USEFUL METM - 193 



mineral in the crystalline schists but magnetite is far more abun- 

 dant. According to C. H. Smyth, Jr., the oolitic hematite con- 

 sists of concentric layers of hematite deposited around grain- 

 of quartz. The ore was formed in the shoal waters of a Silurian 

 MS, presumably upon a sandy bottom. The iron was di.-solvi-d 

 from ferruginous rocks and precipitated from solution by organic 

 matt or, or by oxidation, or by the carbonate of lime. The Clinton 

 ores may be regarded as representing a contemporaneous meta- 

 somatic deposit in which the ore was formed during or imme- 

 diately after the deposition of the original rock. 



Hematite also occurs as a subsequent metasomatic ore body. 

 Here the replacement took place some time after the deposition 

 and consolidation of the original rock. The ferric sulphate 

 traversing limestones may have the iron thrown out of solution 

 by the action of calcium carbonate as hematite, and this mineral 

 will then be deposited along the walls of a fault, or the joint 

 plains of the limestones, or at the junction of two different 

 terranes possessing different possibilities for the migration of 

 solutions. 



Such ore bodies are widely distributed and often associated 

 with limestones. 



There are five hydrous oxides of iron grouped together in this 

 discussion under the name of limonite, because from a commercial 

 standpoint all hydrous oxides of iron giving a brown, yellowish- 

 brown, or reddish-brown streak are classified in the trade as limo- 

 nite. Limonite is far the most abundant mineral of the group 

 and forms large ore bodies. The five hydrous oxides are gothite, 

 Fe 2 3 , H 2 O; xanthosiderite,Fe 2 O3,2H 2 0; (limnite, Fe 2 3 , 3H 2 0) ; 

 turgite, 2Fe 2 3 ,H 2 0; limonite, 2Fe 2 3 ,3H 2 0. 



Gothite is the only mineral in the group that is crystalline. 

 The others are amorphous, and all sorts of admixtures between 

 them may occur. The impurities often found in these ores 

 afford some clue as to origin. They comprise sand, clay, organic 

 matter, the carbonates of iron, calcium and magnesium, the hy- 

 droxides of aluminum and manganese, and the phosphate of 

 iron, vivianite. 



Limonite may result from the hydration of hematite. The 

 decomposition of ferruginous rocks in situ yield laterite which here 

 contains a mixture of the iron and aluminum hydroxides. The 

 limonitizationof pyriteor chalcopyrite is manifested in the gossan 

 caps often reaching to a considerable depth. Waters charged with 



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