44 CLAYS AND CLAY INDUSTRY. 



There are several species of feldspar, which vary somewhat in 

 their chemical composition, and are known by different names, 

 as shown below. 



Composition of Feldspars. 



i Chemical composition. > 



Feldspar species. Silica. Alumina. Potash. Soda. Lime, 



Orthoclase, 64.70 18.40 16.90 .... .... 



Albite, 68.00 20.00 12.00 12.00 



Oligoclase, 62.00 24.00 .... 9.00 5.00 



Labradorite, 53-00 30.00 .... 4.00 13.00 



Anorthite, 43-00 37.00 .... .... 20.00 



Mica. — This is one of the few minerals in clay that can be 

 easily detected with the naked eye, for it occurs commonly in 

 the form of thin, scaly particles, whose bright shining surface 

 renders them very conspicuous, even when small. Very few 

 clays are entirely free from mica, even in their washed condi- 

 tion, for, on account of the light scaly character of the mineral, 

 it floats off with the clay particles. The so-called "kaolins" of 

 the Woodbridge district contain much mica, and it is also notice- 

 able in the hollow-ware clays used along the Raritan river, as 

 well as in Clay Marl I. Mica is rarely seen in the Alloway and 

 Cape May clays. 



There are several species of mica, all of rather complex com- 

 position, but all silicates of alumina, with other bases. Two of 

 the commonest species are the white mica or muscovite, and the 

 black mica or biotite. The former is a silicate of alumina and 

 potash, and the latter a silicate of alumina, iron oxide and 

 magnesia. Of these two, the muscovite is the most abundant in 

 clay, because it is not readily attacked by the weathering agents. 

 The biotite, on the other hand, rusts and decomposes much more 

 rapidly on account of the iron oxide which it contains. The 

 effect of mica in burning is mentioned under alkalies. 



Iron Ores. — This title includes a series of iron compounds, 

 which are sometimes grouped under the above heading, because 

 they are the same compounds that serve as ores of iron, when 

 found in a sufficiently concentrated form. The mineral species 

 included under this head are : 



