Ch. 28] DEPOSITS OF SEDIMENTARY IRON 511 



mina combined with potash, extracted from the sea water, to form the 

 mineral glauconite. This explanation does not account for the large 

 quantities of glauconite not associated with foraminifera tests. 



Galliher (1935, pp. 1351-1366) notes the association of glauconite 

 with partially altered biotite in Monterey Bay off the California coast 

 and suggests that glauconite is formed from biotite by the loss of some 

 alumina, potash, and magnesia, the oxidation of iron, and a gain of 

 water. The granular occurrence of much glauconite is accounted for 

 as follows. The biotite flakes swell greatly during the process of al- 

 teration, so that they do not pass freely through the digestive tracts 

 of mud-dwelling worms, but are molded into coprolite granules. It 

 seems probable that glauconite derived from biotite can be of local 

 significance only, for muscovite (the dominant mica in materials 

 derived from crystalline rocks) is absent or rare in many glauconite 

 deposits, and furthermore some greensand beds are nearly pure glau- 

 conite. It is probable that the silica, alumina, and iron of glauconite 

 were derived from mud and at least part of the potassium and mag- 

 nesium were derived from the marine waters. 



Little is known regarding the origin of chamosite, berthierite, and 

 thuringite, but most writers assume that they are products of diagen- 

 esis. Ailing (1947, p. 1012) suggests that the chamosite in the Clinton 

 formation of New York was formed by solutions percolating through 

 the unconsolidated sediments shortly after deposition. 



The origin of greenalite is not well understood. Van Hise and Leith 

 (1911, pp. 518-529) suggest that the greenalite in the Lake Superior 

 iron formations was deposited as a chemical precipitate in waters, 

 which received their iron and silica by direct contribution from a 

 magma or by the reaction of sea water with hot submarine lavas. 

 Gruner (1922, pp. 407-460) concludes that the greenalite in the Bi- 

 wabik formation of Minnesota was formed in a manner similar to 

 glauconite. He suggests that the iron and silica were derived from a 

 basic igneous terrane through normal weathering. 



DEPOSITS OF SEDIMENTARY IRON 



Sedimentary iron deposits usually consist of one or more iron-bear- 

 ing minerals associated with non-iron-bearing clastic or chemical sedi- 

 ments. Clays, silts, sands, chert, and carbonates of calcium and mag- 

 nesium are often interbedded with or act as diluents of the iron-bear- 

 ing minerals. The various types of sedimentary iron deposits may be 

 classified into four groups: iron oxides and hydroxides, iron carbonate, 

 iron sulphide, and iron silicates; and each group may then be sub- 



