520 tyler. SEDIMENTARY IRON DEPOSITS [Ch. 28 



IRON SILICATES 



Greenalite, glauconite, and chamosite are the most common iron 

 silicates formed in a sedimentary environment. Thuringite, berthierite, 

 and celadonite are relatively rare. 



Greenalite 



Greenalite occurs associated with chert and siderite as an important 

 constituent of the pre-Cambrian Biwabik iron formation of Minnesota, 

 but it is rare to absent in the pre-Cambrian iron formations of Michi- 

 gan and Wisconsin. It has been reported by Hadding (1929, p. 236) 

 from Liassic sandstones of Sweden and by Kennedy (1936, pp. 433- 

 436) from the Ordovician of Scotland; and the writer has observed a 

 greenalite-like mineral to be associated with chert in the iron formation 

 of northern Labrador. Greenalite normally occurs as dark-green el- 

 lipsoidal granules 0.1 to 1.0 millimeter in diameter, embedded in a 

 matrix of chert and siderite. The rock usually ranges from well- 

 bedded to massive and averages about 28 percent iron. 



Glauconite 



Glauconite is the most widely distributed iron silicate. It rarely is 

 found as a pure deposit but occurs in shales, sandstones, and limestones 

 of all ages from Cambrian to present marine sediments. Glauconite 

 occurs as granules and in the form of pigmentary powder. Pure glau- 

 conite rarely contains as much as 20 percent iron, and since it is often 

 associated with clay, sand, or phosphatic shell fragments it forms low- 

 grade, but rather extensive, iron deposits. 



There are three beds of rather pure glauconite in the Cretaceous 

 series of New Jersey which range individually from 10 to 50 feet in 

 thickness and aggregate about 90 feet. The area of these deposits is 

 about 120 miles in length by 40 miles in width. Eckel (1914, p. 56) 

 estimates that the total glauconite in the Cretaceous series contains 

 250 thousand millions of tons of iron oxide. Glauconite beds that have 

 been subjected to surface weathering, such as the Weches horizon of 

 east Texas, may be enriched sufficiently to form limonite ores. 



Chamosite 



Chamosite is often associated with berthierite, thuringite, and sider- 

 ite. Chamosite is present in the Wabana ores of Newfoundland, the 

 Minette ores of Lorraine, the Jurassic ores of the Cleveland Hills, 

 England, and to a limited extent in the Clinton ores of the Appalachian 

 region. 



