IRON 



923 



The following analyses of some carefully-selected samples of the haematite of the 

 carboniferous limestone are by Messrs. Dick and Spiller. (Memoirs of the Geological 

 Survey of Great Britain. The Iron Ores of Great Britain. Part I.) 



The carboniferous limestones of Derbyshire and Somersetshire also contain veins 

 and deposits of haematite, though of a quality not equal to those of Lancashire ; the 

 same ore is also met with in the Devonian series of Devon, West Somerset, and Corn- 

 wall. See HEMATITE. 



Brown Hematite. Under this term are included the following minerals : 



1. Torgite, of the composition 2Fe 2 O s + HO with 5 -6 per cent, of water. 



2. Gothite Fe 2 3 + H0 10- 1 



3. Limonite 2Fe 2 0' + 3HO 14-4 



Of these minerals, the first also known as hydro-hematite, resembles the ordinary 

 red haematite, giving a bright red streak, but decrepitates when heated. It has 

 hitherto been considered to be of rare occurrence ; but latterly it has been 

 generally recognised that the red ore of Bilbao is to a considerable extent of this 

 composition, and probably all hydrated ores giving a red as distinguished from a 

 brown streak may be referred here. Gothite is a finely-crystallised mineral, which 

 has been found at Kestormel in Cornwall in great abundance ; other varieties are 

 known as Lepidocrocite and StUpnosiderite. 



Limonite includes all the massive and earthy varieties. The so-called hydrate of 

 iron is a calcareous brown haematite, occurring at the base of the Cheadle coal-field in 

 North Staffordshire. It is highly esteemed in South Staffordshire for making special 

 classes of pig-iron. 



Brown haematites being in all cases produced by the alteration of other ores, their 

 quality is in great part determined by the composition of the mineral from which they 

 have been derived. Substantially they may be referred to the action of atmospheric 

 air and water upon two minerals namely, carbonate of protoxide of iron, and iron 

 pyrites and in a lesser degree they may be formed from magnetite or silicates 

 containing protoxide of iron. Beds of clay ironstone (argillaceous carbonate) are 

 usually altered at the surface into an earthy brown haematite, and lodes of spathic 

 carbonate are at times changed to considerable depths into a similar substance, which 

 change in composition is often not attended with any alteration of form, but the mass 

 becomes spongy in the interior, giving a free passage to the reducing gases in the 

 furnace, which renders them easily reducible ; a change that is further accelerated by 

 the fissuring produced by the expulsion of the confined water on heating. The great 

 brown iron ore lodes of Bilbao, which occur in cretaceous limestones, are supposed to 

 be likely to produce spathic ores when worked to greater depths. 



The change of iron pyrites into brown iron ore is familiar to all miners under the 

 term of gozzan, where the pyritic contents of lodes have been changed to a red rusty 

 mass, which is taken as an indication of the presence of the sulphides of the more valu- 

 able metals in depth. These gozzans are not workable as iron ores, mainly from 

 the presence of quartz, and sulphides and arsenides of other metals. A very good 

 ore derived from the alteration of iron pyrites is shipped in considerable quantities 



