OXIDKS 



363 



IRON HYDROXIDES 



Limonite. Hydroxide of iron;, Brown hematite; Bog iron 

 ore; 2Fe 2 O, .3H 2 O; Fe 2 O 8 = 85.5, H 2 O = 14.5; Amorphous; 

 Brittle; H. = 5-5.5; G. = 3.6-4; Color, dark brown to ocher- 

 yellow ; Luster, submetallic to dull ; Streak, ocher-yellow. 



B.B. Like hematite, but yields water in the closed tube. 



General description. All the varieties of hydrated oxides of 

 iron are colloids with no constant chemical composition ; their 

 water content varies with 

 their state of dehydration, 

 with the one exception of 

 gothite, FeO(OH), which 

 crystallizes in the ortho- 

 rhombic system. They are 

 all secondary minerals pro- 

 duced by oxidation and hy- 

 dration in the belt of weath- 

 ering ; where deposited from 

 solution in caves and cavi- 

 ties they form stalactites, 

 botryoidal masses, and 

 crusts, with a fibrous or 

 radiated structure which at 

 times have a very character- 

 istic surface as if varnished, 

 or as if having been fused, 

 and in some specimens 

 beautifully iridescent. 



In the oxidation of sulphides, as pyrite or marcasite, ferrous 

 sulphate is formed and carried in solution, while a hydrated oxide 

 of iron remains behind, as the gossans, capping many of the sulphide 

 ore deposits of the West. Many crystals of pyrite have been re- 

 placed in this way by pseudomorphs of limonite and turgite 

 (2Fe 2 O 3 . 2 H 2 O). Large specimens of this nature are obtained from 

 Australia, and small crystals, though perfect in outline, are found 

 near Lancaster, Pennsylvania. Beautiful pseudomorphs of limo- 

 nite after marcasite occur in a clay in Richland County, Wiscon- 

 sin. Many pyrite crystals in Virginia and Maryland have a 

 coat of varying thickness of limonite enclosing the still unoxidized 

 sulphide. 



FIG. 4.m- 



Limonite Stalactite. Ore Hill, 

 Connecticut. 



