Are concretions meteorites?
No. Concretions are rounded lumps formed in sediments on Earth, often iron-rich and heavy, which makes them common meteorwrongs.
Concretions are among the most common rocks mistaken for meteorites. They form within layers of sediment, such as sand or mud, when minerals precipitate from groundwater around a nucleus, gradually cementing grains into a hard, rounded lump.
What they look like. Concretions can be spherical, disc-shaped, knobbly or oddly shaped, ranging from marble-sized to several metres across. Many are made of calcite, silica or iron minerals such as siderite and haematite. Iron-rich ones can be dark brown or black, heavy, and sometimes weakly magnetic, which makes them look meteorite-like.
How to tell them apart. - Inside. A broken concretion often shows concentric layers, like an onion, or a central core, sometimes containing a fossil. Meteorites do not have concentric layers. - Crust. Concretions lack a fusion crust. Their surface may be rough, sandy or coated with iron oxides. - Streak. Iron-rich concretions often leave a red-brown or yellow-brown streak. - Grains. Under a hand lens, sand or silt grains may be visible. - Location. They are often found eroding out of sedimentary rock layers, such as on cliffs, riverbanks and beaches.
Famous examples. Large spherical concretions occur at Moeraki in New Zealand. Small iron concretions found in some deserts resemble the tiny spheres found by NASA's Opportunity rover on Mars in 2004, nicknamed blueberries, which are also concretions, formed in water on Mars.
Concretions are fascinating in their own right, recording groundwater chemistry and sometimes preserving fossils.