510 tyler. SEDIMENTARY IRON DEPOSITS LCh. 28 



during bacteria obtain oxygen for the oxidation of carbon from sul- 

 phates, sulphites, and thiosulphates with the result that these com- 

 pounds are reduced to sulphides. (2) Calcium or magnesium sulphides 

 formed in an analogous manner by bacteria react with carbon dioxide 

 in the presence of water and are changed to carbonates and hydrogen 

 sulphide. The hydrogen sulphide may then react on ferrous salts such 

 as ferrous bicarbonate in the water and precipitate ferrous sulphide. 

 (3) Putrifying bacteria produce hydrogen sulphide from sulphur- 

 bearing proteins. (4) The reduction of sulphur to hydrogen sulphide 

 by bacteria takes place in the presence of decomposing organic matter. 

 The iron sulphides, besides forming as a direct precipitate from 

 ferrous solutions, may be formed by the reduction of originally pre- 

 cipitated ferric hydroxide by decaying organic matter. 



Silicates 



The iron silicates, glauconite, chamosite, and greenalite occur in 

 sedimentary iron deposits as important constituents, whereas celado- 

 nite, berthierite, and thuringite are relatively rare. The formation of 

 these iron silicates, with the exception perhaps of greenalite, appears 

 to be related to diagenesis rather than to direct precipitation from 

 solution. 



Glauconite is found on modern sea bottoms and in rocks that were 

 deposited under marine conditions. Hadding (1932, pp. 44-54) con- 

 cludes that the inferences drawn from geologic evidence indicate that 

 glauconite forms only in a shallow water, sublittoral marine environ- 

 ment in which the waters are agitated and where sedimentation is 

 going on very slowly. The environment seems to be intermediate be- 

 tween strongly reducing and strongly oxidizing. However, the mineral 

 celadonite, which is very similar to glauconite in both appearance and 

 chemical composition, occurs as an alteration product of olivine and 

 also fills vesicules in basalt. Celadonite may be formed from olivine 

 in the late cooling stages (deuteric) of a basalt, as Hendricks and 

 Ross (1941, p. 704) suggest, or it may be of fresh-water origin, as 

 Twenhofel (1939, p. 400) postulates. 



The occurrence of glauconite within foraminifera shells lead Mur- 

 ray and Renard (1891, p. 389) to the conclusion that there was a 

 genetic relationship between the shells and glauconite. They con- 

 cluded that shells of foraminifera became filled with mud containing 

 organic matter. Iron in the mud was altered by the decaying organic 

 matter to iron sulphide. Oxidation of the iron sulphide released sul- 

 phuric acid which decomposed the clay, releasing colloidal silica and 

 alumina. The iron, the colloidal silica, and part of the colloidal alu- 



