IRON 



297 



or more iron. The traditional method is to deposit 

 this waste, which initially is in the form of a slurry, 

 in settling ponds. The waste itself is not chemically 

 reactive — it consists mainly of quartz — but because 

 of the fine grain size the settled material effectively 

 chokes off organic life. No satisfactory alternative 

 for large-scale disposal or utilization of this material 

 has yet been devised, and the problem deserves in- 

 tensive research. 



GEOLOGIC ENVIRONMENT 



Iron is one of the more abundant and ubiquitous 

 elements in the earth's crust. Common sedimentary 

 rocks contain 2-3 percent iron and basalt and gab- 

 bro contain about 8.5 percent; the average content 

 of crustal rocks is about 5 percent. Rock mined eco- 

 nomically ranges in iron content from about 20 

 percent to 69 percent, so the concentration factor 

 ("concentration clarke") relating crustal abundance 

 to minable ore is very low — 4-14, as compared with 

 factors of 10^ or 10=* for elements such as manganese, 

 chromium, copper, and lead (James, 1966) . 



The ability of iron to exist in more than one 

 valence state, coupled with its general abundance, 

 accounts for the presence of iron minerals in de- 

 posits of widely different character and associations. 

 The geochemical cycle of iron is complex, but it is 

 reasonably well known and contains may points at 

 which the element can be trapped to form significant 

 concentrations. Economically minable deposits are 

 formed by magmatic segregation, hydrothermal 

 replacement, direct sedimentation and diagenesis, 

 and weathering processes at the surface and in the 

 subsurface. 



ORE MINERALS 



Nearly 300 minerals contain iron as an essential 

 component, but only about six of them can be classed 

 as ore minerals, and of these only the oxides — 

 magnetite, hematite, and goethite (limonite) — are 

 of major importance (Gross, 1965; James, 1966; 

 United Nations, 1970). Siderite (the iron carbo- 

 nate), pyrite and pyrrhotite (iron sulfides), and 

 chamosite (an iron silicate) are mined locally, but 

 deposits of these minerals are at present of minor 

 economic significance. Ilmenite, the iron-titanium 

 oxide, is a potential source of byproduct iron, but 

 it cannot be classed at this time as an iron-ore 

 mineral. 



The ore minerals are listed as follows : 



Magnetite (FeaOi). — Black, strongly magnetic. 



Iron content, 72.4 percent. Principal ore mineral of 



those parts of Precambrian banded iron-formations 



mined as taconite; associated with quartz and gen- 



erally minor amounts of iron carbonate and iron 

 silicates. Principal ore mineral of replacement de- 

 posits, in which associated minerals vary with type 

 of deposit but commonly include specularite, pyrite, 

 and pyrrhotite, and silicates such as garnet and 

 actinolite. 



Hematite (FeaOs). — Black (if coarse) to red (if 

 fine), depending upon grain size; virtually nonmag- 

 netic. Iron content, 70 percent. As specularite, a 

 common and locally important ore mineral with 

 magnetite in taconite and in replacement deposits. 

 As red iron oxide, a principal constituent of sec- 

 ondary (direct-shipping and semitaconite) deposits 

 in Precambrian iron-formations, associated with 

 goethite (limonite), and in younger sedimentary 

 oolitic ironstones, associated with goethite, calcite, 

 and clastic quartz. 



Goethite (FeaOa-HjO). — Yellow to dark brown, 

 commonly soft and earthy, may be hard and brittle ; 

 nonmagnetic. Iron content, about 60 percent. 

 Limonite is the general term applied to earthy or 

 impure goethite. Principal constituent of ores origi- 

 nating by surface or near-surface weathering proc- 

 esses, as in laterites; associated minerals mainly 

 hematite and clay minerals. 



Siderite (FeCOs). — Gray, but exposed material 

 quickly acquires tan or reddish color owing to sur- 

 face oxidation; nonmagnetic. Iron content of pure 

 mineral is 48 percent, but in most siderite the iron 

 is replaced by significant amounts of manganese, 

 calcium, or magnesium. A major constituent of Pre- 

 cambrian iron-formations and of some younger 

 ironstones, but in North America mined as an iron 

 ore only in the Michipicoten district of Ontario, 

 where it is associated with pyrite. 



Pyrite (FeSj) and pyrrhotite (Fbi-^S). — Yellow, 

 with metallic luster ; pyrrhotite is weakly magnetic. 

 Pyrite is a principal mineral in the relatively scarce 

 sulfide facies of Precambrian iron-formations and 

 younger ironstones; mined as an iron ore in North 

 America only in the Michipicoten district of On- 

 tario (see above). Surface oxidation of pyrite and 

 pyrrhotite bodies gives rise to deposits of iron ox- 

 ides that have been mined locally. 



Chamosite ((Mg,Fe,Al)6(Si,Al)40i4(0H)8). — 

 Greenish, fine-grained, chloritelike ; nonmagnetic. 

 Major constituent of post-Precambrian oolitic iron- 

 stones; locally mined as ore in Europe, associated 

 with siderite and goethite. 



CLASSIFICATION OF DEPOSITS 



The ores of iron fall into four main categories, 

 based on mode of origin (James, 1966). The genetic 

 classification outlined below is basically geologic 



