Oaks said:
Ok. Can you explain what that is please?
There are so many different types that its hard to give an exact answer. Here are a few things I took off of the internet.
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http://meteorites.wustl.edu/id/concretions.htm
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Concretions are hard bodies that form in sediments before they become sedimentary rocks. Slow chemical changes, perhaps related to microbial activity, cause minerals to come out of the groundwater and cement the sediment together. Most often the cementing mineral is calcite, but the brown, iron-bearing carbonate mineral siderite is also common. Iron would give it a redish or brown color. The reason it would not attract a magnet despite containing iron is that iron in combination with other elements is often not magnetic. Some concretions have a central particle, such as a fossil, that triggered the cementation. Others have a void, perhaps where a central object dissolved away, and others have nothing special inside, perhaps because the cementation was imposed from outside. A concretion consists of the same material as the rock around it, plus the cementing mineral, whereas a nodule(like flint nodules in limestone) is composed of different material. Concretions can be shaped like cylinders, sheets, nearly perfect spheres and
everything in between. In size they can range from as small as gravel to as large as truck.
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Dating back to the 18th century concretions have been known as geologic curiosity's due to the various sizes, shapes and compositions. Concretions have also been thought to be dinosaur eggs, extra-terrestrial debris, human artifacts and animal and plant fossils. Due to these curiosity's I will try and enlighten you more on these in the following by explaining the process in which they are formed and explaining some locations where they can be found in large outcrops. "The word "concretion" is derived from the Latin "con"-- meaning "together" -- and "cresco" -- meaning "to grow."" Concretions are hard compact accumulations of mineral matter and are found inside sedimentary rocks. Some examples of this are Sandstone and in some weathered volcanic rock. Concretions come in many different shapes and the most common of the shapes is spherical or disk shaped. Concretions are the most varied-shaped rocks of the sedimentary world. The way concretions come to be is the mineral matter concentrates around the nucleus of a host rock. The nucleus is often organic such as a tooth or leaf or shell or fossil. As the mineral matter concentrates around the nucleus it forms harder zones known as nodules.Concretions are very odd in the sense that they very in size, shape, color and hardness. Often concretions are mistaken for bones, fossils, meteorites and other odd objects. They can be so small that it requires a magnifying glass to be visible or as large as 10 feet in diameter and weigh hundreds of pounds. Concretions can also have somewhat of regular shapes such as boxes, blocks, flat disks, pipes, cannon balls and have even been known to resemble parts of a human body such as a foot or ribs. Concretions are most commonly composed of calcite but sometimes can be composed of iron oxide or iron hydroxide such as goethite. But also concretions can be composed of other minerals ranging from siderite, ankerite, dolomite, pyrite, baryite and gypsum. Concretions form in many different ways. The box shape of some ironstone concretions most often depend on the way a sandstone or shale bed breaks up due to the action of weathering into regular blocks of various sizes. Hematite and magnetite are two iron oxide minerals and most iron ore deposits consist mainly of hematite, magnetite, or both. Hematite concretions or nodules, sometimes called ironstone concretions, are often mistaken for meteorites because their unusual shapes catch people's attention and they are denser than most other rocks. Hematite concretions are formed in sedimentary rocks by oxidation of pyrite (iron sulfide) crystals or precipitation of iron oxide from iron rich solutions. Concretions, 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 a hematite concretion is rich in iron, it is usually not particularly magnetic. Magnetite, on the other hand, is highly magnetic, and it often forms nodules, too. Hematite is easy to identify because it makes a red streak on unglazed tile. 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.
