Through the 4.6 Gyr of Earth’s history, ore deposit has been formed via close linkage with the evolution of this planet. Ore deposit is an abnormally enriched material of targeted elements and its enrichment mechanism macroscopically controlled by “Geology” and microscopically controlled by “Geochemistry.” “Front line of economic geology” is the third collaborative special issue by two Japanese academic journals of Chikyukagaku (Geochemistry) and Shigen-Chishitsu, aiming to review the metallogenesis, recent progresses and unsolved questions in each type of ore deposit for future researches on economic geology and geochemistry. This collaborative special volume is wrapped up by this issue 3.

Chromitites (chromite ores) are reviewed for their importance in magmatism, hydrothermalism, geodynamics and production of resources. The key process for chromitite production is possibly the formation of magmas oversaturated with chromite (or chromian spinel).Production of relatively silica-rich magma during the formation of dunite envelope is coupled with the production of the podiform chromitite. The origin of stratiform chromitites is challenged based on a new idea on the origin of chromite-hosted mineral inclusions, commonly found in all types of chromitites. The chromite-hosted melt inclusions, now characterized by the assemblage pargasite+aspidolite+orthopyroxene, are possibly formed during reaction melting of orthopyroxene within the mantle. Chromite grains in the crustal stratiform chromitites, at least in part, are produced in the mantle stage and transported upward to the magma chamber. Origin of ultrahigh-pressure chromitites has been highly debated, but is still enigmatic. Chromitites of hydrothermal origin have been found and may contribute to redistribution of Cr both in the mantle and in the crust. Production of chromitite ores in Japan is also reviewed. Chromitites serve as a good petrologic marker in both mantle and crustal rocks, and will greatly contribute to our understanding of deep-seated structure and mantle processes in the mantle drilling.

Radiocesium-bearing microparticles (CsMPs) were emitted from the damaged reactors to the environment during the Fukushima Daiichi Nuclear Power Plant accident. This review summarizes current knowledge with respect to the inner structures and physicochemical characteristics of CsMPs. CsMPs are composed of micron-sized spherical silicate glass with Na, Cl, K, Fe, Zn, Rb, Sn, and Cs as major constituent elements. Besides, CsMPs frequently contain nanocrystals such as Cr-rich oxides and chalcogenides. Thermal and dissolution properties of CsMPs have also been elucidated recently. Radiocesium is released from CsMPs to the atmosphere if they were heated at a high temperature. The aqueous dissolution rate of CsMPs depends on pH and dissolved species in solutions, which is similar to that of silica glass. Estimation of the amount and spatial distribution of CsMPs in the environment is also progressing. These findings will lead to the comprehensive elucidation of the dynamics of CsMPs in the environment.