46 CLAYS AND CLAY INDUSTRY. 



This mineral will also change to limonite, if exposed to the 

 weather. 



Pyrite. — This is another mineral, which is not uncommon 

 in some clays, and can be often seen by the naked eye. It is 

 sometimes called iron pyrites, or sulphur, and, chemically, is a 

 sulphide of iron (FeS 2 ). It has a yellow color and metallic 

 lustre, and occurs in large lumps, in small grains or cubes, or 

 again in flat rosette-like forms. Not infrequently it is formed 

 on or around lumps of lignite, and is a familiar object to all 

 the clay miners of the Woodbridge district, and other districts 

 where clays of the Raritan formation are dug, but it is rare in 

 other New Jersey clays. 



When exposed to' weathering action, pyrite is a rather un- 

 stable compound, that is to say, it tends to^ alter, and changes 

 from the sulphide of iron (FeS 2 ) to the sulphate of iron 

 (FeS0 4 ), by taking oxygen from the waters filtering into the 

 clay. This also destroys its form, the yellow metallic particles 

 changing to a white powder)' mineral, which has a bitter taste 

 and is soluble in water. Clays containing pyrite are not, as a 

 rule, desired by the potters. (See Soluble Salts.) 



Glauconite. — This mineral, which is sometimes termed green- 

 sand, and in bulk greensand marl or simply marl, is an im- 

 portant one in some of the New Jersey clays. Chemically, it is 

 a compound containing silica, potash, iron and water (a hydrous 

 silicate of potash and iron), occurring in the form of greenish 

 sandy grains. Its composition is often somewhat variable, and 

 it may contain other ingredients as impurities. Thus a sample 

 from New Jersey 1 analyzed : Silica, 50.70 per cent. ; Alumina, 

 8.03 per cent.; iron oxide, 22.50 per cent.; magnesia, 2.16 per 

 cent; lime, 1.11 per cent.; potash, 5.80 per cent.; soda, 0.75 

 per cent. ; water, 8.95 per cent. It is an easily fusible mineral, 

 and hence a high percentag-e of it is not desired in a clay. 

 Greensand is restricted 2 to the clays of the Clay Marl series, 

 and is most abundant in C^ Marl I. 



Kaolinite. — This mineral is a compound of silica, alumina 

 and water (a hydrated silicate of alumina), represented by the 



1 Dana.. System of Mineralogy, p. 684. 



2 In so far as it occurs in New Jersey clays. 



