512 tyler. SEDIMENTARY IRON DEPOSITS [Ch. 28 



divided on the basis of genesis. This classification, although simple, 

 is difficult to apply to iron deposits of complex mineralogy or to those 

 which have suffered alteration since deposition. 



IRON OXIDES AND HYDROXIDES 



The following oxides and hydroxides of iron are known to form in 

 a sedimentary environment; hematite (Fe 2 3 ), magnetite (Fe 3 4 ), 

 goethite (Fe 2 3 -H 2 0), zanthosiderite (Fe 2 3 -2H 2 0), and limnite 

 (Fe 2 3 -3H 2 0). Limonite (2Fe 2 3 -3H 2 0), according to Posnjak and 

 Merwin (1919, pp. 311-348), is goethite with absorbed and capillary 

 water. Other oxides of iron such as ilmenite and chromite are derived 

 from pre-existing rocks and transported and deposited as elastics. 



Mechanical Deposits 



Magnetite, ilmenite, and hematite are relatively stable minerals in 

 the environment of weathering. Disintegration or decomposition of a 

 parent rock containing these minerals releases them, and they become 

 a potential source of sedimentary iron deposits. Waves and currents 

 remove the associated lighter minerals and concentrate the heavy 

 minerals as "black sand" or "placer" deposits. The requisite condi- 

 tions for a natural segregation of heavy minerals is usually local, and 

 therefore deposits of this type are impure, thin, lenticular, and of only 

 moderate lateral extent. The mineral and chemical composition is 

 usually complex, with rare minerals such as gold, platinum, and dia- 

 monds often of more economic importance than the iron-bearing min- 

 erals. Raeburn and Milner (1927, pp. 90-128) discuss the various 

 factors that control the localization of placer deposits. Black sand 

 deposits have been rather extensively mined in the past along the St. 

 Lawrence River in Quebec and on the coasts of Japan. The deposits 

 on the coast of Florida have been described by Martens (1928, pp. 81- 

 90), and those along the Oregon coast by Twenhofel (1943, 1946). 



More important from the economic viewpoint are the mechanical de- 

 posits of hematite derived from pre-existing iron deposits. To this 

 group belong the Cambrian conglomerate ores of Missouri described 

 by Crane (1912, pp. 111-112, 127-129), the sand, rubble ore, or canga 

 of Minas Geraes, Brazil, described by Leith and Harder (1911, pp. 

 670-686), and many of the low-level laterites. 



Residual Deposits 



Residual deposits of sedimentary iron form on a wide variety of 

 rocks. The original iron content of the parent rock may be low, as in 



