SILK Ml S, T1TANATES I 



In (heir crystallization from the magma they usually follow the 

 pyroace&ee, forming the ground mass in which the crystals and miner- 

 als previously rrystalli/ed arc embedded, as is often the case with 

 (he feldspars; from this relation they are termed the feldspathoids. 

 In >eclions when well formed they are square or six-sided in 

 outline, otherwise rounded and irregular, colorless, greenish to blue. 



FIG. 481. Section of Haiiynite, showing the Dark, Dust-like Inclusions 

 collected near the Margin. 



The relief is slightly rough. Inclusions in sodalite are rare, but 

 fine particles of glass and cavities filled with gas are characteristic 

 of haiiynite. They are often collected at the center, or marginal 

 or in concentric bands in the crystal section. The feldspathoids 

 are easily decomposed; the first step is hydration, forming zeolites , 

 numerous pseudomorphs of natrolite after sodalite occur. When 

 alteration is pushed still further muscovite and kaolin are formed. 



Lazurite is used as an ornamental stone and in inexpensive 

 jewelry, as lapis lazuli. It is particularly prized in Russia for 

 tables, vases, and in the decoration of altars in the churches. It is 

 mined at Badakschan in Siberia. A fine quality also comes from 

 Persia. The pigment ultramarine was formerly the natural mineral 



