Ch. 28] DEPOSITION OF IRON 509 



iron-bearing minerals with light minerals such as quartz and feldspar, 

 but occasionally the iron-bearing minerals form black sand concen- 

 trates on beaches, in river channels, or in sand-dune areas. 



Ferric Oxides 



Colloidal ferric oxide is precipitated, according to Moore and May- 

 nard (1929, p. 278), by various electrolytes, such as sodium chloride, 

 potassium sulphate, magnesium sulphate, potassium chloride, potas- 

 sium nitrate, sodium hydroxide, or sea water with a salt concentration 

 of 34,400 parts per million. If the environment of deposition is one in 

 which oxygen is abundant, ferric hydroxide will also be precipitated 

 from ferrous bicarbonate solutions. An exhaustive study by Harder 

 (1919, p. 77) of the bacteria that precipitate ferric hydroxide indicates 

 that they may be classified into three groups: (1) bacteria that re- 

 quire solutions of ferrous bicarbonate utilizing carbon dioxide set free 

 and the available energy of reaction for their life processes; (2) bacte- 

 ria that do not require ferrous bicarbonate but cause the precipitation 

 of ferric hydroxide when either organic or inorganic iron salts are pres- 

 ent; (3) bacteria that attack iron salts of organic acids, using the or- 

 ganic radical as food and leaving ferric hydroxide, or basic ferric salts 

 that gradually change to ferric hydroxide. 



Ferrous Carbonate 



Iron transported as the bicarbonate may be precipitated as the car- 

 bonate (siderite) in an environment deficient in oxygen. Precipitation 

 may be brought about by agitation, decrease in pressure, increase in 

 temperature, bacteria, or the photosynthesis of green plants. 



Sulphides 



The iron sulphides, marcasite, pyrite, melnikovite, and hydrotroilite, 

 are deposited under reducing conditions from solutions containing vari- 

 ous iron salts. Precipitation may be brought about by hydrogen sul- 

 phide liberated by decaying organic matter or by the reduction of 

 ferrous sulphate, sulphites, or thiosulphates by bacteria, organic mat- 

 ter, or other reducing agents. 



Experiments by Allen, Crenshaw, and Johnston (1912, p. 215) in- 

 dicate that marcasite forms in acid solutions, pyrite in neutral or 

 slightly acid solutions, and melnikovite in alkaline solutions. Hydro- 

 troilite is a black hydrous monosulphide of iron which is probably 

 deposited as a hydrophyllic colloid. 



Harder (1919, p. 41) states that bacteria may bring about the 

 deposition of iron sulphide in four ways. (1) Anaerobic sulphate re- 



