Concretion in the context of Pisolite

Polymetallic nodules, also called manganese nodules, are mineral concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core. As nodules can be found in vast quantities, and contain valuable metals, deposits have been identified as a potential economic interest. Depending on their composition and authorial choice, they may also be called ferromanganese nodules. Ferromanganese nodules are mineral concretions composed of silicates and insoluble iron and manganese oxides that form on the ocean seafloor and terrestrial soils. The formation mechanism involves a series of redox oscillations driven by both abiotic and biotic processes. As a byproduct of pedogenesis, the specific composition of a ferromanganese nodule depends on the composition of the surrounding soil. The formation mechanisms and composition of the nodules allow for couplings with biogeochemical cycles beyond iron and manganese. The high relative abundance of nickel, copper, manganese, and other rare metals in nodules has increased interest in their use as a mining resource.

Nodules vary in size from tiny particles visible only under a microscope to large pellets more than 20 centimetres (8 in) across. However, most nodules are between 3 and 10 cm (1 and 4 in) in diameter, about the size of hen's eggs. Their surface textures vary from smooth to rough. They frequently have botryoidal (mammillated or knobby) texture and vary from spherical in shape to typically oblate, sometimes prolate, or are otherwise irregular. The bottom surface, buried in sediment, is generally rougher than the top due to a different type of growth.

A pisolite (from Ancient Greek πίσον (píson) 'pea') is a sedimentary rock made of pisoids, which are concretionary grains – typically of calcium carbonate which resemble ooids, but are more than 2 mm in diameter. These grains are approximately spherical and have concentric layers reaching 10 mm in diameter.

Bauxites, limonites, and siderites often have a pisolitic structure.

In geology and particularly in sedimentology, a nodule is a small, irregularly rounded knot, mass, or lump of a mineral or mineral aggregate that typically has a contrasting composition from the enclosing sediment or sedimentary rock. Examples include pyrite nodules in coal, a chert nodule in limestone, or a phosphorite nodule in marine shale. Normally, a nodule has a warty or knobby surface and exists as a discrete mass within the host strata. In general, they lack any internal structure except for the preserved remnants of original bedding or fossils. Nodules are closely related to concretions and sometimes these terms are used interchangeably. Minerals that typically form nodules include calcite, chert, apatite (phosphorite), anhydrite, and pyrite.

Nodular is used to describe a sediment or sedimentary rock composed of scattered to loosely packed nodules in a matrix of like or unlike character. It is also used to describe mineral aggregates that occur in the form of nodules, e.g. colloform mineral aggregate with a bulbed surface.

Rūrutu is the northernmost island in the Austral archipelago of French Polynesia, and the name of a commune consisting solely of that island. It is situated 572 km (355 mi) south of Tahiti. Its land area is 32.7 km (12.6 sq mi). It is 10.8 km long and 5.3 km wide. Its highest point (Manureva) is 389 m (1,276 ft). At the 2022 census it had a population of 2,163.

Geologically, Rurutu was initially formed 12 million years ago by the Macdonald hotspot, a hotspot associated with the Macdonald seamount. Over the next 10 million years, erosion shrank the island until it was almost an atoll. Then, just over a million years ago, Rurutu passed over the Arago hotspot, which lifted it roughly 150 meters. Steep sea cliffs of ancient coral lifted by the event—called makatea—now largely encircle the island. These are riddled with caves filled with concretions—indeed, Rurutu is largely unique among islands in French Polynesia in that its historic inhabitants were cave-dwelling.

A Stagnosol in the World Reference Base for Soil Resources (WRB) is soil with strong mottling of the soil profile due to redox processes caused by stagnating surface water.

Stagnosols are periodically wet and mottled in the topsoil and subsoil, with or without concretions and/or bleaching. The topsoil can also be completely bleached (albic horizon). A common name in many national classification systems for most Stagnosols is pseudogley. In the USDA soil taxonomy, many of them belong to the Aqualfs, Aquults, Aquents, Aquepts and Aquolls.