有機鉱物の結晶化学と同位体地球化学における最近の展開 : シュウ酸塩鉱物と多環芳香族炭化水素鉱物による鉱物学の新たな進歩への貢献 Recent advances in crystal chemistry and isotope geochemistry of organic minerals : Contribution of oxalate and polycyclic aromatic hydrocarbon minerals to new developments in mineralogy

  Organic minerals are natural organic compounds with both well-defined chemical composition and crystallographic properties; their occurrences show traces of the high concentration of certain organic compounds in natural environments. Thus the origin and formation process of organic minerals will lead us to understand the fate and behavior of the organic molecules in the lithosphere. This review divides organic minerals into the following two groups to discuss their contribution to new developments in mineralogy: ionic organic minerals in which organic anions and various cations are held together by ionic bonds, and molecular organic minerals in which electroneutral organic molecules are bonded by weak intermolecular interactions. This review is composed of five sections. The first section is concerned with the definition of both organic minerals and the above two groups. The second deals with crystal chemistry and geochemistry of oxalate minerals, which are the most typical ionic organic ones. In this section, the role of (H2O)0 is firstly discussed since most oxalate minerals incorporate (H2O)0 into their crystal structures. Then the phase relationships among hydrous/anhydrous calcium oxalate minerals, namely their structural hierarchy are described, owing to the fact that they are the most abundant ionic organic minerals. In addition, the weak Jahn-Teller effect in humboldtine [Fe2+(C2O4)·2H2O] is mentioned. Fe2+ ion causes octahedral distortion in the organic mineral though the effect has hardly been observed in inorganic minerals. The third describes the origin and formation process of karpatite (C24H12), which is the molecular crystal of coronene and the most typical molecular organic mineral. The fourth and last sections summarize the characteristics of organic minerals and discuss their contribution to earth science, respectively.