44 GENERAL DISCUSSION OF CLAYS. 



metallic scales or as a superficial coating on other 

 mineral grains found in the clay. It dissolves quietly 

 in muriatic acid. Iron may also occur in the clay as 

 a constituent element of many silicates, and indeed 

 the effect which it produces may be caused not so 

 much by the actual amount of iron oxide which is 

 present, but by the condition which ^"t is in. 



Iron oxide is very apt to form concretions in the 

 clay, and these concretions which generally have a 

 shell-like structure, vary in diameter, commonly from 

 a fraction of an inch to several inches. Siderite, the 

 carbonate of iron, which is also to be found in many 

 clays, likewise forms concretions or opaque rounded 

 masses, which effervesce on the addition of warm 

 muriatic acid. The exterior of these siderite concre- 

 tions is not unfrequently altered to limonite, the 

 brown or yellowish hydrated oxide of iron. Such con- 

 cretions are hard and rock-like in their nature, and 

 either have to be separated by screening the clay be- 

 fore using, or crushed by passing the clay between 

 rolls. 



PYRITE. 



This mineral is a compound of iron and sulphur, 

 and the grains of it are easily recognized by their 

 metallic lustre and their yellow color. It is a very 

 common constituent of many fire clays, and occurs 

 either in the form of small grains or concretionary 

 masses of yellow crystals. Its bright metallic surface 

 will serve to distinguish H from limonite which has a 

 dirty appearance. 



DOLOMITE. 



This is a double carbonate of lime and magnesia, 

 and may occur in a clay in the same form as calcite, 

 and the effect of it is practically the same. 



