TALC AND SOAPSTONE. 



BY JOSEPH HYDE PRATT. 



[Joseph Hyde Pratt, geologist and consulting mining engineer; born Hartford, Conn., 

 February 3, 1870; graduated from the Sheffield Scientific school of Yale university; 

 instructor in mineralogy at Yale and later at the University of North Carolina; state 

 geologist of North Carolina, 1897-99 ; assistant geologist of the United States geological 

 survey since 1900. Author of about 125 monographs and other articles on miner- 

 alogy and geology, mostly contributed to scientific periodicals.] 



The name talc has been used very commonly, and yet 

 erroneously, for a number of minerals which are similar to 

 it in physical properties, but distinct mineralogically. Com- 

 mercially, the name talc is usually applied to the fibrous and 

 foliated varieties, which are the purer forms, and the name 

 soapstone confined to the massive varieties. Mineralogically, 

 the name talc refers not only to the foliated and fibrous vari- 

 eties, but also to those which are compact, and soapstone or 

 steatite is simply a variety of this mineral. Talc may be con- 

 sidered as occurring in the varieties, foliated and massive, 

 with a third division known as fibrous talc, which is usually 

 called pseudomorphous, as it has generally resulted from the 

 alteration of the mineral enstatite. The foliated talc is the 

 most valuable, being pure and very free from grit, so that it 

 is suitable for use in the manufacture of talcum powders, etc. 

 Occasionally this variety is so compact that it can be used 

 in the manufacture of tailors' pencils, when it commands the 

 highest price paid for any talc. Certain varieties of the mas- 

 sive talc are also pure enough to be ground into a flour talc, 

 but the greater portion is used in the manufacture of soap- 

 stone articles. 



The properties of talc (exclusive of soapstone) that make 

 it suitable for the purposes for which it is used are its extreme 

 softness, its purity or freedom from grit, its stabihty, and its 

 smooth, sUppery surface. Since the minerals serpentine and 

 pyrophyllite closely approximate many of these properties, 

 they are used to some extent for the same purposes. This is 

 true especially of the latter, which can be used for many of 

 the purposes for which flour talc is employed. 



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