EXPLORATION OF THE MINERAL WORLD BY X-RAYS 441 
(5) We may now consider some properties conferred upon the minerals 
by these characteristic forms of grouping. 
Olivine 
Separate Groups 
(a) 
Felspar 
Three-climensional 
Mica, Sheets Network 
(c) (¢) 
Fic. 4.—The arrangement of the (Si, Al)—O tetrahedra in the common minerals. 
Tetrahedra are silhouetted in black. 
(a) Olivine.—In olivine the separate SiO, tetrahedra are linked together 
by Fe and Mg octahedra. It is geometrically possible to do this in an 
extremely compact way, without wasting any space. The mineral is also 
very uniform in texture, since there are no exceptionally strong bonds 
