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Iron-oxide concretions and nodules
Addtime: 2016/12/16 Read:5624 Font size: Large Small
Hematite and magnetite are two iron oxide minerals. Most iron ore deposits consist mainly of hematite, magnetite, or both.
Iron oxide concretions, iron oxide nodules, and ironstones are often mistaken for meteorites because their unusual shapes catch people's attention and they are denser than most other rocks. Hematite concretions form by precipitation of iron oxide from iron rich solutions. Hematite nodules are often formed in sedimentary rocks by oxidation of pyrite or marcasite (iron sulfide) crystals. Concretions and nodules, which may also be composed in part of the iron oxy-hydroxides, limonite and goethite, come in a wide variety of bizarre shapes. Sometimes they're shiny on the surface, which might give the impression of a meteorite fusion crust. Note that while hematite is rich in iron, it does not attract a simple magnet. Magnetite, on the other hand, is highly magnetic, and it often forms nodules, too. In mixed magnetite-hematite nodules, some parts might attract a magnet and other parts may not.
Hematite is easy to identify because it makes a red streak. Also, hematite concretions are denser (heavier for their size) that any kind of stony meteorite or most other kinds of Earth rocks. For example, the specific gravity of an ordinary chondrite (the most common kind of meteorite) is about 3.9 whereas the specific gravity of a hematite concretion is 4.2-5.
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