TALC. 143 



Banburite. A silicate of lime and boracic acid, of a yellowish white 

 color. H=7. Gr=296. Occurs at Danbury, Ct., with oligoclase. 



3. MAGNESIA. 

 The blowpipe test for distinguishing magnesia when no? 

 disguised by the presence of a metallic oxyd, is given on 

 page 123. None of the silicates of magnesia gelatinize with 

 acids. The species vary in hardness from 1 to 8.* 



1. Hydrous Silicates of Magnesia. 



TALC. 



Trimetric. In right rhombic or hexagonal prisms. M : 

 M = 120 D Usually in pearly foliated masses, separating 

 easily into thin translucent folia. Sometimes stellate, or 

 divergent, consisting of radiating laminae ; often massive, con- 

 sisting of minute pearly scales ; also crystalline granular, oi 

 of a fine impalpable texture. 



Luster eminently pearly, and feel unctuous. Color some 

 shade of light green or greenish white ; occasionally silvery 

 white; also grayish green and dark olive green. R = l — 

 1*5 ; easily impressed with the nail. Gr = 2*5 — 2*9. Lam- 

 inae flexible, but not elastic. 



Varieties. 



Foliated talc. The purest talc, occurring in foliated masses, 

 of a white or greenish white color, and having an unctuous 

 feel. 



Soapstone, or Steatite. A gray or grayish green massive 



talc, snowing often when broken a fine crystalline texture, 



occasionally yellowish or reddish. The Brianqon variety is 



Tnilk- white, with a pearly lustre, very greasy to the feel, or 



like soap. 



Potstone, or Lapis ollaris. An impure talc, of grayish 

 green and dark green colors and slaty structure. Feel 

 unctuous. 



Do any silicates of magnesia gelatinize with acids? Describe talc 

 What is steatite ? What . is potstone 1 



* The base magnesia is replaceable by protoxyd of iron, protoxyd of 

 manganese, or lime, as illustrated in the species pyroxene, and conse 

 quently this group embraces compounds which are not purely silicate 

 of magnesia 



