Siberia. Crystals from 

 Wakefield, Quebec, have the pryamid reduced to almost a line and 

 are terminated by the base. Well-formed crystals, combinations of 

 all seven forms of the type, are found at Poland, Maine, and more 

 complicated combinations are found on the crystals from Vesuvius, 

 in some of which the prism zone is very much reduced, yielding 

 crystals of pyramidal habit. Vesuvianite also occurs radiated, 

 columnar, irregular, granular or massive. A compact, green, jade- 

 like variety is known as californite. 



Chemically the calcium may be replaced in part by manganese, 

 magnesium, or iron ; also fluorine and boron may be present in small 

 amounts. In sections Vesuvianite may appear in crystalline out- 

 lines, rounded or irregular, colorless or pale. The relief is well 

 marked, with prismatic cleavage cracks imperfectly developed. 

 Pleochroism faint, but increasing with the depth of color of the 

 specimen. Interference colors gray of the first order. Basal 

 sections show only the shadow of the interference figure but no 

 colors. Optically negative, rarely positive. 



