IMPORTANT ELEMENTS AND MINERALS 45 
The Mica Group. — In these minerals, of which there 
are numerous species, depending upon differences in 
chemical composition, the one characteristic feature is 
the cleavage. This is so remarkably developed that 
the mineral splits readily mto very thin, elastic plates 
(Fig. 7). The micas are all silicates of alumina, vary- 
ing in color, usually from hght brown to deep black ; 
and they are all so soft, that they are easily scratched 
with a knife, and sometimes even with the finger- 
nail. 
Like the other complex silicates of alumina (for, in 
addition to silicon, oxygen, and aluminum, these contain 
such elements as potassium, magnesium, iron, etc., in 
percentages varying among the different species), the 
micas usually decay easily, forming soluble and insolu- 
ble products, the former passing off in the water, the 
latter often remaining, usually as clayey remnants. 
Seme micas are not so easily decayed as many of the 
rock-forming minerals, and so in the soil, on the beach, 
and in the beds which have been made by the decay 
of other rocks, we often find little glittering particles 
of mica. 
This mineral is common in lavas, granites, and many 
other rocks; and either the fresh mica or its decayed 
product enters into the soil, while the soluble salts 
formed from its decay pass into the waters of the 
earth. 
