310 DESCRIPTIONS OF MINERALS. 



19-09=100-19. From Texas, Pa. ; Webster, N. C. ; MicMpicoton 

 Island, Lake Superior ; Malaga, Spain ; Saasthal, Upper Valois. 

 Bottisite is similar. 



Saponite. 



Soft, clay-like, of the consistence before drying of cheese 

 or butter, but brittle when dry. Color white, yellowish, 

 grayish-green, bluish, reddish. " Does not adhere to the 

 tongue. 



Composition. A hydrous silicate of magnesia containing 

 some alumina. 



From Lizard's Point, Cornwall, in serpentine. Also from 

 geodes of datolite, Roaring Brook, Conn. ; in trap, north 

 shore of Lake Superior. 



Kaolinite. 



Trimetric. I A 1=120°. Occurs massive, clay-like, but 

 consisting usually of thin, microscopic, rhombic or hex- 

 agonal, crystals ; either compact, friable, or mealy. 



Color white, grayish-white, yellowish, sometimes brown- 

 ish, bluish, or reddish. Scales transparent or translucent ; 

 flexible, inelastic, greasy to the touch. H. = l-2*5. Gr.= 

 2-4-2-6. 



Composition. H 2 A1 8 Si 2 4- 1 aq= Silica 46-4, alumina 39*7, 

 water 13*9 = 100. The similarity of the composition to that 

 of serpentine will be seen on comparing the two formulas. 

 B. B. infusible. A blue color with cobalt solution. Yields 

 water in the closed tube. Insoluble in acids. 



Obs. The soapy feel of kaolinite distinguishes a clay con- 

 sisting of it from other kinds of clay ; and when common 

 clays are " unctuous " it is usually owing to the presence of 

 kaolinite. Kaolinite has been made through the decompo- 

 sition of aluminous minerals, and especially the potash and 

 soda feldspars, orthoclase, albite, and oligoclase. In the 

 case of these feldspars the process (1) removes the alkalies ; 

 (2) leaves the alumina, or a large part of it, and part of the 

 silica ; and (3) adds water. So that, with orthoclase, K 2 Al 

 ]6 Si 6 becomes changed to H 2 A1 8 Si 2 + l aq ; half the water 

 which is added replaces K 2 which is removed. Many gran- 

 ites, gneisses, and other feldspar-bearing rocks undergo 

 rapidly this change, so that extensive beds of kaolinite have 

 been formed and are now making in many regions. The 

 kaolinite is usually washed out by streams or the waves from 

 the decomposed material to make the large pure deposits. 



