OCCURRENCE AND ALTERATIONS OF LIMONITE. 233 



so far as known, an original pyrogenic constituent of the igneous rocks. 

 The most important source of bodies of limonite is precipitation from 

 iron-bearing solutions, especially iron carbonate. For iron carbonate the 

 reaction is — 



4FeCO s +20+3H 2 0=2Fe 2 3 .3H 2 0+4C0. 2 +k. 



A second important source for limonite is the oxidation and hydration of 

 solid iron carbonate in rocks, especially siderite, ankerite, parankerite, and 

 iron-bearing limestone or dolomite. The source next in importance is the 

 oxidation and hydration of pyrite, marcasite, and other sulphides. A fourth 

 important source of limonite is the oxidation and hydration of the ferrous 

 iron of silicates. A fifth source is the hydration of hematite. A sixth 

 but unimportant source is the oxidation and hydration of magnetite. All the 

 reactions involve oxidation or hydration, or both, and therefore take place 

 with the liberation of heat. In the production of limonite from iron 

 carbonate there is an important contraction of volume; in the other cases 

 the volume of the limonite is greater than that of the compounds from 

 which it is derived. All the above reactions producing limonite occur in 

 the zone of katamorphism, and the controlling factor is the first part of 

 van't Hoff's law, that of chemical reactions with the liberation of heat. 

 Limonite does not develop in the zone of anamorphism. 



In summary, limonite is derived from the following minerals: Actino- 

 lite, ankerite, anthophyllhe, arfvedsonite, biotite, bronzite, epidote, garnet, 

 greenalite, griinerite, hematite, hornblende, hypersthene, ilmenite, mag- 

 netite, marcasite, olivine, parankerite, pyrite, pyrrhotite, serpentine, and 

 siderite. 



Alterations.; — The important alterations of limonite are into hematite 

 (rhombohedral; sp. gr. 5.20-5.25) and siderite (rhombohedral ; sp. gr. 

 3.83-3.88). Hematite produced from limonite may be earthy or crystalline. 

 The reaction is — 



2Fe 2 3 .3H 2 0=2Fe 2 3 +3H 2 0-k. 



The decrease of volume is 37.78 per cent. The change is therefore one 

 of dehydration, reduction of volume, and crystallization. The transforma- 

 tion takes place on a g'reat scale in the zone of anamorphism, that in which 

 pressure controls whether heat is absorbed or liberated. 



The second important chang*e of limonite is into iron carbonate. 

 Where this change occurs organic compounds are commonly present and 



