8.6.4: Metamorphosed Iron Formations

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8.57.jpg — Figure 8.57: Banded iron formation from Dales Gorge, Western Australia

Ironstone is a general name we give to sedimentary rocks that contain more than 15% iron. These rocks may contain iron hydroxides (limonite), oxides (magnetite and hematite), carbonates (siderite), or silicates (chamosite, Fe-rich chlorite). They generally have a uniform, nonfoliated texture.

Iron formations are similar to ironstones but are mainly Precambrian (ironstones are Phanerozoic). Iron formations generally contain abundant chert and are often well banded with bands ranging from centimeters to meters thick. The bands consist of alternating iron- and chert-rich layers. Figure 8.57 photo shows an example of iron formation from western Australia.

When ironstones and iron formations are metamorphosed, they quickly lose any original hydrous minerals. But any of the other original minerals may persist. At the lowest grades of metamorphism, magnetite and hematite most commonly dominate. If the original rock was rich in carbonate, siderite (Fe-carbonate) will be present. And sometimes pyrite is present as well. At higher grades, greenalite, minnesotaite, and glauconite (all iron silicates) may form. At still higher grades, metamorphism may produce actinolite, grunerite, hedenbergite, or fayalite. The table below summarizes these relationships.

Common Minerals in Metamorphosed Iron Formations
low grade	→	high grade
quartz SiO₂	pyrite FeS₂	actinolite Ca₂(Fe,Mg)₅Si₈O₂₂(OH)₂
hematite Fe₂O₃	greenalite Fe₂_-₃Si₂O₅OH₄	grunerite Fe₇Si₈O₂₂(OH)₂
magnetite Fe₃O₄	minnesotaite Fe₃Si₄O₁₀(OH)₂	hedenbergite CaFeSi₂O₆
siderite FeCO3	glauconite (K,Na)(Fe,Al,Mg)₂(Si,Al)₄O₁₀(OH)₂	fayalite Fe₂SiO₄

The photos below show minerals common in metamorphosed iron formations. The hematite shown in Figure 8.58 is specular hematite (more common hematite has a red earthy color). Actinolite, seen in Figure 8.59, is a calcium-iron amphibole. Grunerite (Figure 8.60) is an iron amphibole. Greenalite (Figure 8.61) is an iron-rich variety of serpentine. Siderite (the brown mineral in Figure 8.62) is an iron carbonate, and pyrite (Figure 8.63) is iron sulfide. The pyrite in Figure 8.63 is somewhat tarnished.

8.58.png — Figure 8.58: Hematite from near Marquette, Michigan

8.59.png — Figure 8.59: Actinolite, northern Wisconsin

8.60.png — Figure 8.60: Grunerite from near Marquette Michigan. 7 cm across

8.62.png — Figure 8.62: Siderite with calcite from near Roxbury, Connecticut. Photo is about 15 cm across

8.63.png — Figure 8.63: Centimeter – sized pyrite crystals in metamorphosed iron formation near Marquette, Michigan