CHEMICAL PROPERTIES OF CLAY. 45 



Limonite, (2Fe20 3 ,3H ;; 0). 

 Hematite (Fe-iOs). 

 Magnetite (FesOO. 

 Siderite (FeCOs). 



The first is an oxide, with three parts of water (a hydrous 

 oxide), the second and third are oxides, and the fourth is a 

 carbonate. 



Limonite. — This has the same composition as iron rust. It 

 occurs in various forms, and is often widely distributed in 

 many clays, its presence being shown by the yellow or brown 

 color of the material. When the clay is uniformly colored, the 

 limonite is evenly distributed through it, sometimes forming a 

 mere film on the surface of the grains; at other times it is col- 

 lected into> small rusty grains, or again forms concretionary 

 masses of spherical or irregular shape; in still other clays it 

 is found in the form of stringers and crusts, extending through 

 the clay in many directions (PI. II, Fig. 2). These crusts are 

 common in some portions of Clay Marl II, as at Collingswood, 

 or occasionally also in the Miocene clays. The concretions are 

 abundant in some of the weathered clays at Lorillard, and the 

 beds of sandstone, found in many of the sand and gravel 

 deposits associated with the clays, are caused by limonite cement- 

 ing the sand grains together. 



Hematite, the oxide of iron, is of a red color and may be 

 found in clays, but it changes readily to' limonite on exposure to 

 the air and in the presence of moisture. 



Magnetite, the magnetic oxide of iron, forms black magnetic 

 grains, and, while not common, is sometimes found when the 

 material is examined microscopically. Lake the hematite, it 

 changes to limonite. 



Siderite, the carbonate of iron, may occur in clay in the fol- 

 lowing forms: 1. As concretionary masses of variable size and 

 shape, often strung out in lines parallel with the stratification 

 of the clay. These are more abundant in shales than in clay, 

 and, if near the surface, the siderite concretions change to 

 limonite on their outer surface. 2. In the form 1 of crystalline 

 grains, scattered through the clay and rarely visible to the naked 

 eye. 3. As a film coating other mineral grains in the clay. 



