48 THE CHICAGO ACADEMY OF SCIENCES. 



The pleochroism is distinct in colored varieties and the axial 

 plane is arranged in the same manner with reference to the 

 percussion figures. Its occurrence is similar to that of biotite. 



Vermiculites. 



\Yhen micae decompose, they give rise to hydrated sili- 

 cates, which closely resemble the original mineral. These 

 alteration products are placed by Dana in the group of VERMIC- 

 ULITES. They retain more or less perfect micaceous cleavage, 

 but are without elasticity, and are softer than mica and of 

 varying specific gravity. Usually they are of very low specific 

 gravity. Their lustre is pearly or bronze-like, and their color 

 yellow to brown, so that they are often brought into the labo- 

 ratory as specimens of gold, in spite of the fact that the indi- 

 vidual scales will float upon water. When heated to 100 C, 

 hygroscopic water is given off. When heated to 300, another 

 portion of water is given off. When the mineral is gradually 

 heated, it opens out slowly, forming long worm-like masses. 



Serpentine. H 4 Mg 3 Si 3 O 9 . 



The decomposition of amphibole and pyroxene often pro- 

 duces finely fibrous, felted and cryptocrystalline granules of 

 serpentine, which under the microscope appear translucent, 

 greasy in lustre and feebly pleochroitic. The general physical 

 properties of this mineral are not well represented in this 

 region, since none of the massive forms appear. 



Talc. H 3 Mg 3 (Si0 3 ) 4 . 



Associated with the fibers of serpentine are often found 

 scales of talc, a sectile, flexible mineral, characterized by its 

 greasy feeling, its softness which is I in the scale, and its. spe- 

 cific gravity of 2.7. Lustre on the cleavage surfaces is pearly. 

 The color is white or green. 



The CLAY GROUP consists of soft earthy crystalline and 

 amorphous minerals which in composition are chiefly 

 hydrated silicates of aluminum. They are usually mixed with 

 great quantities of foreign substances. The pure form of the 

 hydrated aluminum silicate is kaolinite. 



Kaolinite. H 4 Al 2 Si,O 9 . Monoclinic. 



This occurs rarely in small crystals. Sometimes it is in 



scales, usually in brown earthy masses. Cleavage unknown. 



