THE NONMETALLIC MINERALS. 281 



8. Tkona; Ukao. 



This is a hydrous sodium carbonate, corresponding to the fornuda 

 Na,,C03.HNaCO,+2H.,0, = carbon dioxide, 38.9 per cent; soda, 41.2 

 [)er cent; water, 10.9 per cent. 



Found in nature as an efflorescence or incrustation from the evapo- 

 ration of lakes, particularJy those of arid regions. W. P. Blake has 

 recently described ' crude carbonate of soda (Trona) occurring- in the 

 central portion of a basin-shaped depression or dry lake in southern 

 Arizona, near the head of the Gulf of California. The deposit covers 

 an area of some 60 acres to a depth of from 1 to 3 feet, the lower 

 portion being saturated with water from a solution so strong that 

 when exposed to the air soda is deposited at the rate of an inch in 

 thi<-kness for every ten days. In its native condition the soda is 

 naturally somewhat impure, from silt blown in from the surrounding 

 land. The analysis given below shows the general average: 



Sand, silt, etc 13.00 



Iron oxides and alumina 2. 80 



Lime 1-14 



Salt (NaCl.?) 4.70 



Sulphate of soda 4. 70 



Carbonate of soda 73. 66 



100. 00 

 See further mider Thernardite, p. 415. 



VI. SILICATES. 



1. Feldspars. 



The name feldspar is given to a group of minerals resembling each 

 other in being, chemically, silicates of aluminum with varying amounts 

 of lime and the alkalies potash and soda. All members of the group 

 have in common two easy cleavages whereby they split with even, 

 smooth, and shining surfaces along planes inclined to one another at 

 angles of nearly if not quite 90°. (Specimen No. 67301, U.S.N.M.) 

 They vary from transparent through translucent to opaque, the opaque 

 form 1)eing the more frequent. In colors they range from clear and 

 colorless through white and all shades of gray to yellowdsh, pink and 

 red, more rarely greenish. 



On prolonged exposures to the w^eather they become whitish and 

 opaque, gradually decomposing into soluble carbonates of lime and the 

 alkalies, and soluble silica, any one of which may be wholly or in part 

 removed by percolating w^aters, leaving behind a residual product, con- 

 sisting essentially of hydrous silicates of alumina, to which the names 

 kaolin and clay are given (see p. 325). The hardness of the feld- 

 spars varies from 5 to 7 of Dana's scale; specific gravity 2.5 to 2.8. 



^ Engineering and Mining Journal, LXV, 1898, p. 188. 



