Concretion

A hard compact mass found in sediments or sedimentary rocks formed by precipitation of a mineral cement, often nucleating around a pebble, fossil or piece of organic matter.


Common cements include calcium carbonate (mostly calcite, e.g. 'Claystones', 'Clay Dogs'), siderite, silica (e.g. flint), iron oxides (e.g. 'Moqui Marbles' and 'Indian paint pots'), iron sulfides (especially pyrite) or various phosphates.

i. A hard, compact mass or aggregate of mineral matter, normally subspherical, but commonly oblate, disk-shaped, elongate or irregular with odd or fantastic outlines; formed by precipitation from aqueous solution about a nucleus or centre, such as a leaf, shell, bone, or fossil, in the pores of a sedimentary or fragmental volcanic rock, and usually of a composition widely different from that of the rock in which it is found and from which it is rather sharply separated. It represents a concentration of some minor constituent of the enclosing rock or of cementing material, such as quartz (chert), calcite, dolomite, iron oxides, pyrite, or gypsum, and it ranges in size from a small pellet-like object to a large spheroidal body as much as 3 m in diameter. Most concretions were formed during diagenesis, and many (esp. in limestone and shale) shortly after sediment deposition.

Compare with: nodule

ii. A collective term applied loosely to various primary and secondary mineral segregations of diverse origin, including irregular nodules, spherulites, crystalline aggregates, geodes, septaria (see singular septarium), and related bodies. Not recommended usage.