'////; HARE METALS 275 



Ores of the Metal. Zircon, ZrSiO4, is the most important 



source of the element and its compounds. Unlike the other rare 

 minerals to which it is allied, it occurs chiefly as a silicate widely 

 diffused iu the igneous rocks. It is easily distinguished from all 

 other minerals by its crystal form; viz., that of a tetragonal 

 prism terminated by a tetragonal pyramid at either extremity; 

 by its color, which shades through brown and yellow to green; 

 and by its hardness of 7.5. 



Zircon is one of the least alterable of all minerals, for it contains 

 no protoxides, and only the most insoluble of dioxides. It, how- 

 ever, passes into the hydrous state, producing amorphous and iso- 

 tropic species or varieties. This is effected by the loss of silica, 

 and the addition of iron oxides through infiltrating waters. Auer- 

 bachite, calyptolite, cryptolite, malacon, oerstedite, and tachy- 

 aphaltite are all altered varieties of zircon. 



In some instances, zircon seems to have been of pneumatolytic 

 origin. According to F. W. Clarke, it is one of the earliest 

 minerals to crystallize from a cooling magma, and the first of all 

 silicates to thus solidify. The varieties of zircon mentioned 

 above are all of secondary origin, arising through the hydration 

 and metamorphism of zircon. 



Beccarite is an olive-green variety of zircon from Ceylon. 



Braddeleyite, ZrOz, is an oxide of zircon found in Brazil and 

 Ceylon. 



Geographical Distribution. Zircon is one of the commonest 

 constituents of all classes of igneous rocks. It is more abundant, 

 however, in the acid than in the basic intrusives. It is especially 

 abundant in the granites, pegmatites, syenites, gneisses, diorites 

 and pyroxenites, and in the younger eruptives. The most 

 noted American locality is in Burke, McDowell, Henderson, 

 Polk and Rutherford Counties, N. C., where it occurs in the gold- 

 bearing monazite sands, due to the disintegration of granite and 

 gneissoid rocks. At Grenville, Canada, it occurs in a crystalline 

 limestone, in association with wollastonite, titanite and graphite. 



Geological Horizon. Zircon is not restricted to any horizon, 

 for it occurs in the igneous rocks of all ages. 



Method of Extraction. Zircon is separated from its matrix by 

 rough crushing and washing. A clean separation can be made 

 with electrical machinery and by careful washing. A small 

 quantity of zircon is obtained as a by-product from the monazite 

 concentrates. 



