MINERAL CONSTITUENTS OF ROCKS. 



57 



Chrysolite (called also Olivine), occurring in green glassy grains or crystals, and com- 

 mon in many basaltic rocks. Consists of silica, magnesia, and iron. 



Garnet, in crystals of the forms in Figs. 46, 47, disseminated in various crystalline 



Fig. 46. 



Fig. 47. 



rocks ; colors usually red to black ; rarely green. Consists of silica, alumina, Fig. 48. 

 magnesia, lime, and iron. s\ \iN 



Epidote, in yellowish-green prismatic crystals and masses; also of brown K A-^ 

 and gray-white colors. Constituents as in garnet. 



Scapolite, in four-square and eight-sided erect prisms, white to gray, 

 and sometimes greenish or reddish. One of the forms of its crystals is 

 shown in Fig. 48. Constituents, silica, alumina, lime, and usually some 

 soda. 



Andalusite, in whitish, grayish, prismatic crystals, nearly square, imbedded in slaty 

 Fig. 49. Fig. 50. 



rocks. Crystals having the interior tessellated with black, as in figure 49, are called 

 Chiastolite. Composition: silica 37 - 0, alumina 63'0. 



Staurolite, in rhombic prisms of 129° 20/, imbedded in slaty rocks. Usual colors, 

 brown to black. The crystals are often crossed as in Fig. 50, and hence the name, 

 from the Greek for cross. Composition: silica 29*3, alumina 53*5, sesquioxyd of iron 

 17-2 = 100. 



Cyanite (spelt also Kyanite), in thin and often long-bladed crystals of sky-blue to 

 white color. Same composition as Andalusite. Named from the Greek for blue. 



Tourmaline. — Usually in three-sided or six-sided black crystals, showing no distinct 

 cleavage, and thus differing from hornblende. Figs. 51, 52 show two of the forms ; and 



Fig. 53. 



Fig. 52. 



