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Some examples of minerals in iron-rich rocks containing oxides are limonite, hematite, and magnetite. An example of a mineral in iron-rich rock containing carbonates is siderite and an example of minerals in an iron-rich rock containing silicate is chamosite. [2] They are often interbedded with limestones, shales, and fine-grained sandstones.
Ironstone (sandstone with iron oxides) from the Mississippian Breathitt Formation, Mile Marker 166, I-64, Kentucky. Ironstone is a sedimentary rock, either deposited directly as a ferruginous sediment or created by chemical replacement, that contains a substantial proportion of an iron ore compound from which iron (Fe) can be smelted commercially.
Banded iron formation – Distinctive layered units of iron-rich sedimentary rock that are almost always of Precambrian age Breccia – Rock composed of angular fragments Calcarenite – Type of limestone that is composed predominantly of sand-size grains
Iron-rich sedimentary rocks are composed of >15% iron; the most common forms are banded iron formations and ironstones. [ 6 ] Phosphatic sedimentary rocks are composed of phosphate minerals and contain more than 6.5% phosphorus ; examples include deposits of phosphate nodules , bone beds, and phosphatic mudrocks.
The conversion of iron hydroxide and silica gels to banded iron formation is an example of diagenesis, the conversion of sediments into solid rock. There is evidence that banded iron formations formed from sediments with nearly the same chemical composition as is found in the BIFs today.
Iron-rich rocks are common worldwide, but ore-grade commercial mining operations are dominated by the countries listed in the table aside. The major constraint to economics for iron ore deposits is not necessarily the grade or size of the deposits, because it is not particularly hard to geologically prove enough tonnage of the rocks exist.
Iron oxides are derived from mafic igneous rocks and other iron-rich rocks; bauxites are derived from granitic igneous rock and other iron-poor rocks. [15] Nickel laterites occur in zones of the earth which experienced prolonged tropical weathering of ultramafic rocks containing the ferro-magnesian minerals olivine, pyroxene, and amphibole. [10]: 3
Channel iron systems typically form within a depression on the Cenozoic ‘Hamersley Surface’, and form several pods downstream on the palaeodrainage. The channels show typical fluvial sedimentary morphology, with channel scours truncating or incising the channel iron deposits, and rare examples of graded bedding, and so on.