Spherical concretions found in sedimentary rocks are fascinating natural objet trouvés because of their rounded shapes and distinct boundaries. They consist of several minerals, including carbonate minerals, silicate minerals, and iron oxides. Well-preserved fossils are often found in concretions, particularly those composed of calcium carbonate. Concretions are thought to form by diffusion and the development of a syn-depositional reaction front that travels rapidly from the center of the concretion toward its outer margins. Based on the examination of several hundred spherical calcium carbonate concretions, we developed a diffusion-based model to represent the generalized growth conditions of spherical concretions. This model shows that spherical concretions grow rapidly during the first few years of diagenesis. In particular, carbonate concretions consist mainly of CaCO₃, and their permeability is greatly reduced by cementation and sealing by calcite. As a result, any fossils inside the concretion are well preserved, as water is prevented from penetrating the concretion after its formation. This sealing can provide strong resistance to weathering for more than a million years. Based on this model, we have developed synthetic concretion-forming solvents. To test the effectiveness of these solvents in sealing groundwater flow paths, we conducted an in situ experiment in an underground laboratory in Horonobe, Hokkaido. In the experiment, groundwater flow paths in the excavation damaged zone around an underground gallery were successfully sealed. The experiment showed a decrease in permeability by a factor of 1/100 to 1/1,000 over one year. Here we present a detailed model of the concretion formation process and our conclusions about the sealing process. This sealing process can be applied to activities that require long-term containment of material underground; for example, the geological disposal of nuclear waste and underground carbon dioxide storage. These applications will become increasingly important in the near future.

A bed containing large ammonoids (mostly Mesopuzosia yubarensis, 40-60 cm in diameter) and an underlying bed containing huge calcareous concretions in the middle part of the Obisagawa Member (lower Coniacian), Ashizawa Formation, Futaba Group, are exposed in the Iwaki City Ammonite Center. This study investigated these beds to reconstruct their formation on the basis of sedimentary facies and taphonomy, as well as geochemical analyses of the concretions, including major elements, mineral components, and carbon and oxygen isotopic compositions. Numerous shells of M. yubarensis lacking soft tissues may have been transported into the littoral region through postmortem drift from the initial offshore habitat. Subsequently, these shells were partially destroyed (especially body chambers and umbilical whorls), sorted in size, accumulated, and transported offshore by a series of storm waves and storm-induced currents. Finally, the remains were scattered on mounds of hummocky cross-stratification and rapidly buried within amalgamated hummocky cross-stratified very fine sandstone on the offshore side of a lower-shoreface sedimentary environment. Abundant calcareous concretions of 15-194 cm in diameter and with prolate to oblate spheroid shapes are densely and uniformly distributed under the ammonoid bed. Considering the spatial positioning of the ammonoid shells and concretions within the beds, and isotopic values of δ¹³C = −6‰ to −1‰ and δ¹⁸O = −11‰ to −6‰ for 21 concretion samples, the concretions are interpreted to have begun to form in association with the decomposition of organic matter that had accumulated under the influence of storm waves and storm-induced currents, and carcasses of prolific meiobenthos organisms within shallow substrata. Subsequently, the concretions were buried more deeply and enlarged through further filling of calcium carbonate involving bicarbonate ions generated by the methanogenetic decomposition of organic matter.

Carbonate concretions occur in sedimentary rocks of widely varying geological ages throughout the world. Recently, more than 100 gigantic carbonate concretions with diameters ranging from 1 to 9 m have been identified along the Unosaki coast of Oga Peninsula, Akita Prefecture, Japan. The formation process of such gigantic concretions, some of which along the Unosaki coast contain whale bones, remains uncertain. A mineral composition analysis reveals that the major mineral of the concretions is dolomite. Considering the location of dolomite precipitation, their composition implies that the concretions were formed in a reducing environment in which sulfate ions were removed. Stable carbon and oxygen isotopic analysis reveals that the CaCO₃ of whale bone and concretions contains light δ¹³C and heavy δ¹⁸O, suggesting that whale organic matter contributed to the formation of the concretions. The gigantic carbonate concretions were presumably formed by the accumulation and burial of whale carcasses with high sedimentation rates, and subsequent reaction of carbon decomposed by benthic and microbial activity with seawater.

Numerous carbonate concretion samples were collected during dredging around Nagoya Port in the 1960s. Most of the samples contained biological remains, including crabs, sea urchins, and bivalves, which formed their nuclei. In this study, the radiocarbon (¹⁴C) ages of the shells of the biological remains inside the concretions, as well as of the concretions themselves, were determined. Based on the metabolic carbon ratios of the bivalvia and sea urchin shells, which were calculated using δ¹³C values, the ¹⁴C ages of the shells and those of the metabolic carbon were estimated to be 7,350-7,050 cal BP and 9,680-9,430 cal BP, respectively. The ¹⁴C ages of the carbonate concretions were older than those of the shells due to the addition of metabolic carbon with older ages to the carbon of the concretion. The corrected age after removal of the old carbon was estimated to be 7,530-7,270 cal BP. The near-identical corrected ages of the shells and concretions indicate that the carbonate concretions in the Nagoya Port area formed rapidly after the death of the organisms, which is consistent with morphological evidence of rapid concretion formation in the sediment.

Glendonite is a calcite pseudomorph after ikaite and is found in sedimentary rocks worldwide. Glendonite concretions are spherical to ellipsoidal carbonate concretions containing glendonite at their center. Glendonites and glendonite concretions have been widely used as indicators of low temperatures because ikaite, a precursor of glendonite, is stable only in low-temperature environments. The process that forms glendonite concretions means that they are also potential indicators of the chemical environments of sediment porewaters. However, the physicochemical conditions required for the formation of ikaite in sediments remain poorly constrained. As the conditions of formation of both ikaite in modern sediments and glendonite in sedimentary rocks are clarified, the use of glendonite and glendonite concretions as environmental indicators will increase.

There is an amazing array of concretions throughout the sedimentary record of Earth, and now iron oxide examples (“blueberries”) recognized in several regions of Mars. Two questions address the current state of knowledge on nodular cemented mineral masses as well as coloration patterns. Collectively the authigenic cements and patterns chronicle past diagenetic conditions, particularly in clastic rocks.

What do we know? Concretions are cemented mineral masses that occur in many sizes, spanning three orders of magnitude (mm, cm, and m scales). Spheroidal forms are most common, as an minimum free stage dominated by diffusive processes. There are multiple cement mineralogies, sometimes even within single concretions, reflecting different water compositions in open systems. Other concretionary geometries are affected by primary textures such as bedding, grain size, and porosity/permeability, or later textures such as fractures, joints, and faults. Iron cycling is readily apparent where visual coloration patterns indicate histories of early iron reddening, secondary bleaching (removal of iron), and iron replacement or reprecipitation. Interfingering colors may indicate a possible interface of immiscible fluids.

What are the remaining challenges? There are many aspects of concretion diagenesis that are still yet to be deciphered. Non-unique pathways or processes may produce similar-looking end products. Thus, it can be difficult to determine exact histories, as well as the fluid compositions and environmental conditions that initiate concretion formation, particularly if an obvious nucleus is lacking. Microbial life may enhance nucleation and precipitation, and geochemical gradients are potential places to search for biosignatures. Timing and events are mostly relative relationships in these open systems, but newer developments in U-Th/He dating may provide age constraints for iron oxide cements. Continued explorations, field studies, modeling approaches, analytical advances, and instrument precision will enlighten our understanding on the diagenetic histories of both Earth and Mars.

Spherical Fe-oxide concretions have been recognized in both terrestrial and Martian strata and are thought to record a variety of past environmental and alteration conditions. This paper presents a comprehensive review of the occurrence and elemental composition of spherical Fe-oxide concretions on Earth and Mars and their proposed mechanisms of formation. On the basis of geological evidence from Utah and Mongolia, Fe-oxide concretions are considered to form by pH-buffering reactions between Fe-rich acidic water and precursor calcite concretions. By comparing the characteristics of hematite spherules in Meridiani Planum and spherical nodules in Gale Crater, we propose that Martian concretions may also have been formed by interaction between pre-existing carbonates and sulfuric acidic water that infiltrated the rocks early in Martian history. The abundant hematite spherules in Meridiani Planum and spherical nodules in Gale Crater can be considered as relicts of the widespread deposition of carbonate that occurred during the late Noachian-early Hesperian (c. 3.8 to 3.7 Ga) and its dissolution during the late Hesperian (c. 3.5 to 3.2 Ga).

Fe oxide Liesegang bands have often been observed in sedimentary and igneous rocks, and they are formed during weathering and alteration by water-rock interactions. In this study, micro-X-ray fluorescence (μ-XRF) mapping was used to study the Fe bands in an Fe oxide concretion from the Jurassic Navajo Sandstone in Utah, USA, to estimate the duration of their formation. Most of the peaks in Fe concentration are steeper on the inner side than on the outer side, which indicates a supply of ferrous ions (Fe²⁺) from outside the concretion. The precipitation of Fe oxide was controlled by pH buffering that resulted from a reaction between acidic water and alkaline pore water that formed through the dissolution of an earlier calcite concretion. The reaction rate within the Fe oxide concretion was estimated from the width of the Fe peaks and the expected diffusion coefficient for Fe through the rock matrix, and it was found to be no more than years to decades-faster than previously estimated. This demonstrates that μ-XRF mapping is a useful technique to extract quantitative information about water-rock interactions from rocks.

Spherical Fe- oxide concretions are found in the Navajo Sandstone Formation in Utah, USA, which range in size from a few millimeters to a few centimeters. The interior of the concretions is filled with sand and the surface is covered with an Fe oxide rind. The process that formed the Fe rinds, the timescales of formation, and the effect of pores in the concretions and the dissolved oxygen concentration on the rind formation were studied experimentally.

Fe oxide concretions were artificially prepared in a petri dish. An experimental system was established to capture images of the forming Fe rind (brown band). The chemical distribution was investigated by X-ray fluorescence analysis. The results suggest the following two points about the formation of natural Fe oxide concretions. (1) In the early stages of Fe rind formation, there is a rapid increase in the width of the rind. The Fe rind grows rapidly there are many pores in the concretion. (2) Fe oxide concretions with clear rinds were formed at lower dissolved oxygen concentrations.

Spherical iron-oxide concretions are found in the Jurassic Navajo Sandstone in Utah, USA. Based on image analysis of more than a thousand concretions, we found that: (1) the spatial distribution resembles a random distribution, with deviation from the random distribution occurring as concretion clusters; (2) the spacing between concretions is much larger than the concretion sizes; (3) the width of the size distribution increases with increasing concretion size; and (4) the volume fraction of concretions is constant, irrespective of size. These characteristics can be explained by repeating cycles of CaCO₃ dissolution and precipitation, the precursor material of Fe-oxide concretions. In numerical simulations, the width of the size distribution did not depend on the input parameters, including pH and supersaturation.

We investigated the hydraulic and mechanical properties of spherical calcium carbonate concretions and their surrounding matrices in a Neogene accretionary deposit in the Hayama Group (Kanagawa prefecture) and a brackish to marine formation in the Mizunami Group. The concretions in both formations have lower permeabilities and porosities and greater hardness than the surrounding host rocks. Although the concretions in both formations were formed in different depositional environments, they hardened a short time after the death of a living organism. This means that they can be considered stable over long periods and would not be significantly affected by subsequent deformation and tectonic events. This property is useful for engineering applications, as it would yield a long-lasting seal.

This paper introduces two early-stage engineering applications inspired by concretion formation, a naturally occurring elemental and molecular concentration process. First, the precipitation of iron oxide from highly acidic wastewater using cement clinker and its applications to channel control were investigated. Blocking of channels by iron oxide precipitating around the clinker was confirmed using a column test. Second, the production of “Calcium carbonate concrete” using concrete waste and CO₂ in the air was investigated, based on the fact that calcium carbonate is the binding material in calcium carbonate concretions. Understanding the natural mechanism of concretion formation is not only a fascinating topic from a scientific perspective, it also has potential engineering applications.